I i    Mv-   PR 4 tumt
LOCOMOTIVE ENGINE-HINKLEY & DRURY, BUILDERS, BOSTON, MASS.




A
SCHOOL COMPENDIUM
OF
NATURAL AND EXPERIMENTAL
PHILOSOPHY,
EMBRACING THE ELEMENTARY PRINCIPLES OF
MECHANICS, HYDROSTATICS, HYDRAULICS, PNEUMATICS, ACOUSTICS, PYRONOMICS, OPTICS, ELECTRICITY, GALVANISM, MAGNETISM, ELECTROMAGNETISM, MAGNETO-ELECTRICITY, AND ASTRONOMY.
WITH A DESCRIPTION OF THE
STEAM AND LOCOMOTIVE ENGINES.
BY
RICHARD  GREEN  PARKER, A.M.
PRINCIPAL OF THE JOHNSON GRAMMAR SCIIOOL, BOSTON; AUTHOR OF AIDS
TO ENGLISH COMPOSITION, OUTLINES OF GENERAL HISTORY,
ETC., ETC., ETC.
"'Delectando pariterque monendo,
Prodesse quam conspici."
THIRTIETH EDITION, WITH ADDITIONS AND IMPROVEMENTS.
NEW YORK:
PUBLISHED BY A. S. BARNES & CO.,
NO. 51 JOHN-STREET.
CINCINNATI:-H. W. DERBY & CO.
1850.




AT a meeting of the School Committee of the City of Boston, May 8,
1838,
Voted, That Parker's Compendium of Natural and Experimental Philosophy be adopted and used as a text-book in our public schools.
Attest,
S. F. MCCLEARY, Secretary.
TO THE
HON. SAMUEL ATKINS ELIOT,
MAYOR OF THE CITY OF BOSTON, AND CHAIRMAN OF THE SCHOOL
COMMITTEE.
SIR,
The Public Schools of this city are under many obligations to you, for
the interest you have taken in them, and for your disinterested exertions
for their improvement. This volume, designed to supply a want which
they have long felt, affords an opportunity of acknowledging the obligation, which I gladly embrace. The gratification which I feel in seeing
you at the head of our municipal institutions, I beg leave to express in
borrowed language:Tibi ut gratuler non est in animo; sed contra, hanc occasionem, mihi
sic oblatam, nostram civitatem gratulandi, reniti non possum. Qum omnia
solita tua benevolentia ut accipias qumeso.
I am, Sir, very respectfully,
Your obedient Servant,
RICHARD GREEN PARKER
Enitered, according to Act of Congress, in the year 1848.
BY A. S. BARNES & Co.,
In the Clerk's Office of the District Court for the Southern District of
New York.




PRE FACE.
Taz School Committee of the city of Boston having recently furnished
the Grammar Schools with apparatus for exemplifying the principles of
Natural Philosophy, the author of this work, who, for twenty years, has been
at the head of one of these large establishments, and has felt the want of
an elementary treatise unencumbered with extraneous matter, has been
induced to attempt to supply the deficiency.  If he is not deceived in the
result of his labors, the work will commend itself to notice by the following features:
1. It is adapted to the present state of natural science; embraces a
wider field, and contains a greater amount of information on the respective
subjects of which it treats, than any other elementary treatise of its size.
2. It contains engravings of the Boston School set of philosophical
apparatus; a description of the instruments, and an account of many experiments which can be performed by means of the apparatus.
3. It is enriched by a representation and a description of the Locomotire and the Stationary Steam Engines, in their latest and most approved
forms.
4. Besides embracing a copious account of the principles of Electricity
and Magnetism, its value is enhanced by the introduction of the science
of Pyronomics, together with the new sciences Electro-Magnetism and
Magneto-Electricity.
5. It is peculiarly adapted to the convenience of study and of recitation,
by the figures and diagrams being first placed side by side with the Illustratiols, and then repeated on separate leaves at the end of the volume.
rhe number is also given, where each principle may be found, to which
allusion is made throughout the volume.
6. It presents the most important principles of science in a larger type;
while the deductions from these principles, and the illustrations, are contained in a smaller letter  Much useful and interesting matter is also




6                           PREFACE.
crowded into notes at the bottom of the page. By this arrangement, the
pupil can never be at a loss to distinguish the parts of a lesson which are
of primary importance; nor will he be in danger of mistaking theory and
conjecture for fact.
7. It contains a number of original illustrations, which the author has
found more intelligible to young students, than those with which he has
met elsewhere.
8. Nothing has been omitted which is usually contained in an elementary treatise.
A work of this kind, from its very nature, admits but little originality.
The whole circle of the sciences consists of principles deduced from the
discoveries of different individuals, in different ages, thrown into common
stock. The whole, then, is common property, and belongs exclusively to
no one. The merit, therefore, of an elementary treatise on natural science
must rest solely on the judiciousness of its selections. In many of the
works from which extracts have been taken for this volume, the author has
found the same language and expressions without the usual marks of quotation. Being at a loss, therefore, whom to credit for some of the expressions which he has borrowed, he subjoins a list of the works to which he
is indebted, with this general acknowledgment; in the hope that it may
be said of him as it was once said of the Mantuan Bard, that "he has
adorned his thefts, and polished the diamonds which he has stolen."
It remains to be stated, that the Questions at the bottom of the page,
throughout the volume, were not written by the author, but were prepared
by another hand.
R. G P
BOSTON, March, 1848.




ADVERTISEMENT
TO THE SEVENTEENTH EDITION
TEN years have elapsed since this work first appeared
in permanent form from the hands of the stereotyper.
During this time, the author has been gratified to learn
that sixteen editions have been called for by the public; but this gratification has been mingled with regret that he has been unable from time to time to make
such improvements as he knew were needed, and which
the progress of science, as well as a more extended experience, seemed imperiously to demand. IHe gladly
avails himself of the present opportunity, afforded by
the new publishers into whose hands it has fallen, to
make such improvements as in his opinion will render
it more worthy of the liberal patronage it has received;
for although it is a long time since the author has had
any pecuniary interest in the work, he hopes that it is
not true that he has had "no further solicitude."
The necessity of a revision of the work at this time
will appear from the following statement. By a vote
of the School Committee of Boston in 1836, a certain
portion of philosophical apparatus was introduced into
the Grammar Schools, as an experiment. The apparatus was designed to unite economy with simplicity, and
was confined to the departments of Pneumatics and
Electricity. To this apparatus the book was specially
adapted, though not wholly confined. By a recent vote
of the Committee, (August, 1847,) apparatus of superior




8                     ADVERTISEMENT
construction, and embracing a much wider field, was
substituted for the cheap and defective sets that were at
first introduced. It becomes necessary, therefore, in a
volume prepare/iwith special reference to the wants of
the Boston schools, to have regard to the construction
and the character of the instruments by which the principles of physical science are illustrated in these large
establishments; the author has therefore deemed it expedient to make such a revision of the whole work, as
will not only render it a convenient manual to accompany the new apparatus, but also embrace the recent
improvements and discoveries by which the branches of
science of which it treats have been enriched. A schedule of the new apparatus is subjoined; and the author
indulges the expectation that the present edition of this
work will be found more worthy than its predecessors
of the favor which the public have bestowed upon it.
LILAC LODGE,
DEDHAM, October, 1847.
LIST OF WORKS
WHICH HAVE BEEN CONSULTED, OR FROM WIIICII EXTRACTS HAVE BEEN
TAKEN, IN THE PREPARATION OF THIS VOLUME.
Annals of Philosophy; Arnott's Elements of Physics; Bigelow's Technology; Cambridge Physics; Chambers' Dictionary; Enfield's, Olmsted's,
Blair's, Bakewell's, Draper's, GrundY, Jones', Comstock's, and Conversations on Natural Philosophy; Davis' Manual of Magnetism; Encyclopedia Americana; Franklin's Philosophical Papers; Henry's Chemistry;
King's Manual of Electricity; Lardner on the Steam Engine; Library of
"Useful Knowledge; Paxton's Introduction to the Study of Anatomy; Pambour on Locomotive Engines on Railways; Peschel's Elements of Physics;
Philips' Astronomy; Sir John Herschel's Astronomy; Silliman's Journal
of Science; Singer's Electricity; Scientific Class Book; Scientific Dialogues; Smith's Explanatory Key; The Year Book; Turner's Chemistry;
Wilkins' Astronomy; Worcester's and the American School Geography;
Lathtop, McIntire, and Keith on the Globes




SCHEDULE OF PHILOSOPHICAL APPARATUS FOR THE BOSTON
GRAMMAR SCIIOOLS.*
Adopted by the School Committee, August, 1847.
LAWS OF MATTER.
Apparatus for illustrating Inertia.
Pair of Lead Hemispheres, for Cohesion.
Pair of Glass Plates, for Capillary Attraction.
LAWS OF MOTION.
Ivory Balls on Stand for Collision.
Set of eight Illustrations for Centre of Gravity
Sliding Frame for Composition of Forces.
Apparatus for illustrating Central Forces.
MECHANICS.
Complete set of Mechanicals, consisting of Pulleys; Wheel and Axle;
Capstan; Screw; Inclined Plane; Wedge.
HYDROSTATICS.
Bent Glass Tube for Fluid Level.
Mounted Spirit Level.
Hydrometer and Jar, for Specific Gravity.
Scales and Weights, for Specific Gravity.
Hydrostatic Bellows, and Paradox.
HYDRAULICS.
Lifting, or Common Water-pump.
Forcing Pump; illustrating the Fire-engine.
Glass Syphon-cup; for illustrating intermitting Springs.
Glass and Metal Syphons.
PNEUMATICS.
Patent Lever Air-pump and Clamp.
Three Glass Bell Receivers, adapted to the Apparatus.
Condensing and Exhausting Syringe.
Copper Chamber for Condensed Air Fountain.
Revolving Jet and Glass Barrel.
Fountain Glass, Cock, and Jet for Vacuum.
Brass Magdeburg Hemispheres.
* The cost of this apparatus is about two hundred and sixty dollars. It was
made by Mr. Joseph M. Wightman, No. 33 Cornhill, Boston, and in an eminent
degree unites beauty with dlrability.




to       SCHEDULE OF PFILOSOPHICAL APPARATUS.
Improved Weight-lifter, for upward pressure.
Iron Weight of fifty-six pounds and strap, for Weight-lifter
Flexible Tube and Connectors,
Brass Plate and Sliding Rod.
Bolt Head and Jar.
Tall Jar and Balloon.
Hand and Bladder Glasses.
Wood Cylinder and Plate.
India-rubber Bag, for expansion of air
Guinea and Feather Apparatus.
Glass Flask and Stop-cock, for weighing air.
ELECTRICITY.
Plate Electrical Machine.
Pith-ball Electrometer.
Electrical Battery of four Jars.
Electrical Discharger.
Image Plates and Figure.
Insulated Stool.
Chime of Bells.
Miser's Plate, for shocks.
Tissue Figure, Ball and Point.
Electrical Flyer and Tellurian.
Electrical Sportsman, Jar and Birds.
Mahogany Thunder-house and Pistol.
Hydrogen Gas Generator.
Chains, Balls of Pith, and Amalgam.
OPTICS.
Glass Prism, and pair of Lenses.
Dissected Eyeball, showing its arrangement
MAGNETISM.
Magnetic Needle on Stand.
Pair of Magnetic Swans.
Glass Vase for Magnetic Swans.
Horseshoe Magnet.
ASTRONOMY.
Improved School Orrery.
Tellurian, or Season Machine.
ARITHMETIC AND GEOMETRY.
Set of thirteen Geometrical Figures of Solids.
Box of sixty-four one-inch Cubes, for Cube Root, &c.
AUXILIARIES.
Tin Oiler; Glass Funnel; Sulphuric Acid.
Set of Iron Weights for Hydrostatic Paradox.




CONTENTS.
CHAPTER I.
Page
Divisions of the subject.............................................    17
CHAPTER II.
Of Matter...............................................        18
CHAPTER III.
Of Gravity...............................................    25
CHAPTER IV.
Mechanics, or the Laws of Motion.......................................    30
CHAPTER V.
Hydrostatics...............................................  73
CHAPTER VI.
Hydraulics.....................................................................  90
CHAPTER VII.
Pneumatics................................................................... 97
CHAPTER VIII
AcouStics8....................................................................  124
CHAPTER IX.
Pyronomics..............................................          132




12                       CONTENTS.
CHAPTER X.                        Page
Optics...........................................................................  150
CHAPTER XI.
Electricity....................................................................  192
CHAPTER XII.
Galvanism, or Voltaic Electricity........................ 216
CHAPTER XIII.
Magnetism and Electro-Magnetism........................  230
CHAPTER XIV.
Astronomy.................................................................... 257




INT RODUCTION.
THE term Philosophy literally signifies, the love of wisdom;
but, as a general term it is used to denote an explanation of the
reason of things, or an investigation of the causes of all phenomena, both of mind and of matter.
When,tpplied to any particular department of knowledge,
the word Philosophy implies the collection of general laws or
principles, under which the subordinate facts or. phenomena
relating to that subject are comprehended. Thus, that branch
of Philosophy which treats of God, his attributes and perfections, is called Theology;* ihat which treats of the material
world is called Physics, or Natural Philosophy; that which
treats of man as a rational being, is called Ethics, or Moral
Philosophy; and that which treats of the mind is called Intellectual Philosophy, or Metaphysics.t
* The word Theology is derived from two Greek words, the former
of which (e0os) signifies God, and the latter (Xoyos) means a discourse;
and these two words combined in the term Theology, literally imply
a discourse about God. The latter of these two Greek words (:oyoS
or logos) is changed into logy to form English compounds, and it enters
into the composition of many scientific terms. Thus, we have the words
mineralogy, the science of minerals; meteorology, the science which
treats of meteors; ichthyology, the science of fishes; entomology, the
science of insects; lithology, of stones; conchology, of shells, &c.
t The word Metaphysics is composed of two Greek words, Meta, (or
fpra,) which signifies beyond, and phuisis, (or Ovacs,) which signifies nature,
and in composition these words imply something beyond nature. From




1  INTRODUCTION.
All material things are divided into two great classes, called
organized and unorganized matter. Organized matter is that
which is endowed with organs adapted to the discharge of appropriate functions, such as the mouth and stomach of animals,
or the leaves of vegetables. By means of such organs they
enjoy life. Unorganized matter, on the contrary, possesses no
such organs, and is consequently incapable of life and voluntary action.
Physical Science, or Physics, with its subdivisions of Natural
History, (including Zoology, Botany, Mineralogy, Conchology,
Entomology, Ichthyology, &c.,) and Natural Philosophy, including its own appropriate subdivisions, embraces the whole field
of organized and unorganized matter.
The term Natural Philosophy is considered by some authors
as embracing the whole extent of physical science, while others
use it in a more restricted sense, including only the general
properties of unorganized matter, the forces which act upon
it, the laws which it obeys, the results of those laws, and all
those external changes which leave the substance unaffected.
It is in this sense that the term is employed in this work.
Chemistry, on the contrary, is the science which investigates
the composition of material substances, the internal changes
which they undergo, and the new properties which they acthe latter of these words, phusis, (Ovab,,) we obtain the term physics, which
in its most extended sense implies the science of nature and natural objects,
comprehending the study or knowledge of whatever exists. The natural
division of all things that exist is into body and mind-things material and
immaterial, spiritual and corporeal. Physics relates to material things —
Metaphysics to immaterial. Man, as a mere animal, is included in the
science of Physics; but, as a being possessed of a soul, of intellect, of
powers of perception, consciousness, volition, reason, and judgment, he becomes a subject of consideration in the science of Metaphysics.




INTRODUCTION.                    15
quire by such changes. The operations of Chemistry may be
described under the heads of, Analysis or decomposition, and
Synthesis or combination.
Natural Philosophy may be said, pre-eminently, to treat of
motion, while Chemistry particularly relates to change or alteration. By the former we become acquainted with the condition and relations of bodies as they spontaneously arise
without any agency of our own. The latter teaches us how to
alter the natural arrangement of elements to bring about some
particular condition that we desire. To accomplish these objects in both of the departments of science to which we refer,
we make use of appliances called philosophical and chemical
apparatus, the proper use of which it is the office of Natural
Philosophy and Chemistry respectively to explain. All philosophical knowledge proceeds either from observation or experiment, or from both. It is a matter of observation that water,
by cold, is converted into ice; but if, by means of freezing
mixtures, or evaporation, we actually cause water to freeze,
we arrive at the same knowledge by experiment.
By repeated observations, and by calculations based on such
observations, we discover certain uniform modes in which the
powers of nature act. These uniform modes of operation are
called laws;-and these laws are general or particular according to the extent of the subjects which they respectively embrace. Thus, it is a general law that all bodies attract each
other in proportion to the quantity of matter which they contain. It is a particular law of electricity that similar kinds
repel, and dissimilar kinds attract each other.
The collection, combination, and proper arrangement of such
general and particular laws, constitute what is called Science.
Thus, we have the science of Chemistry, the science of Geometry, the science of Natural Philosophy, &c.




16                  INTRODUCTION.
The terms, art and science, have not always been employed
with proper discrimination. In general, an art is that which
depends on practice or performance, while science is the examination of general laws, or of abstract and speculative principles. The theory of music is a science; the practice of it is
an art.
Science differs from art in the same manner that knowledge differs from skill. An artist may enchant us with his skill,
although he is ignorant of all scientific principles. A man of
science may excite our admiration by the extent of his knowledge, though he have not the least skill to perform any operation of art. When we speak of the mechanic arts, we mean
the practice of those vocations in which tools, instruments, and
machinery are employed. But the science of mechanics explains the principles on which tools and machines are constructed, and the effects which they produce. Science, therefore,
may be defined, a collection and proper arrangement of the
general principles or leading truths relating to any subject;
and there is this connection between art and science, namely" A principle in science is a rule of art."




NATURAL PHILOSOPHY
CHAPTER I.
DIVISIONS OF THE SUBJECT.
1. NATURAL PHILOSOPHY is the science which treats
of the powers and properties of natural bodies, their
mutual action on one another, and the laws and operations of the material world.
2. Some of the principal branches of Natural Philosophy are, Mechanics, Pneumatics, Hydrostatics, Hydraulics, Acoustics, Pyronomics, Optics, Astronomy,
Electricity, Galvanism,* Magnetism, Electro-Magnetism,
and Magneto-Electricity.
1. Mechanics is that branch of Natural Philosophy which
relates to motion and the moving powers, their nature and
laws, with their effects in machines, &c.
2. Pneumatics treats of the nature, properties, and effects
of air.
* It may perhaps be questioned whether the subjects of Galvanism,
Electro-Magnetism, and Magneto-Electricity, do not more properly fall
within the province of Chemistry, as they describe effects dependent on
chemical action. As this volume is designed for the diffusion of useful
information, a strict adherence to rigid classification has not been deemed
so important, as to exclude the notice of subjects so intimately connected
with one of the most interesting branches of Natural Philosophy.
1. What is Natural Philosophy?
2. What are the principal branches of Natural Philosophy? What is
Mechanlics? Of what does Pneumatics treat?




18                  NATURAL PHILOSOPHY.
3. Hydrostatics treats of the nature, gravity, and pressure
of fluids.
4. Hydraulics treats of the motion of fluids, particularly of
water; and the construction of all kinds of instruments and
machines for moving them.
5. Acoustics treats of the nature and laws of sound.
6. Pyronomics treats of heat, the laws by which it is governed, and the effects which it produces.
7. Optics treats of light, of color, and of vision, or sight.
8. Astronomy treats of the heavenly bodies, such as the sun,
moon, stars, comets, planets, &c.
9. Electricity treats of thunder and lightning, and the causes
by which they are produced, both naturally and artificially.
10. Galvanism is a branch of Electricity.
11. Magnetism treats of the properties and effects of the
magnet, or loadstone.
12. Electro-Magnetism treats of Magnetism induced by
Electricity.
13. Magneto-Electricity treats of Electricity induced by
Magnetism.
CHAPTER II.
OF MATTER* AND ITS PROPERTIES.
3. Matter is the general name of every thing that
occupies space, or has figure, form, or extension.
* The ancient philosophers supposed that all material substances were
composed of Fire, Air, Earth, and Water, and these four substances were
called the four elements, because they were supposed to be the simple
substances of which all things were composed. Modern science has
proved that not one of these is a simple substance, but that there are at
least fifty-five simple substances, thirty-two of which are metallic and
twenty-four non-metallic. The consideration of those substances which
enter into the composition of all matter, in whatever form, belongs to
Of what does Hydrostatics treat? Hydraulics? Acoustics? Pyronomics?
Optics?  Astronomy?  Electricity? Of what is Galvanism  a branch?
Of what does Magnetism treat?  Electro-Magnetism?  Magneto-Electricity?
3. What is Matter?




MATTER AND ITS PROPERTIES.                     I9
4. There are seven essential properties belonging to
all matter, namely:  1. Impenetrability, 2. Extension,
3. Figure, 4. Divisibility, 5. Indestructibility, 6. Inertia,
and 7. Attraction.
1. These are called essential properties, because no particle
of matter can be deprived of them, or exist without them.
2. There are certain other properties existing in different
bodies, called accidental properties, because they do not necessarily exist in the bodies themselves, but depend upon their
connection with other bodies. Thus, color and weight are accidental properties, because they do not necessarily exist in the
bodies that possess them, but depend upon their connection
with other things.
3. There are also certain terms used to express the state in
which matter exists, such as Porosity, Density, Rarity, Coinpressibility, Expansibility, Mobility, Elasticity, Brittleness, Malleability, Ductility, and Tenacity.
5. Impenetrability is the power of occupying a cer
tain space, so that where one body is, another cannot
be, without displacing it.
1. Impenetrability belongs to fluids as well as solid bodies.
The reason why fluids appear less impenetrable than solid
bodies, is, that the particles of which they are composed move
the science of Chemistry. Bodies which consist of one simple substance
are called homogeneous, while those which consist of two or more sirrmple substances are called heterogeneous. Thus, water is a heterogeneous
substance, being composed of two simple or homogeneous aeriform fluids,
called Hydrogen and Oxygen. An aeriform fluid is a fluid in the form of
air. When the particles of which matter is composed is mentioned, it is
to be understood that the smallest imaginable pordion is meant, not of the
homogeneous substances of which it may be composed, but of the matter
itt elf, whether homogeneous or heterogeneous.
4. How many essential properties of matter are there? What are they?
Why are they called essential properties? What other properties exist in
different bodies? Why are they called accidental properties? Are color
and weight essential or accidental properties? Why? What terms are
used in Philosophy to express the state in which matter exists?
5. What is meant by Ilnpcnetrability? Does impenetrability belong to
fluids?  Why do fluids appear less impenetrable than solid bodies?  What
is supposed to be the form of the particles of fluids?




2)0                 NATURAL PHILOSOPHY.
easily among themselves, on account of their slight degree of
cohesion.*
2. Put some water into a tube closed at one end; and then
insert a piece of wood that fits closely the inside of the tube.
It will be impossible to force the wood to the bottom of
the tube, unless the water be first removed. The same experiment may be made with air instead of water; and proves
that water, air, and all other fluids are equally solid, or impenetrable, with the hardest bodies.
3. The impenetrability of water was shown by an experiment made at Florence many years ago. A hollow globe of
gold was filled with water, and submitted to great pressure.
The water was seen to exude through the pores of the gold,
and covered it with a fine dew.
4. When an open vial, not inverted, is plunged into a basin
of water, the air will rush out in bubbles, to make room for
the water; and if an inverted tumbler or goblet be immersed
in water, the water will not rise far in the tumbler unless it be
inclined so that the air can escape. These are further proofs
of the impenetrability of air.
5. When a nail is driven into wood, or any other substance,
it forces the particles asunder, and makes its way between
them; and the wood is not increased in size by the addition
of the nail, because wood is a porous substance, the particles
of which may be compressed, and thus make way for the
nail.
* It is a well-known fact, that a certain quantity of salt, the particles
of which are supposed to be smaller than those of water, can be put into
a vessel full of water, without causing it to overflow; and as the particles
of which sugar is composed are smaller than those of salt, a portion of
sugar may be added after the fluid is saturated with salt. This may be
accounted for by supposing that the particles of fluids
are round, and therefore touch one another only in a  Fig. 1.
few points. There will be spaces between the particles in the same manner that there are between large
balls which are piled on one another. Between these
spaces other smaller balls may be placed; and these
smaller balls, having spaces between them, will admit others still smaller, as may be seen in Fig. 1.
What follows from this  What figure illustrates this? What example
can you give to prove the impenetrability of water? What of the air?
What of solids?




MATTER AND ITS PROPERTIES.                    21
6. Extension is but another name for bulk, or size;
and it is expressed by the terms length, breadth, width,
height, depth, and thickness.*
7. Figure is the form or shape of a body. Two circles or two balls may be of the same shape or figure,
while they differ in extension. The limits of extension
constitute figure.
8. Divisibility is susceptibility of being divided. To
the divisibility of matter there is no known limit.
1. A single grain of gold may be hammered by a goldbeater until it will cover fifty square inches; each square inch
may be divided into two hundred strips; and each strip into
two hundred parts. One of these parts is only one two-millionth part of a grain of gold, and yet it may be seen with the
naked eye.
2. The particles which escape from odoriferous objects also
afford instances of extreme divisibility.
9. By the Indestructibility of matter is meant that it
cannot be destroyed. It may be indefinitely divided,
or altered in its form, color, and accidental properties,
but it must still continue to exist in some form through
all its changes of external appearance.
1. When water disappears, either by boiling over a fire, or
* Length is the extent from end to end. Breadth or width is the extent from side to side. Height, depth, or thickness, is the extent from the
top to the bottom. The measure of a body from the bottom to the top is
called height; from the top to the bottom is called depth. Thus we speak
of the depth of a well, the height of a house, &c.
6. What is meant by Extension? What terms are used to express the
size of a body?  What is length?  Breadth?  Height, depth, or thickness?  What is the difference between height and depth?
7. What is meant by Figure? May bodies be of the same shape or
figure and of different dimensions? Give an example. What constitutes
figure?
8. What is meant by Divisibility? Is there any known limit to the
divisibility of matter? Mention some examples of the extreme divisibility.
of matter.
9. What is meant by the Indestructibility of matter? May it be
changed in form and in external appearance?  Give examples of such
changes.




22                NATURAL PHILOSOPHY.
evaporating by the heat of the sun, or, in other words, when
" it dries up," it rises slowly in the form of steam or vapor.
This vapor ascends in the air and constitutes clouds; these
clouds again fall to the earth in the shape of rain, snow, or
hail, and form springs, fountains, rivers, &c. The water on or
in the earth, therefore, is constantly changing its shape or
situation, but no particle of it is ever actually destroyed.
2. The simple substances of which fuel is composed are not
destroyed when the fuel is burnt. Parts of them arise in smoke
or vapor, and the remainder is reduced to ashes. A body in
burning undergoes remarkable changes; but the various parts
into which it has been separated by combustion, continue in
existence, and retain all the essential properties of bodies.
10. Inertia is the resistance which inactive matter
makes to a change of state, whether of motion or rest.
A body at rest cannot put itself in motion, nor can
a body in motion stop itself.
A body, when put in motion, will continue to move forever, unless it be stopped. When a stone or ball is thrown
from the hand, there are two forces which continually operate
to stop it, namely, the resistance of the air, and gravitation:
all motion which is caused by animal or
mechanical power, will be destroyed by         Fig. 2.
the combined action of these forces. But
could these forces be suspended, the
body in motion would continue to move
forever.
Fig. 2 represents the simple apparatus
of Mr. Wightman, for illustrating inertia.  A  ball and a card being placed
upon the pillar, motion is given to the
card by means of a spring, but the ball
remains on the pillar.
11. Attraction is the tendency which different bodies
or portions of matter have, to approach or to adhere to
each other.
10. What is meant by Inertia? Can a body at rest put itself in motion? Can a body in motion stop itself? When a stone or ball is thrown
from the hand, how many forces continually operate to stop it? What
are they?  How could a body in motion be made to move forever?  Ex,
plain the apparatus for illustratinga  inertia.




MATTER AND ITS PROPERTIES.                    23
Every portion of matter is attracted by every other
portion of matter, and this attraction increases as the
quantity of matter is increased, and diminishes as the
quantity of matter is diminished.
1-2. There are two kinds of attraction belonging to
all matter, namely, the attraction of gravitation and the
attraction of cohesion.
The attraction of gravitation is the tendency of different bodies to approach each other.
The attraction of cohesion is that which causes the
particles of a body to cohere to each other.
By the attraction of gravity, a stone falls to the
ground. By the attraction of cohesion, the particles
which compose the stone are held together.
1. All matter is composed of very minute particles, which
are connected together in different bodies by different degrees
of cohesion.*
2. The particles of which bodies are composed absolutely
touch one another in few points only. There are small spaces
called porest between the particles; and the proportion of
these pores gives rise to the terms density and rarity. A body
in which the pores are small and few in number, is called a
dense body. When the pores are large and numerous, the
body is said to be rare.
Density, therefore, implies the closeness and compactness of the
* Cohesive attraction is illustrated by means of a pair of leaden hem
ispheres, which being pressed together, will be found to cohere. The experiment may be made with equal success with two bullets scraped smooth
at the points of contact.
t The pores of bodies are generally filled with air.
11. What is attraction? Is every portion of matter attracted by every
other portion of matter?  How does this attraction increase and diminish?
12. How many kindi of attraction are there belonging to all matter?
What is the attraction of gravitation, or gravity? What is the attraction
of cohesion, or cohesive attraction? What causes a stone to fall to the
ground? By what are the particles which compose the stone held together? Of what is matter composed? Is the cohesive power which
unites them the same in all bodies? How may cohesive attraction be illustrated?  Do the particles of matter in bodies absolutely touch each
other? What are the spaces between them called?




24                 NATURAL PHILOSOPHY.
particles of a body, and indicates the quantity of matter contained
in it under a given bulk.
Rarity is the reverse of density, and implies extension of bulk,
without increase of quantity of matter.
13. Compressibility implies the reduction of the limits
of extension. Of this all substances are susceptible if a
sufficient force be applied.*
14. Expansibility  is the reverse of compressibility,
and implies the increase of the limits of extension.
15. Mobility implies susceptibility of motion.
16. Elasticity is the property which causes a body to
resume its shape after being compressed or expanded.
Thus, when a bow is bent, its elasticity causes it to resume
its shape. India-rubber possesses this property in a remarkable degree, but the gases in a still greater. The elasticity of
ivory is very perfect, that is to say, it restores itself after compression with a force very nearly equal to that exerted in
compressing it. Liquids, on the contrary,- have scarcely any
elasticity.
17. Malleability implies susceptibility of extension under the hammer or the rolling-press. This property belongs to some of the metals, as gold, silver, iron, copper,
&c., but not to all; and it is of vast importance to the
arts and conveniences of life. Gold is the most malleable of all metals.
18. Brittleness is the reverse of malleability, and implies aptness to break into irregular fragments. This
property belongs chiefly to hard bodies.
* Sir Isaac Newton conjectured, that if the earth were so compressed
as to be absolutely without pores, its dimensions might not be more than a
cubic inch.
What do you understand by density? What by rarity?
13. What is compressibility? Are all substances susceptible of it?
14. What is expansibility?
15. What is mobility?
16. What is elasticity? What substance possesses this property in a
remarkable degree?
17. What is malleability? Does this property belong to all the metals?
What metal possesses it in the highest degree?
18. What is brittleness?  What bodies are most brittle?




OF GRAVITY.                      25
19. Ductility is that property which renders a substance susceptible of being drawn into wire.
Platina is the most ductile of all metals. It can be
drawn into wire scarcely larger than a spider's web.
20 Tenacity implies a great degree of adhesion among
the particles of bodies. The tenacity of bodies constitutes their strength, or their capability of sustaining
weight. Iron, on account of its fibrous structure,is very
tenacious.
CHAPTER III.
OF GRAVITY.
21. The term Gravity, in Philosophy, expresses the
reciprocal attraction of separate portions of matter.
All matter possesses this attraction, and all bodies
attract each other with a force proportionate to their
size and density, when at a given distance from each
other.
A body unsupported falls to the earth.  This is caused by
the superior attraction of the earth, arising from its density
and size.
22. The attraction of gravitation causes weight.
1. When we say that a body weighs an ounce, a pound, or
a hundred pounds, we express, by these terms, the degree of
attraction by which it is drawn towards the earth.
2. Weight, therefore, is the measure of the earth's attraction.
As this attraction depends upon the quantity of matter there
is in a body, it follows that those bodies which contain the most
nmatter will be most strongly attracted, and will consequently
be the heaviest.
19. What is ductility? Which is the most ductile of the metals?
20. What is tenacity? Which is the most tenacious of the metals?
21. What do you understand by the term gravity? Do all bodies possess this attraction? To what is its force proportional? If a body be uLs
sulpported, will it remain stationary? Why will it fall?
22. What causes weight? When you say that a body weighs an Sunee
what do you mean by it? What, therefore, is weight?
2




i26                 NATURAL PHILOSOPHY.
23. The force of gravity is greatest at the surface of
the earth, and decreases both upwards and downwards,
but in different degrees.*
It decreases above the surface as the square of the
distance from  the centre increases.  From  the surface
to  the centre it decreases, simply as the distance increases. That is, gravity at the surface of the earth
(which is about 4000 miles from  the centre) is four times
more powerful than it would be at double that distance,
or 8000 miles from the centre.
According to the principles just stated, a body which at
the surface of the earth weighs a pound, at the centre of the
earth will weigh nothing.
1000 miles from the centre it will weigh ~ of a pound,
2000    "    "    "    "       "       " ~    of a pound,
3000    "    "    "    "       "       "   -3 of a pound,
4000    "          "     "     "   "       1"  pound,
8000    "    "    "    "       "       "    1
12000   "    "                          I"  "  "   -, &c.t
* The force of gravity is absolutely greatest at the centre of the earth;
tnut at that point it is exerted in all directions, and consequently a body at
that point would remain stationary, because there is no superior attraction
for it to obey.
t It follows from what has been stated, with regard to weight as a consequence of attraction, that if there were but one body in the universe, it
would have no weight, because there would be nothing to attract it. But
cohesive attraction would still exist, and keep the particles which compose
the body united. As the attraction between all bodies is mutual, it follows
that when a stone or any heavy body falls to the earth, the earth will rise
to meet it. But as the attraction is in proportion to the quantity of matter
each contains, the stone will fall as much farther than the earth rises, as
the earth exceeds the stone in mass. Now the earth is one quatrillion,
that is, one thousand million millions times larger than the largest body
which has ever been known to fall through our atmosphere. Supposing,
then, that such a body should fall through a distance of 1000 feet-the
earth would rise no more than the hundred billionth part of an inch, a dis.
tance altogether imperceptible to our senses. The principle of mutual attraction is not confined to the earth. It extends to the sun, the planets,
comets, and stars. The earth attracts each of them, and each of them
23. Where is the force of gravity greatest? How does it change  I How
does it decrease above the surface of the earth? IIow below?




OF GRAVITI.                          27
24. The direction in which falling bodies approach
the surface of the earth, is called a vertical line.*  Such
lines are everywhere perpendicular to the surface, and
when prolonged will meet nearly at the centre of the
earth.
For this reason no two lines suspended
by weights, will be parallel to each other.           Fig. 3.
Even a pair of scales, hanging perpendicular
to the earth, are not exactly parallel, because
they  point to the same spot, namely, the
centre of the earth; but the convergency is             V
so small, that their inclination is not perceptible to our senses.  [See Fig. 3.]  For the
same reason no two bodies can fall to the
earth in parallel lines.
25. According to the laws of attraction, all bodies at
an equal distance from the earth will fall to it in the
same space of time, if nothing impede them. But bodies
of different density fall with different degrees of velocity,
on account of the resistance of the air; and as heavy
bodies overcome this resistance more easily than light
ones, the former will fall with the greater velocity.
The resistance which the air opposes to the fall
of bodies, is proportioned  to their surface, not to their
weight.
attracts the earth, and these mutual attractions are so nicely balanced by
the power of God, as to cause the regular motions of all the heavenly
bodies, the diversity of the seasons, the succession of day and night, summer and winter, and all the grand operations which are described in astronomy.
* A vertical line is sometimes called a plumb-line, because it is formed
by a weight suspended at rest from a string. As the weight thus employed is usually of lead, the term plumb, from the Latin plumbum, lead, is
applied to the line.
24. In what direction will a falling body approach the surface of the
earth? Will the lines of suspension of different bodies ever be parallel?
Where will they meet, if sufficiently produced?
25. Will all bodies at equal distances from the earth, fall to it in the
same time? Why not? What bodies falls fastest? To what is the re
sistance of the air proportional?




28                  N NATURAL PHILOSOPHY.
Heavy bodies can be made to float  in the air, instead of
falling immediately to the ground, by making the extent of
their surface counterbalance their weight. Thus gold, which
is one of the heaviest of all substances, when spread out into
thin leaf, is not attracted by gravity with sufficient force to
overcome the resistance of the air; it therefore floats in the
air, or falls slowly.
26. All substances are influenced by gravity, in exact
proportion to their quantity of matter, and their distance
from the central point of attraction.
1. Even air itself, light as it seems, is subject to this attrac
tion. The air* probably extends to a height of more than
forty-five miles above the surface of the earth. The pressure
of the upper parts of the atmosphere on those beneath, renders the air near the surface of the earth much more dense
than that in the upper regions. This pressure is caused by the
attraction of the earth, or, what is the same thing, the weight
or tne air above; and it would cause the air to fall like other
bodies completely to the earth, were it not for the elasticity of
that portion which is near the surface.
2. The air therefore, of which the atmosphere is composed,
exists in a state of compression, which causes it to be heaviest
near the surface of the earth.
* We have no means of ascertaining the exact height to which the air
extends. Sir John Herschel says, " Laying out of consideration all nice
questions as to the probable existence of a definite limit to the atmosphere,
beyond which there is absolutely and rigorously speaking no air, it is clear,
that for all practical purposes we may speak of those regions which are more
distant above the earth's surface than the hundredth part of its diameter as
void of air, and of course, of clouds, (which are nothing but visible vapors,
diffused and floating in the air, sustained by it, and rendering it turbid, as
mud does water.) It seems probable, from many indications, that the greatest height at which visible clouds ever exist does not exceed ten miles; at
which height the density of the air is about an eighth part of what it is at
the level of the sea." Although the exact height to which the atmosphere
extends has never been ascertained, it ceases to reflect the sun's rays at a
greater height than forty-five miles.
26. In what proportion are all substances influenced by gravity? Is air
affected by it? How far does the air extend above the surface of the
earth? What causes the air to be more dense at the surface of the earth?
What causes this pressure? Why does not the air fall to the earth like
other bodies? Where is the air heaviest? What effect have gravity and
elasticity upon the air?




OF GRAVITY.                         29
27. The specific gravity of bodies is a term used to
xpress the relative weight of equal bulks of different
r)odies.*
1. If we take equal bulks of lead, wood, cork, and air, we
find the lead to be the heaviest, then the wood, then the cork,
and lastly the air. Hence we say that the specific gravity of
cork is greater than that of air, the specific gravity of wood is
greater than that of cork, and the specific gravity of lead
greater than that of wood, &c.
2. From what has now been said with respect to the attraction of gravitation and the specific gravity of bodies, it appears
that although the earth attracts all substances, yet this very
attraction causes some bodies to rise and others to fall.
3. Those bodies or substances, the specific gravity of which
is greater than that of air, will fall, and those whose specific
gravity is less than that of air, will rise; or rather, the air
being more strongly attracted will get beneath them, and, thus
displacing them, will cause them to rise. For the same reason,
cork and other light substances will not sink in water, because
the specific gravity of water being greater, the water is more
strongly attracted, and will be drawn down beneath them.
[For a table of the specific gravity of bodies, see Hydrostatics.]
4. The principle which causes balloons to rise, is the same
which occasions the ascent of smoke, steam, &c. The materials of which a balloon is made, are heavier than air, but their
extension is greatly increased, and they are filled with an elastic fluid of a different nature, specifically lighter than air, so that
on the whole, the balloon when thus filled is much lighter than
* The quantity of matter in a body is estimated, not by its apparent
size, but by its weight. Some bodies, as cork, feathers, &c., are termed
light; others, as lead, gold, mercury, &c., are called heavy. The reason
of this is, that the particles which compose the former are not closely
packed together, and therefore they occupy considerable space; while in
the latter they are joined more closely together, and occupy but little
room. A pound of cork and a pound of lead, therefore, will differ very
much in apparent size, while they are both equally attracted by gravity,
that is, they weigh the same.
27. What is specific gravity? Illustrate this. Does the attraction of
the earth cause all bodies to fall?  What bodies will fall?  What rise?
How does the air cause them to rise? Why do not cork and other light
bodies sink in water? Explain the principle upon which balloons rise




30                NATURAL PHILOSOPHY.
a portion of air of the same size or dimensions, and it will consequently rise.
5. Gravity, therefore, causes bodies which are lighter than
air to ascend, those which are of equal weight with air to remain stationary, and those which are heavier than air to descend; but the rapidity of their descent is affected by the
resistance of the air: which resistance is proportioned to the
extent of the surface of the falling body.
CHAPTER IV.
MECHANICS, OR THE LAWS OF MOTION.
28. Mechanics is that branch of Natural Philosophy
which relates to motion and the moving powers, their
nlature and laws, with their effects in machines, &c.
29. Motion is a continued change of place.
On account of the inertia of matter, a body cannot put itself in motion, nor when it is in motion can it stop itself.
30. The power which puts a body into motion is
called a force; and the power which has a tendency to
stop or impede motion is called resistance.
31. The motion of a body impelled by a single force
is always in a straight line, and in the same direction in
which the force acts.
32. The rapidity with which a body moves is called
its velocity.
What effect has gravity on bodies lighter than the air'? What effect
on bodies of equal weight? What effect on those that are heavier? What
affects the rapidity of their descent? To what is the resistance of the air
proportioned?
28. What is Mechanics?
29. What is motion? Why cannot a body put itself in motion? Why
cannot a body stop itself when in motion?
30. What is force? What is resistance?
31. When is the motion of a body in a straight line? In what direction
will it move?
32. What is meart by velocity?




MECHANICS.                          31
33. The velocity of a given body is proportional to
the force by which it is put in motion.*
34. The velocity of a moving body s determined by
the time that it occupies in passing through a given
space. The greater the space, and the shorter the time,
the greater is the velocity.
Thus, if one body move at the rate of six miles, and another
twelve miles in the same time, the velocity of the latter is
double that of the former.f
35. The velocity of a body is measured by the space
over which it moves, divided by the time which it employs in the motion.
Thus, if a body move one hundred miles in twenty hours,
the velocity is one hundred divided by twenty, that is, five
miles an hour.
* The mean velocity of rivers is about four feet in a second; of a very
rapid stream, about 13 feet; of a moderate wind, about 10 feet; of a
storm, 54 feet; of a violent hurricane, 125 feet; of sound, 1142 feet; of
atmospheric air rushing into a vacuum, 1280 feet; of a musket-ball, 1280
feet; of a rifle-ball, 1600; of a cannon-ball of 24 pounds, 2400 feet; of a
point at the surface of the earth, under the equator, 1500 feet; of the
earth's centre in its orbit round the sun, 101,061 feet.
The average rate of steamers between New York and Albany, exclusive of stoppages, is ten and a half milesper hour; of the mail trains on the
great railroads, about 25 miles an hour; of the fastest sailing vessel, 15
feet in a second; of the swiftest race-horse, 42 feet in a second.
t Velocity is sometimes called absolute, and sometimes relative. Velocity is called absolute when the motion of a body in space is considered
without reference to that of other bodies. When, for instance, a horse goes
a hundred miles in ten hours, his absolute velocity is ten miles an hour.
Velocity is called relative when it is compared with that of another body.
Thus, if one horse travel only fifty miles in ten hours, and another one
hundred in the same time, the absolute velocity of the first horse is five
miles an hour, and that of the latter is ten miles; but their relative velocity
is five miles.
33. To what is the velocity of a moving body proportional?
34. How is the velocity of a moving body determined? If one body go
through six miles in an hour, and another twelve, how does the velocity
of the latter compare with that of the former? What is meant by absolute velocity? Give an example. When is the velocity of a body termed
relative? Give an example.
35. How is the velocity of a body measured? Illustrate this.




32                NATURAL PHILOSOPHY.
36. The time employed by a body in motion may be
ascertained by dividing the space by the velocity.
Thus, if the space be one hundred miles, and the velocity
five miles in an hour, the time will be one hundred divided by
five, which is twenty hours.
37. The space also may be ascertained by multiplying the velocity by the time.
Thus, if the velocity be five miles an hour, and the time
twenty hours, the space will be twenty multiplied by five,
which is one hundred miles.
38. There are three terms applied to motion to express its kind; namely, uniform, accelerated, and retarded motion.
Uniform motion is that of a body passing over equa.
spaces in equal times.
Accelerated motion is that in which the velocity continually increases as the body moves.
Retarded motion is that in which the velocity decreases as the body moves.
39. Uniform motion is produced by a force having
acted on a body, and then ceasing to act
A ball struck by a bat, or a stone thrown from the hand, is
in theory an instance of uniform motion; and if both the at
traction of gravity and the resistance of the air could be entirely removed, it would proceed onwards in a straight line,
and with a uniform motion forever. But as the resistance of
the air and gravity tend to deflect it, it in fact becomes an instance first of retarded and then of accelerated motion.
40. Accelerated motion is produced by the continued
action of one or more forces.
36. How do you ascertain the time employed by a body in motion? Illustrate this.
37. How can you ascertain the space? Illustrate this.
38. How many terms are applied to motion to express its kind? What
are they? What is uniform motion? Accelerated? Retarded?
39. How is uniform motion produced? Why is not a ball struck by a
bat, or a stone thrown from the hand, an ikhstance of uniform motion?
How can it be made an instance?
40. How is accelerated motion produced?




MECHANICS.                         33
1. Thus, when a stone falls from a height, the impulse which
it receives from gravity would be sufficient to bring it to the
ground with a uniform velocity. But the stone while falling
at this rate is still acted upon by gravity with an additional
force, which continues to impel it during the whole time of its
descent.
2. In the first second it falls sixteen feet, three times that distance
in the next, five times in the third, seven times in the fourth, and
so on, regularly increasing its velocity according to the number
of seconds consumed in falling.
3. The height of a building, or the depth of a well, may thus
be measured by observing the length of time which a stone
takes in falling from the top to the bottom.*
41. Retarded motion is produced when a body in
motion encounters a force operating in an opposite direction.
1. Thus, when a stone is thrown perpendicularly upwards,
the force of gravity is continually operating in the opposite
direction, and attracting it downwards to the earth. The stone
moves upwards slower and slower, until the upward motion
ceases, and the body returns with accelerated motion to the
earth. It is found that a body thrown perpendicularly upwards, takes the same length of time in ascending that it takes
in descending.
2. Perpetual motion has never yet been produced by art;
and the principles of mechanics seem to prove that such a
motion is impossible; for although in many cases of bodies
acting upon one another, there is a gain of absolute motion,
yet the gain is always equal in opposite directions, so that the
* The spaces through which a body falls in equal successive portions of
time, increase as the odd numbers 1, 3, 5, 7, &c.; that is, a falling body
descends in the 2d second of its fall through 3 times, and in the 3d second
through 5 times the space passed over in the first second. But the entire
spaces through which a body will have fallen in any given number of sec.
oends, increase as the squares of the times.
Give an instance of accelerated motion. How far does a stone fall the
first second of time? The second? Third? Fourth? How can you
measure the height of a building, or the depth of a well?
41. How is retarded motion produced? Give an example. How does
the time of the ascent of a body thrown perpendicularly upwards, compare
with that of its descent? Why cannot perpetual motion be produced'
2*




34                 NATURAL PHILOSOPHY.
quantity of direct motion is never increased. But nature
abounds with examples of perpetual motion, as for instance,
the motion of the heavenly bodies, described in the science of
astronomy.
42. The momentum of a body is its quantity of motion, or the force with which it would strike against
another body. It, is measured by multiplying its weight
by its velocity.*
Thus, if a body weighing six pounds move at the rate of
two miles in a second of time, its momentum may be represented by six multiplied by two, which is twelve. Hence a
small or a light body may be made to strike against another
body with a greater force than a heavy one, simply by giving
it sufficient velocity.
43. The action of a body is the effect which it produces upon other bodies. Reaction is the effect which
it receives from the body on which it acts.
Thus, when a body in motion strikes against another body,
it acts upon it, or produces action; but it also meets with resistance from the body which is struck, and this resistance is
the reaction of the body.
44. Action and reaction are always equal, but in opposite directions.
1. Experiments to show the mutual action and reaction of
bodies, are made with both elastic and non-elastic bodies. Fig. 4
* The quantity of motion communicated to a body does not affect the
duration of the motion. If but little motion be communicated, the body
will move slowly. If a great degree be imparted, it will move rapidly.
But in both cases the motion will continue until it is destroyed by some
external force.
42. What is the momentum of a body? How can the momentum of a
body be ascertained? Note. Does the quantity of motion communicated
to a body affect the duration of the motion? If but little motion is communicated, how will the body move? If a great degree?  How long will
the motion continue? How can a light body be made to have a greater
momentum than a heavy one? Give an instance of this.
43. What is meant by action? Reaction? Illustrate this.
44 How do action and reaction compare?




MECHANICS.                       35
represents two ivory balls, A and B, of equal     Fig. 4.
weight, &c., suspended by threads. If the ball
A be drawn a little on one side and then let
go, it will strike against the other ball B, and
drive it off to a distance equal to that through   A  T
which the first ball fell; but the motion of A
Vrill be stopped, because when it strikes B it
receives in return a blow equal to that which it gave, but in a
contrary direction, and its motion is thereby stopped, or rather,
given to B. Therefore, when a body strikes against another,
the quantity of motion communicated to the second body is
lost by the first; but this loss proceeds, not from the blow
given by the striking body, but from the reaction of the body
which it struck.
2. Fig. 5 represents six ivory balls, of equal weight, suspended by threads. If the ball A be drawn out of the perpendicular, and let fall against B, it will communicate its
motion to B, and receive a reaction from it
which will stop its own motion.  But the ball     Fig. 5.
B cannot move without moving C; it will therefore communicate the motion which it received
from  A to C, and receive from  C a reaction
which will stop its motion.  In like manner the
motion and reaction are received by each of the  A  BCDE F
balls, D, E, F; but as there is no ball beyond F
to act upon it, F will fly off.
N. B. This experiment can be accurately performed by those
bodies only which are perfectly elastic.
3. Fig. 6 represents two balls of clay, (which are not
elastic,) of equal weight, suspended by strings.  If
the ball D be raised and let fall against E, only   Fig. 6.
part of the motion of D will be destroyed by it,  eAA
(because the bodies are non-elastic,) and the two
balls will move on together to d and e, which are
less distant from  the vertical line than the ball D    d 
was before it fell. Still, however, action and reaction are equal, for the action on E is only enough to make it
move through a smaller space, but so much of D's motion is
now also destroved.*
* Figs. 4 and 5, as has been explained, show the effect of action and
reaction in elastic bod.es, and Fig. 6 shows the same effect in non-elastic
Explain Fig. 4. Fig. 5  Fig. 6.




36                  NATURAL PHILOSOPHY.
4. It is upon the principle of action and reaction, that birds
are enabled to fly. They strike the air with their wings, and
the reaction of the air enables them to rise, fall, or remain
stationary at will, by increasing or diminishing the force of
the stroke of their wings.*
5. It is likewise upon the same principle of action and reaction, that fishes swim, or, rather, make their way through
the water; namely, by striking the water with their fins.t
6. Boats are also propelled by oars on the same principle,
and the oars are lifted out of the water, after every stroke, so
as completely to prevent any reaction in a backward direction.
45. Motion may be caused either by action or reaction. When caused by action it is called incident, and
when caused by reaction it is called reflected motion.4
bodies. When the elasticity of a body is imperfect, an intermediate effect
will be produced; that is, the ball which is struck iill rise higher than in
case of non-elastic bodies, and less so than in that of perfectly elastic
bodies; and the striking ball will be retarded more than in the former case,
but not stopped completely, as in the latter. They will, therefore, both
move onwards after the blow, but not together, or to the same distance;
but in this, as in the preceding cases, the whole quantity of motion destroyed in the striking ball, will be equal to that produced in the ball struck.
Connected with " the Boston school apparatus" is a stand with ivory balls,
to give a visible illustration of the effects of collision.
* Thq muscular power of birds is much greater in proportion to their
weight than that of man. If a man were furnished with wings sufficiently
large to enable him to fly, he would not have sufficient strength, or muscular power, to put them in motion.
t The power possessed by fishes, of sinking or rising in the water, is
greatly assisted by a peculiar apparatus furnished them by nature, called
an air-bladder, by the expansion or contraction of which they rise or fall,
on the principle of- specific gravity.
t The word incident implies falling upon, or directed towards. The
word reflected implies turned back. Incident motion is motion directed
towards any particular object, against which a moving body strikes. Reflected motion is that which is caused by the reaction of the body which
is struck. Thus, when a ball is thrown against a surface, it rebounds or
Upon what principle do birds fly? Explain how. Upon what principle
do fishes swim?  Upon what principle do boats move upon the water?
Explain how.
45. How may motion be caused? When caused by action what is it
called? When caused by reaction what is it termed?




MECHANICS.                           37
46. The angle* of incidence is the angle formed by
the line which  the incident body makes in its passage
towards any object, with  a line perpendicular to the surface of the ob-                   Fig. 7.   A
ject.
Thus, in Fig. 7, the line A B C repre-   p.
sents a wall, and P B a line perpendicular
to its surface.  0 is a ball moving in the   R       "
direction of the dotted line, 0 B. The angle 0 B P is the angle of incidence.
is turned back. This return of the ball is called reflected motion. As reflected motion is caused by reaction, and reaction is caused by elasticity,
it follows, that reflected motion is always greatest in those bodies which
are most elastic. For this reason, a ball filled with air rebounds better
than one stuffed with bran or wool, because its elasticity is greater. For
the same reason, balls made of caoutchouc, or India-rubber, will rebound
more than those which are made of most other substances.
* As this book may fall into the hands of some who are unacquainted
with geometrical figures; a few explanations are here subjoined.
1. An angle is the opening made by two lines which meet each other in a
point. The size of the angle depends upon the opening, and not upon the
length of the lines.
2. A circle is a perfectly round figure, ev-     Fig. 8.
ery part of the outer edge of which, called the      R
circumference, is equally distant from a point                   T
within, called the centre. (See Fig. 8.)
3. The straight lines drawn from the centre to the circumference are called radii.                    A
[The singular number of this word, is radius.]
Thus, in Fig. 8, the lines CD, CO, CR, and
CA, are radii.
4. The lines drawn through the centre,
and terminating in both ends at the circumference, are called diameters.
Thus, in the same figure, D A is a diameter of the circle.
5. The circumference of all circles is divided into 360 equal parts, called
degrees. The diameter of a circle divides the circumference into two,
equal parts of 180 degrees each.
46. What is the angle of incidence?  (Note.-1. What is an angle?
Upon what does the size of an angle depend? 2. What is a circle?
3. What are radii?  What lines in Fig. 8 are radii?  4. What are diameters?  In Fig. 8, what line is the diameter?  5. How is the circuniference of all circles divided? Into how many parts does the diameter of a
circle divide it?




38                  N NTURAL PHILOSOPHY.
47. The angle of reflection is the angle formed by
the perpendicular with the line made by the reflected
body as it leaves the surface against which it struck.
Thus, in Fig. 7, the angle P B R is the angle of reflection.
48. The angles of incidence and reflection are always
equal to one another.
1. Thus, in Fig. 7, the angle of incidence, 0 B P, and the
angle of reflection P B R, are equal to one another; that is,
they contain an equal number of degrees.
6. All angles are measured by the number of degrees which they contain. Thus, in Fig. 8, the angle R C A, as it includes one quarter of the
circle, is an angle of 90 degrees, which is a quarter of 360. And the angles R C O and O C D are angles of 45 degrees.
7. Angles of 90 degrees are right angles; angles of less than 90 degrees,
acute angles, and angles of more than 90 degrees are called obtuse angles.
Thus, in Fig. 8, R C A is a right angle, O C R acute, and O C A an obtuse angle.
8. A perpendicular line is a line which makes an angle of 90 degrees on
each side of any other line or surface; therefore, it will incline neither to
the one side nor to the other. Thus, in Fig. 8, R C is perpendicular to D A.
9. The tangent of a circle is a line which touches the circumference,
without cutting it when lengthened at either end. Thus, in Fig. 8, the
line R T is a tangent.
10. A square is a figure having four equal sides, and four equal angles.
These will always be right angles. (See Fig. 9.)
11. A parallelogram is a figure whose opposite sides are equal and parallel. (See Figs. 10 and 11.) A square is also a parallelogram.
12. A rectangle is a parallelogram whose angles are right angles.
13. The diagonal of a square, of a parallelogram, or a rectangle, is a
line drawn through either of them, and terminating at the opposite angles.
Thus, in Figs. 9, 10, and 11, the line A C is the diagonal of the square,
parallelogram, or rectangle.
6. How are all angles measured? Illustrate this by Fig. 8. 7. How
many degrees do right angles contain? Acute? Obtuse? Illustrate these
angles by Fig. 8. 8. What is a perpendicular line? What line is perpendicular in Fig. 8?  9. What is a tangent?  What line is a tangent in
Fig. 8? 10. What is a square? 11. What is a parallelogram? 12. A
rectangle? 13. What is a diagonal? What lines are diagonals in Figs.
9, 10, and 11?) Explain the angle of incidence by Fig. 7.
47. What is the angle of reflection? Illustrate this by Fig. 7.
48. How do the angles of incidence and reflection compare with each
other? Illustrate this by Fig. 7.




MECHANICS.                        39
2. From what has now been stated with regard to the
angles of incidence and reflection, it follows, that when a ball
is thrown perpendicularly against an object which it cannot
penetrate, it will return in the same direction; but if it be
thrown obliquely, it will return obliquely on the opposite side
of the perpendicular. The more obliquely the ball is thrown,
the more obliquely it will rebound.*
COMPOUND MOTION.
49. Compound motion is caused by the operation of
two or more forces at the same time.
50. When a body is struck by two equal forces in
opposite directions, it will remain at rest.
51. A body struck by two forces in different directions, will move in a line between them. This line will
be the diagonal of a parallelogram, having for its sides
the lines through which the body would pass, if urged
by each of the forces separately.
1. Let Fig. 9 represent a ball struck by the two equal
forces, X and Y. In this figure, the forces are inclined to
each other at an angle of 90 degrees, or          Fig. 9.
a right angle.  Suppose that the force X                 x
would send it from  C to B, and the force   D             C
Y, from C to D.  As it cannot obey both,
it will go between them to A, and the line
C A, through which it passes, represents
the diagonal of the square, A B C D.
The time occupied in its passage from C
* It is from a knowledge of these facts that skill is acquired in many
different sorts of games, as Billiards, Bagatelle, &c. A ball may also, on
the same principle, be thrown from a gun against a fortification, so as to
reach an object out of the range of a direct shot.
What follows from what has been stated with regard to the angles of
incidence and reflection?
49. What is compound motion?
50. In what direction will a body, struck by two equal forces in opposite directions, move?
51. When struck by two forces inclined to each other, how will it move?
What is this line called? Illustrate these, first, by Fig. 9, which represents
a ball struck by two equal forces in different directions.




40                NATURAL PHILOSOPHY.
to A will be the same as the force X would require to send it
to B, or the force Y to send it to D.
2. If the two forces acting on a body are    Fig. 10.
unequal, but still operate at right angles to  D         c
each other, the body will move from C to A
as represented in Fig. 10; in which it is to.
be observed that the force Y is as much
greater than the force X, as the length of   A          B
the side C D of the rectangle A B C D, ex-      Fig. 11.
ceeds the length of the side C B.          D
3. When two forces operate in the direction of an acute angle, (see Fig. 11,) the
body will move as represented by C A, in.la
the parallelogram A B C D.                A 
4. If the forces operate in the direction of an obtuse angle,
the body will move as represented by D B in the same figure.
52. Circular motion is motion around a central point,
and is caused by two forces operating at the same time,
by one of which it is projected forward in a straight
line, while by the other it is deflected towards a fixed
point.
The whirling of a ball, fastened to a string held by the hand,
is an instance of circular motion. The ball is urged by two
forces, of which one is the force of projection, and the other the
string which confines it to the hand. The two forces act at
right angles to each other, and (according to No. 51) the ball
will move in the diagonal of a parallelogram. But, as the force
which confines it to the hand only keeps it within a certain
distance, without drawing it nearer to the hand, the motion of
the ball will be through the diagonals of an indefinite number
of minute parallelograms, formed by every part of the circumference of the circle.
53. There are three different centres which require
to be distinctly noticed; namely, the centre of magnitude, the centre of gravity, and the centre of motion.
Second, by Fig. 10, which represents a ball struck by two unequal
forces, acting at right angles. Third, by Fig. 11, where the forces operate
in the direction of an acute angle. Fourth, by Fig. 11, where the forces
operate in the direction of an obtuse angle.
52. What is circular motion? How is it caused? Illustrate this.
53. How many different centres are there which require to be noticed
Define each of them.




MECHANICS.                           41
The centre of magnitude is the central point of the
bulk of a body.
The centre of gravity is the point about which all
the parts balance each other.
The centre of motion is the point around which all
the parts of a body move.
When the body is not of a size nor shape to allow
every point to revolve in the same plane, the line around
which it revolves is called the axis of motion.*
54. The centre or the axis of motion is generally supposed to be at rest.
Thus the axis of a spinning top is stationary, while every
other part is in motion around it. The axis of motion and the
centre of motion are terms which relate only to circular motion.
55. The two forces by which circular motion is produced, are called central forces. Their names are the
centripetal force and the centrifugal force.t
56. The centripetal force is that which confines a
body to the centre around which it revolves.
The centrifugal force is that which impels the body
to fly off from the centre.
57. If the centrifugal force of a revolving body be
destroyed, the body will immediately approach the centre which attracts it; but if the centripetal force be
* Circles may have a centre of motion; spheres or globes have an axis
of motion. Bodies that have only length and breadth may revolve around
their own centre, or around axes; those that have the three dimensions of
length, breadth, and thickness, must revolve around axes.
4 The word centripetal means seeking the centre, and centrifugal
means flying from the centre. In circular motion, these two forces constantly balance each other; otherwise the revolving body will either approach the centre or recede from it, according as the centripetal or centrifugal force is the stronger.
54. Is the centre or axis of motion supposed to be at rest, or does it
move? To what do the terms centre of motion and axis of motion relate 7?
55. What are the two forces called which produce circular motion?
What is the name of each? What do the words centripetal and centrifugal mean?
56. Define a centripetal force. Also a centrifugal force.
57. If the centrifugal force be destroyed, to what point will the body tend'?




L42               NATURAL PHILOSOPHY.
destroyed, the body will fly off in the direction of a tangent to the curve which it describes in its motion.
Thus, when a mop filled with water is turned swiftly round
by the handle, the threads which compose the head will fly
off from the centre; but being confined to it at one end, they
cannot part from it; while the water they contain, being unconfined, is thrown off in straight lines.
58. The parts of a body which are farthest from the
centre of motion, move with the greatest velocity; and
the velocity.of all the parts diminishes, as their distance
from the axis of motion diminishes.
Fig. 12 represents the vanes of a windmill.  The circles
denote the paths in which the different parts of the vanes
move. M is the centre or axis of motion around which all the parts revolve.        Fig. 12.
The outer part revolves in the circle             G
D EF G, another part revolves in the
circle H  I J K, and the inner part in         \'
the circle LN O    P.  Consequently, as   F J o(          D
they all revolve around M in the same                 /
time, the velocity of the parts which re-'
volve in the outer circle is as much greater than the velocity of the parts which
revolve in the inner circle, L NO P, as the diameter of the
outer circle is greater than the diameter of the inner.
59. As the earth revolves round its axis, it follows,
from the preceding illustration, that the portions of the
earth which move most rapidly are nearest to the equator, and that the nearer any portion of the earth is to
the poles, the slower will be its motion.
60. Curvilinear motion requires the action of two
forces; for, the impulse of one single force always produces motion in a straight line.
What would be its direction if the centripetal force were destroyed?
Give an example.
58. What parts of a body move with the greatest velocity? In what
proportion does the velocity of all the parts diminish? What does Fig. 12
represent?
59. What follows, with regard to the motion of the earth, from the illustration of Fig. 12?
60. Of what is curvilinear motion always the result? Why?




MECHANICS.                          43
61. A  ball thrown  in  a horizontal direction  is influenced by three forces; namely, first, the force of
projection, (which   gives  it a  horizontal  direct,'r;)
second, the resistance of the air through which it passes,
which diminishes its velocity, without changing its direction; and third, the force of gravity, which finally
brings it to the ground.
62. The force of gravity is neither increased nor diminished by the force of projection.*
Fig. 13 represents a cannon, loaded with a ball, and placed
on the top of a tower, at such a height as to require just three
seconds for another ball to descend
perpendicularly.  Now suppose the                 Fig. 13.
cannon to be fired in a horizontal            c
direction, and at the same instant
the other ball to be dropped towards      "li "      —.!l
the ground.  They will both reach          I                  6
the horizontal line at the base of the
tower at the same instant.  In this          a
figure C a represents the perpendicular line of the falling ball.
C b is the curvilinear path of the projected ball, 3 the horizontal
line at the base of the tower. During the first second of time,
the falling ball reaches 1, the next second 2, and at the end
* The action of gravity being always the same, the shape of the curve
of every projectile (see No. 63) depends on the velocity of its motion; but,
whatever this velocity be, the moving body, if thrown horizontally from
the same elevation, will reach the ground at the same instant. Thus, a
ball from a cannon, with a charge sufficient to throw it half a mile, will
reach the ground at the same instant of time that it would had the charge
been sufficient to throw it one, two, or six miles, from the same elevation.
The distance to which a ball will be projected, will depend entirely on the
force with which it is thrown, or on the velocity of its motion. If it moves
slowly, the distance will be short-if more rapidly, the space passed over
in the same time will be greater; but in both cases the descent of the
ball towards the earth, in the same time, will be the same number of feet,
whether it moves fast or slow, or even whether it move forward at all, or
not.
61. How many forces act upon a ball thrown in a horizontal direction?
What are they? Why do bodies fall to the ground?
62. Does the force of gravity either increase or decrease the force of
projection? Give an illustration.




44                 NATURAL PHILOSOPHY.
of the third second it strikes the ground. Meantime, that projected from the cannon, moves forward with such velocity, as
to reach 4 at the same time that the falling ball reaches 1.
But the projected ball falls downward exactly as fast as the
other, since it meets the line 1 4, which is parallel to the horizon, at the same instant.  During the next second the ball
from the cannon reaches 5, while the other falls to 2, both
having an equal descent. During the third second the projected ball will have spent nearly its whole force, and therefore its
downward motion will be greater while the motion forward will
be less than before.  Hence it appears that the horizontal motion
does not interfere with the action of gravity, but that a projectile
descends with the same rapidity while moving forward that it
would if it were acted on by gravity alone.  This is the necessary result of the action of two forces.
63. A projectile is a body thrown into the air, as a
rocket, a ball from a gun, or a stone from the band.
The force of gravity and the resistance of the air
cause projectiles to form a curve both in their ascent
and descent; and in descending, their motion is gradually changed from an oblique towards a perpendicular
direction.
In Fig. 14 the force of projection would carry a ball from A
to D, while gravity would bring it to C. If       Fig. 14
these two forces alone prevailed, the ball  D              A
would proceed in the dotted line to B.
But as the resistance of the air operates in
direct opposition to the force of projection,  /
instead of reaching the ground at B, the  B
ball will fall somewhere about E.*                       C
64. When  a body is thrown in a horizontal direction, or upwards or downwards obliquely, its course will
* It is calculated that the resistance of the air to a cannon-ball of two
pounds weight, with the velocity of two thousand feet in a second, is more
than equivalent to sixty times the weight of the ball.
63. What is a projectile? What lines do projectiles describe? From
what cause? Give the illustration. How great is the resistance of the air
calculated to be to a canaon-ball of two pounds weight, with the velocity
of 2000 feet in a second?




MECHANICS.                          45
be in the direction of a curve-line, called
a parabola;* (see Fig. 15,) but when it is            Fig. 15.
thrown perpendicularly upwards or downwards, it will move perpendicularly, because the force of projection and that of
gravity are in the same line of direction.  A
* The science of gunnery is founded upon the laws relating to proJectiles. The force of gunpowder is accurately ascertained, and calculations
are predicated upon these principles, which enable the engineer to direct his
guns in such a manner as to cause the fall of the shot or shells in the very
spot where he intends. The knowledge of this science saves an immense
expenditure of ammunition, which would otherwise be idly wasted without
producing any effect. In attacks upon towns and fortifications, the skilful
engineer knows the means he has in his power, and can calculate, with
great precision, their effects. It is in this way that the art of war has
been elevated into a science, and much is made to depend upon skill,
which, previous to the knowledge of these principles, depended entirely
upon physical power.
The force with which balls are thrown by gunpowder is measured by an
instrument called the Ballistic pendulum. It consists of a large block of
wood suspended by a rod in the manner of a pendulum. Into this block
the balls are fired, and to it they communicate their own motion. Now
the weight of the block and that of the ball being known, and the motion
or velocity of the block being determined by machinery, or by observation,
the elements are obtained by which the velocity of the ball may be found;
for, the weight of the ball is to the weight of the block as the velocity of
the block is to the velocity of the ball. By this simple apparatus, many
facts relative to the art of gunnery may be ascertained. If the ball be
fired from the same gun, at different distances, it will be seen how much
resistance the atmosphere opposes to its force at such distances. Rifles
and guns of smooth bores may be tested, as well as the various charges of
powder best adapted to different distances and different guns. These, and
a great variety of other experiments, useful to the practical gunner or
sportsman, may be made by this simple means.
The velocity of balls impelled by gunpowder from a musket with a
common charge, has been estimated at about 1650 feet in a second of time,
when first discharged. The utmost velocity that can be given to a cannon-ball, is 2000 feet per second; and this only at the moment of its leaving the gun.
In order to increase the velocity from 1650 to 2000 feet, one half more
64. When a body is thrown horizontally, or upwards or downwards
obliquely, in what curve will it move? In what line will it move when
thrown upwards or downwards obliquely?




46                   NATURAL PtILOSOPHY.
65. The random of a projectile is the horizontal distance from the place whence it is thrown, to the place
where it strikes. The greatest random takes place at an
angle of 45 degrees —that is, when a gun is pointed at
this angle with the horizon, the ball is
thrown to the greatest distance.                      Fig. 16.
Let Fig. 16 represent a gun or a carronade, from which a ball is thrown at an angle of 45 degrees with the horizon.  If the
ball be thrown at any angle above 45 degrees, the random  will be the same as it
would be at the same number of degrees
below 45 degrees.*
66. When the centre of gravity of a body is supported, the body itself will be supported; but when the
centre of gravity is unsupported, the body will fall.t
powder is required; and even then, at a long shot, no advantage is gained;
since, at the distance of 500 yards, the greatest velocity that can be obtained is only 1200 or 1300 feet per second. Great charges of powder are
therefore not only useless, but dangerous; for, though they give little additional force to the ball, they hazard the lives of many by their liability
to burst.
Experiment has also shown, that, although long guns give a greater velocity to the shot than short ones, still, that on the whole, short ones are
preferable; and, accordingly, armed ships are now almost invariably furnished with short guns, called carronades.
The length of sporting guns has also been greatly reduced, of late years.
Formerly, the barrels were from four to six feet in length; but the best
fowling-pieces of the present day have barrels of two feet, or two and a
half, only, in length. Guns of about this length are now universally employed for such game as woodcocks, partridges, grouse, and such birds as
are taken on the wing, with the exceptions of ducks and wild geese, which
require longer and heavier guns.
* A knowledge of this fact, and calculations predicated on it, enables
the engineer so to direct his guns, as to reach the object of attack when
within the range of shot.
t The Boston School Apparatus contains a set of eight illustrations for
65. What is the random of a projectile? At what angle does the greatest random take place?
66. When the centre of gravity of a body is supported, will the body
stand or fall? What if the centre be unsupported?




MECHANICS.                         47
A line drawn from the centre of gravity, perpendicular to the horizon, is called the line of direction.*
67. When the line of direction falls within the baset
of any body, the body will stand; but when that line
falls outside of the base, the body will fall or be overset.
1. Fig. 18 represents a loaded wagon on the declivity of a
hill. The line C F represents a horizontal line,
D E the base of the wagon.  If the wagon be           Fig. 18.
loaded in such a manner that the centre of
gravity be at B, the perpendicular B D will fall
within the base, and the wagon will stand.
But if the load be altered so that the centre of
gravity be raised to A, the perpendicular A C   C             F
will fall outside of the base, and the wagon
will be overset. From this it follows that a wagon, or any
carriage, will be most firmly supported when the line of direction
of the centre of gravity falls exactly between the wheels; and
that is the case on a level road.  The centre of gravity in the
human body, is between the hips, and the base is the feet.
So long as we stand uprightly, the line of direction falls
within this base. When we lean on one side, the centre of
gravity not being supported, we no longer stand firmly.
2. A rope-dancer performs all his feats of agility, by dexterously supporting the centre of gravity. For this purpose
he carries a heavy pole in his hands, which he shifts from side
to side as he alters his position, in order to throw the weight
to the side which is deficient; and thus, by changing the
the purpose of giving a clear idea of the centre of gravity, and showing
the difference between the centre of gravity and the centre of magnitude.
* The line of direction is the line which the centre of gravity would
describe if the body were permitted to fall.
t The base of a body is its lowest side. The base  Fig. 17.
of a body standing on wheels or legs, is represented by lines drawn from the lowest part of one
wheel or leg, to the lowest part of the other wheel
or leg.
Thus, in Figs. 17 and 18, D E represents the base    D     E
of the wagon and of the table.
What is the line of direction?
67. If the line of direction falls within the base, will the body stand or
fall? Give an illustration.




48                  NATURAL PHILOSOPHY.
situation of the centre of gravity, he keeps the line of direction within the base, and he will not fall.*
A spherical body will roll down a slope, because the centre
of gravity is not supported.t
68. When a body is of uniform density, the centre
of gravity is in the same point with the centre of mag
nitude.
When one part of the body is composed of heavier
materials than another part, the centre of gravity (being
the centre of the weight of the body) no longer corresponds with the centre of magnitude. Thus, the centre
of gravity of a cylinder plugged with lead, is not in the
same point as the centre of magnitude.
Bodies, therefore, consisting of but one kind of substance, as
wood, stone, or lead, and whose densities are consequently
uniform, will stand more firmly than bodies composed of a
variety of substances, of different densities.
* The shepherds in the south of France afford an interesting instance
of the application of the art of balancing'to the common business of life.
These men walk on stilts from three to four feet high, and their children,
when quite young, are taught to practise the same art. By means of
these odd additions to the length of the leg, their feet are kept.out of the
water, or the heated sand, and they are also enabled to see their sheep at
a greater distance. They use these stilts with great skill and care, and
run, jump, and even dance on them with great ease.
t A cylinder can be made to roll up a slope, by plugging one side of it
with lead; the body being no longer of a uniform density, the centre of
gravity is removed from the middle of the body to some point in the lead,
as that substance is much heavier than wood. Now, in order that the
cylinder may roll down the plane, as it is here situated, the centre of gravity must rise, which is impossible; the centre of gravity must always descend in moving, and will descend by the nearest and readiest means,
which will be by forcing the cylinder up the slope, until the centre of
gravity is supported, and then it stops.
A body also in the shape of two cones united at their bases, can be
made to roll up an inclined plane formed by two bars with their lower ends
inclined towards each other. This is illustrated by a simple contrivance in
the " Boston School Set," and the fact illustrated is called " the mechanical paradox."
68. If a body is of uniform density, what centres will coincide? Give
an example in which the centres of magnitude and gravity will not coinlcide




MECHANICS.                          49
69, Bodies that have a narrow base are easily overset; for if they are in the least degree inclined, the line
of direction will fall outside of the base, and their centre of gravity will not be supported.*
The broader the base, and the nearer the centre of
gravity to the ground, the stronger will be the edifice.
For this reason a pyramid,t having a broad base and but
little elevation, is the firmest of all structures.
70. When two bodies are fastened together, they are
to be considered as forming but one body, and have but
one centre of gravity. If the two bodies be of equal
weight, the centre of gravity will be in the middle of
the line which unites them. But if one be heavier than
the other, the centre of gravity will be as much nearer
to the heavier one as the heavier exceeds the light one
in weight.
1. Fig. 19 represents a bar with an
equal weight fastened at each end: the             Fig. 19.
centre of gravity is at A, the middle of       W E           w
the  bar, and  whatever supports this
centre will support both the bodies and             Fe. 0.
the pole.
2. Fig. 20 represents a bar with an
unequal weight at each end.  The centre of gravity is at C nearer to the larger
body.
* A person can carry two pails of water more easily than one, because
the pails balance each other, and the centre of gravity remains supported
by the feet. But a single pail throws the centre of gravity on one side,
and renders it more difficult to support the body.
t A cone has also the same degree of stability; but, strictly speaking,
a cone is a pyramid with an infinite number of sides.
69. What shaped bodies are easily overturned? What bodies must stand
more firmly than others?  Why? Why do bodies which have a narrow
base overturn more easily than those which have broad bases? Why can
a person carry two pails of water more easily than one? Why is a pyra.
niod the firmest of all structures?
70. If two bodies of equal weight are fastened together, where is th4
centre of gravity?  If one be heavier than the other? What does Fig.
19 represelnt  Fig. 20? Fig. 21?
3




50                NATURAL PHILOSOPHY.
3. Fig. 21 represents a bar with un-          Fig. 21.
equal weights at each end, but the larger
weight exceeds the less in such a degree        w ---
that the centre of gravity is within the
larger body at C.
RESULTANT MOTION.
71. Resultant motion is the effect or result of two
motions compounded into one.
If two men be sailing in separate boats, in the same direction, and at the same rate, and one toss an apple to the other,
the apple would appear to pass directly across from one to
the other, in a line of direction perpendicular to the side of
each boat.  But its real course is through the air in the
diagonal of a parallelogram, formed by the lines representing
the course of each boat, and perpendiculars drawn to those
lines from  the spot where each man stands as the one tosses
and the other catches the apple.  In Fig. 22
the lines A B and C D represent the course         Fig. 22.
of each boat; E the spot where the man   A
stands who tosses the apple; while the apple.C       l
is in its passage, the boats have passed from    c    E     D
E and G, to H and F respectively.  But the
apple having a motion, with the man, that would carry it from
E to H and likewise a projectile force which would carry it
from E to G, cannot obey them both, but will pass through
the dotted line E F, which is the diagonal of the parallelogram
EG F H.*
* On the principle of resultant motion, if two ships in an engagement
be sailing before the wind, at equal rates, the aim of the gunners will be
exactly as though they both stood still. But if the gunner fire from a ship
standing still, at another under sail, or a sportsman fire at a bird on the
wing, each should take his aim a little forward of the mark, because the
ship and the bird will pass a little forward while the shot is passing to
them.
71. What is resultant motion? Give the examples of this kind of motion.




MECHANICS.                          51
THE PENDULUM.
72. The Pendulum* consists of a weight or ball suspended by a rod, and made to swing backwards and
forwards.
73. The motions of a pendulum are called its vibrations, and they are caused by gravity.t The part of a
circle through which it moves, is called its arc.
74. The vibrations of pendulums of equal length, are
* The pendulum was invented by Galileo, a great astronomer of Florence, in the beginning of the seventeenth century. Perceiving that the
chandeliers suspended from the ceiling of a lofty church vibrated long and
with great uniformity, as they were moved by the wind or by any accidental disturbance, he was led to inquire into the cause of their motion,
and this inquiry led to the invention of the pendulum. From a like apparently insignificant circumstance arose the great discovery of the principle
of gravitation. During the prevalence of the plague, in the year 1665,
Sir Isaac Newton retired into the cofintry to avoid the contagion. Sitting
in his orchard, one day, he observed an apple fall from a tree. His inquisitive mind was immediately led to consider the cause which brought
the apple to the ground, and the result of his inquiry was the discovery
of that grand principle of gravitation, which may be considered as the first
and most important law of material nature. Thus, out of what had been
before the eyes of men, in one shape or another, from the creation of the
world, did these philosophers bring the most important results.
t When a pendulum is raised from its perpendicular position, its weight
will cause it to fall, and, in the act of falling, it acquires a degree of motion which impels it to a height beyond the perpendicular almost as great
as that to which it was raised. Its motion being thus spent, gravity again
acts upon it to bring it to its original perpendicular position, and it again
acquires an impetus in falling which carries it nearly as high on the opposite side. It thus continues to swing backwards and forwards, until the
resistance of the air wholly arrests its motion. In the construction of
clocks, an apparatus connected with the weight or the spring is made to
act on the pendulum with such a force, as to enable it to overcome the
resistance of the air, and keep up a continued motion.
72. Of what does a pendulum consist? By whom was the pendulum
invented?  What led him to the discovery? By whom was the principle
of gravitation d;scovered? What led him to the discovery?
73. What are the movements of the pendulum called? What is meant
by its arc? Whlat causes its vibrations?




52                  NATURAL PHILOSOPHY.
very nearly equal, whether they move through a greater
or less part of their arcs.
In Fig. 23, A B represents a pendulum.             Fig. 23.
D F E C the are in which it vibrates.  If the              A A
pendulum be raised to E it will return to F,
if it be raised to C it will return to D in
nearly the same length of time, because that  D                 c
in proportion as the arc is more extended, the
steeper will be its beginnings and endings,              B
and, therefore, the more rapidly will it fall.
75. The time occupied in the vibration of a pendulum, depends upon its length.  The longer the pendulum, the slower are its vibrations.
76. The length of a pendulum which vibrates sixty
times in a minute (or, in other words, which vibrates
seconds) is about 39 inches; But in different parts of
the earth this length must be varied. A pendulum to
vibrate seconds at the equator must be shorter than one
which vibrates seconds at the poles.*
77. A clock is regulated by lengthening or shortening the pendulum. By lengthening the pendulum, the
clock is made to go slower; by shortening it, it will go
faster.t' The equatorial diameter of the earth exceeds the polar diameter by
about 34 miles; consequently, the poles must be nearer to the centre of
the earth's attraction than the equator, and'gravity must also operate with
greater force at the poles than at the equator. Hence, also, the length of
a pendulum, to vibrate in Any given time, must vary with the latitude of
the place.
t The pendulum of a clock is made longer or shorter, by means of a
74. How do the vibrations of pendulums of equal length compare?
Illustrate by Fig. 23.
75. Upon what does the time of the vibrations of a pendulum depend?
76. What is the length of a pendulum which vibrates sixty times in a
minute? Do different situations affect the vibrations?  How can a pendulum which vibrates seconds at the equator be made to vibrate seconds
at the poles?
77. How is a clock regulated? What effect has the lengthening of the
pendulum? The shortening? What is a clock? Of what use is the
weight? What do the wheels show? Why do clocks go slower in summer than in winter? How does a watch differ from a clock?




MI:CITANICS.                         53
THE MECHANICAL POWERS.
78. The Mechanical Powers are certain contrivances
designed to increase or to diminish force, or to alter its
direction.
There are five things in mechanics which require a
distinct consideration, namely:
First, the power that acts.
Secondly, the resistance which is to be overcome by
the power.
Thirdly, the centre of motion, or, as it is sometimes
called, the fulcrum.*
Fourthly, the respective velocities of the power and
the resistance; and,
Fifthly, the instruments employed in the construction
of the machine.
1. The power that acts is the muscular strength of men, or
screw beneath the weight or ball of the pendulum. The clock itself is
nothing more than a pendulum connected with wheel-work, so as to record the number of vibrations. A weight is attached, in order to counteract the retarding effect of friction, and the resistance of the air. The
wheels show how many swings or beats of the pendulum have taken place
in a given time, because, at every beat, the tooth of a wheel is allowed to
pass. Now if this wheel have sixty teeth, it will turn round once in sixty
vibrations of the pendulum, or in sixty seconds; and a hand, fixed on the
axis of the wheel projecting through the dial-plate, will be the second-hand
of the clock. Other wheels are so connected with the first, and the number of teeth in them is so proportioned, that the second-wheel turns sixty
times slower than the first, and to this is attached the minute-hand; and
the third wheel, moving twelve times slower than the second, carries the
hour-hand. On account of the expansion of the pendulum by heat, and
its contraction by cold, clocks will go slower in summer than in winter,
because the pendulum is thereby lengthened at that season.
A watch differs from a clock, in having a vibrating wheel instead of a
pendulum. This wheel is moved by a spring, called the hair-spring.
The place of the weight is supplied by another larger spring, called the
main-?pring.
* The word fulcrum means a prop, or support.
78. What are the mechanical powers? How many things are to be
considered in order to understand the power of a machine? What is the
first? Second? Third? Fourth? Fifth?




NATURAL PIIILOSOPIIY.
animals, the weight and momentum of solid bodies, the elastic
force of steam, springs, the pressure of the air, &c.
2. The resistance to be overcome is the attraction of gravity,
or of cohesion, the inertness of matter, &c.
3. The centre of motion, or the fulcrum, is the point about
which all the parts of the body move.
4. The velocity is the rapidity with which an effect is produced.
5. The instruments are the mechanical powers which enter
into the construction of the machine.*
79. There  are  six mechanical powers, namely, the
Lever, the Pulley, the WhVteel and Axle, the Inclined
Plane, the Wedge, and the Screw.
80. The Levert is an inflexible bar, moveable on a
fulcrum, or prop.
There are three kinds of levers, called the first, second, and third kinds, according to the respective position
of the fulcrum, the power, and the weight.
81. In a lever of the first kind, the weight is at one end,
the power at the other, and the fulcrum between them.
Fig. 24 represents a lever of the first kind, resting on the
fulcrum  F, and moveable upon it.  W  is the weight to be
moved, and P is the power which
moves it.  The advantage gained in              Fig. 24.
raising a weight by the use of this kind   B   F \ /P
of lever, is in proportion as the distance
of the power from the fulcrum exceeds                       w
that of the weight from  the fulcrum.
Thus, in this figure, if the distance
between P and F be double that be* All machines and instruments are constructed on the principle of
some one or more of the mechanical powers.
t The lever is made in a great variety of forms, and of many different
What is the power that acts? What is the resistance to be overcome?
What is the fulcrum? What is the velocity?
79. EIow many mechanical powers are there? What are they?
80. What is a lever? How many kinds of levers are there? How dc
they differ?
81. What is a lever of the first kind?  What figure illustrates this?
Explain it by the figure. To what is the advantage gained by this lever
proportional?




MECHANICS.                          55
tween W and F, then a man, by the exertion of a force of 100
pounds with the lever, can move a weight of 200 pounds. From
this it follows that the nearer the power is applied to the end of the
lever, the greater is the advantage gained.  Thus, a greater
weight can be moved by the same power, when applied at B,
than when it is exerted at P.*
2. The common steelyard, an instrument for weighing articles, is constructed on the principle of the lever of the first
lkind. It consists of a rod or bar, marked with notches to
designate the pounds and ounces, and a weight which is moveable along the notches. The bar is furnished with three hooks,
materials; and is much used in almost every kind of mechanical operation.
Sometimes it is detached from the fulcrum, but most generally the fulcrum
is a pill or rivet by which the lever is permanently connected with the
framework of other parts of the machinery.
* It is a fundamental principle in mechanics, that what is gained in
power is lost in time. To illustrate this principle, (Fig. 25,) W represents
the weight, F the fulcrum, P the power, and the
bar W F P the lever. To raise the weight W to       Fig. 25.
w, the power P must descend to p. But as the
radius of the circle in which the power P moves
is double that of the radius of the circle in which
the weight W moves, the arc Pp is double the
arc W wto; or, in other words, the distance Pp is
double the distance of V w. Now, as these distances are traversed in the same time by the
power and the weight respectively, it follows,
that the velocity of the power must be double the velocity of the weight;
that is, the power must move at the rate of two feet in a second, in order
to move the weight one foot in the same time.
This principle applies not only to the lever, but to all the mechanical
powers, and to all machines constructed on mechanical principles.
When two weights are equal, and the fulcrum is placed exactly in the
centre of the lever between them, they will mutually balance each other;
or, in other words, the centre of gravity being supported, neither of the
weights will sink. This is the principle of the common scale for weighing.
To gain power by the use of the lever, the fulcrum Ihust be placed near
the weight to be moved, and the power at the greater distance from it.
The force of the lever, therefore, depends on its length, together with
the power applied, and the distance of the weight from the fulcrum.
What follows from this?  What is meant by all inflexible bar?  Note
What is a fundamental principle in Mechanics?  Illustrate this by the
figure. Does this principlo apply to all tlie mechanical powers?




56              NATURAL PHIILOSOPHY.
on the longest of which, the article to be weighed is always to be
hung. The other two hooks serve for the handle of the in-,&~s ~ ~ Fig. 26.
strument when in use. The pivot of each of these two hooks
serves for the fulcrum. When suspended by the hook C, as in
Fig. 26, it is manifest that a pound weight at E will balance as
many pounds at W, as the distance between the pivot of D, and
the pivot of C, is contained in the space between the pivot of
C and the ring from which E is suspended.
E               )5~~~~ ~~~ ~~n
Dt




MECHANICS.                       57
3. The same instrument may be used to weigh heavy articles, by using the middle hook for a handle, where, as will be
seen in the figure, the space between the pivot of F (which in
this case is the fulcrum) and the pivot of D (from which the
weight is suspended) being lessened, is contained a greater
number of times in the distance between the fulcrum and the
notches on the bar. The steelyard is furnished with two sets
of notches on opposite sides of the bar. An equilibrium will
always be produced when the product of the weights on the
opposite sides of the fulcrum into their respective distances
from it, are equal to one another.
4. A balance, or pair of scales, is a lever of the first kind,
with equal arms. Steelyards, scissors, pincers, snuffers, and a
poker used for stirring the
fire, are all levers of the              Fig. 28.
first kind. The longer the..l....
handles of scissors, pincers,                     B   
&c., and the shorter the
points, the more easily are
they used. A compound lever, represented in Fig. 28, consists
of several levers, so arranged that the shorter arm of one may
act on the longer arm of the other. Great power is obtained
in this way; but its exercise is limited to a very small space.
82. In a lever of the second kind, the fulcrum  is at
one end, the power at the other, and the weight between
them.
1. Let Fig. 29 represent a lever of the second kind. F is the
fulcrum, P the power, and W  the weight.          Fig. 29
The advantage gained by a lever of this
kind is in proportion as the distance of the  Prr
power from the fulcrum exceeds that of the               la
weight from the fulcrum.  Thus in this figure, if the distance from P to F, is four times        w
the distance from W to F, then a power of
one pound at P will balance a weight of four pounds at W.
2. On the principle of this kind of lever, two persons carryExplain the common steelyard in Fig. 26. Also in Fig. 27. What is a
balance, or pair of scales? Give some examples of levers of the first kind.
82. What is a lever of the second kind? What figure illustrates this?
To what is the advantage gained by this lever proportional?'Give some
examples of levers of the second kind.
3*




58                NATURAL PHILOSOPHY.
ing a heavy burden, suspended on a bar, may be made to bear
unequal portions of it, by placing it nearer to the one than the
other.
3. Two horses, also, may be made to draw unequal portions
of a load, by dividing the bar attached to the carriage in such
a manner that the weaker horse may draw upon the longer
end of it.
4. Oars, rudders of ships, doors turning on hinges, and
cutting-knives, which are fixed at one end, are constructed upon the principle of levers of the second kind.*
83. In a lever of the third kind, the fulcrum is at one
end, the weight at the other, and the power is applied
between them.
1. In levers.of this kind the power must always exceed the
weight, in the same proportion as the distance of the weight from
the fulcrum exceeds that of the power from the fulcrum.
2. In Fig. 30, F is the fulcrum, W the weight, and P the
power between the fulcrum and the weight;
and the power must exceed the weight in the        Fig. 30.
same proportion that the distance between W    F
and F exceeds the distance between P and F.
3. A  ladder which is to be raised by the             C
strength of a man's arms, represents a lever
of this kind, where the fulcrum  is that end              w
which is fixed against the wall: the weight
may be considered as at the top part of the ladder, and the
power is the strength applied in raising it.
4. The bones of a man's arm, and most of the moveable
bones of animals, are levers of the third kind. But the loss of
power in limbs of animals is compensated by the beauty and
compactness of the limbs, as well as the increased velocity of
their motion. The wheels in clock and watch work, and in
various kinds of machinery, may be considered as levers of
* It is on the same principle that, in raising a window, the hand should
be applied to the middle of the sash, as it will then be easily raised; whereas, if the hand be applied nearer to one side than the other, the centre of
gravityhbeing unsupported, will cause the further side to bear against the
frame, and obstruct its free motion.
83. What is a lever of the third kind? In what proportion must the
power exceed the weight in this lever? Explain Fig. 30. Give some examples of levers of the third kind.




MECHANICS.                         59
this kind, when the power that moves them acts on the pinion,
near the centre of motion, and the resistance to be overcome
acts on the teeth at the circumference. But here the advantage gained is the change of slow into rapid motion. The sails
of vessels are constructed on the principle of the lever.*
84. The Pulley is a small wheel turning on an axis,
with a string or rope in a groove running around it.
There are two kinds 6f pulleys —the fixed and the
moveable. The fixed pulley is a pulley that has no
other motion than a revolution on its axis, and it is used
only for changing the direction of motion.
Fig. 31 represents a fixed pulley. P is a small wheel turning on its axis, with a string running round it in a
groove. W  is a weight to be raised, F is the force or  Fig. 31.
power applied.  It is evident that, by pulling the
string at F, the weight must rise just as much as the         P
string is drawn down.  As, therefore, the velocity of
the weight and the power is precisely the same, it is   i
manifest that they balance each other, and that no
mechanical advantage is gained.  But this pulley is
very useful for changing the direction of motion.  If,
for instance, we wish to raise a weight to the top of a
high building, it can be done with the assistance of a fixed
pulley, by a man standing below.t  A curtain, or a sail, also,
can be raised by means of a fixed pulley, without ascending
with it, by drawing down a string running over the pulley.
85. The moveable pulley differs from the fixed pulley
* It may perhaps assist the memory to retain the rela've positions of
the *eight the power and the fulcrum in the three kinds oi levers, if the
initials be presented to the eye as follows:
First kind,   W. F. P.
Second"      F. W. P.
Third  "     F. P. W.
t The fixed pulley operates on the same principle as a lever of the first
kind with equal arms, where the fulcrum being in the centre of -gravity,
the power and the weight are equally distant from it, and no mechanical
advantage is gained.
84. What is a pulley?  How many kinds of pulleys are there?  What
are they?  What is a fixed pulley? Explain Fig. 31. What advantage
is gained by this pulley? What is the use of this pulley?  Upon what
principle does the fixed pulley operate?




60               NATURAL PIIILOSOPHY.
by being attached to the weight; it therefore rises and
falls with the weight.
Fig. 32 represents a moveable pulley, with the   Fig. 32.
weight W  attached to it by a hook below.  One   F
end of the rope is fastened at F; and as the power
P draws the weight upwards, the pulley rises with    B
the weight. Now, in order to raise the weight one     C
inch, it is evident that both sides of the string must   M
be shortened; in order to do which, the power P
must pass over two inches. As the velocity of the
power is double that of the weight, it follows that
a power of one pound will balance a weight on the
moveable pulley of two pounds.
86. The power gained by the use of pulleys is ascertained by multiplying the number of moveable pulleys
by 2.*
1. A weight of 72 pounds may be balanced by a power of
9 pounds with four pulleys; by a power of 18 pounds with
two pulleys; or by a power of 36 pounds with one pulley.
But in each case the space passed over by the power must be
double the space passed over by the weight, multiplied by the
number of moveable pulleys. That is, to raise the weight one
foot, with one pulley, the power must pass over two feet, with
two pulleys four feet, with four pulleys eight feet.
2. Fig. 33 represents a system of fixed and moveable pulleys
In the block F, there are four fixed pulleys, and
in the block M there are four moveable pulleys,    Fig. 33.
all turning on their common axis, and rising and  Hf
falling with the weight W. The moveable pulleys
are connected with the fixed ones by a string at-  F
tached to the hook H, passing over the alternate
grooves of the pulleys in each block, forming
eight cords, and terminating at the power P.
Now to raise the weight one foot, it is evident
that each of the eight cords must be shortened
one foot, and, consequently, that the power P     w
must descend eight times that distance. The          i P
* This rule applies only to the moveable pulleys in the same block.
85. How does the moveable pulley differ from the fixed pulley? Explain Fig. 32.
86. How car} the power g.ained by the use of the moveable pulley be ascertained? What illustration of this is given? What does Fig. 33 reprsenlt?




MECIANICS.                       61
power, therefore, must pass over eight times the distance that
the weight moves.
87. Pulleys act on the same principle with the lever,
the deficiency of the strength of the power being compensated by its superior velocity.
Now, as we cannot increase our natural strength, but can
increase the velocity of motion, it is evident that we are
enabled, by pulleys and other mechanical powers, to reduce
the resistance or weight of any body to the level of our
strength.
1. Practical use of Pulleys. Pulleys are used to raise goods
into warehouses, and in ships, &c. to draw up the sails. Both
kinds of pulleys are in these cases advantageously applied; for
the sails are raised up to the masts by the sailors on deck, by
means of the fixed pulleys, while the labor is facilitated by the
mechanical power of the moveable ones.
2. Both fixed and moveable pulleys are constructed in a great
variety of forms, but the principle on which all kinds are constructed, is the same. What is generally called a tackle and
fall, or a block and tackle, is nothing more than a pulley.
Pulleys have likewise lately been attached to the harness of a
horse, to enable the driver to govern the animal with less
exertion of strength.
3. It may be observed, in relation to the mechanical powers
in general, that power is always gained at the expense of time
and velocity; that is, the same power which will raise one
pound in one minute, will raise two pounds in two minutes, six
pounds in six minutes, sixty pounds in sixty minutes, &c.; and
that the same quantity of force used to raise two pounds one
foot, will raise one pound two feet, &c. And, further, it may
be stated that the product of the weight, multiplied by the
velocity of the weight, will always be equal to the product of
the power multiplied by the velocity of the power. Hence
we have the following rule. The power is in the same proportion to the weight as the velocity of the weight is to the velocity
of the power.
87. Upon what principle do pulleys act?  What advantage is gained
by the use of pulleys and other mechanical powers? What are some of
the practical uses of the pulley? What is a tackle and fall? Is there
any time or velocity gained by the power in the mechanical powers? To
what is the product of the weight, multiplied by its velocity, always equal?
What rule is given?




62                 NATU'RAL PHILOSOPHY.
88. The wheel and axle consists of two wheels of
different sizes, revolving together around the same centre of motion.
The place of the smaller wheel is generally supplied
by a cylinder,* which forms the axle.
1. The wheel and axle, though made in many forms, will easily
be understood by inspecting Figs. 34 and 35. In Fig. 34, P
represents the larger wheel,
where the power is applied;
C the smaller wheel or Fig. 34.
cylinder, which is the axle,
and W  the weight to be
raised. The advantage gained is in proportion as the
circumference of the wheel is
greater than that of the axle.
That is, if the circumference
of the wheel be six times
the circumference of the
axle, then a power of one
pound applied at the wheel
will balance a power of six
pounds on the axle.
2. Sometimes the axle is
constructed with a winch or
handle, as in Fig. 35, and sometimes the wheel has projecting spokes, as in Fig. 34.
3. The principle upon which the wheel and axle is constructed is the same with that of the other mechanical powers,
the want of power being compensated by velocity. It is
evident (from the Figs. 34 and 35) that the velocity of the
circumference of the wheel is as much greater than that of
the axle as it is further from the centre of motion; for the
wheel describes a great circle in the same time that the
* A cylinder is a long circular body of uniform diameter, with extremities forming equal and parallel circles.
88 Of what does the wheel and axle consist? What is a cylinder?
What figures illustrate the wheel and axle?  Explain. To what is the
advantage gained in proportion? What does Fig. 34 represent? Fig. 35?
Upon what principle is the wheel and axle constructed? Explain by Figs.
34 and 35.




MECHIANICS.                        63
axle describes a small one;
therefore the power is in-                  Fig. 35.
creased in the same. proportion  as the circumference of the wheel is greater
than that of the axle.  If                                   P
the velocity of the wheel be
twelve times greater than
that of the axle, a power                     t
of one pound on the wheel
will support a weight of
twelve pounds on the axle.
89. The wheel and axle are constructed on the same
principle with the lever; the axle acting the part of the
shorter arm  of the lever,,the wheel that of the longer
arm.
1. The capstan,* on board of ships and other vessels, is
constructed on the principle of the wheel and axle. It consists
of an axle placed uprightly, with a head or drum, pierced with
holes for the lever, or levers, which supply the place of the
wheel.
2. Windmills, lathes, the common windlass,* used for drawing water from wells, and the large wheels in mills, are all constructed on the principle of the wheel and axle.
3. Wheels are a very essential part to most machines; they
are applied in different ways, but when affixed to the axle their
mechanical power is always in the same proportion; that is, as
the circumference of the wheel exceeds that of the axle, so
much will the power be increased. Therefore the larger the
wheel and the smaller the axle, the greater will be the power
obtained.
4. Fly-wheels are heavy wheels used to accumulate power
* The difference between a capstan and a windlass lies only in the
position of the wheel. If the wheel turn horizontally it is called a capstan; if vertically, a windlass.
89. Upon what principle are the wheel and axle constructed? Explain
how. Upon what principle is the capstan on board of vessels constructed?
Of what does it consist? What other things are mentioned as constructed
upon this principle? Are wheels an essential part to most machines? Aro
they applied in more than one way? When they are affixed to the axle,
in what proportion is the power increased?




V,4                NATURAL PIIILOSOPHY.
and distribute it equally among all the parts of a machine.
They are caused to revolve by a force applied to the axle;
and when once set in motion continue by their inertia to move
for a long time. As their motion is steady and without sudden
jerks, they serve to steady the power, and cause a machine to
work with regularity.
5. Cranks are sometimes connected with the axle of a
wheel, either to give or to receive its motion. They are made
by bending the axle in such a manner as to form      Fig 36.
four right angles facing in different directions, as
is represented in Fig. 36.  This is seen in lathes
and many other kinds of machinery. Cranks are
often used to change the motion from rectilinear
to circular, or from circular to rectilinear.
6. In connexion with the wheel and axle, it is proper to
mention the subject of complex wheel-work. It has already
been stated that the velocity of the wheel is greater than that
of the axle; and that this velocity is in proportion to the
relative size of the wheel compared with that of the axle.
Advantage is taken of this circumstance in the construction of
machinery, by such an arrangement of the parts as will enable
us to increase or lessen the speed at pleasure. For it is
evident that if the power be applied to the axle, and machinery
attached to the wheel, rapid motion will be produced; and on
the contrary, if the power be applied to the wheel and the
machinery to the axle, slow motion will be produced.
7. Fig. 37 represents four wheels with their axles, each
wheel acting on the axle of the adjoining
wheel.  F is the power applied to the             Fig. 37.
axle of the wheel d.  Now, supposing the        d  c  b  a
circumference of each wheel to be six times
the circumference of each axle, it is evident  
that each time the wheel d revolves it must
cause the wheel c to make six revolutions,     
because the circumference of the wheel d is
What are fly-wheels, and for what are they used? How are they made
to revolve? When once set in motion, what causes them to move on for
some time?  Of what service are they in a machine?  For what are
cranks sometimes. connected with the axle of a wheel?  How are they
made?  What does Fig. 36 represent?  For what are cranks often used?
How does the velocity of the wheel compare with that of the axle? To
what is this velocity in proportion?  Is any advantage taken of this in
driving machinery where the speed is to be increased or diminished?




MECHANICS.                       65
six times the circumference of the axle of c. In like manner
the circumferences of the wheels c and b, acting respectively
on the circumferences of the axles of the adjoining wheel, will
communicate a velocity six times greater than their own, and
while the wheel d makes one revolution the wheel c will
make six, b tlhrty-six, and a two hundred and sixteen revolutions.
8. Reversing the figure, and applying the power at S which
communicates with the circumference of the wheel a, it follows
that a must perform six revolutions while b is performing one,
thirty-six while c, and two hundred and sixteen, while d performs
one revolution. It will thus be perceived that a rapid or a slow
motion may be communicated by various combinations of the
wheel and axle.
9. The usual way of transmitting the action of the axles to
the adjoining wheels is by means of teeth or cogs, raised on
their surfaces. The cogs on the surface of the wheels are
generally called teeth, and those on the surface of the axle are
called leaves. The axle itself, when furnished with leaves, is
called a pinion.
10. Fig. 38 represents
a connexion  of cogged                   Fig. 38.
wheels.  The wheel B                        D       C
being moved by a string
around its circumference
is a simple wheel with-    t
out teeth. Its axle being   B
furnished with  cogs or
leaves, to which the teeth
of the wheel D are fitted,
communicates its motion    6P
to D, which, in like manner, moves the wheel C.
The power P and the weight W  must be attached to the circumference of the wheel or of the axle according as a slow or
a rapid motion is desired.
11. Wheels are sometimes turned by bands, as in Fig. 39;
How would rapid motion be produced?  Slow motion?  Explain Fig.
37. What is the usual way of transmitting the action of the axles to the
adjoining wheels? What are the cogs on the surface of the wheel called?
Those on the axle'? What is a pinion? Explain Fig. 38. By what are
wheels sometimes turned?




6)i6               NATURAL PHILOSOPIHY.
and the motion communicated may be direct
or reversed by attaching the band as repre-        Fig. 39.
sented in Figs. 39 and 40.  When the wheel
and the axle from which it receives motion are
intended to revolve in the same direction, the
strap is not crossed, but is applied as in Fig. *    Fig. 40.
39. But when the wheel is to revolve in a
direction contrary to the revolution of the
axle, the strap is crossed, as in Fig. 40.
12. Different directions may be given to the motion proFig. 41.                    Fig. 42.
duced by wheels, by varying the position of their axles, and
causing them to revolve in different planes, as in Fig. 41; or
by altering the shape and position of the cogs, as in Fig 42.
90. The inclined plane consists of a plain surface inclined to the horizon, and the advantage gained by the
inclined plane is in proportion as the length of the plane
exceeds its perpendicular height.
1. Fig. 43 represents an inclined plane. C A its height, C B
its length, and W  a weight which is to
be moved on it.  If the length C B be            Fig. 43.
four times the height C A, then a power a
of one pound at C will balance a weight
of four pounds on the inclined plane  B
C B.
What figure represents one? In what way can the motion be made
direct or reversed?  What does Fig. 39 represent?  Fig. 40?  In what
way can different directions be given to the motion produced by wheels?
What does Fig. 41 represent? Fig. 42?
90. What is an inclined plane?  What figure represents an inclined
plane?  Explain the figure.  To what is the advantage gained by the
use of the inciined piane in proportion?




MECHANICS.                       67
2. The greater the inclination of the plane, the greater must
be its perpendicular height, compared with its length; and, of
course, the greater must be the power to elevate a weigrht
along its surface.
3. Instances of the application of the inclined plane are very
common. Sloping planks or pieces of timber leading into a
cellar, and on which casks are rolled up and down; a plank or
board with one end elevated on a step, for the convenience of
trundling wheelbarrows, or rolling barrels into a store, &c.,
are inclined planes.
4. The advantage gained by the use of the inclined plane,
like that of the other mechanical powers, is attended by a loss
of time; for the weight, instead of moving directly up the
ascent, must move the whole length of the plane.
5. Chisels and other cutting instruments, which are chamfered or sloped only on one side, are constructed on the principle of the inclined plane.
91. The wedge consists of two inclined planes united
at their bases; and the advantage gained by the use of
the wedge is in proportion as the length exceeds one
half the thickness of its converging sides.
1. Fig. 44 represents a wedge. The line a b repre-   Fig. 44.
sents the base of each of the inclined planes of which  a
it is composed, and at which they are united.
2. The wedge is a very important mechanical
power, used to split rocks, timber, &c., which could    V
not be effected by any other power.                     b
3. Axes, hatchets, knives, and all other cutting instruments chamfered, or sloped on both sides, are constructed
on the principle of the wedge.
92. The screw consists of an inclined plane, wound
round a cylinder, thus producing a circular inclined
What follows from the greater or less inclination of the plane? Give
some instances of the application of the inclined plane. Is any time gained
by the use of the inclined plane? Upon what principle are chisels and
other cutting instruments, which are sloped only on one side, constructed?
91. Of what does the wedge consist? What does Fig. 44 represent?
To what is the advantage gained by the wedge in proportion? Of what
use is the wedge? GiVe some examples of the wedge.
92. Of what does the screw consist?




6)8                 NATURAL PIIIOSOPIIY.
plane, and forming what is called the threads of the
screw.
The advantage gained in the use of the screw is in
proportion as the circumference described by the handle
exceeds the distance between the threads of the screw.
The screw is generally composed of two parts, the
screw and the nut; or, as they are generally called, the
convex and concave screw.
1. Fig. 45 represents the screw and the nut.  S is the conFig. 45.    vex screw, (which is an inclined plane wound
round a cylinder,) N is the nut, or concave screw,
which has a spiral groove, to which the thread
of the convex screw is accurately fitted.  L is a
lever attached to the nut, to which the power is
applied.  By turning the lever
in one direction the nut ascends,        Fig. 46.
and by turning it in the opposite
direction, the nut descends on the screw.*  In
this figure the screw  is fixed, and the nut is
moveable.
2. Fig. 46 represents another screw, which
is moveable. The nut is fixed to the frame, and
the screw ascends or descends as the lever L is turned.
* Although the screw is mentioned as one of the six mechanical powers,
it is, in reality a compound power, consisting of a lever and an inclined
plane. The power of the screw being estimated by the distance of the
threads, the closer the threads the greater is the power; but here, again,
the increase of power is procured by an increase of velocity, for a loss of
time. For if the threads be a quarter of an inch apart; the power must
move through the whole circumference of the circle described by the
lever, in order to move the resistance a quarter of an inch. The screw,
with its appendage the lever, is therefore used for the purpose of moving
large or heavy bodies through small distances. Its power may be increased
by lengthening the lever. The screw is applied to presses of all kinds
where great power is required, such as bookbinders' presses, cider and
wine presses, &c.
Of how many parts is it generally composed? What are they? What
figure represents the screw and the nut? Explain the figure. How does
Fig. 45 differ from Fig. 46?  Note. Is the screw a simple or compound
power?  How is the power of the screw estimated?,How does the closeness of the thread affect the power? What is the use of the screw? How
can its power be increased? To what is the screw applied?




MECHANICS.                           69
3. By friction in machinery is meant the resistance which
bodies meet with in rubbing against each other.
4. There are two kinds of friction, the rolling and the
sliding. The rolling friction is caused by the rolling of a
circular body. The sliding friction is produced by the sliding
or dragging of a body over a flat surface.  The sliding friction
is overcome with more difficulty than the rolling. In calculating the power of a machine, an allowance must always be
made for friction. It is usually computed that friction destroys one-third of the power of a machine.*
5. Friction is caused by the unevenness of the surfaces
which come into contact;t and it is diminished in proportion
as the surfaces are smooth and well polished. Oil, grease,
black-lead, or powdered soap-stone, is used. to lessen friction,
* When finely polished iron is made to rub on bell metal, the friction
is said to be reduced to about one-eighth. Mr. Babbit of Boston has prepared a composition for the wheel-boxes of locomotive engines and other
machinery, which it is said has still further reduced the amount of friction.
rhis composition is now much in use. As the friction between rolling bodies
is much less than in those that drag, the axle of large wheels is sometimes
made to move on small wheels or rollers. These are called friction wheels,
or friction rollers. They turn round their own centre as the wheel continues its motion.
t All bodies, how well soever they may be polished, have inequalities in
their surfaces, which may be perceived by a microscope. When, therefore, the surfaces of two bodies come into contact, the prominent parts of
the one will often fall into the hollow parts of the other, and cause more
or less resistance to motion.' Friction increases, 1st, as the weight or pressure is increased; 2d, as the areas of the surfaces in contact are increased;
3d, as the roughness of the surface is increased. Friction may be diminished,
1st, by lessening the weight of the body in motion; 2d, by mechanically
reducing the asperities'of the sliding surfaces; 3d, by lessening the amount
of surface of homogeneous bodies in contact with each other; 4th, by converting a sliding into a rolling motion; 5th, by applying some suitable
unguent.
What is meant by friction in machinery? How many kinds of friction
are there? What are they? How is the rolling friction produced? The
sliding? Which is overcome with the less difficulty, the rolling or sliding?
What allowance must always be made, in calculating the power of a machine? What proportion of the power is usually computed to be destroyed
by friction? Between which is friction the less, rolling bodies or those
that slide?  What causes friction?  In what proportion is it diminished?
In what manner can it be lessened?




70                 NATURAL PrIILOSor Y.
because they act as a polish by filling up the cavities of the
rubbing surfaces, and thus making them slide more easily over
each other.
6. From what has been stated with regard to the mechanical
powers, it appears that by their aid a man is enabled to perform works to which his unassisted natural strength is wholly
inadequate. But the power of all machines is limited by the
strength of the materials of which they are composed. Iron,
which is the strongest of all substances, will not resist a strain
beyond a certain limit. Its cohesive attraction may be destroyed, and it can withstand no resistance which is stronger
than its cohesive attraction. Besides the strength of the materials, it is necessary, also, to consider the time which is expended in the application of mechanical assistance. Archimedes
is said to have boasted to Hiero, king of Syracuse, that, if he
would give him a place to stand upon, he would move the
whole world.  In order to do this, Archimedes must himself
have moved over as much more space than he moved the
world, as the weight of the world exceeded his own weight;
and it has been computed that he must have moved with the
velocity of a cannon ball for a million of years, in order to
move the earth the twenty-seven millionth part of an inch.
7. Wheels are used on vehicles to diminish the friction of
the road.  The larger the circumference of the wheel, the
more readily it will overcome any obstacles, such as stones, or
inequalities in the road.*
93. A mediumt is the substance, solid or fluid, which
surrounds a body.
* In descending a steep hill, the wheels of a carriage are often locked,
(as it is called,) that is, fastened in such a manner as to prevent their
turning; and thus the rolling is converted into the sliding friction, and the
vehicle descends more safely.
Castors are put on the legs of tables and other articles of furniture, to
facilitate the moving of them; and thus the sliding is converted into the
rolling friction.
t The plural of this word is media.
What is the use of wheels? In what proportion do they overcome the
obstacles, such as stones, &c., in the road?  Why, in descending a steep
hill, are the wheels of a carriage often locked? How do castors, which
are put upon furniture, facilitate the moving of it? How is the motion of
all bodies influenced?
93. What is meant by a medium? Give examples.




MECHANICS.                        71
Thus, air is the medium which surrounds a bird when flying;
water is the medium which surrounds the fish when swimming, &c.
94. The motion of all bodies is influenced by the
medium in which they move; and the resistance of a
medium is in exact proportion to its density.
A body falling through water meets with more resistance
than when falling through air, because water is a denser
medium than air. If a machine could be worked in vacuo,
(that is, in a vacuum, or a space where there is neither air nor
any thing else to impede it,) and without friction, it would be
perfect.
95. The main-spring of a watch consists of a long
ribbon of steel, closely coiled, and contained in a round
box. It is employed instead of a weight, to keep up the
motion.
1. As the spring, when closely coiled, exerts a stronger force
than when it is partly loosened, in order to correct this inequality, the chain through which it acts is wound upon an
axis surrounded by a spiral groove, (called afusee,) gradually
increasing in diameter from the top to the bottom; so that, in
proportion as the strength of the spring is diminished, it may
act on a larger lever, or a larger wheel and axis.
2. Fig. 47 represents a
spring  coiled in a round                Fig. 47.     A
box. A B is the fusee, surrounded by a spiral groove,
on wlich the chain C  is'
wound. When the watch is
recently wound, the spring
is in the greatest state- of                           X
tension, and will, therefore,
turn the fusee by the smallest groove, on the principle of the
wheel and'axle. As the spring loses its force by being partly
unwound, it acts upon the larger circles of the fusee; and the
94. To what is the resistance of a medium in proportion? What illustration is given? When would a machine be perfect?
95. Of what does the main-spring of a watch consist? What is its use?
Does the spring exert a stronger force when closely coiled, or when partly
loosened? What is done in order to correct this inequality? What does
Fig. 47 represent? Explain.




72                 NATURAL PHILOSOPHY.
want of strength in the spring is compensated by the mechanical aid of a larger wheel and axle in the larger grooves.
By this means the spring is made at all times to exert an equal
power upon the fusee.  The motion is communicated from
the fusee by a cogged wheel which turns with the fusee.
96. The name of governor has been given to an ingenious piece of mechanism, invented by Mr. James
Watt, which is used to regulate the supply of power in
machinery; as that of steam in steam-engines, and of
water in water-mills.
Fig. 48 represents a governor. A B
and A C are two levers or arms, loaded           Fig. 48.
with heavy balls at their extremities
B and C, and suspended by a joint at                0
A  upon the extremity of a revolving
shaft, A D. At a is a collar, or sliding
box, connected with the levers by the
rods b a and c a., with joints at their
extremities.  When the shaft A D re-        B
volves rapidly, the centrifugal force of
the balls B and C will cause them  to
diverge in their attempt to fly off, and
thus raise the collar a, by means of         l              D
the rods b a and c a.  On the contrary,
when the shaft A D revolves slowly,
the weights B and C will fall by their own weight, and the
rods b a and c a will cause the collar a to descend. The steam.
valve in a steam-engine, or the sluice-gate of a water-wheel,
being connected with the collar a, the supply of steam or
water, which puts the works in motion, is thus regulated.*; In manufactures, there is one certain and determinate velocity with
which the machinery should be moved, and which, if increased or diminished, would render the machine unfit to perform the work it is designed to
execute. Now, it frequently happens that the resistance is increased or
diminished by some of the machines which are worked, being stopped, ol
others put on. The moving power, having this alteration in the resistance,
would impart a greater or less velocity to the machinery, were it not fo:
the regulating power of the governor, which increases or diminishes the
supply of water or of steam, which is the moving power.
96. What is a governor?  Explain Fig. 48. What is said in the note
of the use of the governor?




HYDROSTATICS.                         73
97. The knee-joint, or, as it is sometimes called, the
toggle-joint, consists of two rods or bars connected by a
joint, and increasing rapidly in power as the two rods
approach to the direction of a straight line.
1. Fig. 49 represents a toggle-joint.  A  C          Fig. 49.
and B C are the two rods connected by a joint               A
C.  A moving force applied in the direction             \.
C D acts with great and constantly increasing   D -— C
power to separate the parts A and B.
2. The operation of the toggle-joint is seen             B
in the iron joints which are used to uphold the
tops of chaises. It is also used in various kinds of printingpresses, to obtain the greatest power at the moment of impression.
CHAPTER V.
HYDROSTATICS.*
98. Hydrostatics treats of the nature, gravity, and
pressure of fluids.
99. A fluid is a substance which yields to the slightest
pressure, and the particles of which, having but a slight
degree of cohesion, move easily among themselves.
100. A liquid differs from a fluid in its degree of compressibilityt and elasticity.
* Hydrostatics treats of the properties of fluids at rest; Hydraulics
treats of fluids in motion.
t The experiments made at Florence many years ago seem to prove
that some kinds of liquids-water, for instance-are wholly incompressible
Later experiments, particularly those of Mr. Jacob Perkins, of Newbury97. Of what does the knee-joint, or toggle-joint, consist?  In what proportion does it increase in power?  What does Fig. 49 represent?  Explain the figure. Give an instance of the operation of the toggle-joint.
What is its use in printing-presses?
98. Of what does Hydrostatics treat?
99. What is a fluid?  Does the attraction of cohesion have much influence on the particles of fluids? What follows from this?
100. How do fluids differ from liquids? Can water be compressed;
4




74                  NATURAL PHILOSOPHY.
1. The particles of fluids gravitate among themselves in a
more perfect manner than solids; because the strong cohesion
of the particles of solid bodies in some measure counteracts
the effects of gravity.
2. From the slight degree of cohesion in the particles of
fluids, it is inferred that they must be small, smooth, and
globular; smooth, because there appears to be no friction
among them; and globular, because their touching each other
but by a point will account for the slightness of their cohesion.
3. Fluids cannot be formed into figures, or preserved in
heaps, on account of their want of cohesion.
4. Fluids are subjected to a kind of attraction called capillary* attraction, by which they are raised above their levels in
capillary tubes, or tubes the bores of which are exceedingly
small.  Thus, if a small glass tube be placed in water, the
water on the inside will rise above the level of that on the
outside of the tube.
5. The cause of this seems to be nothing more than the ordinary attraction of the particles of matter for each other. The
sides of a small orifice are so near to each other as to attract
the particles of the fluid on their opposite sides; and as all
attraction is strongest in the direction of the greatest quantity
of matter, the water is raised upwards, or in the direction of
port, (now in London,) have proved that water is capable of a considerable
degree of compression. Fluids, in general, have a voluntary tendency to
expand when at liberty; but liquids will not expand without a change of
temperature. Heat is supposed to be the primary cause of the fluid form
of bodies. It insinuates itself between the particles of bodies, and forces
them asunder. Thus, for instance, ice, without heat, is a solid; with heat
it becomes water, and, with a greater degree of heat;, it expands into an
elastic fluid, called steam.
* The word capillary is derived from the Latin word capilla, (hair,)
and it is applied to this kind of attraction, because it is exhibited most
prominently in tubes, the bores of which are as fine as a hair, and hence
called capillary tubes.
What is supposed to be the primary cause of the fluid form of bodies?
What effect has heat upon bodies?  What illustration is given? Why do
fluids gravitate in a more perfect manner than solids? What is inferred
from the slight degree of cohesion in the particles of fluids? Why smooth.
Why globular? Why cannot fluids be formed into figures, or preserved
in heaps?  What do you understand by capillary attraction?  Explaiu
the reason of it




HYDROSTATICS.                          75
the length of the tube.   On the outside of the tube, the
opposite surfaces cannot act on the same column of water,
and therefore the influence of attraction is here imperceptible
in raising the fluid.
6. All porous substances, such as sponge, bread, linen,
sugar, &c., may'be considered as collections of capillary
tubes; and, for this reason, water and other liquids will rise
in them, when they are partly immersed.
7. It is on the same principle that the wick of a lamp will
carry up the oil to supply the flame, although the flame is
several inches above the level of the oil.* If the end of a
towel happen to be left in a basin of water, it will empty the
basin of its contents. On the same principle, when a dry
wedge of wood is driven into the crevice of a rock, as the
rain falls upon it, it will absorb the water, swell, and sometimes split the rock.  In this manner, mill-stone quarries are
worked in Germany.
8. A beautiful experiment, dependent on the same principle
of capillary attraction, may be thus performed. Take two
* The reason why well-filled lamps will sometimes fail to give light, is,
that the wick is too large for its tube, and being thus compressed, the capillary attraction is impeded by the compression. The remedy is to reduce
the size of the wick. Another cause, also, that prevents a clear light, is, that
the flame is too far from the surface of the oil. As capillary attraction acts
only at short distances, the surface of the oil should always be within a short
distance of the flame. But another reason which requires particular attention, is, that all kinds of oil usually employed for lamps contain a glutinous
matter, of which no treatment can wholly divest them. This matter fills the
pores or capillary tubes of the wick, and prevents the ascent of the oil to feed
the flame. For this reason the wicks of lamps should be often renewed.
A wick that has been long standing in a lamp, will rarely afford a clear
and bright light. Another thing to be noticed by those who wish the
lamp to perform its duty in the best possible manner, is, that the wick be
not of such size as, by its length as well as its thickness, to fill the cup, and
thereby leave no room for the oil. It must also be remembered, that although the wick when first adjusted may be of the proper size, the glutinous matter of the oil, filling its capillary tubes, causes the wick to swell,
and thereby become too large for the tube, producing the same difficulty
as has alrcidy been noticed in cases where the wick is too large to allow
the free operation of capillary attraction
Explain why the same takes place in all porous substances. Explain
all the circumstances attending the burning of a lamp. Explain the expe.imeint with the glass plates.




76i                NATURAL PiIILOSOP11Y.
pieces of flat glass, joined together at one side, and separated
at the other by a thin strip of' wood, card, or other substance.
When thus prepared, immerse the glass in colored water,
having previously wet the inner surface.  The water will then
rise between the pieces of glass, forming a beautiful curve,
the higher part appearing where the pieces of glass are in
contact. This is exemplified by the glass plates in the " Boston School Set."
101. The level, or equilibrium of fluids, is the tendency
of the particles so to arrange themselves that every
part of the surface shall be equally distant from  the
centre of the earth; that is, from the point to which
gravity tends.
All fluids have a tendency to preserve this equilibrium.
Hence the surface of all fluids, when in a state of rest must
partake of the spherical form of the earth. This level or
equilibrium of fluids is the natural result of the independent
gravitation of each particle. The particles of a solid body
being united by cohesive attraction; if any one of them be
supported, it will uphold those also with which it is united.
But when any particle of a fluid is unsupported, it is attracted
down to the level of the surface of the fluid; and the readiness
with which fluids yield to the slightest pressure will enable the
particle by its own weight, to penetrate the surface of the fluid
and mix with it.
102. Fluids of different densities all preserve their
own equilibrium.
If a quantity of mercury, water, oil, and air, be put into the
same vessel, they will arrange themselves in the order of their
specific gravities. The mercury will sink to the bottom, the
water will stand above the mercury, the oil above the water,
and the air above the oil; and the surface of each fluid will
partake of the spherical form of the earth, to which they all
respectively gravitate.
103. A water-level is an instrument constructed on the
101. What is meant by the level or equilibrium of fluids? Have all
fluids a tendency to preserve this equilibrium?  What follows from this?
Of what is this level or equilibrium of fluids the natural result? How does
the gravitation of solid bodies differ from that of fluids?
102. Do fluids of different densities all preserve their own equilibrium?
What illustration is given to prove this?




HYDROSTATICS.                       77
principle of the equilibrium of fluids. It consists of a
glass tube, partly filled with water, and closed at both
ends. When the tube is not perfectly horizontal,that is, if one end of the tube be lower than the other, —
the water will run to the lower end. By this means the
level of any line to which the instrument is applied may
be ascertained.
Fig. 50 represents a water-level.  A B          Fig. 50.
is a glass tube partly filled with water.  C      A  C  B
is a bubble of air occupying the space not   CoL
filled by the water. When both ends of
the tube are on a level, the air-bubble will remain in the centre of the tube; but if either end of the tube be'depressed,
the water will descend and the air-bubble will rise. The glass
tube when used is generally set in a wooden or a brass box.
It is an instrument much used by carpenters, masons, surveyors, &c.
104. Solid bodies gravitate in masses, their parts
being so connected as to form a whole, and their weight
may be regarded as concentrated in a point called the
centre of gravity; while each particle of a fluid may be
considered as a separate mass, gravitating independently.
It is for this reason that a body of water, in falling, does
less injury than a solid body of the same weight.  But if the
water be converted into ice, the particles losing their fluid
form, and being united by cohesive attraction, gravitate unitedly in one mass.
105. Fluids not only press downwards like solids, but
also upwards, sidewise,* and in every direction.
* If the particles of fluids were arranged in  Fig. 51.
regular columns as in Fig. 51, there would be           Fig. 52.
no lateral pressure; for when one particle is
perpendicularly above the other, it can press
only downwards. But if the particles be ar103. Upon what principle is a water-level constructed? Of what does
it consist? For what is it used? What figure represents a water-level?
Explain the figure.
104. In what manner do solid bodies gravitate? What is the centre of
gravity?  What effect has gravity on the particles of fluids?
105. How is the pressure of fluids exerted?




/i'S8              NATURAL PIIILOSOPHY.
So  long as the equality of pressure is undisturbed.
every particle will remain at rest. If the fluid be disturbed  by agitating it, the equality of pressure will be
disturbed, and the fluid will not rest until the equilibrium
is restored.
1. The downward pressure of fluids is shown by making all
aperture in the bottom of a vessel of water. Every particle
of the fluid above the aperture will run downwards through
the opening.
2. The lteral pressure is shown by making the aperture at
the side of the vessel.  The fluid will then escape through the
aperture at the side.
3. The upward pressure is shown by taking a glass tube,
open at both ends, inserting a cork in one end, (or scopping it
with the finger,) and immersing the other in the water.  The
water will not rise in the tube. But the moment the cork is
taken out, (or the finger removed,) the fluid will rise in the
tube to a level with the surrounding water.
106. The pressure of a fluid is in proportion to the
perpendicular distance from the surface; that is, the
deeper the fluid the greater will be the pressure. This
pressure is exerted in every direction, so that all the
parts at the same depth press each other with equal
force.
1. A bladder, filled with air, being immersed in water, will
be contracted in size, on account of the pressure of the water
ranged as in Fig. 52, where a particle presses between two particles beneath, these last must suffer a lateral pressure. In whatever manner the
particles are arranged, if they be globular, as is suppoed, there must be
spaces between them. See Fig. 1, page 20.
How long will the particles of fluids remain at rest?  Explain Fig. 51.
What does Fig. 52 represent? If the equality of the pressure be undisturbed, what will follow? If the fluid be agitated, when will it again come
to a state of rest?  How is the downward pressure of fluids shown? The
lateral pressure? The upward pressure?
106. To what is the pressure of a fluid in proportion?  In what direction is this pressure exerted?  What illustrations are given so prove this?
Why can a bottle, filled with water, or any other liquid, be let down to
any depth without injury?




HYDROSTATICS.                         79
in all directions; and the deeper it is immersed, the more will
it be contracted.
2. An empty bottle, being corked, and by means of a weight
let down to a certain depth in the sea, will either be broken by
the pressure, or the cork will be driven into it, and the bottle
be filled with water. This will take place even if the cork be
secured with wire and sealed. But a bottle filled with water,
or any other liquid, may be let down to any depth without
damage, because; in this case, the internal pressure is equal to
the external.*
* " Experiments at sea.-We are indebted to a friend, who has just arrived from Europe, says the Baltimore Gazette, for the following experiments made on board the Charlemagne:
" 26th of September, 1836, the weather being calm, I corked an empty
wine-bottle, and tied a piece of linen over the cork; I then sank it into
the sea six hundred feet; when drawn immediately up again, the cork was
inside, the linen remained as it was placed, and the bottle was filled with
water.
" I next made a noose of strong twine around the bottom of the cork,
which I forced into the empty bottle, lashed the twine securely to the neck
of the bottle, and sank the bottle six hundred feet. Upon drawing it up
immediately, the cork was found inside, having forced its way by the twine,
and in so doing had broken itself in two pieces; the bottle was filled with
water.
" I then made a stopper of white pine, long enough to reach to the bottom of the bottle; after forcing this stopper into the bottle, I cut it off about
half an inch above the top of the bottle and drove two wedges, of the same
wood, into the stopper. I sank it six hundred feet, and upon drawing it
up immediately the stopper remained as I placed it, and there was about
a gill of water in the bottle, which remained unbroken. The water must
have forced its way through the pores of the wooden stopper, although
wedged as aforesaid; and had the bottle remained sunk long enough, there
is no doubt that it would have been filled with water."
Similar experiments were made by the author of this work, in a voyage
to the West Indies in the year 1839, first with an empty bottle and then
with a bottle filled with water from the tanks'on the deck; in both cases
the bottle being closely stopped and the cork covered with canvass. The
empty bottle was drawn from a depth of six hundred feet filled with water,
and the full bottle with brackish water, the water from the tank having
been compressed, and water from the depths of the ocean mixing with it.
It is the opinion of some philosophers, that the pressure at very great
What experiment is mentioned in the note? What opinion have some
philosophers expressed?




80                 NNATURAL PI1ILOSO)P.IY.
107. From Pwhat has now  been stated, it appears that
the lateral pressure proceeds entirely from the pressure
downwards, or, in other words, from  the weight of the
liquid above; and that consequently the lower an orifice
is made in a vessel containing water or any other liquid,
the greater will be the force and velocity with which
the liquid will rush out.
Fig. 53 represents a vessel of water,           Fig. 53.
with orifices at the side at different dis-      A
tances from  the surface.  The different    |
curves in the figure, described by the liquid    |
in running out of the vessel, show the action of gravity and the effects produced
by the force of the pressure on the liquid
at different depths.  At A the pressure is the least, because
there is less weight of fluid above. At B and C the fluid is
driven outwards by the weight of that portion above, and the
force will be strongest at C.
108. As the lateral pressure arises solely from the
downward pressure, it is not affected by the width nor
the length of the vessel in which it is contained, but
merely by its depth; for as every particle acts independently of the rest, it is only the column of particles above
the orifice that can weigh upon and press out the water.
109. The lateral pressure on one side of a cubical vessel will be equal only to half of the pressure downwards;
for every particle at the bottom of the vessel is pressed
upon by a column of the whole depth of the fluid,
while the lateral pressure diminishes from the bottom
upwards to the surface, where the particles have no
pressure.
depths of the sea is so great, that the water is condensed into a solid state;
and that at or near the centre of the earth, if the fluid could extend so
deeply, this pressure would convert the whole into a solid mass of fire.
107. What causes the lateral pressure? What follows froln this? Explain Fig. 53.
108. Does the length or the width of the vessel in which a fluid is conltained have any effect upon the lateral pressure?  By what is it affected?
109. How does the lateral pressure on one side of a cubical vessel comlpare with the pressure downwards?  How would you explain this?




HYDROSTATICS.                     81
110. The  upward  pressure of fluids, although appatently in opposition to the principles of gravity, is but
a necessary consequence of the operation of that principle; or, in other words, the pressure upwards as well as
the pressure downwards is caused by gravity.
When water is poured into a vessel with a spout, (like a
tea-pot, for instance,) the water rises in the spout to a level
with that in the body of the vessel. The particles of water at
the bottom of the vessel are pressed upon by the particles
above them, and to this pressure they will yield, if there is
any mode of making way for the particles above them. As
they cannot descend through the bottom of
the vessel they will change their direction       Flg. 54.
and rise in the spout.  Fig. 54 represents a
tea-pot, and the columns of balls represent
the particles of water magnified. From an
inspection of the figure it appears that the
particle numbered 1, at the bottom, will be
pressed laterally, by the particle numbered 2, and by this
pressure forced into the spout, where meeting with the particle
3 it presses it upwards, and this pressure will be continued
from 3 to 4, from 4 to 5, and so on till the water in the spout
has risen to a level with that in the body of the vessel. If
water be poured into the spout the water will rise in the same
manner in the body of the vessel; from which it appears that
the force of pressure depends entirely on the height, and not on
the length or breadth of the column of fluid.
111. One principle in hydrostatics, is so remarkable,
that it is named the hydrostatic paradox. It is this.
That any quantity of fluid, however small, may be made
to balance and support any other quantity, however
large,
110. What causes the upward and downward pressure? Illustrate this
by Fig. 54.
111. Upon what does the force of pressure depend? What is meant by
the hydrostatic paradox? What is the use of the hydrostatic bellows?
What figure represents the hydrostatic bellows?  Explain the figure.
What is the fundamental principle of Mechanics? Is this the principle
of the hydrostatic bellows?
4*




8i2                 NATURAL PHILOSOPHY.
Fig. 55 represents the hydrostatic bellows.*
A B is a long tube, one inch square.  CD E F A    Fig. 5
are the bellows, consisting of two boards,
eight inches square, connected by broad pieces
of leather, or india-rubber cloth, in the manner of a pair of common bellows. One pound
of water poured into the tube will raise 64
pounds on the bellows.  If a smaller tube be
used the same quantity of water will fill it
higher, and consequently will raise a greater
weight; but if a larger tube be used it will of B
course not fill it so high, and consequently
will not raise so great a weight; because it          E    -
is the height not the quantity which causes the          D       F
pressure.t
* This is the form of the Hydrostatic bellows in the original "Boston
School Set." By means of a straight jet substituted for the tube, it was
designed to illustrate a principle in Hydraulics also.
t The Hydrostatic bellows may be constructed in a variety of forms,
the simplest of which consists, as in the figure, of two boards connected
together by broad pieces of leather, or india-rubber cloth, in such a manner as to allow the upper board to rise and fall like the common bellows.
A perpendicular tube is so adjusted to this apparatus, that water poured
into the tube, passing between the boards, will separate them by its upward pressure, even although the upper board is loaded with a considerable
weight.
[N. B. A small quantity of water must be poured into the bellowsto separate the surfaces before they are loaded with the weight.]
The force of pressure exerted on the bellows by the water poured
into the tube, is estimated by the comparative size of the tube and the beliows. Thus, if the tube be one inch square, and the top of the bellows
twelve inches, thus containing 144 square inches, a pound of water poured
into the tube will exert a pressure of 144 pounds on the bellows. Now it
will be clearly perceived that this pressure is caused by the height of the
column of water in the tube. A pound, or a pint of water will fill the
tube 144 times as high as the same quantity would fill the bellows. To
raise a weight of 144 pounds on the bellows to the height of one inch, it
will be necessary to pour into the tube as much water as would fill the
tube were it 144 inches long. It will thus be perceived that the fundamental principle of the laws of motion is here also in full force; namely,
that what is gained in power is lost either in time or in space; for,
while the water in the bellows is rising to the height of one inch, that in
the tube passes over 144 inches.




HYDROSTATICS.                         b3
Fig. 56 is an apparatus* to show that liquids press according
to the height and not
the quantity. A and                      Fig. 56
B are two vessels of                            A        E   c
unequal size but of
the  same  length.
These may successively be screwed to
the apparatus and
filled with water.
Weights may then
be added to the suspended  scale  until the pressure is
counterbalanced. It
will then  be perceived   that   although  A  is  ten
times larger than B,
the water will stand
at the same height
in both, because they are of the same length.  If C be used instead of A or B, the apparatus may be used as the hydrostatic
Dellows.
112. If water be confined in any vessel, and a pressure
to  any  amount be exerted  on  a square  inch  of that
water, a pressure to an equal amount wi'l be transmitted to every square inch of the surface of the vessel in
which the water is confined.t
* This apparatus, belonging to the " Boston Fo-hool Set," unites simplicity with convenience. Instead of two boards, connected with leather,
an india-rubber bag is placed between two boards, and the boards are
made to rise or fall as the water runs into or out of the bag. It is an apparatus easily repaired, anlt the bag may also be used for gas, or for experiments in Pneumatics.
t This property of fluids seems to invest us with a power of increasing
the intensity of a pressure exerted by a comparatively small force, without
any other limit than that of the strength of the materials of which the engine itself is constructed. It also enables us with great facility to transmit
112. What fact is mentioned in this number with regard to the pressure on water?




34                 NATURAL PHILOSOPHY.
1. It is upon this principle that Bramah's hydrostatic press,
represented in Fig. 57, is constructed. The main features of
this apparatus are
as follows: a is a
narrow  and  A  a                      Fig. 57
large metallic cyl-    _:
inder having com-                 R
munication    one -
Water  stands in
both  the  cylin- 
ders.  The piston                                             d
S carries a strong                      
head   P,  which
works in a frame
opposite to a similar plate, R. Between  the  two
plates  the  substance W to be
compressed is placed.  In the narrow tube a is a piston p,
worked by a lever c b d, its short arm c b driving the piston,
while the power is applied at d. The pressure exerted by
the small piston p on the water at a is transmitted with equal
force throughout the entire mass of the fluid, while the surface
at A presses up the piston S with a force proportioned to its
area. For instance, if the cylinder a of the force-pump has
an area of half an inch while the great cylinder has an
area of 200 inches, then the pressure of the water in the latter on the piston S, will be (1 half inch multiplied by 400
half inches) equal to 400 times that on p.
2. Next, suppose the arms of the lever to be to each other
as 1 to 50, and that at d, the extremity of the longer arm, a
the motion and force of one machine to that of another, in cases where local
circumstances preclude the possibility of instituting any ordinary mechanical connection between the two machines. Thus, merely by means of
water-pipes, the force of a machine may be transmitted to any distance,
and over inequalities of ground, or through any other obstructions.
Upon what principle is Bramah's hydrostatic press constructed? What
figure represents this? Explain the figure. To what uses is this press
applied?




HYDROSTATICS.                         85
man works with a force of 50 pounds, the piston p will consequently descend on the water with a force of 2000 pounds.
Deducting I for the loss of power caused by the different impediments to motion, and one man would still be able to exert
a force of three quarters of a million of pounds by means
of this machine. This press is used in pressing paper, cloth,
hay, gunpowder, &c.; also  in uprooting trees, testing the
strength of ropes, &c.
113. A fluid specifically lighter than another fluid will
float upon its surface.*
114. A body specifically lighter than a fluid will sink
in the fluid until it has displaced a portion of the fluid
equal in weight to itself.
If a piece of cork is placed in a vessel of water, about one
third part of the cork will sink below, and the remainder will
stand above the surface of the water; thereby displacing a
portion of water equal in bulk to about a third part of the
cork, and this quantity of water is equal in weight to the whole
of the cork; because the specific gravity of water is about
three times as great as that of cork.t
* The slaves in the West Indies, it is said, steal rum by inserting the
tong neck of a bottle, full of water, through the top aperture of the rumcask. The water falls out of the bottle into the cask, while the lighter
rum ascends in its stead.
t It is on the same principle that boats, ships, &c., although composed
of materials heavier than water, are made to float. From their peculiar
shape, they are made to set lightly on the water. The extent of the surface presented to the water counterbalances the weight of the materials,
and the vessel sinks to such a depth as will cause it to displace a portion
of water equal in weight to the whole weight of the vessel. From a knowledge of the specific gravity of water, and the materials of which a vessel
is composed, rules have been formed by which to estimate the tonnage of
vessels; that is to say, the weight which the vessel will sustain without
sinking.
113. When will one fluid float upon another?
114. What is stated with regard to a body specifically lighter than a
fluid? What illustration is given of this? How do the specific gravities
*of water and cork compare with each other? Upon what principle is it
that boats, ships, &c., are made to float upon the water?  What rules
have been formed from the knowledge of the specific gravity of water and
the materials of which vessels are composed?




86                   NATURAL PHILOSOPHY.
115. The standard which has been adopted to estimate
the specific gravity of bodies, is rain or distilled water.*
Taking a certain quantity of rain or distilled water, we find
that a quantity  of gold, equal in  bulk, will weigh  nearly
twenty times as much as the water; of lead, nearly twelve
times as much; while oil, spirit, cork, &c., will weigh less than
the water.t
* As heat expands and cold condenses all metals, their specific gravity
cannot be the same in summer that it is in winter. For this reason they
will not serve as a standard to estimate the specific gravity of other bodies.
The reason that distilled water is used is, that spring, well, or river water
is seldom perfectly pure; and the various substances mixed with it affect
its weight. The cause of the ascent of steam, or vapor, may be found in
its specific gravity. It may here be stated that rain, snow, and hail are
formed by the condensation of the particles of vapor in the upper regions
of the atmosphere. Fine watery particles coming within the sphere of
each other's attractions, unite in the form of a drop, which being heavier
than the air, falls to the earth. Snow and hail differ from rain only in the
different degrees of temperature at which the particles unite. When rain,
snow, or hail fall, part of it reascends in the form of vapor, and forms
clouds; part is absorbed by the roots of vegetables, and part descends into
tihe earth and forms springs. The springs form brooks, rivulets, rivers, &c.,
and descend to the ocean, where being again heated by the sun, the water
rising in the form of vapor, again forms clouds, and again descends in rain,
snow, hail, &c. The specific gravity of the watery particles which constitute vapor, is less than that of the air near the surface of the earth;
they will, therefore, ascend until they reach a portion of the atmosphere of
the same specific gravity with themselves. But the constant accession of
fresh vapor from the earth, and the loss of heat, causes several particles to
come within the sphere of each other's attraction, as has been stated above,
and they unite in the form of a drop, the specific gravity of which being
greater than that of the atmosphere, it will fall in the form of rain. Water,
as it descends in rain, snow, or hail, is perfectly pure, but when it has
fallen to the earth, it mixes with the various substances through which it
passes, which gives it a species of flavor, without affecting its transparency.
t The following table shows the specific gravity of the substances therein mentioned. It is to be understood that all substances whose specific
gravity is greater than water, will sink when immersed in it, and that all
115. What standard has been adopted to estimate the specific gravity of
substances in general? Why c ould not metals have been adopted? Why
is distilled water used?  What bodies will sink when immersed in water?
What will float?




HYDROSTATICS.                            8 
116. The specific gravity of bodies that will sink in
water is ascertained by weighing them first in water,
and then out of the water, and dividing the weight out
of the water by the loss of weight. in water.
1. Fig. 58 represents the scales for ascertaining the specific gravity of bodies.              Fig. 58.
One scale is shorter than the other, and
a hook is attached to the bottom of the                        "
scale to which substances, whose specific gravity is sought, may be attached
and sunk in water.
2. Suppose  a  cubic  inch  of gold
weighs 19 ounces when weighed out
whose specific gravity is less than that of water, will float in it. Let us
then take a quantity of water which will weigh exactly one pound; a
quantity of the substances specified in the table, of the same bulk, will
weigh as follows:
Platinum,  - 23.  lbs. Chalk, - - - 1.793 lbs. Livingmen, -.891 lbs.
Fine Gold, - 19.640 "   Coal,  - - - 1.250 "   Ash, -. - -.800
Mercury,  - 14.019 "   Mahogany, - 1.063 "'  Beech, - - -.700
Lead,    -- I1.525 "   Milk,  - - - 1.034 "   Elm, - - - -.600"
Silver,  - - 11.091"   Box-wood, - 1.030      Fir,'   - -  - -.500 
Copper, - -  9.000 "   Rain water, - 1.000, Cork, - - - -.240 
Ironl, - - -  7.645     Oil, - - - -.920 "   Common air, -.0011"
Glass,- - -   3.000 "   Ice, - - -..908 "   Hydrogen gas,.000105
Marble, - -  2.705 "   Brandy,  - -.820 
A cubic foot of water weighs one thousand avoirdupois ounces.  By
multiplying the number opposite to any substance in the above table by one
thousand, we obtain the weight of a cubic foot of that substance, in ounces
Thus, a cubic foot of platinum is 23000 ounces in weight.
In the above table it appears that the specific gravity of living men is
about one ninth less than that of common water. So long, therefore, as the
lungs can be kept free from water, a person, although unacquainted with
the art of swimming, will not completely sink, provided the hands and arms
be kept under water.
The specific gravity of sea water is greater than that of the water of
lakes and rivers, on account of the salt contained in it. On this account
the water of lakes and rivers has less buoyancy, and it is more diffiult to
swim in it.
What is the weight of a cubic foot of water? What is the use of the
above table? How does the specific gravity of living men compare with
that of water? Which is the greater, the specific gravity of sea water, or
of lakes and rivers? Why?
116. How is the specific gravity of bodies that will sink in water ascerrained?  What illustration is given?  Explain Fig 58




188                 NATURAL PIIILOSOPIIY.
of the water, and but 18 ounces* when weighed in water-the
loss in water is one ounce. The weight out of water, 19
ounces, being divided by one (the loss in water) gives 19.
The specific gravity of gold, then, would be 19, or, in other
words, gold is nineteen times heavier than water.
117. The specific gravity of a body that will not sink
in water, is' ascertained by dividing its weight b}y the
sum of its weight, added to the loss of weight which
it occasions in a heavy body previously balanced in
water. 
* The gold will weigh less in the water than out of it, on account of the
upward pressure of the particles of water, which in some measure supports
it, and by so doing diminishes its weight. Now, as the upward presst_ 
of these particles is exactly sufficient to balance the downward pressure of
a quantity of water of exactly the same dimensions with the gold, it follows that the gold will lose exactly as much of its weight in water, as a
quantity of water of the same dimensions with the gold will weigh. And
this rule applies to all bodies heavier than water, that are immersed
in it. They will lose as much of their weight in water as a quantity of water of their own dimensions weighs. All bodies, therefore, of
the same size, lose the same quantity of their weight in water. Hence,
the specific gravity of a body is the weight of it compared with that
of water. As a body loses a quantity of its weight when immersed in
water, it follows that when the body is lifted from the water, that portion
of its weight which it had lost will be restored. This is the reason that
a bucket of water, drawn from a well, is heavier when it rises above the
surface of the water in the well, than it is while it remains below the surface. For the same reason our limbs feel heavy in leaving a bath.
t The method of ascertaining the specific gravities of bodies was discovered accidentally by Archimedes. He had been employed by the king
of Syracuse to investigate the metals of a golden crown which he suspectWhy will gold weigh less in the water than out of it? How does this
upward pressure of the particles compare with the downward pressure of
a quantity of water of the same dimensions?  What follows from this?
What rule is given with regard to all bodies heavier than water that are
immersed in it?  What is the specific gravity of a body?  What is the
reason that a bucket of water, drawn from a well, is heavier when it
rises above the surface of the water, than while it is below it?
117. How can the specific gravity of a body that will not sink in water
be ascertained? What illustration is given? By whom was the method
of ascertaining the specific gravities of bodies discovered? In what mal)ner did he ascertain it?




IIYDROSTATICS.                        89
If a body lighter than water weighs six ounces, and on being attached to a heavy body, balanced in water, is found to
occasion it to lose twelve ounces of its weight, its specific
gravity is determined by dividing its weight (six ounces) by
the sum of its weight, added to the loss of weight it occasions
in the heavy body, namely, six added to twelve, which, in
other words, is 6 divided by 18, or 6s, which is 3.
118. An hydrometer is an instrument to ascertain the
specific gravity of liquids.
1. The hydrometer is constructed on the principle, that
the greater the weight of a liquid, the greater will be its
buoyancy.
2. The hydrometer is made in a variety of forms, but it
generally consists of a hollow ball of silver, glass, or other
material, with a graduated scale rising from the upper part.
A weight is attached below the ball. When the instrument,
thus constructed, is immersed in a fluid, the specific gravity
of the fluid is estimated by the portion of the scale that remains above the surface of the fluid. The greater the specific
gravity of the fluid, the less will the scale sink.
ed had been adulterated by the workmen. The philosopher labored at the
problem in vain, till going one day into the bath, he perceived that the
water rose in the bath in proportion to the bulk of his body. He instantly
perceived that any other substance of equal size would raise the water just
as much, though one of equal weight and less bulk could not produce the
same effect. He then obtained two masses, one of gold and one of silver,
each equal in weight to the crown, and having filled a vessel very accurately with water, he first plunged the silver mass into it, and observed the
qujantity of water that flowed over; he then did the same with the gold,
and found that a less quantity had passed over than before. Hence he
inferred that, though of equal weight, the bulk of the silver was greater
than that of the gold, and that the quantity of water displaced was, in
each experiment, equal to the bulk of the metal. He next made trial
with the crown, and found that it displaced more water than the gold, and
less than the silver, which led him to conclude, that it was neither pure
gold nor pure silver.
118. What is an hydrometer? Upon what principle is it constructed?
Explain its construction. In what proportion does the scale sink?




90                   NATURAL PIIILOSOPHY
CHAPTER VI.
HYDRAULICS.
119. Hydraulics treats of the motion of fluids, particu.
larly of water; and the construction of all kinds of instruments and machines for moving them.
Water, in its motion, is retarded by the friction of the
bottom and sides of the channel through which it passes.
For this reason the velocity of the surface of a running stream
is always greater than that of any other part.*
120. A fluid running from an orifice in a vessel is discharged with the greater rapidity when the vessel from
which it flows is kept constantly full.
This is a necessary consequence of the law, that pressure is
proportioned to the height of the column above.
121. When a fluid spouts from several orifices in the
side of a vessel, it is thrown  to the  greatest distance
from  the orifice nearest to the centre.t
122. A vessel filled with any liquid will discharge a
greater quantity of the liquid through an orifice to which
* In consequence of the friction of the banks and beds of rivers, and the
numerous obstacles they meet in their circuitous course, their progress is
slow. If it were not for these impediments, the velocity which the waters
would acquire would produce very disastrous consequences. An inclination of three inches in a mile, in the bed of a river, will give the current a
velocity of about three miles an hour.
t This is true only on the condition that the vessel be not elevated. If
the vessel be elevated, the lowest orifice will discharge the fluid to the
greatest distance, but when the vessel is placed low, the fluid will reach
the plane before its projectile force is expended.
119. Of what does Hydraulics treat? What retards the motion of water?
Why does the surface of a canal or river have a greater velocity than any
other part?
120, Does the fulness of a vessel from the orifice of which a fluid is
running, have any effect upon its velocity?
121. When a fluid spouts from several orifices in the side of a vessel,
frolmu which is it thrown to the greatest distance?




HYDRAULICS.                          91
a short pipe of peculiar shape is fitted, than through an
orifice of the same size without a pipe.* But if the pipe
project into the vessel, the quantity discharged will be
diminished instead of increased by the pipe.
The quantity of a fluid discharged through a pipe or an
orifice is increased by heating the liquid; because heat diminishes the cohesion of the particles, which exists, to a certain
degree, in all liquids.
123. The velocity of a current of water may be ascertained  by immersing in it a bent
tube, shaped like a tunnel at the end
which is immersed.
Fig. 59 is a tube shaped like a tunnel, with the larger end immersed in an
opposite direction to the current.  The
rapidity of the current is estimated by
the height to which the water is forced
into the tube, above the surface of the
current.   By such an instrument the  =            --
comparative velocity of different streams,
or the same stream  at different times,
may be estimated.t
* This is caused by the cross-currents made by the rushing of the water
from different directions towards the sharp-edged orifice. The pipe smooths
the passage of the liquid.
t To measure the velocity of a stream  at its surface, hollow floating
bodiAs are used; as, for example, a glass bottle filled with a sufficient
quartity of water to make it sink just below the level of the current,
and having a small flag projecting from the cork. A wheel may also be
caused to revolve by the current striking against boards projecting from
the circumference of the wheel, and the rapidity of the current may be
estimated by the number of the revolutions in a given time.
122. What 6ffect will a pipe, fitted to an orifice, have with regard to the
quantity discharged?  What will be the effect if the pipe project into the
vessel?  How can the quantity discharged through a pipe or orifice, be
increased?  Why will heat increase it?
123. How can the velocity of a current of water be ascertained? Wlhat
does Fig. 59 represent? How is the rapidity of the current estimated,
What is the use of the instrument?




92                 NATURAL PHILOSOPIY.
124. Waves are caused by the friction between air
and water.*
125. The instruments used for raising or drawing
water or other liquids, are the common pump,t the
chain pump, the forcing pump, the siphon, and the screw
of Archim6des.
126. The chain-pump is a ma-                Fig. 60.
chine by which the water is lifted
through a box or channel, by boards
fitted to the channel and attached
to a chain.  It has been used principally on board of ships.
Fig. 60 represents a chain-tump. It    E
consists of a square box through which
a number of square boards or buckets,
connected by a chain, is made to pass.
The chain passes over the wheel C and
under the wheel D, which is under
water.  The buckets are made to fit
the box, so as to move with little friction.  The upper wheel, C, is turned
by a crank, (not represented in the Fig.)        e
which causes the chain with the buckets
attached to pass through the box. Each
bucket, as it enters the box, lifts up the
water above it, and discharges it at the
top.
127. The screw  of Archimedes is a machine said to
have been invented by the philosopher Archimedes, for
* When oil is poured on the windward side of a pond, the whole surface
will become smooth. The oil protects the water from the friction of the
wind or air. It is said that boats have been preserved in a raging surf, in
consequence of the sailors having emptied a barrel of oil on the water.
t The common pump, and the forcing pump, will be explained in connection with Pneumatics.
124. What causes waves? What is sometimes done to remove this
friction?
125. What instruments are used for raising liquids?
126. Where is. the chain-pump used? What figure represents it? Explain the figure.




HYDRAULICS.                     93
raising water and draining the lands of Egypt, about
200 years before the Christian era.
Fig. 61 represents the              Fig. 61.
screw  of Archimedes.
A  single tube, or two
tubes, are wound in the
form of a screw around                      I
a shaft or cylinder, supported by the prop and
the pivot A, and turned
by the handle n. As
the end of the tube dips
into the water, it is filled  T'
with the fluid, which is  a
forced up the tube by
every successive revolution, until it is discharged at the upper
end.
128. Springs and rivulets are formed by the water,
from rain, snow, &c., which penetrates the earth, and
descends until it meets a substance which it cannot penetrate. A reservoir is then formed by the union of small
streams under ground, and the water continues to accumulate until it finds an outlet.
Fig. 62..?77........... — -- ----------
Fig. 62 represents a vertical section of the crust of the
earth. a, c, and e are strata. either porous, or full of cracks,
127. What i Pidc of tne screw of Archimedes? Explain the use of the
screw by Fir. 61
128. dow are springs and rivulets formned? Explain Fig. 62.




94                 NATURAL PHILOSOPHY.
which permit the water to flow through, while b, d, and f, are
impervious to the water.  Now according to the laws of hydrostatics, the water at b will descend and form a natural
spring at g; at i it will run with considerable force, forming a
natural jet; and at 1, p, and g, artesian wells may be dug, in
which the water will rise to the respective heights g A, p h, and
1 m, the water not being allowed to come in contact with the
porous soil through which the bore is made, but being brought
in pipes to the surface; at n the water will ascend to about o,
and there will be no fountain. This explains also the manner
in which water is obtained by digging wells.
129. A spring will rise nearly as high, but cannot rise
higher than the reservoir from whence it issues. Water
may be conveyed over hills and valleys in bent pipes
and tubes, or through  natural passages, to any height
which is not greater,than the level of the reservoir from
whence it flows.*
130. Fountains are formed by water               Fig. 63.
carried  through  natural  or artificial
ducts from a reservoir.  The water will
spout through the ducts to nearlyt the
height of the surface of the reservoir.
A simple method of making an artificial,       lj l 
fountain may be understood by Fig. 63.
A glass siphon a b c is immersed in a vessel
of water, and the air being exhausted from'
the siphon, a jet will be produced at a, proportioned to the fineness of the bore and the
length of the tube.
* The ancient Romans, ignorant of this property of fluids, constructed
vast aqueducts across valleys, at great expense, to convey water over them.
The moderns effect the same object by means of wooden, metallic, or
stone pipes.
t The resistance of the air prevents the fluids from rising to quite the
same height with the reservoir.
129. How high will a spring rise?
130. How are fountains formed? How high will the water spout
through the ducts? What prevents the fluid from rising to the same
height with the reservoir?




HYDRAUrLICS.                      95
131. The siphon is a tube bent in the form of the
letter U, one side being a little longer than the other.
1. Fig. 64 represents a siphon.  A siphon is
used by filling it with water or some other fluid, Fis 64.
then stopping both ends, and in this state immersing the shorter leg or side into a vessel containing a liquid. The ends being then unstopped,
the liquid will run through the siphon until the
vessel is emptied. In performing this experiment,
the end of the siphon which is out of the water must
always be below the surface of the water.
2. This instrument may also be used to show
the equilibrium of fluids. For, if the tube be inverted and two liquids, poured into it, they will
rise in each side or leg of the siphon to different heights-the higher fluid standing at the highest level.
The specific gravity of mercury being thirteen times greater than
that of water, will balance thirteen times its bulk of water.
Consequently water will rise thirteen times as high on one side
of the siphon as the mercury on the other; But if one liquid
only is poured into the siphon it will rise to the same height in
both sides or legs of the siphon.
3. Tantalus' cup consists of a goblet containing a small
figure of a man. A siphon is concealed within
the figure, which empties the water from  the   Fig. 65.
goblet as fast as it is poured in, so that the glass
can never be filled.
4. Fig. 65 represents the cup with the siphon.
The figure of the man is omitted, in order that
the position of the siphon may be seen.
132. Water, by means of its weight or its force when
in motion, becomes a mechanical agent of great power.
It is used to propel or turn wheels of different construction, which, being connected with machinery of various
kinds, form  mills, &c.
131. What is the siphon? In what manner is the siphon used? How
can the siphon be used to. show the equilibrium of fluids? How high will
the liquid rise in each side of the siphon? What is Tantalus' cup? What
does Fig. 65 represent?
132. How, and for what purposes is water used as a mechanical agent?




96                rNATURAL PHILOSOPIIY.
There are three kinds of water-wheels, called undershot, overshot, and breast wheels.
133. The Overshot wheel is a wheel set in motion bv
the weight of water flowing upon it. It receives its
motion at the top.
Fig. 66 represents the overshot              Fig. 66.
wheel.  It consists of a wheel turn       ds
ing on an axis, (not represented in
the Fig.,) with compartments called
buckets, abcd, &c., at the circumference, which are successively filled
with water from the stream S.  The
weight of the water in the buckets
causes the wheel to turn, and the
buckets being gradually inverted are
emptied as they descend. It will be seen, from an inspection
of the figure, that the buckets in the descending side of the
wheel are always filled, or partly filled, while those in the opposite or ascending part are always empty until they are again
presented to the stream. This kind of wheel is the most
powerful of all the water-wheels.
134..The Undershot wheel-is a wheel which is moved
by the motion of the water. It receives its impulse at
the bottom.
Fig. 67 represents the                 Fig. 67.
undershot wheel.  Instead
of buckets at the circum-       _ 
ference, it is furnished with
plane surfaces, called floatboards, a b cd, &c., which
receive the impulse of, the
water, and cause the wheel 
to revolve.
How many kinds of water-wheels are there? What are they?
133. What is the overshot wheel?  Where does it receive its motion?
Explain Fig. 66. What causes the wheel to turn? How does this wheel
compare in power with the other water-wheels?
134. What is the undershot wheel? Where does it receive its motion?
What does Fig. 67 represent? How does this wheel differ from the
overshot?




PNEUMATICS.                           97
135. The Breast-wheel is a wheel which receives the
water at about half
its  own  height, or                        Fig. 68.
af the level of its
axis.  It is moved
both by the weight
and  the  motion  of
the water.*
Fig. 68 represents a
breast-wheel.   It  is
furnished either with
buckets, or with floatboards, fitting the water-course.
CHAPTER VII.
PNEUMATICS.
136. Pneumatics treats of the nature, mechanical
properties, and effects of air and similar fluids, distinguished by the name of aeriform fluids. t
137. The air. we breathe is an elastic fluid which surrounds the  earth, extending  to  an  indefinite  distance
above its surface, and constantly decreasing upwards in
density.1
* In all the wheels which have been described, the motion given to the
wheel is communicated to other machinery or gearing, as it is called, by
Ather wheels or pinions attached to the axis, such as have been described
ander the head of Mechanics.
t An adriform fluid is a fluid in the form of air, and, like air, generally
alvisible.
t The terms "rarefaction" and " rarefied" are applied to air when it is
mxpanded; and "condensation," or "' condensed," when it is compressed.
135. What is the breast-wheel? How is it set in motion? What figure
represents the breast-wheel? To what is the motion given to the wheels
which have been described, communicated?
136. Of what does Pneumatics treat?
137. What is the air which we breathe? 1How far does it extend above
the surface of the earth?
5




98                  NATURAL PHILOSOPHY.
It possesses many of the properties belonging to liquids in
general, besides several others, the result, or, perhaps, the
cause of its elasticity.  Its specific gravity is eight hundred
times less than that of water.*
138. Air, steam, vapor, gas, are all elastic fluids possessing the same mechanical properties.t Whatever,
therefore, is stated in relation to air, belongs in common
to all of these fluids.
139. Aeriform fluids have weight, but no cohesive
attraction.
140. Air has two principal properties, namely, GRAVITY and ELASTICITY.1
It has already been stated, that the air near the surface of the earth bears
the weight of that which is above it. Being compressed, therefore, by the
weight of that above it, it must exist in a condensed form near the surface
of the earth, while in the upper regions of the atmosphere, where there is
no pressure, it is highly rarefied. This condensation, or pressure, is very
similar to that of water at great depths in the sea.
* The air is necessary to animal and vegetable life, alid to combustion.
It is a very heterogeneous mixture, being filled with vapors of all kinds
It consists, however, of two principal ingredients, called oxygen and nitrogen, or azote; of the former of which there are 28 parts, and of the latter,
72 in a hundred. The air is not visible, because it is perfectly transparent.
It may be felt when it moves in the form of wind, or by swinging the hand
rapidly backwards and forwards.
t The chemical properties of liquids, fluids, &c., are not treated in the
sciences of Pneumatics, Hydraulics, or Hydrostatics, but belong peculiarly
to the science of Chemistry. They are not, therefore, described in this
work. But fluids possess all the properties of liquids, and the laws of
Hydrostatics and Hydraulics apply to them as well as to liquids.
t Besides these two principal properties, the operations of which produce most of the phenomena of Pneumatics, it will be recollected that
as air, although an invisible is yet a material substance, possessing all the
Does it possess properties common to liquids in general I How does its
specific gravity compare with that of water? Of what two principal ingredients does the air consist? What is the proportion of these parts to each
other?
138. What other fluids are named belonging to the class of elastic fluids?
139. Have the air and other similar fluids weight? With what power
alone has heat to contend in airiform fluids?
140 What two principal properties has the air?




PNEUMATICS.                          99
141. A column of air, having a base an inch square,
and reaching to the top of the atmosphere, weighs
about fifteen pounds. This pressure, like the pressure
of liquids, is exerted equally in all directions.*
142. The elasticity of air and other aeriform fluids is
that property by which they are increased or diminished in extension, according as they are compressed.t
This property exists in a much greater degree in air and
other similar fluids than in any other substance. In fact it
has no known limit; for when the pressure is removed from
any portion of air, it immediately expands to such a degree
that the smallest quantity will diffuse itself over an indefinitely
large space. And, on the contrary, when the pressure is increased, it will be compressed into indefinitely small dimensions.$
common properties of matter, it possesses also the common property of
impenetrability. This will be illustrated by experiments.
* It has been computed that the weight of the whole atmosphere is
equal to treat of a globe of lead sixty miles in diameter, or to five thousand
billions of tons.
t The pressure of the atmosphere caused by its weight is exerted on all
substances, internally and externally, and it is a necessary consequence of
its fluidity. The body of a man of common stature has a surface of about
2000 square inches; whence the pressure at 15 pounds per square inch
will be 30,000 pounds. The reason why this immense weight is not felt,
is, that the air within the body and its pores counterbalances the weight
of the external air. When the external pressure is artificially removed
from any part, it is immediately felt by the reaction of the internal air.
I Heat insinuates itself between the particles of bodies, and forces them
asunder, in opposition to the attraction of cohesion and of gravity; it therefore exerts its power against both the attraction of gravitation and the attraction of cohesion. But as the attraction of cohesion does not exist in
aeriform fluids, the expansive power of heat upon them has nothing to
contend with but gravity. Ally increase of temperature, therefore, expands an elastic fluid prodigiously, and a diminution of heat condenses it.
141. What is the weight of a column of air one inch square at the base,
and reaching to the top of the atmosphere? Is the pressure exerted equally
in all directions?
142. What is meant by the elasticity of the air? How do the adriforni
fluids differ from liquids? When is the air said to be rarefied? When
condensed?  Is the air, near the surface of the earth, rare or dense?




100                 NATURAL PrIILOSOrPHY.
143. Air becomes a mechanical agent by means of its
weight, its elasticity, its inertia, and its fluidity.*
144. A vacuum is a space from which air and every
other substance has been removed.
The Torricellian vacuum was discovered by Torricelli, and
was obtained in the following manner.  A  tube, closed at
one end, and about 32 inches long, was filled with mercury;
the open end was then covered with the finger so as to prevent the escape of the mercury, and the tube inverted and
plunged into a vessel of mercury; the finger was then removed
and the mercury permitted to run out of the tube. It was
found, however, that the mercury still remained in the tube to
the height of about thirty inches, leaving a vacuum at the top
of about two inches. This vacuum, called from the discoverer
the Torricellian vacuum, is the most perfect that has been discovered.t
* The fluidity of air invests it, as it invests all other liquids, with the
power of transmitting pressure. But it has already been shown, under
the head of Hydrostatics, that fluidity is a necessary consequence of the independent gravitation of the particles of a fluid. It may, therefore, be
included among the effects of weight.
The inertia of air is exhibited in the resistance which it opposes to motion, which has already been noticed under the head of Mechanics. This
is clearly seen in its effect upon failing bodies, as will be exemplified in the
experiments with the air-pump.
t As this is one of the most important discoveries of the science of
Pneumatics, it is thought to be deserving of a labored explanation. The
whole phenomenon is the result of the equilibrium of fluids. The atmosphere pressing by its weight (15 pounds on every square inch) on the surface of the mercury. in the vessel, counterpoised the column of mercury in
the tube when it was about 30 inches high, showing thereby that a column
of the atmosphere is equal in weight to a column of mercury of the same
base, having a height of 30 inches. Any increase or diminution in the
density of the air produces a corresponding alteration in its weight, and consequently, in its ability to sustain a longer or a shorter column of mercury.
Had water been used instead of mercury, it would have required a height
of about 33 feet to counterpoise the weight of the atmospheric column.
Other fluids may be used, but the perpendicular height of the column of
any fluid, to counterpoise the weight of the atmosphere, must be as much
143. How does the air become a mechanical agent?
144. What is a vacuum?




PNEUMATICS.                         101
115. The barometer is an instrument to measure the
weight of the atmosphere, and thereby to indicate the
variations of the weather.*
1. Fig. 69 represents a barometer. It consists of a long
glass tube, about thirty-three inches in length,
closed at the upper end and filled with mercury.    Fig. 69
The tube is then inverted in a cup, or leather bag,
of mercury, on which the pressure of the atmo-             30
sphere is exerted.  As the tube is closed at the           28
top, it is evident that the mercury cannot de-,27
scend in the tube without producing a vacuum.
The pressure of the atmosphere (which is capable
of supporting a column of mercury of about 30
inches in height) prevents the descent of the
mercury; and the instrument, thus constructed,
becomes an implement for ascertaining the weight
of the atmosphere.  As the air varies in weight
or pressure, it must, of course, influence the mercury in the tube, which will rise or fall in exact proportion with
the pressure. When the air is thin and light, the pressure is
less, and the mercury will descend; and when the air is dense
and heavy, the mercury will rise. At the side of the tube
there is a scale, marked inches and tenths of an inch, to note
the rise and fall of the mercury.t
greater than that of mercury as the specific gravity of mercury exceeds
that of the fluid employed.
* The elasticity of the air causes all increase or diminution of its bulk,
according as it is affected by heat and cold; and thia increase and diminution of bulk materially affect its specific gravity. The height of a column
of mercury that can be sustained by a column of the atmosphere must,
therefore, be affected by the state of the atmosphere. The instrument
used to indicate these changes is called a barometer, from two Greek
words signifying a measure of the weight, that is, of the atmosphere. A
Thermometer is a measure of the heat, and a Hygrometer a measure of
the moisture of the atmosphere.
t Any other fluid may be used as well as mercury, provided the length
145. What is a barometer? What does the word barometer mean?
What is a thermometer? What does the word thermometer mean? What
is a hygrometer?  What does the word hygrometer mean?  What figure
represents a barometer? Explain its construction. What height of mercury is the pressure of the atmosphere capable of sustaining? What effect
has the pressure of the atmosphere on the mercury in the tube 7




1-02                NATURAL PIIILOSOPHY.
2. The pressure of the atmosphere on the mercury, in the bag
or cup of a barometer, being exerted on the principle of the
equilibrium of fluids, must vary according to the situation in
which the barometer is placed. For this reason it will be the
greatest in valleys and low situations, and least on the top of
high mountains. Hence the barometer is often used to ascertain the height of mountains and other. places above the level
of the sea.*
of the tube be extended in proportion to the specific gravity of the fluid,
Thus, a tube filled with water must be 33 feet long, because the atmosphere will support a column of water of that height.'Mercury is used,
therefore, in the construction of the barometer, because it does not require
so long a tube as any other fluid, It may here be remarked, that the aix
is the heaviest in dry weather, and that, consequently, the mercury will then
rise highest. In wet weather the dampness renders the air less salubrious,
and it appears, therefore, more heavy then, although it is, in fact, much
lighter. The greatest depression of the barometer occurs daily at about
4 o'clock, both in the morning and in the afternoon, and its highest elevation at about 10 o'clock morning and night. In summer, these extreme
points are reached an hour or two earlier in the morning, and as much
later in the afternoon.
* As the air diminishes in density, upwards, it follows, that it must be
more rare upon a hill than on a plain. In very elevated situations it is
so rare that it is scarcely fit for respiration, or breathing; and the expansion which takes place in the more dense air contained within the body is
often painful: it occasions distention, and sometimes causes the bursting
of the smaller blood-vessels, in the nose and ears. Besides, in such situations, we are more exposed both to heat and cold; for, though the atmosphere is itself transparent, its lower regions abound with vapors and
exhalations from the earth, which float in it, and act in some degree as a
covering, which preserves us equally from the intensity of the sun's rays,
and from the severity of the cold.
In what proportion does the mercury rise and fall? In what way can
barometers be made of other fluids? Why is mercury used in preference
to any other fluid? Is the air heaviest in wet or dry weather?  On what
principle is the pressure of the atmosphere on the mercury, in the cup of
a barometer, exerted? What follows from this? For what other purpose,
besides measuring the pressure of the atmosphere, and foretelling the variations of the weather, is the barometer used?  Is the air the more dense at
the surface of the earth or upon a hill? What is a thermometer?  What
figure represents a thermometer? Explain its construction.




PNEUMATICS.                        103
3. The thermometer is an instrument used to   Fig. 7O.
indicate the temperature of the atmosphere.  In
appearance it resembles a barometer, but it is
constructed on a different principle, and for a
different purpose.   It consists of a capillary
tube, closed at the top, and terminated with
a bulb, which is filled with mercury.*  As heat
expands and cold contracts most substances, it
follows, that in warm weather the mercury must
be expanded and will rise in the tube, and that
in cold weather it will contract and sink.  Hence
the instrument becomes a correct measure for the
heat and cold of the air.  A scalet is placed at
the side of the tube, to mark the degree of heat
or cold, as it is indicated by the rise and fall of
the mercury in the capillary tube.                     j
4. The hygrometer, for measuring the degree
of moisture in the air,t may be constructed of
any thing which contracts and expands by the moisture or
dryness of the atmosphere, such as most kinds of wood; catgut, twisted cord the beard of wild oats, &c.
* Ally other liquid which is expanded by heat and contracted by cold,
such as spirits of wine, &c., will answer instead of mercury.
t There are several different scales applied to the thermometer, of
which those of Fahrenheit, Reaumur, Delisle, and Celsius are the principal. The thermometer in common use in this country, is graduated by
Fahrenheit's scale, which, commencing with 0, or zero, extends upwards
to 212 degrees, the boiling point of water, and downwards to 20 or 30 degrees. The scales of Reaumur and Celsius fix zero at the freezing point
of water; and that of Delisle at the boiling point.
t By the action of the sun's heat upon the surface of the earth, whether
land or water, immense quantities of vapor are raised into the atmosphere,
supplying materials for all the water which is deposited again in the various forms of dew, fog, rain, snow, and hail. Experiments have been
made to show the quantity of moisture thus raised from the ground by the
heat of the sun. Dr. Watson found that an acre of ground, apparently
dry, and burnt up by the sun, dispersed into the air sixteen hundred galWhat effect have heat and cold on most substances?  What follows
from this? Whose scale is generally used in this country?  Foi what is
the hygrometer used? Of what kind of substances may it be construced?
What experiment is given in the note to show the quantity of moisture
raised from the ground by the heat of the sun?




104                NATURAL PHILOSOPHY.
146. The impenetrability of air prevents the ascent
of water into any inverted vessel, unless the air is first
permitted to escape.
1. If a tube, closed at one end, or an inverted tumbler, be
inserted at its open end, in a vessel of water, the water will
not rise in the tube or tumbler, to a level with the water in
the vessel, on account of the impenetrability of the air within
the tube. But if the tube be open at both ends, the water will
rise, because the air can escape through the upper end. It is
on this principle that the diving-bell (or the diver's bell, as it
is sometimes called) is constructed.
2. Fig. 71 represents a diving-bell. It consists of a large
heavy vessel, formed like a bell, (but may be made of any other
shape,) with the mouth open. It descends into the water with
ions of water in the space of twelve hours. His experiment was thus
made: he put a glass, mouth downwards, on a grass-plot, on which it had
not rained for above a month. In less than two minutes the inside was
covered with vapor; and in half an hour drops began to trickle down its
inside. The mouth of the glass was 20 square inches. There are 1296
square inches in a square yard, and 4840 square yards in an acre. When
the glass had stood a quarter of an hour, he wiped it with a piece oI
muslin, the weight of which had been previously ascertained. When
the glass had been wiped dry, he again weighed the muslin, and found
that its weight had been increased six grains by the water collected from
20 square inches of earth; a quantity equal to 1600 gallons, from an acre,
in 12 hours. A'nother experiment, after rain had fallen, gave a much
larger quantity. (She No. 9.)
When the atmosphere is colder than the earth, the vapor, which arises
from the ground, or a body of water, is condensed and becomes visible
This is the way that fog is produced. When the earth is colder than the
atmosphere, the moisture in the atmosphere condenses in the form of dew,
on the ground, or other surfaces.
Clouds are nothing more than vapor, condensed by the cold of the upper
regions of the atmosphere.
Rain is produced by the sudden cooling of large quantities of watery
vapor.
Snow and hail are produced in a similar manner, and differ from rain
only in the degree of cold which produces them.
146. Is air impenetrable, like other substances?  How is this shown?
Upon what principle is the diving-bell constructed? What figure represents the diving-bell? Why does not the water rise in the bell? Explain
the figure.




PNEUMATICS.                       105
its mouth downwards.  The air within            Fig. 71.
it having no outlet is compelled, by the
order of specific gravities, to ascend in
the bell, and thus (as water and air           P
cannot occupy the same space at the
same time) prevents the water from
rising in the bell. A person, therefore,
may descend with safety in the bell to
a great depth in the sea, and thus recover valuable articles that have been
lost. A constant supply of fresh air is
sent down, either by means of barrels,  D          A
or by a forcing-pump.  In the Fig., B
represents the bell with the diver in it.                B
C is a bent metallic tube attached to
one side and reaching the air within;
and P  is the forcing-pump  through
which air is forced into the bell.  The  C
forcing-pump is attached to the tube
by a joint at D.  When the bell descends to a great depth, the pressure
of the water condenses the air within the bell, and causes the
water to ascend in the bell. This is forced out by constant
accessions of fresh air, supplied as above mentioned. Great
care must be taken that a constant supply of fresh air is sent
down, otherwise the lives of those within the bell will be endangered.  The heated and impure air is alloved to escape
through a stop-cock in the upper part of theebell.
147. Water is raised in the common pump by means
of the pressure of the atmosphere on the surface of the
water. A vacuum being produced by raising the piston
or pump-box,* the water below is forced up by the atmospheric pressure, on the principle of the equilibrium
* In order to produce such a vacuum, it is necessary that the piston or
box should be accurately fitted to the bore of the pump; for if the air
above the piston has any means of rushing in to fill the vacuum, as it is
produced by the raising of the piston, the water will not ascend. The piston is generally worked by a lever, which is the handle of the pump, not
represented in the figure.
147. By what means is water raised in the common pump? How is
the pressure removed?
5*




106                NATURAL PHILOSOPIIY.
of fluids. On this principle the water can be raised
only to the height of about thirty-three feet, because
the pressure of the atmosphere will sustain a column of
water of that height only.
Fig. 72 represents the common pump, called    Fig. 72
the sucking-pump.  The body consists of a large
tube, or pipe, the lower end of which is to be
immersed in the water which it is designed to
raise.  P is the piston, V a valve* in the piston,
which, opening upwards, admits the water to
rise through it, but prevents its return. Y is a
similar valve in the body of the pump, below the
piston.  When the pump is not in action the
valves are closed by their own weight; but when
the piston is raised it draws up the column of
water which rested upon it, producing a vacuum
between the piston and the lower valve Y. The
water below, immediately rushes through the
lower valve, and fills the vacuum. When the
piston descends a second time, the water in the
body of the pump passes through the valve V,
and on the ascent of the piston is lifted up by
the piston, and a vacuum is again formed below,
which is immediately filled by the water rushing
through the lower valve Y. In this manner the body of the
pump is filled with water, until it reaches the spout S, where
it runs out in an interrupted stream.t
* A valve is a lid or cover, so contrived as to open a communication in
one way and close it in the other. Valves are made in different ways, according to the use for which they are intended. In the common pump,
they are generally made of thick leather partly covered with wood. In
the air-pump they are made of oiled silk, or thin leather softened with
oil. The clapper of a pair of bellows is a familiar specimen of a valve.
The valves of a pump are commonly called boxes.
t Although water can be raised by the atmospheric pressure only to the
height of 33 feet above the surface, the common pump is so constructed,
that after the pressure of the atmosphere has forced the water through the
What figure represents the common pump? Explain it. Which of the
mechanical powers is the handle of the pump? How high call water be
raised by the common pump? Why? Why is the common pump sometimes called the lifting-pump?




FNEUMATICS.                         107
148. The  forcing-pump  differs  from   the  common
pump in having  a forcing  power added  to raise the
water to any desired height.
1. Fig. 73 represents the forcing-            Fig. 73
pump.  The body and lower valve
V are similar to those in the com-  A
mon pump.  The piston P has no
valve, but is solid; when, therefore,              H
the vacuum  is produced above the
lower valve, the water on the de-           v
scent of the piston is forced through;     T
the tube into the reservoir or airvessel R, where it compresses the
air above it.  The air, by its elasticity, forces the water out through
the jet J in a continued stream  and
with great force. It is on this principle that fire-engines are constructed.
2. Sometimes a pipe with a valve in it is substituted for the
air-vessel; the water is then thrown out in a continued stream,
but not with so much force.
149. Wind is air put in motion.*
When any portion of the atmosphere is heated, it becomes
rarefied, its specific gravity is diminished, and it consequently
rises. The adjacent portions immediately rush into its place
to restore the equilibrium. This motion produces a current
which rushes into the rarefied spot from all directions. This is
what we call wind. The portions north of the rarefied spot,
valve in the body of the pump, and the descent of the piston has forced it
through the valve in the piston, it is lifted up, when the piston is raised.
For this reason, this pump is sometimes called the lifting pump. The
distance of the lower valve from the surface of the water must never exceed 32 feet; and in practice it must be much less.
* There are twp ways in which the motion of the air may arise. It
may be considered as an absolute motion of the air, rarefied by heat and
condensed by cold; or it may be only an apparent motion, caused by the
superior velocity of the earth in its daily revolution.
148. How does the forcing-pump differ from the common pump? What
figure represents the forcing-pump?  Explain it.
149. What is wind?  In what two ways may the motion of the air be
explained?  Explain the manner in which the air is put in motion.




108                 NATURAL P1IILOSOPfiY.
produce a north wind; those to the south produce a south
wind; while those to the east and west, in like manner, form
currents moving in opposite directions. At the rarefied spot,
agitated as it is by winds from  all directions, turbulent and
boisterous weather, whirlwinds, hurricanes, rain, thunder and
lightning prevail. This kind of weather occurs most frequently in the torrid zone, where the heat is greatest. The air being more rarefied there than in any other part of the globe, is
lighter, and, consequently, ascends: that about the polar
regions is continually flowing from the poles to the equator, to
restore the equilibrium; while the air rising from the equator
flows in an upper current towards the poles, so that the polar
regions may not be exhausted.* A regular east wind prevails
i From what has now been said, it appears that there is a circulation
of air in the atmosphere; the air in the lower strata flowing from the
poles to the equator, and in the upper strata flowing back from the equator
to the poles. It may here be remarked, that the periodical winds are
more regular at sea than on the land; and the reason of this is, that the
land reflects into the atmosphere a much greater quantity of the sun's rays
than the water; therefore, that part of the atmosphere which is over the
land is more heated and rarefied than that which is over the sea. This
occasions the wind to set in upon the land, as we find it regularly does on
the coast of Guinea and other countries in the torrid zone. There are certain winds called trade-winds, the theory of which may be easily explained
on the principle of rarefaction, affected as it is by the relative position of
the different parts of the earth with the sun, at different seasons of the
year and at various parts of the day. A knowledge of the laws by which
these winds are controlled, is of importance to the mariner. When the
place of the sun, with respect to the different positions of the earth at
the different seasons of the year is understood, it will be seen that they all
depend upon the same principle. The reason that the wind generally subsides at the going down of the sun, is, that the rarefaction of the air, in
the particular spot which produces the willd, diminishes as the sun declines, and consequently, the force of the wind abates. The great variety
of winds in the temperate zone is thus explained. The air is an exceedingly elastic fluid, yielding to the slightest pressure; the agitations in it,
therefore, caused by the regular winds, whose causes have been explained,
must extend every way to a great distance; and the air, therefore, ill all
climates will suffer more or less perturbation, according to the situation of
the country, the position of mountains, valleys, and a variety of other
causes. Hence every climate must be liable to variable winds. The
How are the north, south, east, and west winds produced?




PNEUMATICS.                       109
about the equator, caused by the rarefaction of the air produced by the sun in his daily course from east to west. This
wind, combining with that from the poles, causes a constant
northeast wind, for about thirty degrees north of the equator,
and a southeast wind at the same distance south of the
equator.
OF THE AIR-PUMP.
150. The air-pump is an instrument for exhausting
the air from a vessel prepared for the purpose. This
vessel is called a receiver, and is made of glass in order
that the effects of the removal of the air may be seen.
Air-pumps are made in a great variety of forms;
but all are constructed on the principle, that when any
portion of confined air is removed, the residue immediately expanding, by its elasticity fills the space occupied
by the portion that has been withdrawn.
1. From this statement it will appear that a perfect vacuum
can never be obtained by the air-pump as at present constructed. But so much of the air within a receiver may be exhausted
that the residue will be reduced to such a degree of rarity
as to subserve most of the practical purposes of a vacuum.
Fig. 74 represents a single barrel air-pump, used both for
condensing and exhausting.  A D is the stand or platform
of the instrument, which is screwed down to the table by
quality of winds is affected by the countries over which they pass; and
they are sometimes rendered pestilential by the heat of deserts, or the putrid exhalations of marshes and lakes. Thus, from the deserts of Africa,
Arabia, and the neighboring countries, a hot wind blows, called Samniel,
ol Simoom, which sometimes produces instant death. A similar wind
blows from the desert of Sahara, upon the western coast of Africa, called
the Harnmattan, producing a dryness and heat which is almost insupportipble, scorching like the blasts of a furnace
150. What is an air-pump? What is the vessel called from which the
air is exhausted? On what principle are all air-pumps constructed? Can
a perfect vacuum ever be obtained? Descrine the air-pump represented
by Fig. 74




110              NATURAL PHILOSOPHY.
means of a clamp, underneath, which is not repre-             Fig 74
sented in the figure. R is
the glass vessel or bulbed
receiver from  which the
air is to be exhausted. P
is a solid piston, accurate-                  R
ly fitted to the bore of
the cylinder, and H the
handle  by which  it is
moved.  The dotted line          B
T, represents the commu-              T
nication between the re- P
ceiver R and the barrel
B; it is a tube through
which the air, entering
at the opening I, on the plate of the pump, passes into the
barrel, through the exhausting valve E v. c v is the condensing valve, communicating with the barrel B by means of an
aperture near E, and opening outwards through the condensing
pipe p.
2. The operation of the pump is as follows: The piston P being
drawn upwards by the handle H, the air in the receiver R, expanding by its elasticity, passes by the aperture I through the
tube T, and through the exhausting valve E v into the barrel.
On the descent of the piston, the air cannot return through
that valve, because the valve opens upwards only: it must,
therefore, pass through the aperture, by the side of the valve,
and through the condensing valve c v into the pipe p, where
it passes out into the open air. It cannot return through the
condensing valve c v, because that valve opens outwards only.
By continuing this operation, every ascent and descent of the
piston P must render the air within the receiver R more and.
more rare, until its elastic power is exhausted. The receiver
is then said to be exhausted; and although it still contains a
small quantity of air, yet it is in so rare a state that the space
within the receiver is considered a vacuum.*
3. From the explanation which has been given of the operation of this air-pump, it will readily be seen that, by removing
the receiver R, and screwing any vessel to the pipe p, the air
* The only known method of procuring a perfect vacuum, is that pursued by Torricelli, which has already been explained.




PNEUMATICS.                         111l
may be condensed in the vessel. Thus the pump is made to
exhaust or to condense, without alteration.*
4. The double air-pump differs from the single air-pump, in
having two barrels and two pistons; which, instead of being
moved by the hand, are worked by means of a toothed wheel,
playing in notches of the piston rods.
5. Fig. 75 represents Wightman's patent lever air-pump,
belonging to "the Boston School Set."  This instrument is of
Fig. 75.
an improved construction, and differs from others in the facility with which it is worked. In this pump the piston is sta* Air-pumps in general are not adapted for condensation; that office
being performed by an instrument called " a condensiwg syringe," which
is an air-pump reversed, its valves being so arranged as to force air into
a chamber, instead of drawing it out. For this purpose, the valves open
illnwards in respect to the chamber, while in air-pumps they open outwards.
A gauge, constructed on the principle of the barometer, is sometimes
adjusted to the air-pump for the purpose of exhibiting the degree of exhaustion




112              NATURAL PIIILOSOPI1Y.
tionary, while motion is given to the barrel by means of the
lever H. The barrel is kept in a proper position by means of
polished steel guides.*
151. By means of the air-pump, many interesting
experiments may be performed, illustrating the gravity,
elasticity, fluidity, and inertia of air.
EXPERIMENTS ILLUSTRATING THE GRAVITY OF AIR.
1. Having adjusted the receiver to the plate of the airpump, exhaust the air and the receiver will be held firmly on
the plate. The force which confines it, is nothing more than
the weight of the external air, which, having no internal pressure to contend with, presses with a force of nearly 15 pouw
on every square inch of the external surface of the receiver.
N. B. The exact amount of pressure depends on the degree
of exhaustion, being at its maximum of 15 pounds when there
is a perfect vacuum. On readmitting the air the receiver may
be readily removed.
2. THE MAGDEBURGH CUPS OR HEMISPHERES. Fig. 76 represents the Magdeburgh cups or hemispheres. They consist of two
hollow brass cups, the edges of which are
accurately fitted together. They each have   Fig. 76.
a handle, to one of which a stop-cock is
fitted. The stop-cock, being attached to
one of the cups, is to be screwed to the
plate of the  air-pump, and left open.
Having joined the other cup to that on
the pump, exhaust the air from  within
them, turn the stop-cock to prevent its readmission, and screw the handle that had
been removed to the stop-cock. Two persons may then attempt to draw the cups
asunder.  It will be found that great
power is required to separate them; but,
on readmitting the air between them, by
turning the cock, they will fall asunder by
* Mr. Wightman has published a small volume, entitled, "A Con
panion to the Air-pump," which will be found a very convenient guide fo
the management of the pump, and the skilful performance of experiments
151. Give, in succession, the experiments made by means of the air
pump. Of the receiver Of the Magdeburgh cups.




PNEUMATICS.                      113
their own weight. When the air is exhausted from within
them, the pressure of the surrounding air upon the outside
keeps them united. This pressure being equal to a pressure
of fifteen pounds on every square inch of the surface, it follows,
that the larger the cups or hemispheres the more difficult it
will be to separate them.*
3. THE HAND-GLASS.  Fig. 77 is nothing
more than a tumbler, open at both ends, Fig. 77.
with the top and bottom ground smooth, so
as to fit the brass plate of the air-pump.
Placing it upon the plate, cover it closely
with the palm  of the hand, and work the
plump.  The air within the glass being thus
exhausted, the hand will be pressed down
by the weight of the air above it: on readmitting the air, the hand may be easily removed.
4. THE BLADDER-GLASS. Fig. 78 is a bell-         Pig. 78.
shaped glass, covered with a piece of bladder,
which is tied tightly around its neck.  Thus
prepared, it may be screwed to the plate of
the air-pump, or connected with it by means
of an elastic tube.  On exhausting the air
from the glass, the external pressure of the
air on the bladder will burst it inwards with
a loud explosion.
5. THE INDIA-RUBBER GLASS.  Fig. 79 is         Fig. 79.
a glass similar to the one represented in the 
last figure, covered with india-rubber.  The
same experiments may be made with this as
were mentioned in the last article, but with
different results.  Instead of bursting, the
india-rubber will be pressed inwards the whole depth of the
glass.
* Otto Guericke, the inventor of the air-pump, prepared two hemispheres, two feet in diameter, and having accurately fitted them together,
and exhausted the air, 30 horses harnessed to them were unable to sep.
arate them. When more horses were added, the hemispheres parted with
a loud report.
Explain the experiment with the hand-glass. Of the bladder-glass. Of
the india-rubber glass.




114               NATURAL PHILOSOPHY.
6. THE FOUNTAIN-GLASS AND JET. Fig. 80 represents the
jet, which is a small brass tube. Fig. 81 is the fountainglass. The experiment with these inFig. 80. struments is designed to  show  the   Fig. 81.
pressure of the atmosphere on the
surface of liquids.  Screw the straight
jet to the stop-cock, the stop-cock to
the fountain-glass, with the straight jet
inside of the fountain-glass, and the
lower end of the stop-cock to the plate
of the air-pump, and then open the
stop-cock.  Having exhausted the air
from the fountain-glass, close the stopcock, remove the glass from the pump, and, immersing, it in a vessel of water, open the stopcock. The pressure of the air on the surface of the water will
cause it to rush up into the glass like a fountain.'7. PNEUMATIC SCALES FOR WEIGHING AIR. Fig. 82 represents the flask or glass vessel and scales for weighing air.
Weigh the flask when full of air: then
exhaust the air and weigh the flask
again. The difference between its present and former weight is the weight
of the air that was contained in the          Fig.
flask.
8. THE SUCKER.  A  circular piece
of wet leather, with a string attached
to the centre, being pressed upon a
smooth surface, will adhere with considerable tenacity, when drawn upwards by the string. The
string in this case must be attached to the leather so that no
air can pass under the leather.
9. THE MERCURIAL OR WATER TUBE. Exhaust the air from
a glass tube three feet long, fitted with a stop-cock at one
end, and then immerse it in a vessel containing mercury or
water. On turning the stop-cock, the mercury will rise to the
height of nearly 30 inches; or, if immersed in water, the
water will rise and fill the tube, and would fill it were it 30
feet long. This experiment shows the manner in which water
is raised to the boxes or valves in common water-pumps.
Explain the experiment of the foultain-glass and jet. Explain the
Dneu:natic scales f)r weighing air. Explain the sucker. Mercurial tube.




PNEUMATICS.                     115
EXPERIMENTS SHOWING THE ELASTICITY OF AIR.
1. Place an india-rubber bag, or a bladder, partly inflated,
and tightly closed, under the receiver, and on exhausting the air,
the air within the bag or bladder expanding, will fill the bag.
On readmitting the air, the bag will collapse. The experiment
may also be made with some kinds of shrivelled fruit, if the
skin be sound.  The internal air expanding will give the fruit
a fresh and plump appearance, which will disappear on the readmission of the air.
2. The same principle may be illustrated by the indiarubber and bladder glasses, if they have stop-cocks to confine
the air.
3. A small bladder partly filled with air may be sunk in a
vessel of water by means of a weight, and placed under the
receiver. On exhausting the air from the receiver, the air in
the bladder will expand, and its specific gravity being thus
diminished, the bladder with the weight will rise. On readmitting the air the bladder will sink again.
4. AIR CONTAINED IN WATER AND IN WOOD. Place a vessel
of water under the receiver, and on exhausting the air from the
receiver, the air in the water previously invisible will make its
appearance in the form of bubbles, presenting the semblance
of ebullition.
5. A piece of light porous wood being immersed in the
water below the surface, the air will be seen issuing in bubbles from the pores of the wood.
6. THE PNEUMATIC BALLOON. Fig. 83 repre-    Fg. 83
sents a small glass balloon with its car immersed in a jar of water, and placed under a
receiver.  On exhausting the air, the air within
the balloon expanding, gives it buoyancy, and it
will rise in the jar. On readmitting the air the
balloon will sink.
7. The experiment may be performed without the air-pump by covering the jar with some
elastic substance, as india-rubber. By pressing on the elastic covering with the finger
the air will be condensed, the water will rise
in the balloon, and it will sink. On removing
the pressure, the air in the balloon expandExplain the first experiment.-2d, 3d. 4th. 5th. 6th. 7th.




1 16                NATURAL PHILOSOPHY.
ing, will expel part of'the water and the balloon will rise. This
is the more convenient mode of performing the experiment, as
it can be repeated at pleasure without resort to the pump.*
8. The following is a full explanation: —The pressure on
the top of the vessel first condenses the air between the cover
and the surface of the water; —this condensation presses upon
the water below, and as this pressure affects every portion
of the water throughout its whole extent, the water, by its
upward pressure, compresses the air within the balloon, and
makes room for the ascent of more water into the balloon so
as to alter the specific gravity of the balloon, and cause it to
sink. As soon as the pressure ceases, the elasticity of the air
in the balloon drives out the lately entered water, and restoring
the former lightness to the balloon causes it to rise. If, in the
commencement of this experiment, the balloon be made to
have a specific gravity too near that of water, it will not rise
of itself, after once reaching the bottom, because the pressure
of the water then above it will perpetuate the condensation of
the air which caused it to descend. It may even then, however, be made to rise, if the perpendicular height of the water
above it be diminished by inclining the vessel to one side.
9. This experiment proves many things; namely:
First. The materiality of air, by the pressure of the hand
* This experiment exhibits the principle on which the well-known glass
figure, called the Cartesian Devil, is constructed; and it may be thus explained: several images of glass, hollow within, and each having a small
opening at the heel by which water may pass in and out, may be made to
manceuvre in a vessel of water. Place them in a vessel in the same manner
with the balloon, but by allowing different quantities of water to enter the
apertures in the images, cause them to differ a little from one another in
specific gravity. Then, when a pressure is exerted on the cover, the
heaviest will descend first, and the others follow in the' order of their specific gravity; and they will stop or return to the surface in reverse order,
when the pressure ceases. A person exhibiting these figures to spectators
who do not understand them, while appearing carelessly to rest his hand
on the cover of the vessel, seems to have the power of ordering their movements by his will. If the vessel containing the figures be inverted, and
the cover be placed over a hole in the table, through which, unobserved,
pressure call be made by a rod rising through the hole, and obeying the foot
of the exhibiter, the most surprising evolutions may be produced among
the figures, in perfect obedience to the word of command.
8. Explain, in full, the experiment of the glass figures. 9. Explain all
that this experiment proves. Explain the condensing jar.




PNEUMATICS.                    117
on the top being communicated to the water below through
the air in the upper part of the vessel.
Secondly. The compressibility of air, by what happens in the
globe before it descends.
Thirdly. The elasticity, or elastic force of air, when the
water is expelled from the globe, on removing the pressure.
Fourthly. The lightness of air, in the buoyancy of the globe.
Fifthly. It shows that the pressure of a liquid is exerted in
all directions, because the effects happen in whatever position
the jar be held.
Sixthly. It shows that pressure is as the depth, because less
pressure of the hand is required, the farther the globe has descended in the water.
Seventhly. It exemplifies many circumstances of fluid support. A person, therefore, who is familiar with this experiment, and can explain it, has learned the principal truths of
Hydrostatics and Pneumatics.
EXPERIMENTS WITH CONDENSED AIR.
1. THE CONDENSING AND EXHAUSTING SYRINGE. The condensing syringe is the air-pump reversed.  The exhausting
syringe is the simple air-pump without its plate or stand.
These implements are used respectively with such parts of the
apparatus as cannot conveniently be attached to the airpump; and as an addition to
such pumps as do not perform               Fig. 84.
the double  office of exhaustion
and condensation.  In some sets
of apparatus the condensing and
exhausting syringes are united, and
are made to perform  each office
respectively, by merely reversing
the part which contains the valve.
2. THE  AIR-CHAMBER.   The
air-chamber, Fig. 83, is a hollow
brass globe prepared for the reception of a stop-cock, and is designed for the reception of condensed air. It is made in different
forms in different sets, and is used
by screwing it to a condensing
pump or a condensing syringe.




1 ]  NATURAL PHILOSOPHY.
3. STRAIGHT AND REVOLVING JETS FROM CONDENSED AIR.
Fill the air-chamber (Fig. 84) partly with water and then condense the air. Then confine the air by turning the cock;
after which unscrew it from the air-pump, and screw on the
straight or the revolving jet. Then open the stop-cock, and the
water will be thrown from the chamber in the one case, in
a straight continued stream, in the other in the form of a
wheel.  Figs. 85 and 86            Fig. 85.          Fig. 86.
represent a view of the
straight and the revolving
jets. In the revolving jet
the water is thrown from "
two small apertures made
at each end on opposite sides, to assist the revolution.
The circular motion is caused by the reaction of the
water on the sides of the arms opposite the jets; for
as the water is forced into the tubes, it exerts an equal
pressure on all sides of the tubes, and as the pressure is re
lieved on one side by the jet-hole, the arm is caused to revolvt
in a contrary direction. This experiment performed with tht
straight jet, illustrates the principle on which " Hero's ball'"
and Hero's fountain are constructed.
4. THE PRINCIPLE OF THE AIR-GUN. With the air-chamber
as in the last experiments, a small brass cylinder or gun-barrel,
Fig. 87, may be substituted for the jets, and loaded
with a small shot or paper ball.  On turning the  Fig.87.
cock quickly, the condensed air rushing out will throw
the shot to a considerable distance.  In this way the
air-gun operates, an apparatus resembling the lock of
a gun being substituted for the stop-cock, by which a 
small portion only of the condensed air is admitted to
escape at a time, so that the chamber being once filled    |
will afford two or three dozen discharges.  The force
of the air-gun has never been equal to more than a
fifteenth of the force of a common charge of powder, and the
loudness of the report made in its discharge is always as great
in proportion to its force as that of the common gun.
5. Condensed air may be weighed in the air-chamber; but
in estimating its weight, the temperature of the room must always be taken into consideration, as the density of air is
materially affected by heat and cold.
Explain the jets. The air-gun.




PNEUMATICS.                       119
EXPERIMENTS SHOWING THE INERTIA OF AIR.
THE GUINEA AND FEATHER DROP. The inertia of air is
shown by the guinea and feather drop, exhibiting the resistance
which the air opposes to falling bodies. This apparatus is made
in different forms, some having shelves    Fig. 89
Fig. 88. on which the guinea and feather rest;
and when the air is exhausted they
are made to fall by the turning of
a handle.  A better form is that rep4)O   resented  in Fig. 88, in which  the
guinea and  feather (or a piece of
brass substituted for the guinea) are
enclosed, and the  apparatus  being
screwed to the plate of the pump,
the air is exhausted;  a stop-cock
turned to prevent the readmission of
the air, and the apparatus being then
unscrewed, the experiment may be repeatedly
showed by one exhaustion of the air.  It will
then appear that every time the apparatus is
inverted, the guinea and the feather will fall
simultaneously.  The two forms of the guinea
and feather drop are exhibited in Figs. 88 and
89, one of which, Fig. 88, is furnished with a stop-cock,* the
other, Fig. 89, with shelves.
EXPERIMENTS SHOWING THE FLUIDITY OF AIR.
1. THE WEIGHT-LIFTER. The upward pressure of the air,
one of the properties of its fluidity, may be exhibited by an
apparatus called the weight-lifter, made in different forms,
but all on the same principle. The one represented in Fig. 90
* Most sets of philosophical apparatus are furnished with stop-cocks
and elastic tubes, for the purpose of connecting the several parts with the
pump or with one another. In selecting the apparatus, it is important
to have the screws of the stop-cocks, and of all the apparatus of similar
thread, in order that every article may subserve as many purposes as
possible. This precaution is suggested by economy as well as by convenience.
Explain the experiment with the guinea and feather. Also the weightlifter.




120              NATURAL PHILOSOPHY.
consists of a glass tube, of large
bore, set in a strong case or stand,      Fig. 90.
supported by three legs. A piston is
accurately fitted to the bore of the
tube, and a hook is attached to the bottom of the piston, from which weights
are to be suspended. One end of the
elastic tube is to be screwed to the
plate of the pump, and the other end
attached to the top of this instrument.
The air being then exhausted from the       w
tube, the weights will be raised the
whole length of the glass. The number of pounds weight that can be raised
by this instrument may be estimated by multiplying the num.ber of square inches in the bottom of the piston by fifteen.
2. THE PNEUMATIC SHOWER-BATH. On the principle of the
upward pressure of the air, the pneumatic shower-bath is
constructed. It consists of a tin vessel perforated with holes
in the bottom for the shower, and having an aperture at the
top, which is opened or closed at pleasure by means of a spring
valve. [Instead of the spring valve, a bent tube may be
brought round from the top down the side of the vessel, with
an aperture in the tube below the bottom of the vessel which
may be covered with the thumb.] On immersing the vessel
thus constructed in a pail of water, with the valve open, and
the tube (if it have one) on the outside of the pail, the water
will fill the vessel. The aperture then being closed with the
spring or with the thumb, and the vessel being lifted out of
the water, the upward pressure of the air will confine the
water in the vessel. On removing the thumb, or opening the
valve, the water will descend in a shower until the vessel is
emptied.
MISCELLANEOUS EXPERIMENTS DEPENDING ON TWO OR
MORE OF THE PROPERTIES OF AIR.
1. TIIE BOLT-HEAD AND JAR. Fig. 91, a glass globe with
a long neck, called a bolt-head, (or any long-necked bottle,)
partly filled with water, is inverted in a jar of water, (colored
with a few drops of red ink or any coloring matter, in order that
Explain the pneumatic shower-bath.




PNEUMIA'TICS.                   121
the effects may be more distinctly visible,) and   Fig. 91
placed under the receiver. On exhausting the air
in the receiver, the air in the upper part of the bolthead expanding, expels the water, showing the
elasticity of the air.  On readmitting the air to
the receiver, as it cannot return into the bolt-.head, the pressure on the surface of the water in
thlle jar, forces the water into the bolt-head,
showing the pressure of the air caused by its
weight.  The experiment may be repeated with
the bolt-heiad without any water, and on the rer;dnission of the air, the water will nearly fill
the bolt-head, affording an accurate test of the
degree of exhaustion.
2. T'HE TRANSFER OF FLUIDS FROM ONE VESSEL TO ANOTHER.
The experiment may be made with two bottles tightly closed.
Let one be partly filled with water, and the two connected by a
bent tube, connecting the interior of the empty bottle with the
water of the other, and extending nearly to the bottom of the
water. On exhausting the air from the empty bottle, the
water will pass to the other, and on readmitting the air the
water will return to its original position, so long as the lower
end of the bent tube is above the surface.
EXPERIMENTS WITH THE SIPHON.
1. Close the shorter end of the siphon with the finger or
with a stop-cock, and pour mercury or water into the longer
side. The air contained in the shorter side will prevent the
liquid from rising in the shorter side.  But if the shorter end
be opened, so as to afford free passage outwards for the air,
the fluid will rise to an equilibrium in both arms of the siphon.
2., Pour any liquid into the longer arm of the siphon until
the shorter arm is filled. Then close the shorter end, to prevent the admission of the air; the siphon may then be turned
in anv direction and the fluid will not run out, on account of
the pressure of the atmosphere against it. But if the shorter
end be unstopped, the fluid will run out freely.
Explain the transfer of fluids. Explain the first experiment with the
siphon. Also the second.
6




122               NATURAL PHILOSOPHY.
AIR ESSENTIAL TO ANIMAL LIFE.
If an animal be placed under the receiver, and the air exhausted, it will immediately droop, and if the air. be not
speedily readmitted it will die.
AIR ESSENTIAL TO COMBUSTION.
Place a lighted taper, cigar, or any other substance that
will produce smoke, under the receiver, and exhaust the air;
the light will be extinguished, and the smoke will fall, instead
of rising. If the air be readmitted, the smoke will ascend.
THE PRESSURE OF THE AIR RETARDS EBULLITION.*
1. Ether, alcohol, and other distilled liquors, or boiling
water, placed under the receiver, will appear to boil when
the air is exhausted.
2. The existence of many bodies in a liquid form depends
on the weight or pressure of the atmosphere upon them. The
same force, likewise, prevents the gases which exist in fluid
and solid bodies from disengaging themselves. If, by rarefying the air, the pressure on these bodies be diminished, they
either assume the form of vapors, or else the gas detaches
itself altogether fiom the other body. The following experiment proves this: place a quantity of lukewarm water, milk,
or alcohol, under a receiver, and exhaust the air, and the liquid
will either pass off in vapor, or will have the appearance of
boiling.
3. An experiment to prove that the pressure of the atmosphere preserves some bodies in the liquid form, may thus be
performed: fill a long vial, or a tube closed at one end, with
water, and invert it in a vessel of water. The atmospheric
pressure will retain the water in the vial. Then by means
of a bent tube introduce a few drops of sulphuric ether, which,
by reason of their small specific gravity, will ascend to the top
* EBULLITION. The operation of boiling. The agitation of liquor by
heat, which throws it up into bubbles.
What takes place when an animal is placed under an exhausted receiver? Is air essential to combustion? How is this proved?
How is it shown that air prevents ebullition? Give the 1st, 2d, and 3d
experiments.




PNEUMATICS.                    123
of the vial, expelling an equal bulk of water. Place the whole
under the receiver, and exhaust the air, and the ether will be
seen to assume the gaseous form, expanding in proportion to
the rarefaction of the air under the receiver, so that it gradually
expels the water froin the vial, and fills up the entire space
itself. On readmitting the air, the ether becomes condensed,
and the water will reascend into the vial.
4. A simple and interesting experiment connected with the
science of chemistry, may thus be performed by means of the
air-pump. A watch-glass, containing water, is placed over a
small vessel containing sulphuric acid, and put under the
bulbed receiver.  When the air is exhausted, vapor will freely
rise from the water, and be quickly absorbed by the acid. An
intense degree of cold is thus produced, and the water will
freeze.
5. In the above experiment, if ether be used instead of the
acid, the ether will evaporate instead of the water, and in the
process of evaporation, depriving the water of its heat, the
water will freeze. These two experiments, apparently similar
in effects, namely, the freezing of the water, depend upon two
different principles which pertain to the science of chemistry.
THE PNEUMATIC PARADOX.
An interesting experiment, illustrative of the pneumatic
paradox, may be thus performed:-Pass a small open tube,
(as a piece of quill,) through the centre of a circular card two
or three inches in diameter, and cement it, the lower end passing down, and the upper just even with the card. Then pass
a pin through the centre of another similar card, and place it
on the former with the pin projecting into the tube to prevent
the upper card from sliding off. It will then be impossible to
displace the upper card by blowing through the quill, on account of the adhesion produced by the current passing between
the discs. On this principle, smoky chimneys have been remedied, and the office of ventilation more effectually performed.
Give tha 4th experiment to show that air prevents ebullition. Also the
fifth. Explain the pneumatic paradox.




121               NATURAL PHILOSOPHY.
CHAPTER VIII.
ACOUSTICS.
152. Acoustics is the science which treats of the
nature and laws of sound. It includes the theory of
musical concord or harmony.
153. Sound is caused by a tremulous or vibratory
motion of the air.
1. If a bell be rung under an exhausted receiver, no sound
can be heard from it; but when the air is admitted to surround
the bell, the vibrations immediately produce sound.
2. Again, if the experiment be made by enclosing the bell in
a small receiver, full of air, and placing that under another
receiver, from which the air can be withdrawn; though the bell,
when struck, must then produce sound, as usual, yet it will
not be heard if the outer receiver be well exhausted, and care
be taken to prevent the vibrations from being communicated
through any solid part of the apparatus; because there is no
medium through which the vibrations of the bell, in the smaller
receiver, can be communicated to the ear.
154. Sounds are louder when the air surrounding the
sonorous body is dense, than when it is in a rarefied
state.
For this reason the sound of a bell is louder in cold than in
warm weather; and sound of any kind is transmitted to a
greater distance in cold, clear weather, than in a warm sultry
day. On the tops of mountains, where the air is rare, the human voice can be heard only at the distance of a few rods;
and the firing of a gun produces a sound scarcely louder than
the cracking of a whip.
152. What is that science called which treats of the nature and laws of
sound? What does it include?
153. What causes sound? What illustrations are given to prove this?
154. In what proportion are sounds loud or faint? Why does a bell
sound louder in cold than in warm weather? Why is sound fainter on the
top of a mountain than near the surface of the earth?




ACOUSTICS.                       125
155. Sonorous bodies are those which produce clear,
distinct, regular, and durable sounds, such as a bell, a
drum, wind instruments, musical strings and glasses.
These vibrations can  be communicated to a distance
not only through the air, but also through liquids and
solid bodies.
156. Bodies owe their sonorous property to their
elasticity.*
157. The sound produced by a musical string is
caused by its vibrations; and the height or depth of the
tone depends upon the rapidity of these vibrations.
Long strings vibrate with less rapidity than short onest
and for this reason the low tones in a musical instrument proceed from the long strings, and the high tones
from the short ones.
1. Fig. 92, A B represents a musical string. If it be drawn
up to G, its elasticity will not on-                   Fig. 92
ly carry it back...again, but will               --   -     -—..        -
give it a momen-.........
tum  which will  A — D 
carry  it to  H,              "'-. 
from  whence it
will successively
return to T, F, C, D, &c., until the resistance of the air entirely
destroys its motion.
2. The vibrations of a sonorous body give a tremulous motion to the air around it. similar to the motion communicated
tc smooth water when a stone is thrown into it.
i Although it is undoubtedly the case that all sonorous bodies are elastie, it is not to be inferred that all elastic bodies are sonorous.
155. What are sonorous bodies?
156. To what do sonorous bodies owe their sonorous property? Are all
elastic bodies sonorous?
157. What causes the sound produced by a musical string? Upon what
does the height and depth of the tone depend?  Which strings, in a musical instrument, produce the low tones?  Why? Explain Fig. 92.




126                NATURAL PHILOSOPHY.
158. The science of harmony is founded on the relation which the vibrations of sonorous bodies have to
each other.
Thus, when the vibrations of one string are double those of
another, the chord of an octave is produced. If the vibrations
of two strings be as two to-three, the chord of a fifth is produced.*   When the vibrations of two strings frequently coincide, they produce a musical chord; and when the coincidence
of the vibrations is unfrequent, discord is produced.
159. The quality of the sound produced by strings
depends upon their length, thickness, weight, and degree
of tension. The quality of the sound produced by wind
instruments depends upon their size, their length, and
their internal diameter.
Long and large strings, when loose, produce the lowest
tones; but different tones may be produced from the same
string, according to the degree of tension. Large wind instruments, also, produce the lowest tones; but different tones
may be produced from the same instrument, according to the
distance of the aperture for the escape of the wind, from the
aperture where it enters.
160. The quality of the sound of all musical instruments is affected by the changes in the temperature and
specific gravity of the atmosphere.
As heat expands and cold contracts the materials of which
* When music is made by the use of strings, the air is struck by the
body, and the sound is caused by the vibrations; when it is made by
pipes, the body is struck by the air; but as action and reaction are equal,
the effect is the same in both cases.
158. Upon what is the science of harmony founded? How is the chord
of an octave produced? How is the chord of a fifth produced? How is a
musical chord produced? A discord?
159. Upon what does the quality of the sound produced by strings depend? Upon what does that produced by wind instruments depend?
What strings produce the lowest tones?  How may different tones be produced from the same string? How may different tones be produced from
the same wind instrument?
160. What, in some degree, affects the quality of the sound of all musical instruments?  What effect have heat and cold on the materials of
which the instrument is made? What follows from this?




ACOUSTICS.                       127
the instrument is made, it follows, that the strings will have a
greater degree of tension, and that pipes and other wind instruments will be contracted, or shortened, in cold weather.
For this reason, most musical instruments are higher in tone
(or sharper) in cold weather, and lower in tone (or more flat)
in warm weather.
161. Sound is communicated more rapidly and with
greater power through solid bodies, than through the
air, or fluids. It is conducted by water about four
times quicker than by air, and by solids about twice as
rapidly as by water.
1. If a person lay his head on a long piece of timber, he
can hear the scratch of a pin at the other end, while it could
not be heard through the air.
2. If the ear be placed against a long, dry, brick wall, and
a person strike it once with a hammer, the sound will be heard
twice, because the wall will convey it with greater rapidity
than the air, though each will bring it to the ear.
162. The Stethescope is an instrument depending on
the power of solid bodies to convey sound.
It consists of a wooden cylinder, one end of which is applied
firmly to the breast, while the other end is brought to the ear.
By this means the action of the lungs may be distinctly heard.
The instrument, therefore, becomes useful in the hands of a
skilful physician, to ascertain the state of those organs.
163. Sound, passing through the air, moves at the
rate of 1142 feet in a second of time. This is the case
with all kinds of sound.
1. The softest whisper flies as fast as the loudest thunder,
Why are most musical instruments higher in tone, or sharper, in cold
weather?
161. Through which is sound communicated more rapidly, and with
greater power, through solid bodies, or the air? How fast is it conducted
by water?  How fast by solids? What examples are given to show that
sound is communicated more rapidly through solid bodies than the air or
fluids?
162. What is a stethescope? Of what does it consist?  For what is it
used?
163. How fast does sound move? Does the force or direction of the
wvv.id make any difference in its velocity?




128                 NATURAL PHILOSOPHY.
and the force or direc-tion of the wind makes but slight difference in its velocity.
2. This uniform velocity of sound enables us to determine
the distance of an object from  which it proceeds.  If, for instance, the light of a gun, fired at sea, be seen a half of a
minute before the report is heard, the vessel must be at the
distance of six miles and a half. In the same manner, the distance of a thunder-cloud may be ascertained, by counting the
seconds between the appearance of the lightning and the noise
of the thunder, and multiplying them by 1142 feet.
164. An echo is produced by the vibrations of the air
meeting a hard and regular surface, such as a wall, a
rock, a mountain, and  being  reflected  back  to the ear,
thus producing the same sound a second, and sometimes
a third and fourth time.*
1. Speaking-trumpets are constructed on the principle of
the reflection of sound.
2. The voice, instead of being diffused in the open air, is
* From this it is evident that no echo can be heard at sea, or on an extensive plain; because there is no object there to reflect the sound. An
echo is heard only when a person stands in such a situation as to hear
both the original and the reflected sound. The pupil will doubtless recollect what has been said in Mechanics with respect to the angles of incidence and reflection. Sound (as well as light, as will be explained under
the head of Optics) is communicated and reflected by the same law,
namely, that the angles of incidence and reflection are always equal. It
is not difficult, therefore, to ascertain the direction in which sound will proceed, whether it be direct or reflected. It is related of Dionysius, the
tyrant of Sicily, that he had a dungeon, (called the ear of Dionysius,) in
which the roof was so constructed as to collect the words, and even the
whispers, of the prisoners confined therein, and direct them along a hidden
conductor to the place where he sat to listen: and thus he became acquainted with the most secret expressions of his unhappy victims.
What advantage results from this uniform velocity of sound? How can
the distance of a thunder-cloud be ascertained?
164. How is an echo produced? Note. Why cannot an echo be heard
at sea, or on an extensive plain?  How must a person stand in order to
hear an echo? By what law is sound communicated and reflected? What
anecdote is related of Dionysius?  Upon what principle are speakinagtrumpets constructed? Explain the manner in which the vibrations of the
air are reflected.




ACOUSTICS.                         129
confined within the trumpet; and the vibrations which spread
and fall against the sides of the instrument are reflected according to the angle of incidence, and fall in the direction of
the vibrations, which proceed straight forward. The whole
of the vibrations are thus collected into a focus; and if the ear
be situated in or near that spot, the sound will be prodigiously
increased.
3. Hearing-trumpets, or the trumpets used by deaf persons,
are also constructed on the same principle; but as the voice
enters the large end of the trumpet, instead of the small ofie,
it is not so much confined, nor so much increased.
4. The musical instrument called the trumpet, acts, also, on
the same principle with the speaking-trumpet, so far as its
form tends to increase the sound.*
165. Sound, like light, after it has been  reflected
from  several surfaces, may be collected into one point,
as a focus, where it will be more audible than in any
other part; and on this principle whispering-galleries
may be constructed.
The famous whispering-gallery in the dome of St. Paul's
church, in London, is constructed on this principle.t Persons
at very remote parts of the building can carry on a conversation in a soft whisper, which will be distinctly audible to one
another, while others in the building cannot hear it; and the
ticking of a watch may be heard from side to side.
* The smooth and polished surface of the interior parts of certain kinds
of shells, particularly if they be spiral or undulating, fit them to collect and
reflect the various sounds which are taking place in the vicinity. Hence
the Cyprias, the Nautilus, and some other shells, when held near the ear,
give a continued sound, which resembles the roar of the distant ocean.
t There is a church in the town of Newburyport, in Massachusetts,
which, as was accidentally discovered, has the same property as a whisperinlg-gallery. Persons in opposite corners of the building, by facing the
wall, may carry on a conversation in the softest whisper, unnoticed by
others in any other part of the building. It is the building which contains
in its cemetery the remains of the distinguished preacher, Whitfield.
Upon what principle are hearing-trumpets constructed? How far does
the musical instrument, called the trumpet, act upon the principle of the
speaking-trumpet? Note. How can the continued sound, given by some
shells, when held near the ear, be explained?
165. Upon what principle may whispering-galleries be constructed?
/4*




130               NATURAL PHILOSOPHY.
166. Sounds may be conveyed to a much greater
distance through continuous tubes than through the
open air.
1. The tubes used to convey sounds are called acoustic
tubes. They are much used in public houses, stores, counting-rooms, &c., to convey communications from one room to
another.
2. The quality of sound is affected by the furniture of a
room, particularly the softer kinds, such as curtains, carpets,
&c., because, having little elasticity, they present surfaces unfavorable to vibrations.
3. For this reason, music always sounds better in rooms with
bare walls, without carpets, and without curtains. For the
same reason, a crowded audience increases the difficulty of
speaking.
4. As a general rule, it may be stated, that plane and
smooth surfaces reflect sound without dispersing it; convex surfaces lisiperse it, and concave surfaces collect it.
5. The air is a better conductor of sound when it is humid
than when it is dry.
6. A bell can be more distinctly heard just before a rain;
and sound is heard better in the night than in the day, because
the air is generally more damp in the night.
7. The distance to which sound may be heard depends upon
various circumstances, on which no definite calculations can be
predicated. Volcanoes, among the Andes, in South America,
have been heard at the distance of three hundred miles; naval
engagements have been heard two hundred; and even the
watchword " All's well," pronounced by the unassisted human
voice, has been heard from Old to New Gibraltar, a distance of
twelve miles.
167. The sound of the human voice is produced by
the vibration of two delicate membranes, situated at the
top of the windpipe, and between which the air from the
lungs passes.
166. In what way can sounds be conveyed to a much greater distance
than through the air? What are the tubes, used to convey sounds, called?
Why do the softer kinds of furniture in a room affect the quality of the
sound? What general rule is given with regard to the reflection of sound?
Is the air a better conductor when it is humid, or when it is dry? Why
can a sound be heard better in the night than in the day?
167. How is the sound of the human voice produced?




ACOUSTICS.                        131
1. The tones are varied from grave to acute, by opening or
contracting the passage; and they are regulated by the muscles belonging to the throat, by the tongue, and by the cheeks.
2. The management of the voice depends much upon cultivation; and although many persons can both speak and sing
with ease, and with great power, without much attention to its
culture, yet it is found that they who cultivate their voices by
use, acquire a degree of flexibility and ease in its management,
which, in a great measure, supplies the deficiency of nature.*
3. Ventriloquismt is the art of speaking in such a manner
as to cause the voice to appear to proceed from a distance.
* The reader is referred to Dr. Rush's very valuable work on the
" Philosophy of the Human Voice," for plain and practical instructions on
this subject. Dr. Barber's " Grammar of Elocution," and Parker's " Progressive Exercises in Rhetorical Reading," likewise contain the same instructions in a practical form. To the work of Dr. Rush, both of the
latter-lnentioned works are largely indebted.
t The word ventriloquism literally means, " speaking from the belly,"
and it is so defined in Chambers' Dictionary of Arts and Sciences. The
ventriloquist, by a singular management of the voice, seems to have it in
his power " to throw his voice" in any direction, so that the sound shall appear to proceed from that spot. The words are pronounced by the organs
usually employed for that purpose, but in such a manner as to give little
or no motion to the lips, the organs chiefly concerned being those of the
throat and tongue. The variety of sounds which the human voice is capable of thus producing is altogether beyond common belief, and, indeed,
is truly surprising. Adepts in this art will mimic the voices of all ages
and conditions of human life, from the smallest infant to the tremulous
voice of tottering age, and from the intoxicated foreign beggar to the
high-bred, artificial tones of the fashionable lady. Some will also imitate
the warbling of the nightingale, the loud tones of the whip-poor-will, and
the scream of the peacock, with equal truth and facility. Nor are these
arts confined to professed imitators; for in many villages boys may be
found, who are in the habit of imitating the brawling and spitting of cats,
in-such a manner as to deceive almost every hearer.
The human voice is also capable of imitating almost every inanimate
sound. Thus, the turning and occasional creaking of a grindstone, with
the rush of the water, the sawing of wood, the trundling and creaking of a
wheelbarrow, the drawing out of bottle-corks, and the gurgling of the
flowing liquor, the sound of air rushing through a crevice on a wintry
How are the toues varied and regulated? Upon what does the management of the voice depend?  What is ventriloquism?




132                NATURAL PHILOSOPHY.
4. The art of ventriloquism  was not unknown to the ancients; and it is supposed by some authors that the fafious
responses of the oracles at Delphi, at Ephesus, &c., were delivered by persons who possessed this faculty.  There is no
doubt that many apparently wonderful pieces of deception,
which, in the days of superstition and ignorance, were considered as little short of miracles, were performed by means
of ventriloquism. Thus houses have been made to appear
haunted, voices have been heard from tombs, and the dead
have been made to appear to speak, to the great dismay of
the neighborhood, by means of this wonderful art.
5. Ventriloquism is, without doubt, in great measure the
gift of nature; but many persons can, with a little practice,
utter sounds and pronounce words without opening the lips or
moving the muscles of the face; and this appears to be the
great secret of the art.
CHAPTER IX.
PYRONOMICS, OR THE LAWS OF HEAT.
168. Pyronomics is the science which treats of the
laws, the properties, and operations of heat.*
1. The nature of heat is unknown; but it has been proved
that the addition of heat to any substance produces no perceptible alteration in the weight of that substance.  Hence it
is inferred that heat is imponderable.
2. Heat pervades all bodies, insinuating itself, more or less,
night, and a great variety of other noises of the same kind, are imitated by
the voice so exactly, as to deceive any hearer who does not know whence
they proceed.
* Heat is undoubtedly a positive substance or quality. Cold is merely
negative, being only the absence of heat
Was this art known to the ancients? What is supposed, by some authors, concernilg the responses at Delphi, Ephesus, &c.? Is ventriloquism
a natural gift, or an acquired one?
168. What is Pyronomics?  What is said in regard to the nature of
heat? Is it ponderable or imponderable?




r PRONOMICS.                         133
betlween thei particles, and forcing them  asunder.  Heat, and
the attraction of cohlesion, constantly act in opposition to each
other;'hence the moire a body is heated, the more its particles
will be separated.
3. The effect of heat in separating the particles of different
kinds of substances is seen in the melting of solids, such as
metals, wax, butter, &c.  The heat insinuates itself between
the particles, and forces them asunder. These particles then
are removed from that degree of proximity to each other within which cohesive attraction exists, and the body is reduced to
a fluid form. When the heat is removed the bodies return to
their former solid state.*
4. Heat passes through some bodies with more difficulty
* Of all the effects of heat, that produced upon water is, perhaps, the
most remarkable. The particles are totally separated and converted into
steam or vapor, and their extension is wonderfully increased. The steam
which arises from boiling water is nothing more than portions of the water
heated. The heat insinuates itself between the particles of the water, and
-forces them asunder. When deprived of the heat, the particles will unite
in the form of drops of water. This fact can be seen by holding a cold
plate over boiling water. The steam rising from the water will be condensed into drops on the bottom of the plate. The air which we breathe
generally contains a considerable portion of moisture. On a cold day, this
moisture condenses on the glass in the windows, and becomes visible. We
see it also collected into drops on the outside of a tumbler or other vessel
containing cold water in warmn  weather. Heat also produces most remarkable effects upon air, causing it to expand to a wonderful extent,
while the absence of heat causes it to shrink or contract into very small
dimensions. The attraction of cohesion causes the small watery particles
which compose mist or vapor to unite together in the form of drops of
water. It is thus that rain is produced. The clouds consist of mist or
vapor expanded by heat. They rise to the cold regions of the skies, where
the particles of vapor lose their heat, and then, uniting in drops, fall to the
earth. But so long as they retain their heat, the attraction of cohesion can
have no influence upon them, and they will continue to exist in the form
of steam, vapor, or mist.
What effect has heat upon bodies?  What two forces continually act
in opposition to each other? In what can the effect of heat be seen? How
does it separate the particles?  What would be the effect were the heat
removed? Upon what has heat the most remarkable effect?  How does
it affect it?  What effect has heat upon air?  How is rain produced?
What is stated with regard to heat?




134                 NATURAL PLHILOSOPIIY.
than through others; but there is no kind of matter which can
completely arrest its progress.*
169. The principal effects of heat are three, namely:
1st. Heat expands most substances.
2d. It converts them  from  a solid to a fluid state.
3d  It destroys their texture by combustion.t
* The thermometer, an instrument designed to measure degrees of heat,
has already been described, in connexion with the barometer, under the
head of Pneumatics. Heat, under the name of caloric, is properly a subject of consideration in the science of Chemistry. It exists in two states,
called, respectively, free heat and latent heat. Free heat, or free caloric,
is that which is perceptible to the senses, as the heat of a fire, the heat of
the sun, &c. Latent heat is that which exists in most kinds of substances,
but is not perceptible to the senses, until it is brought out by mechanical
or chemical action. Thus, when a piece of cold iron is hammered upon
an anvil, it becomes intensely heated; and when a small portion of sulphuric acid, or vitriol, is poured into a vial of cold water, the vial and the
liquid immediately become hot. A further illustration of the existence of
latent or concealed heat is given at the fireside every day. A portion of cold
fuel is placed upon the grate or hearth, and a spark is applied to kindle the
fire which warms us. It is evident that the heat given out by the fuel,
when ignited, does not all proceed from the spark, nor can we perceive it
in the fuel; it must, therefore, have existed somewhere in a latent state.
It is, however, the effects of free heat, or free caloric, which are embraced
in the science of Pyronomics. The subject of latent heat belongs more
properly to the science of Chemistry.
The terms heat and cold, as they are generally used, are merely relative terms; for a substance which in one person would excite the sensation
of heat, might, at the same time, seem cold to another. Thus, also, to the
same individual, the same thing may be made to appear, relatively, both
warm and cold. If, for instance, a person were to hold one hand near to
a warm fire, and the other on a cold stone, or marble slab, and then plunge
both into a basin of lukewarm water, the liquid would appear cold to the
warm hand and warm to the cold one.
t These effects do not take place in all substances. Some substances
Can the progress of heat be arrested? What is caloric? In what two
states does heat exist?  What is free heat?  Give some examples of free
heat. What is latent heat?  Give some examples of latent heat. How
are the terms heat and cold generally used? What illustration of this is
given?
169.'What are the three principal effects of heat on bodies to which it
is applied? Give an example of each'effect.




PYRONOMICS.                       135
170. Heat tends to diffuse itself equally through all
substances.
1. If a heated body be placed near a cold one, the temperature of the former will be lowered, while that of the latter will
be raised.
2. All substances contain a certain quantity of heat; but,
on account of its tendency to diffuse itself equally, and the
difference in the power of different substances, to conduct it,
bodies of the same absolute temperature appear to possess
different degrees of heat.
3. Thus, if the hand be successively applied to a woollen
garment, a mahogany table, and a marble slab, all of which
have stood for some time in the same room, the woollen galrment will appear the warmest, and the marble slab the coldest
of tl.e three articles; but if a thermometer be applied to each,.1o difference in the temperature will be observed.
4. From this it appears, that some substances conduct heat
readily, and others with great difficulty.  The reason that the
marble slab seems the coldest, is, that marble, being a good
conductor of heat, receives the heat from the hand so readily
that the loss is instantly felt by the hand; while the woollen
garment, being a bad conductor of heat, receives the heat
from the hand so slowly that the loss is imperceptible.
171. The different power of receiving and conducting
heat, possessed by different substances, is the cause of
the difference in the warmth of various substances used
for clothing.
are incombustible; others cannot be transformed to a fluid state by any
degree of heat yet produced artificially. The expansive effect of heat has
but one known exception. The sources from which heat is derived are1st. From the sun in connexion with light;
2dly. From mechanical operations, such as friction, percussion, and
compression;
3dly. From chemical operations, especially combustion;
4thly. From living animals and vegetables.
What are the sources of heat?
170. In what way does heat tend to diffuse itself? Why do bodies of
the same absolute temperature appear to possess different degrees of heat?
What illustration of this is given? What appears from this?
171. What causes the difference in the warmth of substances used for
clothing?




] 36               NATURAL PIlLOSOPIIY.
1. Thus, woollen garments are warm garments, because they
part slowly with the heat which they acquire from the body,
and, consequently, they do not readily convey the warmth of
the body to the air; while, on the contrary, a linen garment is
a cool one, because it parts with its heat readily, and as readily receives fresh heat from the body. It is, therefore, constantly receiving heat from the body and throwing it out
into the air, while the woollen garment retains the heat which
it receives, and thus encases the body with a warm covering.
2. For a similar reason ice, in summer, is wrapped in woollen
cloths. It is then protected from the heat of the air, and will
not melt.
172. Heat is propagated in two ways, namely, by
conduction and by radiation. Heat is propagated bi
conduction when it passes from one substance to another
in contact with it. Heat is propagated by radiation
when it passes through the air or any other elastic
fluid.
173. Different bodies conduct heat with different degrees of facility. The metals are the best conductors,
and among metals silver is the best conductor.
1. For this reason any liquid may be heated in a silver vessel
more readily than in any other of the same thickness. The
metals stand in the following order, with respect to their conducting power; namely, silver, gold, tin, copper, platina, steel,
iron, and lead.
2. It is on account of the conducting power of metals* that
* Metals, on account of their conducting power, cannot be handled
when raised to a temperature above 120 degrees of Fahrenheit. Water
becomes scalding hot at 150 degrees, but air, heated far beyond the temperature of boiling water, may be applied to the skin without much pain.
Sir Joseph Banks, with several other gentlemen, remained some time in a
room when the heat was 52 degrees above the boiling point; but, though
they could bear the contact of the heated air, they could not touch any
metallic substance, as their watch-chains, money, &c. Eggs, placed on
172. In what two ways is heat propagated? When is it propagated by
conduction? When is it propagated by radiation?
173. Do all bodies conduct heat with the same degree of facility? What
-bodies are the best conductors? In what order do the metals stand with
respect to their conducting power?




PYRONOMICS.                         137
the handles of metal tea-pots and coffee-pots are commonly
made of woed; since, if they were made of metal, they would
become too hot to be grasped by the hand, soon after the vessel
is tilled with heated fluid.  Wood conducts heat very imperfectly. For this reason wooden spoons and forks are preferred
for ice. Indeed, so imperfect a conductor of heat is wood, that
a stick of'wood may be grasped by the hand while one end of
the stick is a burning coal. Animal and vegetable substances,
of a loose texture, such as fur, wool, cotton, &c., conduct heat
very imperfectly; hence their efficacy in preserving the warmth
of the body.
174. Heat is reflected from bright surfaces; while
black or dark colored bodies absorb the heat that falls
on them.
1. This is the reason why the bright brass andirons, or any
other bright substances, placed near a hot fire, seldom become
heated; while other dark substances, further removed from the
fire, become too hot for the hand.
2. Snow or ice will melt under a piece of black cloth, when
it will remain perfectly solid under a white one. The farmers
in some of the mountainous parts of Europe, are accustomed
to spread black earth, or soot, over the snow, in the spring, to
hasten its melting, and enable them to commence ploughing
early.
175. All bodies, when  violently  compressed  or extended, become warm.
a tin frame, were roasted hard in twenty minutes; and a beef-steak was
overdone in thirty-three minutes.
Chantrey, the celebrated sculptor, had an oven which he used for drying
his plaster cuts and moulds. The thermometer generally stood at 300 degrees in it, yet the workmen entered, and remained in it some minutes
without difficulty; but a gentleman once entering it with a pair of silvermlounted spectacles on, had his face burnt where the metal came in contact with the skin.
Is wood a good conductor of heat? Why are wool, fur, &c., so efficacious in preserving the warmth of the body? What is related in the
note with regard oto the conducting power of heat?
174. What bodies reflect the heat?  What bodies absorb the heat?
Why do bright bodies, when placed near the fire, seldom become heated?
Will snow melt most readily under white or black cloth?
175. What effect is produced on all bodies when violently compressed
or extended?




138                 NATURAL PHILOSOPHY.
1. If a piece of india-rubber be quickly stretched and applied to the lip, a sensible degree of heat will be felt. An iron
bar, on being hammered, becomes red-hot; and even water,
when strongly compressed, gives out heat.
2. When air is forcibly compressed by driving down the
piston of a syringe, nearly closed at the end, great heat is produced. Syringes have been constructed on this principle for
procuring fire, the heat, thus produced, being sufficient to
kindle dry tinder.
176. All substances, as they are affected by heat,
may be divided into combustible and incombustible
bodies.*
177. The pyrometert is an instrument to show the
expansion of bodies by the application of heat.
It consists of a metallic bar or wire, with an index connected
with one extremity. On the application of heat the bar expands and turns the index to show the degree of expansion.
178. The most obvious and direct effect of heat on a
body, is to increase its extension in all directions.
1. Coopers, wheelwrights, and other artificers, avail themselves of this property in fixing iron hoops on casks, and the
tires or irons on wheels. The hoop or tire having been heated,
expands, and being adapted in that state to the cask or the
wheel, as the metal contracts in cooling, it clasps the parts
very firmly together.J
*- Vegetable substances, charcoal, oils, most animal substances, as hair,
wool, horn, fat, and all metallic bodies, are combustible. Stones, glass,
salts, &c., are incombustible.
t Wedgewood's pyrometer, the instrument commonly used for high
temperatures, measures heat by the contraction of clay.
t From what has been stated above, it will be seen, that an allowance
should be made for the alteration of the dimensions in metallic beams or
What experiments are here related to illustrate this? What is said or
the air when strongly compressed?
176. Into what classes are all substances, as affected by heat, divided?
What substances are combustible?  What substances are incombustible?
177. What is a pyrometer? Of what does it consist? How does Wedgev ood's pyrometer measure high temperatures?
17. What is the most obvious and direct effect of heat on a body
1W iat vpplieation of this principle is related in the note?




PYRONOMICS.                        139
2. The effect of heat and cold,* in the expansion and contraction of glass, is an object of common observation; for it is
this expansion and contraction which cause so many accidents
with glass articles. Thus, when hot water is suddenly poured
into a cold glass, of any form, the glass, if it have any thickness, will crack; and, on the contrary, if cold water be poured
into a heated glass vessel, the same effect will be produced.
The reason of which is this: heat makes its way but slowly
through glass; the inner surface, therefore, when the hot
water is poured into it, becomes heated, and, of course, distended before the outer surface, and the irregular expansion
causes the vessel to break. There is less danger of fracture,
therefore, when the glass is thin, because the heat readily
penetrates it, and there is no irregular expansion.t
supporters, caused by the dilatation and contraction effected by the weather.
In the iron arches of Southwark bridge, over the Thames, the variation of
the temperature of the air causes a difference of height, at different times,
amounting to nearly an inch. A happy application of this principle to the
mechanic arts was made, some years ago, at Paris. The weight of the
roof of a building, in the Conservatory of Arts and Trades, had pressed
outwards the side walls of the structure, and endangered its security. The
following method was adopted to restore the perpendicular direction of the
structure. Several apertures were made in the walls, opposite to each
other, through which iron bars were introduced, which, stretching across
the building, extended beyond the outside of the walls. These bars terminated in screws, at each end, to which large broad nuts were attached.
Each alternate bar was then heated by means of powerful lamps, and
their lengths being thus increased, the nuts on the outside of the building
were screwed up close to it, and the bars were suffered to cool. The powerful contraction of the bars drew the walls of the building closer together,
and the same process being repeated on all the bars, the walls were gradually and steadily restored to their upright position..
* Cold is merely the absence of heat; or rather, more properly speaking, inferior degrees of heat are termed cold.
t The glass chimneys, used for oil and gas burners, are often broken by
being suddenly place~d, when cold, over a hot flame. The danger of fracture may be prevented (it is said) by making a minute notch on the bottom of the tube with a diamond. This precaution has been used in an
What is said of the effect of heat and cold on glass?  When hot water
is suddenly poured into a cold glass, why will the glass crack?  When
cold water is applied to a heated glass, why will the glass crack?




140                 NATURAL PHIILOSOPIiY.
179. The expansion caused by heat in solid and
liquid bodies differs in different substances; but aeriform fluids all expand alike, and undergo uniform degrees of expansion at various temperatures.
The expansion of solid bodies depends, in some degree, on
the cohesion of their particles; but as gases and vapors are
destitute of cohesion, heat operates on them without any opposing power.
180. The density of all substances is augmented by
cold, and diminished by heat.
There is a remarkable exception to this remark, and that is
in the case of water; which, instead of contracting, expands at
the freezing point, or when it is frozen. This is the reason why
pitchers, and other vessels, containing water and other similar
fluids, are so often broken when the liquid freezes in them.
For the same reason, ice floats* instead of sinking in water;
for as its density is diminished, its specific gravity is consequently diminished.
181. Different bodies require different quantities of
heat to raise them to the same temperature; and those
establishment where six lamps were lighted every day, and not a single
glass has been broken in nine years.
* Were it not for this remarkable property of water, large ponds and
lakes, exposed to intense cold, would become solid masses of ice; for if the
ice, when formed on the surface, were more dense (that is, more heavy)
than the water below, it would sink to the bottom, and the water above,
freezing in its turn, would also sink, until the whole body of the water would
be frozen. The consequence would be the total destruction of all creatures in the water. But the specific gravity of ice causes it to continue on
the surface, protecting the water below from congelation.
179. Is the expansion caused by heat in solid and liquid bodies the same
in all substances? How do adriform fluids differ, in this respect, fromn
solid and liquid bodies?  Upon what does the expansion of solid bodies in
some degree depend? Why has heat more power over gases and vapors?
180. What effect has heat and cold ppon the density of all substances?
What exception is there to this remark?  Why are the vessels, containing water and other similar fluids, so often broken when the liquid freezes
in them?  Why does ice float upon the water, instead of sinking in it?
What is stated in the note with regard to this property of water?




PYRONOMtCS.                          141
which are heated with  most difficulty retain their heat
the longest.
Thus oil becomes heated more speedily than water, and it
likewise cools more quickly.
182. When heat is thrown upon a bright or polished
surface it is reflected,* and the angle of reflection will
be equal to the angle of incidence.
183. When a certain degree of heat is applied to
water it converts it into steam or vapor.
184. The temperature of steam is always the same
with that of the liquid from which it is formed, while it
remains in contact with that liquid.  When closely confined, its elastic power is often sufficient to burst the
vessel in which it is confined.
185. The elastic force of steam  is increased by heat,
and diminished by cold. The amount of pressure,
therefore, which it will exert depends on the temperature at which it is formed.
186. The great and peculiar property of steam, on
which its mechanical agencies depend, is its power of
* Advantage has been taken of this property of heat in the construction
of a simple apparatus for baking. It is a bright tin case, having a cover
inclined towards the fire in such a manner as to reflect the heat downwards. In this manner use is made both of the direct heat of the fire, and
the reflected heat, which would otherwise pass into the room. The whole
apparatus, thus connected with the culinary department, is called, in New
England, " The Connecticut baker."
181. Can all bodies be raised to the same temperature by the same
quantities of heat? What bodies retain their heat the longest?
182. What becomes of the heat which is thrown upon a bright or polished surface? How do the angles of incidence and reflection compare
with each other?
183. When is water converted into steam or vapor?
184. How does the temperature of the steam compare with that of the
liquid from which it is formed while it remains in contact with that liquid?
185. By what is the elasticity of steam increased and diminished?
Upon what does the amount of pressure, which steam exerts, depend?
186. What is the great and peculiar property of steam, on which its
mechanical agencies depend?




142                NATURAL PHILOSOPHY.
exerting a high degree of elastic force, and losing it instantaneously.
187. The steam-engine is a machine moved by the
expansive force of steam.*
188. Steam occupies a space about 1700 times larger
than it will when converted into water. If, therefore,
the steam in a cylinder be suddenly converted into
water, it will occupy a much smaller space, and produce a vacuum in the cylinder.
1. The mode in which steam is made to act, is by causing
its expansive force to raise a solid piston accurately fitted to
the bore of a cylinder, like that in the forcing-pump.
2. The piston-rod rises by the impulse of expanding steam,
admitted into the cylinder below. When the piston is thus
raised, if the steam below it be suddenly condensed, or withdrawn from under it, a vacuum will be formed, and the pressure of the atmosphere on the piston above will drive it down.
The admission of more steam below it will raise it again, and
thus a continued motion of the piston, up and down, will be
produced. This motion of the piston is communicated to wheels,
levers, and other machinery, in such a manner as to produce
the effect intended.t
3. The celebrated Mr. James Watt introduced two important
improvements into  the steam-engine.   Observing that the
cooling of the cylinder by the water thrown into it to condense
the steam, lessened the expansibility of the steam; he contrived a method to withdraw the steam from the principal cylinder, after it had performed its office, into a condensing* Steam, as it issues into the air, is visible, and resembles smoke in itt
appearance, because the coldness of the air instantly condenses it inte
minute watery globules; but while performing its office, it is perfectly dry,
that is, it contains no watery particles, but is expanded into so rare a state
as to be absolutely invisible.
t This is the mode in which the engine of Newcomen and Savery,
commonly called the atmospheric engine, was constructed.
187. What is the steam-engine?
188. How much larger space does steam occupy than water? By
what mode is steam made to act? By what impulse does the piston rise?
What causes the piston to descend?  What improvement did Mr Watt
introduce into the steam-engine?




PYRONOMICS.                      143
chamber, where it is reconverted into water, and conveyed
back to the boiler.
4. The other improvement consists in substituting the expansive power of steam for the atmospheric pressure. This
was performed by admitting the steam into the cylinder above
the raised piston, at the same moment that it is removed from
below it; and thus the power of steam is exerted in the descending as well as in the ascending stroke of the piston; and a
much greater impetus is given to the machinery than by the
former method. From the double action of the steam above,
as well as below the piston, and from the condensation of the
steam, after it has performed its office, this engine is called
Watt's double-acting condensing steam-engine.
5. Fig. 93 represents that portion of the steam-engine in
which steam is made to act, and propel such machinery as may
be connected with it. The principal parts are the boiler, the
cylinder and its piston,
the condenser, the air-                 Fig. 93.
pump, the  steam-pipe,                    nl
the  eduction-pipe, and
the cistern. In this figure,        D
A represents the boiler,          Ia   c 
C the cylinder, with H                            K
the piston, B the steam- _
pipe, with two branches*        A    F           P
communicating with the                                  U
cylinder, the one above                                   N
and the other below the
piston. This pipe has two valves, F and G, which are opened
and closed alternately by machinery connected with the piston.
The steam is carried through this pipe by the valves, when
open, to the cylinder both above and below the piston. K is
the eduction-pipe, having two branches, like the steam-pipe,
furnished with valves, &c., which are opened and shut by the
same machinery. By the eduction-pipe. the steam is led off
from the cylinder as the piston ascends and descends.
* The steam and the eduction pipes are sometimes made in forms differing from those in the figure, and they differ much in different engines.
What does Fig. 93 represent? What are the principal parts? What
does A represent? What does C represent? What does B represcnt?
What does K represent? By what is the steam led off from the cylinder?




144               NATURAL PHILOSOPHY.
L is the condenser, and 0 a stop-cock for the admission of
cold water. M is the air-pump. N is the cistern of cold water
in which the condenser is immersed. R is the safety-valve.
When the valves are all open, the steam issues freely from the
boiler, and circulates through all the parts of the machine, expelling the air.* Now, the valves F and Q, being closed, and
G and P remaining open, the steam presses upon the piston
and forces it down. As it descends, it draws with it the end
of the working-beam, which is attached to the piston-rod J,
(but which is not represented in the figure.) To this workingbeam, (which is a lever of the first kind,) bars or rods are attached, which, rising and falling with the beam and the piston,
open the stop-cock 0, admitting a stream of cold water, which
meets the steam from the cylinder and condenses it, leaving no
force below the piston to oppose its descent. At this moment
the rods attached to the working-beam close the stop-cocks G
and P, and open F and Q. The steam then flows in below the
piston, and rushes from above it into the condenser, by which
means the piston is forced up again with the same power as
that with which it descended.  Thus the steam-cocks G and
P and F and Q are alternately opened and closed; the steam
passing from the boiler drives the piston alternately upwards
and downwards, and thus produces a regular and continued
motion. This motion of the piston, being communicated to the
working-beam, is' extended to other machinery, and thus an
engine of great power is obtained.
The air-pump M, the rod of which is connected with the
working-beam, carries the water from the condenser back into
the boiler, by a communication represented in Fig. 94.
* This process is called blowing out, and is heard when a steamboat is
about starting.
What does L represent? What does O represent? What does M rep
resent? What does N represent?  What does R represent?  When the
valves are all open, what becomes of the steam? When the valves F and
Q are closed, and G and P open, upon what does the steam press? What
does the cylinder draw with it in its descent? Which of the mechanical
powers is this working-beam?  What are attached to this working-beam?
What is their use? What becomes of the steam when the stop-cocks G
and P are closed, and F and Q are open?  How is the regular and continued motion produced?  To what is this motion of the piston communicated? What is the use of the air-pump M? For what is the safoetyvvalve R used?




PYRONOMICS.                         l 1)
The safety-valve R, connected with a lever of the second
xind, is made to open when the pressure of the steam  within
the boiler is too great.  The steam then rushing through the
aperture under the valve, removes the danger of the bursting
of the boiler.
189. The power of a steam-engine is generally expressed by the power of a horse, which can raise
33,000 lbs. to the height of' one foot in a minute. An
engine of 100 horse power, is one that will raise
3,300,000 lbs. to the height of one foot in one minute.
190. The steam-engine* is constructed in various
forms; the principal of which are the high and the low
pressure engines; or, as they are sometimes called, the
non-condensing and the condensing engines.
1. The non-condensing or high-pressure engines differ from
the low pressure or condensing engines in having no condenser.
The steam, after having moved the piston, is let off into the
open air. As this kind of engine occupies less space, and is
much less complicated, it is generally used on railroads.
2. In the low pressure or condensing engines, the steam, after
having moved the piston, is condensed, or converted into water,
and then conducted back into the boiler.
* The steam-engine, as it is constructed at the present day, is the result of the inventions and discoveries of a number of distinguished individuals, at different periods. Among those who have contributed to its
present state of perfection, and its application to practical purposes, may
be mentioned the names of Somerset, the Marquis of Worcester, Savery,
Newcomen, Fulton, and especially Mr. James Watt.
To the inventive genius of Watt, the engine is indebted for the. condenser, the appendages for parallel motion, the application of the governor,
and for the double action. In the words of Mr. Jeffrey it may be added,
that, " by his admirable contrivances, and those of Mr. Fulton, it has become a thing alike stupendous for its force and its flexibility; for the pro189. How is the power of a steam-engine expressed?  What is an engine of 100 horse power?
190. What are the principal forms in which the steam-engine is constructed? How do they diffei from each other? What becomes of the
steam  after having moved the piston in the non-condensing engines?
What kind of engines is ginerally used on railroads?  What becomen of
thi steamn after having lmoved the piston in the condensing engines?
7




146                 NATURAL PHILOSOPHY.
WATT'S DOUBLE-ACTING CONDENSING STEAM-ENGINE.
Fig. 94.
L                       l
7
3. Fig. 94 represents Watt's double-acting condensing steamengine, in which A represents the boiler, containing a large
quantity of water, which is constantly replaced as fast as portions are converted into steam. B is the steam-pipe, conveying the steam to the cylinder, having a steam-cock b to admit
or exclude the steam at pleasure.
C is the cylinder, surrounded by the jacket cc, a space kept
constantly supplied with hot steam, in order to keep the
cylinder from being cooled by the external air. D is the
digious power it can exert, and the ease and precision and ductility with
which it can be varied, distributed, and applied. The trunk of an elephant, that can pick up a pin, or rend an oak, is as nothing to it. It can
engrave a seal, and crush masses of obdurate metal before it; draw out,
without breaking, a thread as fine as gossamer, and lift up a ship of wal
like a bauble in the air. It can embroider muslin, and forge anchors; cut
steel into ribands, and impel loaded vessels against the fury of the winds
and waves."




PYRONOMICS.                      147
eduction-pipe, communicating between the cylinder and the
condenser.  E is the condenser, with a valve e, called the
injection-cock, admitting a jet of cold water, which meets the
steam  the instant that the steam enters the condenser.  F is
the air-pump, which is a common suction-pump, but is here
called the air-pump because it removes from the condenser not
only the water, but also the air, and the steam that escapes
condensation. G G is a cold-water cistern, which surrounds
the condenser, and supplies it with cold water, being filled
by the cold-water pump, which is represented by H. I is the
hot well, containing water from the condenser. K is the hotwater pump, which conveys back the water of cbndensation
from the hot well to the boiler.
L L are levers, which open and shut the valves in the channel between the steam-pipe, cylinder, eduction-pipe, and condenser; which levers are raised or depressed by projections
attached to the piston-rod of the pump. M M is an apparatus
for changing the circular motion of the working-beam into
parallel motion, so that the piston-rods are made to move in a
straight line. N N is the working-beam, which being moved
by the rising and falling of the piston, attached to one end,
communicates motion to the fly-wheel by means of the crank
P, and from the fly-wheel the motion is communicated by
bands, wheels, or levers, to the other parts of the machinery
0 0 is the governor.
The governor being connected with the fly-wheel, is made
to participate the common motion of the engine, and the balls
will remain at a constant distance from the perpendicular
shaft, so long as the motion of the engine is uniform; but
whenever the engine moves faster than usual, the balls will recede farther from the shaft, and, by raising a valve connected
with the boiler, will let off such a portion of the force as to
reduce the speed to the rate required.
The steam-engine, thus constructed, is applied to boats to
turn wheels having paddles attached to their circumference,
which answer the purpose of oars. It is used also in workWhat does Fig. 94 represent? What does A represent? What does B
represent?  What does C represent?  What does D represent?  What
does E represent? What does F represent? What does G G represent?
What does I represent?  What does K represent?  What does LL represent? What does M M represent? What does N N represent? Wllat
does O O represent? What is said of the governor?




148              NATURAL P IiljOSOPlI Y.
shops, factories, &c.; and different directions and velocities
may be given to the motion produced by the action of the
steam on the piston, by connecting the piston to the beam with
wheels, axles, and levers, according to the principles stated
under the head of Mechanics.
191. The locomotive engine is a high-pressure steamengine, mounted on wheels, and used to draw  loads
on a railroad, or other level roads. It is usually accompanied by a large wagon, called a tender, in which the
wood and water, used by the engine, are carried.
Fig. 95 represents a side view of the internal construction
of a locomotive steam-engine; in which F represents the firebox, or place where the fire is kept; D the door through which
the fuel is introduced. The spaces marked B are the interior
of the boiler, in which the water stands at the height indicated
by the dotted line. The boiler is closed on all sides, all its
openings being guarded by valves. The tubes marked p p
conduct the smoke and flame of the fuel through the boiler to
the chimney C C, serving, at the same time, to communicate
the heat to the remotest part of the boiler. By this arrangement none of the heat is lost, as these tubes are all surrounded by the water. S S S is the steam-pipe, open at the top
V S, having a steam-tight cock, or regulator, V, which is
opened and shut by the lever L, extending outside of the
boiler, and managed by the engineer.
The operation of the machine is as follows: the steam being
generated in great abundance in the boiler, and being unable
to escape out of it, acquires a considerable degree of elastic
force. If at that moment the valve V be opened, by the
handle L, the steam entering the pipe S passes in the direction
of the arrow, through the tube, and enters the valve-box at
X. There a sliding-valve, which moves at the same time with
the machine, opens for the steam a communication successively
with each end of the cylinder below. Thus, in the figure, the
entrance on the right hand of the sliding-valve is represented
as being open, and the steam follows in the direction of the
arrows into the cylinder, where its expansive force will move
191. Describe the locomotive steam-engine. In Fig. 95, what do F
and D represent? What do the following references respectively represent, namely, SSS?  BBB? pppp?  CC? X? L? P? NN?
RGKK?




p B     A- -,  _
A~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I "   a x  P[..,,                         C
z~~~~~~~~~~~~~~~~~~~~: -  _-        _ _-  S 1,. —,.-L
U            0__-_H~ _   H
o~~~~~~~~~~ ~
III
VIEW' OF THE INTERNAL CONSTRUCTION OF HINKLEY & DRURY'S LOCOMOTIVE STEAMNI-ENGINE.




150                NATURAL PHILOSOPHY.
the piston P in the direction of the arrow. The steam or air
on the other side of the piston passes out in the opposite
direction, and is conveyed, by a tube passing through C C,
into the open air.
The motion of the piston in the direction of the arrow,
causes the levers N N to close the sliding-valve on the right,
and open a communication for the steam on the opposite side
of the piston p, where it drives the piston back towards the
arrow, at the same time affording a passage for the steam on
the right of the piston to pass into the open air.
Motion being thus given to the piston, it is communicated, by
means of the rod R and the beam G, to the cranks K K, which,
being connected with the axle of the wheel, causes it to turn,
and thus move the machine.
Thus constructed, and placed on a railroad, the locomotive
steam-engine is advantageously used as a substitute for horse
power, for drawing heavy loads.*
THE STATIONARY STEAM-ENGINE.
This engine is generally a high-pressure, or non-condensing
engine, used to propel machinery in workshops and factories.
As it is designed for a labor-saving machine, it is desirable to
combine simplicity and economy with safety and durability in
its construction; and that form of this engine is to be preferred
which in the greatest degree unites these qualities. The figure
on page 151 represents Tuft's stationary steam-engine,t with sec* The apparatus of safety-valves and other appliances for the management of the power produced by the machine, are the same in principle,
though differing in form, with those used in other steam-engines; for a
particular description of which, the student is referred to practical treatises
upon the subject.
t This engine was constructed by Mr. Otis Tufts of East Boston, Massachusetts. It is the engine used to propel the machinery at the late Fair
of the Massachusetts Mechanic Association, where it was very highly and
justly commended for its beauty and simplicity of construction, and the
perfectly "noiseless tenor of its way."  The figure which represents it
is an electrotype copy of a steel plate, designed by Brown & Harbrys.
under the direction of Mr. Tufts. The electrotype copy was taken by
Mr. A. Wilcox, Washington-street, Boston. The electrotype process will
be noticed inl a subsequent page of this volume.




PYRONOMICS.                    151
il
tM




152               NATURAL PHILOSOPIIY.
tions of the interior. Like the double-acting condensing engine
of Mr. Watt, described in Fig. 94, it is furnished with a governor,
by which the supply of steam is regulated; and like the locomotive, Fig. 95, the cylinder with its piston has a horizontal position. The steam is admitted into the valve-box through an aperture at E, in the section, and from thence passes into the cylinder
through a sliding-valve, alternately to each side of the piston P,
as is represented by the direction of the arrows, the sliding-valve
being moved by the rod V, communicating with an " eccentric"
apparatus attached to the axis of the fly-wheel. The direction
of the current of steam to the valve-box is represented by the
arrow at I, and its passage outward from the cylinder, after it
has moved the piston, is seen at 0. In this engine there is no
working-beam as in Watt's engine, Fig. 94, but the motion is
communicated from. the piston-rod to a crank connected with
the fly-wheel, which, turning the wheel, will move all machinery connected either with the axis or the circumference of
that wheel.
CHAPTER X.
OPTICS.
192. Optics is the science which treats of light, of
colors, and of vision.
1. The science of optics divides all substances into the
following classes; namely, luminous, transparent, and translucent; reflecting, refracting, and opaque.
2. Luminous bodies are those which shine by their own light;
such as the sun, the stars, a burning lamp, or a fire.
3. Transparent substances are those which allow light to
pass through them freely, so that objects can be distinctly
seen through them; as glass, water, air, &c.
192. Of what does Optics treat? Into what classes does the science of
Optics divide all substances? What are luminous bodies? Give an example of a luminous body. What are transparent bodies?




OPTICS.                        153
4. Translucent bodies are those which permit a portion of
light to pass through them; but render the object behind
them indistinct; as horn, oiled paper, colored glass, &c.
5. Reflecting substances are those which do not permit light
to pass through them; but throw it off in a direction more or
less oblique, according as it falls on the reflecting surface; as
polished steel, looking-glasses, polished metal, &c.
6. Refracting substances are those which turn the light
from its course, in its passage through them; and opaque substances are those which permit no light to pass through them;
as metals, wood, &c.
7. It is not known what light is. Sir Isaac Newton supposed it to consist of exceedingly small particles, moving from
luminous bodies; others think that it consists of the undulations of an elastic medium, which fills all space. These undulations (as is supposed) produce the sensation of light to the
eye, in the same manner as the vibrations of the air produce
the sensation of sound to the ear. The opinions of philosophers
at the present day are inclining to the undulatory theory.
193. A ray of light is a single line of light proceeding
from a luminous body.
194. Rays of light are said to diverge when they
separate more widely, as they pro-                Fig. 96.
ceed from  a luminous body.
Fig. 96 represents the rays of light diverging as they proceed from  the luminous body, from F to D.
195. Rays of light are called converging when they
approach each other. The point at
which converging rays meet is called              Fig. 97.
the focus.
Fig. 97 represents converging rays of
light, of which the point F is the focus.
Give an example of a transparent body. What are translucent bodies?
Give an example of a translucent body. What are reflecting substances?
G'ive an example of a reflecting body. What are refracting substances?
What are opaque substances? What is light? What did Sir Isaac Newton suppose it to be? What other opinions have been formed concerning it?
193. What is a ray of light?
194. When are rays of light said to diverge? What does Fig. 96 represent?
7*




154               NATUAJ rAL PHLOSOPFY.
196. A beam  of light consists
of many rays running in parallel              Fig. 98.
lines.                                        z
Fig. 98 represents a beam of light.
197. A pencil of rays is a
collection of diverging or converging rays.
198. A medium is any substance, solid or fluid,
through which light can pass; as water, glass, air, &c.
199. The rays of light which issue from terrestrial
bodies, continually diverge, until they meet with a refracting substance; but the rays of the sun diverge so
little, on account of the immense distance of that luminary, that they are considered parallel.
200. Light, proceeding from a luminous body, is projected forward in straight lines in every possible direction.   It moves with a rapidity but little short of
200,000 miles in a second of time.
201. Every point of a luminous body is a centre,
from which light radiates in every direction.    Rays
of light proceeding from different bodies, cross each
other without interfering.
202. A shadow is the darkness produced by the intervention of an opaque body, which prevents the rays
of light from reaching an object behind the opaque
body.
Shadows are of different degrees of darkness, because the
light from other luminous bodies reaches the spot where the
195. When are rays of light called converging? What is the point, at
which converging rays meet, called?
196. What is a beam of light? What does Fig. 98 represent?
197. What is a pencil of light?
198. What is a medium?
199. In what manner do the rays of light proceed from terrestrial bodies?
In what kind of lines do the rays of light proceed from the sun?
200. In what way is light projected forward from any luminous body?
With what rapidity does it move?
201. From what point, in a luminous body, does light radiate?
202. How is a shadow produced? Why are shadows of different degrees of darkness'!




OPTICS.                          155
shadow is formed. Thus, if a shadow be formed when two
candles are burning in a room, that shadow will be both
deeper and darker if one of the candles be extinguished. The
darkness of a shadow is proportioned to the intensity of the
light, when the shadow is produced by the interruption of the
rays from a single luminous body.*
203. When a luminous body is larger than an opaque
body, the shadow of the opaque body will gradually
diminish in size till it terminates in a point. The form
of the shadow of a spherical body                Fig. 99.
will be that of a cone.                   A
Fig. 99. A represents the sun, and, 
B the moon.  The sun, being much
larger than the moon, causes it to
cast a converging shadow, which terminates at E.
204. When the luminous body is smaller than the
opaque body, the shadow
of the opaque body grad-                    i 
ually  increases  in  size                   B
with the distance, without limit. 
In Fig. 100 the shadow
of the object, A, increases
in size at the different distances, B, C, D, E, or, in other words, it constantly diverges.
205. When several luminous bodies shine upon the
* As the degree of light and darkness can be estimated only by comparison, the strongest light will appear to produce the deepest shadow.
Hence, a total eclipse of the sun occasions a more sensible d trkness than
midnight, because it is immediately contrasted with the stLong light of
day.
To what is the darkness of a shadow proportioned, when the shadow is
produced by the interruption of the rays from a single luminous body!
203. What is said of the shadow of the opaque body when the luminous
body is the larger? Explain Fig. 99.
204. What is said of the shadow of the opaque body when the luminous
body is the smaller? Explain Fig. 100.
205. How many shadows are produced when several luminous bodies
shine upon the same. object?




156                NATURAL PIRILOSOPHY.
same object, each one will
produce a shadow.                          Fig. 101.
Fig. 101 represents a ball,
A, illuminated by the three   d
candles, B, C, and D.  The         I
light B produces the shadow         I i       A
b, the light C, the shadow c,  c I   II
and the light D, the shadow
d; but as the light from each              lD
of the candles shines upon all  b                           n
the shadows, except its own,
the shadows will be faint.
206. When rays of light fall upon an opaque body,
part of them are absorbed, and part are reflected.
Light is said to be reflected when it is thrown off from  the
body on which it falls; and it is reflected in the largest quantities from the most highly polished surfaces. Thus, although
most substances reflect it in a degree, polished metals, lookingglasses, or mirrors, &c., reflect it in so perfect a manner as to
convey to our eyes, when situated in a proper position to receive them, perfect images of whatever objects shine on them,
either by their own, or by borrowed light.
207. That part of the science of Optics which relates
to reflected light, is called Catoptrics.
208. The laws of reflected light are the same as those
of reflected motion. Thus, when light falls perpendicularly on an opaque body, it is reflected back in the
same line, towards the point whence it proceeded. If
it fall obliquely, it will be reflected obliquely in the opposite direction; and in all cases the angle of incidence
Explain Fig. 101.
206. What is the consequence when rays of light fall upon an opaque
body which they cannot pass? What is meant by the reflection of light?
207. What are Catoptrics?
208. By what laws is light governed?  How is light reflected when it
falls perpendicularly on an opaque body'? How is it reflected when it falls
obliquely? How do the angles of incidence and reflection compare with
sach other?  By what light are opaque objects seen?  By what light are
luminous objects seen?




OPTICS.                          157
will be equal to the angle of reflection. This is the
fundamental law of Catoptrics, or reflected light.*
Opaque objects are seen only by reflected light. Luminous
bodies are seen by the rays of light which they send directly
to our eyes.
209. All bodies absorb a portion of the light which
they receive; therefore the intensity of light is diminished every time that it is reflected.
* The angles of incidence and reflection have already been described
under the head of Mechanics; but as all the phenomena of reflected light
depend upon the law stated above, and a clear idea of these angles is necessary in.order to understand the law, it is deemed expedient to repeat
in this connexion the explanation already given.
An incident ray is a ray proceeding to, or falling on any surface; and
a reflected ray is the ray which proceeds from any reflecting surface.
Fig. 102 is designed to show the angles of incidence and of reflection. In this figure M A M is a     Fig 102.
mirror, or reflecting surface. P is a line perpefidic-.
ular to the surface. I A represents an incident ray,
falling on the mirror in such a manner as to form,
with the perpendicular P, the angle I A P. This is
called the angle of incidence. The line RA is to
be drawn on the other side of P A in such a manner
as to have the same inclination with P A as I A
has: that is, the angle RAP is equal to IA P.   M
The line R A will then show the course of the reflected ray; and the angle R A P will be the angle of reflection.
From whatever surface a ray of light is reflected, whether it be a plain
surface, a convex surface, or a concave surface, this law invariably prevails; so that if we notice the inclination of any incident ray, and the
situation of the perpendicular to the surface on which it falls, we can always determine in what manner, or to what point it will be reflected.
This law explains the reason why, when we are standing on one side of a
mirror, we can see the reflection of objects on the opposite side of the
room, but not those on the same side on which we are standing. It also
explains the reason why a person can see his whole figure in a mirror not
more than half of his height. It also accounts for all the apparent peculiarities of the reflection of the different kinds of mirrors.
Explain the angles of incidence and reflection. Is the same principle
applicable to all kinds of surfaces? Explain its application to mirrors.
209. Why is the intensity of light diminished every time it is reflected?




158               NATURAL PrIIILOSO(HY.
210. Every portion of a reflecting surface reflects an
entire image of the luminous body shining upon it.
1. When the sun or the moon shines upon a sheet of water,
every portion of the surface reflects an entire image of the luminary; but as the image can be seen only by reflected rays,
and as the angle of reflection is always equal to the angle of
incidence, the image for any point can be seen only in the reflected ray prolonged.
2. Objects seen by moonlight appear fainter than when seen
by daylight, because the light by which they are seen has been
twice reflected; for, the moon is not a luminous body, but its
light is caused by the sun shining upon it. This light, reflected from the moon and falling upon any object is again reflected by that object. It suffers, therefore, two reflections; and
since a portion is absorbed by each surface that reflects it, the
light must be proportionally fainter. In traversing the atmosphere, also, the rays, both of the sun and moon, suffer diminution; for, although pure air is a transparent medium, which
transmits the rays of light freely, it is generally surcharged
with vapors and exhalations, by which some portion of light
is absorbed.
211. All objects are seen by means of the rays of
light emanating or reflected from them; and therefore
when no light falls upon an opaque body it is invisible.
This is the reason why none but luminous bodies can be
seen in the dark. For the same reason, objects in the shade,
or in a darkened room appear indistinct, while those which are
exposed to a strong light can be clearly seen.
212. When rays of light, proceeding from any object,
enter a small aperture, they cross one another and form
an inverted image of the object.*
* This is a necessary consequence of the law that light always moves
in straight lines.
210. Does every portion of a reflecting surface reflect an entire image
of the luminous body shining upon it? When the sun or moon shines upon
a sheet of water, why do we not see an image reflected from every portion
of the surface? Why do objects, seen by moonlight, appear fainter than
when seen by daylight? By what light does the moon shine? What absorbs some of the rays of light in traversing the atmosphere?
211. How are all objects seen? Why can none but luminous bodies be
seen in the dark?




OPTICS.                         1 59
Fig. 103 represents the rays from an object, a c, entering
an aperture. The ray from a passes down
through the aperture to d, and the ray from        Fig. 103.
c passes up to b, and thus these rays, cross-             a
ing at the aperture, form an inverted image  
on the wall.  The room  in which this experiment is made should be darkened, and
no light permitted to enter, excepting through
the aperture. It then becomes a camera
obscura.*
213. The angle of vision is the angle formed at the
eye by two lines drawn from opposite parts of an object.
1. The angle C, in                   Fig. 104.           A
Fig.  104, represents                                D
the  angle  of vision.
The line A C proceed-,c         C
ing from one extremity of the object meets                        G
the line B C proceeding                              E
from the opposite extremity, and forms an angle C at the eye; —this is the angle
of vision.
2. Fig. 105 represents the different angles, made by the
same object, at different distances. From an inspection of the
* These words signify a darkened chamber. In the future description
which will be given of the eye, it will be seen that the camera obscura is
constructed on the same principle as the eye. If a convex lens be placed
in the aperture, an inverted picture, not only of a single object, but of the
entire landscape, will be found on the wall. A portable camera obscura is
made by admitting the light, into a box of any size, through a convex
lens, which throws the image upon an inclined mirror, from whence it is
reflected upwards to a plate of ground glass. In this manner a beautiful
but diminished image of the landscape, or of any group of objects, is presented on the plate in an erect position.
212. What kind of an image is formed when rays of light, proceeding
from an object, enter a small aperture? Illustrate this by Fig. 103. What
is a camera obscura? How can a portable camera obscura be made?
213. How is the angle of vision formed?  Explain Fig. 104. What
does Fig. 105 represent?




160                NATURAL PHILOSOPHY.
figure it is evident, that the nearer an
object is to the eye, the wider must             Fig. 105.
be the opening of the lines to admit             F    D    A
the extremities of the object; and,  
consequently, the  larger the  angle   C
under which it is seen; and, on the
contrary, that objects at a distance             C   E    B
will form small angles of vision. Thus,
in this figure, the three crosses, F G, D E, and A B, are all of
the same size; but A B, being the most distant, subtends the
smallest angle* A C B, while D E and F G, being nearer to the
eye, situated at C, form  respectively the larger angles, D C E
and F C G.
214. When an object, at any distance, does not subtend an angle of more than two seconds of a degree, it
is invisible.
At the distance of four miles a man of common stature will
thus become invisible, because his height at that distance will
not subtend an angle of two seconds of a degree. The size of
* The apparent size of an object depends upon the size of the angle of
vision. But we are accustomed to correct, by experience, the fallacy of
appearances; and, therefore, since we know that real objects do not vary
in size, but that tho angles under which we see them do vary with the
distance, we are not deceived by the variations in the appearance of objects. Thus, a house at a distance appears absolutely smaller than the
window through which we look at it; otherwise we could not see it
through the window; but our knowledge of the real size of the house prevents our alluding to its apparent magnitude. In Fig. 104 it will be seen
that the several crosses, AB, DE, FG, and HI, although very different in
size, on account of their different distances, subtend the same angle A C B;
they, therefore, all appear to the eye to be of the same size, while, in Fig.
105, the three objects AB, DE, and FG, although of the same absolute
size, are seen at a different angle of vision, and they, therefore, will seem
of different sizes, appearing larger as they approach the eye.
It is upon a correct observance of the angle of vision-that the art of perspective drawing is indebted for its accuracy.
What effect has the nearness of the object to the eye, on the angle?
Illustrate this by the figure. Upon what does the apparent size of an ob.
ject depend?  Why do objects appear so large?  To what is the art o,
perspective drawing indebted for its accuracy?
214. How large an angle must a body subtend to be visible?




OPTICS.                         161
the apparent diameter of the heavenly bodies is generally
stated by the angle which they subtend.
215. When the velocity of a moving body does not
exceed twenty degrees in an hour, its motion is imperceptible to the eye.
1. It is for this reason that the motion of the heavenly bodies
is invisible, notwithstanding their immense velocity.
2. The real velocity of a body in motion ro nd a point, depends on the space comprehended in a degree.  The more
distant the moving body from the centre,
or, in other words, the larger the circle         Fig. 106.
which it has to describe, the larger will be
the degree.                                             -..A
3. In Fig. 106, if the man at A, and the
man at B, both start together, it is manifest             3B
that A must move more rapidly than B, to
arrive at C at the same time that B reaches,
D; because the arc AC is the arc of a   ce           3 3     E
larger circle than the arc BD. But to
the eye at E, the velocity of both appears to be the same, because both are seen under the same angle of vision.
216. There are three kinds of mirrors,* namely, the
plain, the concave, and the convex mirror.
Plain mirrors are those which have a flat surface,
such as a common looking-glass; and they neither
* A mirror is a smooth and polished surface, that formns images by the
reflection of the rays of light- Mirrors (or looking-glasses) are made of
glass, with the back covered with an amalgam, or mixture of mercury and
tinfoil. It is the smooth and bright surface of the mercury that reflects
the rays, the glass acting only as a transparent case, or covering, through
which the rays find an easy passage. Some of the rays are absorbed in
their passage through the glass, because the purest glass is not free from
imperfections. For this reason, the best mirrors are made of fine and
highly-polished steel.
215. When is the motion of a body invisible? Why is the motion of
the heavenly bodies invisible? Upon what does the real velocity of a
body, in motion round a point, depend? Explain Fig. 106. Why does
the velocity of both, to an eye at E, appear to be the same?
216. How many kinds of mirrors are there? What are plain mirrors?
Do they magnify or diminish the object?




162               NATURAL, PIIILOSOPHY.
magnify nor diminish the image of objects reflected
from them.
A convex mirror is a portion of the external surface
of a sphere. Convex mirrors have therefore a convex
surface.
A concave mirror is a portion of the inner surface oI
a hollow sphere. Concave mirrors have therefore a
concave surface.
In Fig. 107, M N represents both a convex and a concave
mirror. They are both a portion of a sphere of which O is the
centre.  The outer part
of M N is a convex, and               Fig. 107.G
the inner part is a concave
mirror. Let AB, CD, EF, /
represent rays falling on
the convex mirror M N.                               A
As the three rays are  0o ~  c
parallel, they would all be                          E
perpendicular to a plane:rE i
or flat mirror; but no ray
can fall perpendicularly                      It
ona a concave or convex mirror, which is not directed towards the centre of the sphere of which
the mirror is a portion. For this reason the ray C D is perpendicular to the mirror; while the other rays A B and E F fall
obliquely upon it. The middle ray therefore falling perpendicularly on the mirror, will be reflected back in the same line,
while the two other rays falling obliquely will be reflected
obliquely; namely, the ray A B will be reflected to G, and the
ray E F to H, and the angles of incidence A B P and E F T
will be equal to the angles of reflection P B G and T F H, and
since we see objects in the direction of the reflected rays, we shall
see the image at L, which is the point at which the reflected
rays if continued through the mirror would unite and form thed
image. This point is equally distant from the surface, and the
centre of the sphere, and is called the imaginary focus of the
mirror.  It is called the imaginary focus, because the rays do
What are convex mirrors? What part of a sphere is a convex mirror? What are concave mirrors? What part of a sphere is a coneave
mirror? In Fig. 107, which part of the sphere represents a convex Inirror? Which part a concave mirror?  Explain the figure.




OPTICS.                        163
not really-unite at that point, but only appear to do so; for
the rays do not pass through the mirror, since they are reflected by it.
217. The image of an object reflected from a convex
mirror is smaller than the object.
This is owing to the divergence of the reflected rays. A
convex mirror converts, by reflection, parallel rays into divergent
rays; rays that
fall upon the mir-                  Fig. 108.
ror diveryent, are
Drendered still more
divergent by refiection, and convergent rays are
reflected   either >
parallel, or less
convergent.  If,   D
then, an object,
A B, be  placed
before  any part                                           C
of a convex mirror, the two rays A and B proceeding from  the extremities,
falling convergent on the mirror, will be reflected less convergent, and will not come to a focus until they arrive at C;
then an eye placed in the direction of the reflected rays will
see the image formed in (or rather behind) the mirror at a b;
and as the image is seen under a smaller angle than the object,
it will appear smaller than the object.
218. The true focus of a concave mirror is a point
equally distant from the centre and the surface of the
sphere, of which the mirror is a portion.
219. When an object is further from the concave
mirror than its focus, the image will be inverted; but
when the object is between the mirror and its focus,
217. How does the image of an object reflected from a convex mirror
compare with the object? Give the illustration.
218. What is the focus of a concave mirror?
219. When an object is further from the concave mirror than the focus,
how will the image appear? When the object is between the mirror and
the focus?




164                 NATURAL PHILOSOPHY.
the image will be upright, and grow larger in proportion
as the object is placed nearer to the mirror.*
220. The image reflected by a concave mirror is larger than the object, when the object is placed between
the mirror and its focus.t
1. This is owing to the convergent property of the concave
mirror.  If the object AB be placed between the concave
mirror and its focus f, the rays A                     Fig. 109.
and B from its ex-      a
tremities will fall
divergent on the
mirror, and, on
being   reflected,                           -
become  less divergent, as if they
proceeded  from
C.   To  an  eye                   t
placed in that situation, namely, at
* Concave mirrors have the peculiar property of forming images in the
air. The mirror and the object being concealed behind a screen, or a wall,
and the object being strongly illuminated, the rays from the object fall
upon the mirror, and are reflected by it through an opening in the screen
or wall, forming an image in the air. Showmen have availed themselves
of this property of concave mirrors, in producing the appearance of apparitions, which have terrified the young and the ignorant. These images
have been presented with great distinctness and beauty, by raising a fine
transparent cloud of blue smoke, by means of a chafing-dish, around the
focus of a large concave mirror.
t There are three cases to be considered with regard to the effects of
concave mirrors:
1. When the object is placed between the mirror and the principal
focus.
2. When it is situated between its centre of concavity and that focus.
3. When it is more remote than the centre of concavity.
1st. In the first case, the rays of light dlvrging after reflection, but in a
How must the object be placed that the image may appear upright?
And as the object is removed towards the mirror?
220. If the object be placed between the mirror and the focus, how does
the image compare with the object?




OPTICS.                          1 (t5
C, the image will appear magnified behind the mirror, at a b,
sincle it is seen under a larger angle than the object.
Q. The following facts result from the operation of the law
already stated as the fundamental law of Catoptrics, namely,
that the angles of incidence and reflection are always equal.
3. In estimating these angles, it must be recollected, that no
line is perpendicular to a convex or concave mirror, which will
not, when sufficiently prolonged, pass through the centre of the
sphere of which the mirror is a portion.
4. The truth of these statements may be illustrated by simple
drawings; always recollecting, in drawing the figures, to make
the angles of incidence and reflection equal. The whole may
also be shown by the simple experiment of placing the flame
of a candle in various positions, before both convex and concave
mirrors.
FACTS WITH REGARD TO CONVEX MIRRORS.
1. Parallel rays reflected from a coNVEx surface, are made
to diverge.
less degree than before such reflection took place, the image will be larger
than the object, and appear at a greater or smaller distance from the surface of the mirror, and behind it. The image in this case will be erect.
2d. When the object is between the principal focus and the centre of the
mirror, the apparent image will be in front of the mirror, and beyond the
centre, appearing very distant when the object is at or just beyond the
focus, and advancing towards it as it recedes towards the centre of concavity, where, as already stated, the image and the object will coincide.
During the retreat of the object, the image will still be inverted, because
the rays belonging to each visible point will not intersect before they reach
the eye. But in this case, the image becomes less and less distinct, at the
same time that the visual angle is increasing; so that at the centre, or
rather a little before, the image becomes confused and imperfect, owing to
the small parts of the object subtending angles too large for distinct vision,
just as happens when objects are viewed too near with the naked eye.
3d. In the cases just considered, the images will appear erect; but in the
case where the object is further from the mirror than its centre of concavity, the image will be inverted. The more distant the object is from
the centre, the less will be its image; but the image and object will coincide when the latter is stationed exactly at'the centre.
What peculiar properties have concave mirrors? What facts are stated
with regard to convex mirrors, as resulting from the fundamental law of
Catoptrics 7




166                NATURAL PHILOSOPHY.
2. Diverging rays reflected from a CONVEX surface, are made
more diverging.
3. When converging rays tend towards the focus of parallel
rays, they will become parallel when reflected from a CONVEX
surface.
4. When converging rays tend to a point nearer the surface
than the focus, they will converge less when reflected from a
CONVEX surface.
5. If converging rays tend to a point between the focus and
the centre, they will diverge as from a point on the other side
of the centre, farther from it than the point towards which
they converged.
6. If converging rays tend to a point beyond the centre,
they will diverge as from a point on the contrary side of the
centre, nearer to it than the point towards which they convergred.
7. If converging rays tend to the centre, when reflected, they
will proceed in a direction as far from the centre.
FACTS WITH REGARD TO CONCAVE MIRRORS.
1. Parallel rays, reflected from a CONCAVE surface, are made
converging.
2. Converging rays, falling upon a CONCAVE surface, are
made to converge more.
3. Diverging rays, falling upon a CONCAVE surface, if they
diverge from the focus of parallel rays, become parallel.
4. If from a point nearer to the surface than that focus, they
diverge less than before reflection.
5. If from a point between that focus and the centre, they
What is said of parallel rays? What is said of diverging rays? What
is said of converging rays, when they tend towards the focus of parallel
rays? What is said of converging rays, when they tend to a point nearer
the surface than the focus? What is said of converging rays, when they
tend to a point between the focus and the centre? What is said of converging rays, when they tend to a point beyond the centre? What is
said of converging rays, when they tend to the centre?
What is said with regard to parallel rays, when reflected from a concave surface? What is said of converging rays? What is said of diverging rays, if they diverge from a focus of parallel rays? What, if from
a point nearer to the surface than that focus?




OPTICS.                          167
converge, after reflection, to some point on the contrary side of
the centre, and farther from the centre than the point from
which they diverged.
6. If from a point beyond the centre, the reflected rays will
converge to a point on the contrary side, but nearer to it than
the point from which they diverged.
7. If from the centre, they will be reflected thither again.*
REFRACTION OF LIGHT.
221. That part of the -science of Optics which treats
of refracted light is called Dioptrics.
222. By the refractiont of light is meant its being
turned or bent from its course; and this always takes
place when it passes obliquely from one medium to
another.
223. By a medium,t in Optics, is meant any substance
through which light can pass. Thus, air, glass, water,
and other fluids, are media.
* Concave mirrors, by the property which they possess of causing parallel rays to converge to a focus, are sometimes used as burning-glasses.
M. Dufay made a concave mirror of plaster of Paris, gilt and burnished,
20 inches in diameter, with which he set fire to tinder at the distance of
50 feet. But the most remarkable thing of the kind on record, is the compound mirror constructed by Buffon. He arranged 168 small plane mirrors
in such a manner as to reflect radiant light and heat to the same focus,
like one large concave mirror. With this apparatus, he was able to set
wood on fire at the distance of 209 feet, to melt lead at 100 feet, and silver
at 50 feet.
t The power of being refracted is called refrangibility.
t The plural number of this word is media, although mediums is sometimes used. A medium is called dense or rare, in optics, according to its
refractive power, and not according to its specific gravity. Thus, alcohol,
and many of the essential oils, although of less specific gravity than water,
What, if from a point between that focus and the centre? If from a
point beyond the centre? If from the centre?
221. What is Dioptrics?
222. What is meant by the refraction of light?  When does this take
place?
223. What is a medium, in Optics? Give some examples of media.
Note. In what proportion is a medium dense or rare?




168                NATURAL PHIILOSOPHY.
424. There are three fundamental laws of Dioptrics,
on which all its phenomena depend, namely:
1st. When light passes from one medium to another, in
a direction perpendicular to the surface, it continues on
in a straight line without altering its'course.
2d. When light passes in an oblique direction, from a
rarer to  a denser medium, it will be turned  from  its
course, and  proceed  through  the denser medium  less
obliquely, and in a line nearer to a perpendicular to its
surface.
3d. When light passes from a denser to a rarer medium,
it passes through the rarer medium in a more oblique
direction, and in a line further from  a perpendicular to
the surface of the denser medium.
1. In Fig. 110, the line A B repre-           Fig. 110.
sents a ray of light passing from air into     G
water, in a perpendicular direction.  According to the first law, stated above, it
will continue on in the same line through': "the denser medium  to E.  If the ray   "         _
were to pass upward through the denser
medium, the water, in the same perpendicular direction to the air, by the same          E    F ]
law it would also continue on in the same straight line to A.
2. But if the ray proceed from a rarer to a denser medium,
in an oblique direction, as from C to B, when it enters the
denser medium it will not continue on in the same straight line
to D, but, by the second law, stated above, it will be refracted
or bent out of its course, and proceed in a less oblique direction
to F, which is nearer the perpendicular A B E than D is.
3. Again, if the ray proceed from the denser medium, the
have a greater refracting power, and are, therefore, called denser media
than water. In the following list, the various substances are enumerated
in the order of their refractive power, or, in other words, in the order of
their density, as media; the last-mentioned being the densest, and the first
the rarest, namely: air, ether, ice, water, alcohol, alum, olive oil, oil of
turpentine, amber, quartz, glass, melted sulphur, diamond.
224. What are the three fundamental laws of Dioptrics? IllustratP the
first law by the line A B, in Fig. 110. Illustrate the second law by the
line C B. Illustrate the third law by the line F B.




OPTICS.                         169
water, to the rarer medium, the air, namely, from F to B,instead of pursuing its straight course to G, it will be refracted
according to the third law above stated, and proceed in a more
oblique direction to C, which is further from the perpendicular
EB A than G is.
4. The refraction is more or less in all cases in proportion
as the rays fall more or less obliquely on the refracting surface.
5. From what has now been stated, with regard to refraction, it will be seen that many interesting facts may be explained. Thus, an oar or a stick, when partly immersed in water,
appears bent, because we see one part in one medium, and the
other in another medium: the part which is in the water appears
higher than it really is, on account of the refraction of the
denser medium.
6. For the same reason, when we look obliquely upon a body
of water it appears more shallow than it really is. But when
we look perpendicularly downwards, we are liable to no such
deception, because there will be no refraction.
7. Let a piece of money be put into a cup or a bowl, and
the cup and the eye be placed in such a position that the side
of the cup will just hide the money from the sight; then keeping the eye directed to the same spot, let the cup be filled with
water,-the money will become distinctly visible.
225. The refraction of light prevents our seeing the
heavenly bodies in their real situation.*
* There is another reason, also, why we do not see the heavenly bodies
in their true situation. Light, though it move with great velocity, is about
eight and a half minutes in its passage from the sun to the earth, so that
when the rays reach us, the sun has quitted the spot he occupied on their
departure; yet we see him in the direction of those rays, and, consequently, in a situation which he abandoned eight minutes and a half before. The
refraction of light does not affect the appearance of the heavenly bodies
when they are vertical, that is, directly over our heads, because the rays
then pass vertically, a direction incompatible with refraction.
In what proportion does the refraction increase or diminish? Why does
an oar or a stick, when partly immersed in water, appear bent? Why
does the part which is in the water appear higher than it really is? Why
does a body of water, when viewed obliquely, appear more shallow than it
really is? In what direction can we look so as to cause no refractioul
What experiment is here related?
225. Why do we not see the heavenly bodies in their real situation?
8




170                 NATURAL PHIILOSOPHY.
The light which they send to us is refracted in passing
through the atmosphere, and we see the sun, the stars, &c., in
the direction oft the refracted ray. In consequence of this atmospheric refraction the sun sheds his light upon us earlier in
the morning and later in the evening, than we should otherwise
perceive it. And when the sun is actually below the horizon,
those rays which would otherwise be dissipated through space,
are refracted by the atmosphere towards the surface of the
earth, causing twilight. The greater the density of the air the
higher is its refractive power, and, consequently, the longer the
duration of twilight.
226. When a ray of light passes from one medium to
another, and through that into the first again, if the two
refractions be equal, and in opposite directions, no sensible effect will be produced.
This explains the reason why the refractive power of flat
window-glass produces no effect on objects seen through it.
The rays suffer two refractions, which, being in contrary directions, produce the same effect as if no refraction had taken
place.
227. A lens is a glass, which, owing to its peculiar
form, causes the rays of light to converge to a focus, or
disperses them according to the laws of refraction.
It may here also be remarked, that it is entirely owing to the reflection
of the atmosphere that the heavens appear bright in the daytime. If the
atmosphere had no reflective power, only that part would be luminous in
which the sun is placed; and on turning our back to the sun, the whole
heavens would appear as dark as in the night; we should have no twilight, but a sudden transition from the brightest sunshine to darkness, immediately upon the setting of the sun.
In what direction do we see them?  What causes twilight?  Upon
what does the duration of twilight depend? What other reason is given,
in the note, why we do not see the heavenly bodies in their true situation?
When does the refraction of light not affect the appearance of tile heavenly bodies? Why do the heavens appear bright in the daytime?
226. What effect is produced when a ray of light passes from one medium to another, and through that into the first again? Why does the
refractive power of flat window-glass produce no effect on objects seen
through it?
227. What Is a lens?




OPTICS.                          171
There are various kinds of lenses, named according
to their focus; but they are all to be considered as portions of the internal or external surface of a sphere.
1. A single convex lens has one side                     Fig. 111.
flat and  the  other        A       B        C        D    E
convex; -s A in Fig.
2. A  single con-   G
cave lens is flat on
one side and concave
on the other, as B in
Fig. 111.
3. A double convex lens is convex on both sides, as C,
Fig. 111.
4. A double concave lens is concave on both sides, as D,
Fig. 111.
5. A meniscus* is convex on one side and concave on the
other, as E, Fig. 111.
6. The axis of a lens is a line passing through the centre;
thus, F G, Fig. 111, is the axis of all the five lenses.
228. The peculiar form of the various kinds of lenses
causes the light which passes through them to be refiacted  from  its course, according  to the laws of Dioptrics.
* The word meniscus is derived from the Greek language, and means
literally a little moon. This term is applied to a concavo-convex lens,
from its similarity to a moon in its early appearance. To this kind of lens
the term periscopic has recently been applied, from the Greek language,
meaning literally viewing on all sides. When the concave and convex
sides of periscopic glasses are even or parallel, they act as plane glasses;
but when the sides are unequal, or not parallel, they will act as concave
or convex lenses, according as the concavity or the convexity is the greater.
How are all lenses to be considered?  What is a single convex lens?
What part of Fig. 111 represents a single convex lens?  What is a single
concave lens?  What part of Fig. 111 represents a single concave lens?
What is a double convex lens?  What part of Fig. 111 represents a double
convex lens?  What is a double concave lens?  What part of Fig. 111
represents a double concave lens?  What is a meniscus?  What part of
Fig. 111 represents a meniscus?  What is the axis of a lens?  What line,
in Fig. 111, represents the axis of all the five lenses?
228. What is stated with egard to the form of the lenses?




1  w2                NATURAL PHIILOSOPIHY.
It will be remembered that, according to these laws, light,
in passing from  a rarer to a denser medium  is refracted towards the perpendicular; and, on the contrary, that in passing from a denser to a rarer medium, it is refracted further
from the perpendicular. In order to estimate the effect of a
lens, we must consider the situation of the perpendicular, with
respect to the surface of the lens. Now, a perpendicular, to
any convex or concave surface, must always, when prolonged,
pass through the centre of sphericity; that is, in a lens, the
centre of the sphere of which the lens is a portion.  By an attentive observation, therefore, of the laws above stated, and of
the situation of the perpendicular on each side of the lens, it
will be found in general,1. That convex lenses collect the rays into a focus, and magnify objects at a certain distance.
2. That concave lenses dispersd the rays, and diminish objects
seen through them.
229. The focal distance of a lens is the distance from
the middle of the glass to the focus. This, in a single
convex  lens, is equal to the diameter of the sphere ot
which the lens is a portion; and in a double convex
lens is equal to the radius of a sphere of which the lens
is a portion.
230. When-parallel rays* fall on a convex lens, those
only which fall in the direction of the axis of the lens are
perpendicular to its surface, and those only will continue
on in a straight line through the lens. The other rays,
falling obliquely, are refracted towards the axis and will
meet in a focus.
* The rays of the sun are considered parallel at the surface of the earth.
How is light refracted in passing from a rarer to a denser medium?
How, in passing from a denser to a rarer? What must be considered in
estimating the effect of lenses? Through what must a perpendicular, to
any convex or concave surface, always, when prolonged, pass?  What is
stated with regard to convex lenses? What, with regard to concave
lenses?
229. What is the focal distance of a lens? To what is this equal in a
single convex lens?  To what is it equal in a double convex lens?
230. When parallel rays fall on a convex lens, which one is perpendicular to its surface? How are the other rays, falling obliquely, refracted?




OPTICS.                         173
It is this property of a convex lens which gives it its power
as a burning-glass. All the parallel rays of the sun which pass
through the glass, are collected together in the focus; and,
consequently, the heat at the focus is to the common heat of' the
sun, as the area of the glass is to the area of the focus. Thus, if
a lens, four inches in diameter, collect the sun's rays into a
focus, at the distance of twelve inches, the image will not be
more than one-tenth of an inch in diameter; the surface of this
little circle is 1600 times less than the surface of the lens, and,
consequently, the heat will be 1600 times greater at the focus
than at the lens.*
231. The following effects result from the laws of re
fraction.
FACTS WITH REGARD TO CONVEX SURFACES.
1. Parallel rays passing out of a rarer into a denser medium.
through a CONVEX surface, will become converging.
2. Diverging rays will be made to diverge less, to become
parallel, or to converge, according to the degree of divergency
before refraction, or the convexity of the surface.
3. Converging rays, towards the centre of convexity, will
suffer no refraction.
4. Rays converging to a point beyond the centre of convexity, will be made more converging.
* The following effects were produced by a large lens, or burning-glass,
two feet in diameter, made at Leipsic in 1691. Pieces of lead and tin
were instantly melted; a plate of iron was soon rendered red-hot, and
afterwards fused, or melted; and a burnt brick was converted into yellow
glass. A double convex lens, three feet in diameter, and weighing 212
pounds, made by Mr. Parker, in England, melted the most refractory substances. Cornelian was fused in 75 seconds, a crystal pebble in 6 seconds,
and a piece of white agate in 30 seconds. This lens was presented by the
king of England to the emperor of China.
What property of a convex lens gives it its power as a burning-glass?
Where are all the parallel rays of the sun, which pass through the glass,
collected? How does the heat at the focus compare with the common
heat of the sun? What is related in the note with regard to the effects of
lenses produced by burning-glasses?
231. What is the first effect related as resulting from the laws of.refraction with regard to convex surfaces? What is said of diverging rays?
What is said of rays converging towards the centre of convexity? What
of rays converging to a point beyond the centre of convexity?




174                 NATURAL PIILOSOPIIY.
5. Converging rays towards a point nearer the surface than
the centre of convexity, will be made less converging by refraction.
[When the rays proceed out of a denser into a rarer medium,
the reverse occurs in each case.]
FACTS WITH REGARD TO CONCAVE SURFACES.
1. Parallel rays, proceeding out of a rarer into a denser
medium, through a CONCAVE surface, are made to diverge.
2. Diverging rays are made to diverge more,-to suffer no
refraction, —or to diverge less, according as they proceed from
a point beyond the centre, from the centre, or between the
centre and the surface.
3. Converging rays are made less- converging, parallel, or
diverging, according to their degree of convergency before refraction.*
[When the rays proceed out of a denser into a rarer medium,
the reverse takes place in each case.]
232. Double convex, and double concave glasses, or
lenses, are used in spectacles, to remedy the defects of
the eye; the former, when by age it becomes too flat, or
loses a portion of its roundness; the latter, when by
* The above eight principles are all the necessary consequence of the
operation of the three laws mentioned as the fundamental laws of Dioptrics.
The reason that so many different principles are produced by the operation
of those laws, is, that the perpendiculars to a convex or concave surface
are constantly varying, so that no two are parallel. But in flat surfaces
the perpendiculars are parallel; and one invariable result is produced by
the rays when passing from a rarer to a denser, or from a denser to a
rarer medium, having a flat surface.
What of rays converging to a point nearer the surface than the centre
of convexity? When the rays proceed out of a denser into a rarer medium, what occurs?
What is stated of parallel rays, proceeding from a rarer into a denser
medium, through a concave surface?  What is said of diverging rays?
What is said of converging rays? Of what are the above eight principles the necessary consequlece? What is the reason that so many different principles are produced by the operation of these laws?
232. For what are double convex and concave glasses, or lenses, used in
spectacles?




OPTICS.                        175
any other cause it assumes too round a form, as in the
case of short-sighted (or, as they are sometimes called,
near-sighted) persons. Convex glasses are used when
the eye is too flat, and concave glasses when it is too
round.*
THE EYE.
233. The eye is composed of a number of coats, or
coverings, within which are enclosed a lens, and certain
humors, in the shape, and performing the office of con
vex lenses.
1. The different parts of the eye, are:
1. The Cornea.               6. The Vitreous Humor.
2. The Iris.                 7. The Retina.
3. The Pupil.                8. The Choroid.
4. The Aqueous Humor.   9. The Sclerotica.
5. The Crystalline Lens.   10. The Optic Nerve.
2. Fig. 112 represents a front
view of the eye, in which a a rep-           Fig. 112.
resents the  Cornea, or, as it is
commonly called, the white of the  
eye; e e is the Iris,t having a circular opening in the centre, called
the pupil, p, which contracts in a
strong light, and expands in a faint      a
light, and thus regulates the quantity which is admitted to the tender parts in the interior of the eye.
* These lenses or glasses are generally numbered, by opticians, according to their degree of convexity or concavity; so that by knowing the
number that fits the eye, the purchaser can generally be accommodated,
without the trouble of trying many glasses.
t It is the iris which gives the peculiar color to the eye.
What glasses are used when the eye is too flat? What are used when
the eye is too round?
233. Of what is the eye composed? What are the different parts of
the eye? First? Second? Third? Fourth? Fifth? Sixth? Seventh?
Eighth? Ninth? Tenth? What does Fig. 112 represent?  Explain the
figure.




176                NATURAL PHILOSOPHY.
3. Fig. 113 represents a side              Fig. 113.
view of the eye, laid open, in                        a
which b b represents the cor- 
nea, e e the iris, d d the pupil,    "                       b
ff the aqueous humor, gg the           i          
crystalline lens, h h the vitreous   A
humor, i i i i i the retina, c c
the choroid, a a a a a the sele-E                           d
rotica, and n the optic nerve.    a
4. The cornea forms the
anterior portion  of the eye.            S
It is set in the sclerotica in the
same manner as the crystal of a watch is set in the case.  Its
degree of convexity varies in different individuals and in different periods of life. As it covers the pupil and the iris, it protects them from injury. Its principal office is to cause the
light which reaches the eye to converge to the axis. Part of
the light, however, is reflected by its finely polished surface,
and causes the brilliancy of the eye.
5. Tile iris is so named from its being of different colors. It
is a kind of circular curtain, placed in the front of the eye to
regulate the quantity of light passing to the back part of the
eye. It has a circular opening in the centre, which it involuntarily enlarges or diminishes.
6. The pupil is merely the opening in the iris, through which
the light passes to the lens behind. It is always circular in
the human eye, but in quadrupeds it is of different shape.
When the pupil is expanded to its utmost extent, it is capable
of admitting ten times the quantity of light that it does when
most contracted.* In cats and other animals, which are said
* When we come from a dark place into a strong light, our eyes
suffer pain, because the pupil being expanded, admits a larger quantity of
light to rush in, before it has had time to contract. And when we go
from a strong light into a faint one, we at first imagine ourselves in darkness, because the pupil is then contracted, and does not instantly expand.
What does Fig. 113 represent?  Explain the figure. What part of the
eye does the cornea form? Is its degree of convexity the same in all persons and all periods of life? What is its principal office? From what
does the iris take its name? What is the use of the iris? What is the
pupil?  What is its form in the human eye?  How much more light is
the pupil capable of admitting, when expanded to its utmost extent, tharn
when most contracted?




OPTICS.                        177
to see in the dark, the power of dilatation and contraction is
much greater; it is computed, that their pupils may receive
one hundred times more light at one time than at another.
That light only, which passes the pupil, can be of use in vision;
that which falls on the iris being reflected, returns through
the cornea, and exhibits the color of the iris.
7. The aqueous humor is a fluid, as clear as the purest
water. In shape it resembles a meniscus, and, being situated
between the cornea and the crystalline lens, it assists in collecting and transmitting the rays of light from external objects to
that lens.
8. The crystalline lens is a transparent body, in the form of
a double convex lens, placed between the aqueous and vitreous
humors. Its office is not only to collect the rays to a focus,
on the retina, but also to increase the intensity of the light
which is directed to the back part of the eye.
9. The vitreous humor (so called from its resemblance to
melted glass) is a perfectly transparent mass, occupying the
globe of the eye. Its shape is like a meniscus, the convexity
of which greatly exceeds the concavity.
10. In Fig. 114 the shape of the              Fig. 114.
aqueous and vitreous humors and the
crystalline lens is presented.  a is the       _         /1
aqueous humor, which is a meniscus, b                  I/l/I
the crystalline lens, which is a double
convex lens, and c the vitreous humor,
which is, also, a meniscus, whose concavity has a smaller radius than its
convexity.
11. The retina is the seat of vision. The rays of light being
refracted in their passage by the other parts of the eye, are
brought to a focus in the retina, where an inverted image of the
object is represented.
12. The choroid is the inner coat or covering of the eye. Its
outer and inner surface is covered with a substance called the
What is said of those animals which are said to see in the dark? What
light, only, is of use in vision? What becomes of the light which falls on
the iris? What is the aqueous humor? What is its form? Of what use
is it? What is the crystalline lens?  What is its office? What is the
vitreous humor? Why do persons sometimes experience pain when passing from a dark place into strong light? What is the shape of the vitreous
humnor?  Explain Fig 11:   What is the retinal? What is the Ghoroid?
8*




17138              NATURAL PHILOSOPHY.
pigmentum nigrum, (or black paint.)  Its office is, apparently,
to absorb the rays of light immediately after they have fallen
on the retina.  It is the opinion of some philosophers, that it
is the choroid and not the retina, which conveys the sensation
produced by rays of light to the brain.
13. The sclerotica is the outer coat of the eye. It derives
its name from  its hardness.  Its office is to preserve the
globular figure of the eye, and defend its more delicate internal
structure. To the sclerotica are attached the muscles which
move the eye. It receives the cornea, which is inserted in it
somewhat like a watch-glass in its case. It is pierced by the
optic nerve, which, passing through it, expands over the inner
surface of the choroid, and thus forms the retina.
14. The optic nerve is the organ which carries the impressions made by the rays of light, (whether by the medium of the
retina, or the choroid,) to the brain, and thus produces the
sensation of sight.*
234. The eye is a natural camera obscura,t and the
images of all objects seen by the eye are represented on
the retina, in the same manner as the forms of external
objects are delineated in that instrument.
1. Fig. 115 represents only those parts of the eye which
are most essential for the explanation of the phenomena of
Fig. 115.
* For the above description of the eye and its parts, the author is mainly indebted to Paxton's Introduction to the Study of Anatomy, edited by
Dr. Lewis of this city.
t The camera obscura is explained in a note on page 157.
By what is its outer and inner surface covered? What is its office?
What is the opinion of some philosophers with regard to the choroid?
What is the sclerotica? From what does it derive its name? What is its
office? Wlhat are attached to the sclerotica? What is the optic nerve?




OPTICS.                          179
vision. The image is formed thus. The rays from the object
c d, diverging towards the eye, enter the cornea c, and cross one
another in their passage through the crystalline lens d, by
which they are made to converge on the retina, where they
form the inverted* image, fe.
2. The convexity of the crystalline humor is increased or
* Although the image is inverted on the retina, we see objects erect,
because all the images formed on the retina have the same relative position
which the objects themselves have; and as the rays all cross each other,
the eye is directed upwards, to receive the rays which proceed from the
upper part of an object, and downwards, to receive those which proceed
from the lower part.
A distinct image is also formed on the retina of each eye; but as the
optic nerves of the two eyes unite, or cross each other before they reach
the brain, the impressions received by the two nerves are united, so that
only one idea is excited, and objects are seen single. Although all object
may be distinctly seen with only one eye, it has been calculated that the
use of both eyes makes a difference of about one-twelfth. From the description now given of the eye, it may be seen what are the defects which
are remedied by the use of concave and convex lenses, and how the use of
these lenses remedies them. When the crystalline humor of the eye is
too round, the rays of light which enter the eye converge to a focus
before they reach the retina, and, therefore, the image will not be distinct;
and when the crystalline humor is too flat, (as is often the case with old
persons,) the rays will not converge on the retina, but tend to a point
beyond it. A convex glass, by assisting the convergency of the crystalline lens, brings the rays to a focus on the retina, and produces distinct
vision.
The eye is also subject to iniperfection by reason of the humors losing
their transparency, either by age or disease. For these imperfections no
glasses offer a remedy without the aid of surgical skill. The operation of
couching and removing cataracts from the eye, consists in making a puncture or incision through which the diseased part may escape. Its office is
then supplied by a lens. If, however, the operator, by accident or want
of skill, permit the vitreous humor to escape, the globe of the eye immediately diminishes in size, and total blindness is the inevitable result.
234. What philosophical instrument does the eye resemble in its construction?  Explain Fig. 115. Note. Why do the objects appear erect
when the images are inverted?  Why do we see only one image when
an imarge is formed on both eyes?  What are the defects which are
remedied by the use of concave and convex lenses?  In what other -way
is the eye subject to imperfection? Is there any remedy for this?




180               NATURAL PHILOSOPHY.
diminished by means of two muscles, to which it is attacbed.
By this means the focus of the rays which pass through it,
constantly falls on the retina; and an equally distinct image is
formed, both of distant objects and those which are near.
235. A single microscope consists simply of a convex
lens, commonly called a magnifying-glass; in the focus
of which the object is placed, and through which it is
viewed.
1. By means of a microscope the rays of light from an object are caused to diverge less; so that when they enter the
pupil of the eye, they fall parallel on the crystalline lens, by
which they are refracted to a focus on the retina.
2. Fig. 116 represents a convex lens, or single microscope,
C P. The diverging
rays from  the ob-                   Fig. 116.
ject A B are refracted in their passage           c
through  the  lens   A
C P, and made to 
fall parallel on the
crystalline lens, by
which they are re-                                   J
fracted to a focus            P
on the retina R R;
and the image is
thus magnified, because the divergent rays are collected by the
lens and carried to the retina.
3. Those lenses or microscopes which have the shortest
focus, have the greatest magnifying power; and those which
are the most bulging or convex, have the shortest focus.
Lenses are made small because a reduction in size is necessary
to an increase of curvature.
236. A double microscope consists of two convex
lenses, by one of which a magnified image is formed,
By what is the convexity of the crystalline humor increased or diminished? What is effected by this means?
235. What is a single microscope? What is the use of this microscope I
What figure represents a microscope? Explain the figure. What lenses
have the greatest magnifying power? What lenses have the shortest
focus?
236. Of what does a double microscope consist?




OPTICS.                         181
and by the other this image is carried to the retina of
the eye.
Fig. 117 represents the effect produced by the lenses of a
double microscope.  The rays which diverge from the object
A B are collected by the lens L M, (called the object-glass, because it is nearest to the object,) and form an inverted magniFig. 117.
N
fled image at C D. The rays which diverge from this image
are collected by the lens N 0, (called the eye-glass, because
it is nearest to the eye,) which acts on the principle of the
single microscope, and forms a still more magnified image on
the retina R R.
237. The solar microscope, is a microscope with a
mirror attached to it, upon a moveable joint, which can
be so adjusted as to receive the sun's rays and reflect
them upon the object. It consists of a tube, a mirror or
looking-glass, and two convex lenses. The sun's rays
are reflected by the mirror through the tube upon the
object; the image of which is thrown upon a white
screen, placed at a distance to receive it.
1. The microscope, as above described, is used for viewing
transparent objects only. When opaque objects are to be
viewed, a mirror is used to reflect the light on the side of the
What is the use of these two lenses? What does Fig. 117 represent?
Explain the figure.
237. What is the solar microscope?  Of what does it consist?  By
what, in this microscope, are the sun's rays reflected, and upon what?
For viewing what objects, only, is the microscope, above described, used?
How do those microscopes, used for viewing opaque objects, differ from
these?




182                NATURAL PHILOSOPHY.
object; the image is then formed by light reflected from th.
object, instead of being transmitted through it.
2. The magnifying power of a single'microscope is ascertain
ed by dividing the least distance at which an object can be dis
tinctly seen by the naked eye, by the focal distance of the lens.
This, in common eyes, is about 7 inches. Thus, if the focal
distance of a lens be  only  1 of an inch, then the diametek
of an object will be magnified 28 times, (because 7, divided by
4, is the same as multiplying., 7 by 4,) and the surface will be
magnified 784 times.
3. The magnifying power of the compound microscope ik
found in a similar manner, by ascertaining the magnifying
power, first of one lens, and then of the other.
4. The magnifying power of the solar microscope is in proportion as the distance of the image, from the object-glass, is
greater than that of the object itself from it. Thus, if the distance of the object from the object-glass be - of an inch, and
the distance of the image, or picture, on the screen, be ten
feet, or 120 inches, the object will be magnified in length 480
times, or, in surface, 230,000 times.*
238. The magic lantern is an instrument constructed
on the principle of the solar microscope, but the light is
supplied by a lamp instead of the sun.
1. The objects to be viewed by the magic lantern are
generally painted with transparent colors, on glass slides, which
are received into an opening in the front of the lantern. The
light from the lamp, in the lantern, passes through them, and
carries the pictures, painted on the slides, through the lenses,
by means of which a magnified image is thrown upon the wall,
on a white surface prepared to receive it.
* A lens may be caused to magnify or to diminish an object. If the
object be placed at a distance from the focus of a lens, and the image be
formed in or near the focus, the image will be diminished; but if the object be placed near the focus, the image will be magnified.
How is the image then formed?  How is the magnifying power of a
single microscope ascertained? Illustrate this. How is the magnifying
of the compound microscope ascertained? In what proportion is the magnifying power of the solar microscope? Illustrate this. Note. How may
a lens be made to magnify.or diminish an object?
2:38. What is the magic lantern? How are objects, viewed by the magic
lanu!ter,  geuerally represented?




OPTICS..                         183
2. Fig. 118 represents the magic lantern.  The rays of light
from the lamp are received upon the concave mirror e, and reFig. 118.
flected to the convex lens c, which is called the condensing lens,
because it concentrates a large quantity of light upon the object
painted on the slide, inserted at b. The rays from the illuminated object at b, are carried divergent through the lens a,
forming an image on the screen atf. The image will increase
or diminish in size, in proportion to the distance of the screen
from the lens a.
239. A telescope is an instrument for viewing distant
objects.
There are two kinds of telescopes, namely, the refracting telescope and the reflecting telescope.
A refracting telescope is one in which the object itself
is viewed, through the medium of a number of lenses.
A  reflecting  telescope  is one in  which  the  image
of the object is reflected from a concave mirror, within
the tube of the telescope, and viewed through a number
of lenses.*
* The image of the object seen through a refracting telescope is never
so clear and perfect as that obtained by the reflecting telescope; because
What figure represents a magic lantern? Explain the figure. In what
proportion will the size of the image increase or diminish?
239. What is a telescope? How many kinds of telescopes are there?
What are they? What is a refracting telescope? What is a reflecting
telescope? Note. Why is the image of an object, seen through a refracting telescope, less clear and perfect than when seen through a reflecting
telescope?




184                NATURAL PHILOSOPHY.
1. There are two kinds of refracting telescopes, called the
astronomical telescope, or night-glass, and the terrestrial telescope, or day-glass.* In the former, or night-glass, there are
but two lenses or glasses, but the object is viewed in an inverted position. As the glass is used principally for viewing the
heavenly bodies, the inversion of the image produces no inconvenience. In the latter, or day-glass, two additional lenses are
introduced to give the image its natural position.
2. Fig. 119 represents a night-glass, or astronomical telescope. It consists of a tube, A B C D, containing two glasses,
or lenses. The lens, A B, having a longer focus, forms the
object-glass; the other lens, D C, is the eye-glass. The rays
Fig. 119.
O    c                                       N
from a very distant body, as a star, and which may be considered parallel to each other, are refracted by the object-glass
A B to a focus at K. The image is then seen through the eyeglass D C, magnified as many times as the focal length of the
eye-glass is contained in the focal length of the object-glass.
Thus, if the focal length of the eye-glass D C, be contained
100 times in that of the object-glass A B, the star will be seen
magnified 100 times. It will be seen by the figure, that the
image is inverted; for the ray MA, after refraction, will be
seen in the direction C O, and the ray N B, in the direction D P.
3. Fig. 120 represents a day-glass or terrestrial telescope,
commonly called a spy-glass.  This, likewise, consists of a
tube, A B H G, containing four lenses, or glasses, namely,
the dispersion of colors which every lens produces, in a greater or less degree, renders the image dull and indistinct, in proportion to the number of
lenses employed.
* Some glasses or telescopes are marked "Night and Day."  These
have four glasses, two of which maybe removed when the heavenly bodies
are viewed.
How many kinds of refracting telescopes are there? What are they?
HIow do they differ the one from the other? What does Fig. 119 represent? Explain the figure.




OPTICS.                      1S5
A B, C D, E F, and G H.  The lens A B is the object-glass,
and G H  the eye-glass.  The two additional eye-glasses,
E F and CD, are of the same size and shape, and placed
Fig. 120.
A
oE...... B
at equal distances from each other, in such a manner that the
focus of the one meets that of the next lens. These two eyeglasses, E F and C D, are introduced for the purpose of collecting the rays proceeding from the inverted image M N, into
a new upright image, between G H and E F, and the image is
then seen through the last eye-glass G H, under the angle of
vision, P 0 Q.
4. Fig. 121 represents a reflecting telescope. This consists
also of a large tube, containing two concave metallic mirrors,
A B and C, with two plano-convex eye-glasses. The mirrors
Fig. 121.
are placed at a little more than the sum of their focal distance
from each other. The parallel rays r r, coming from a distant
object, are reflected to a focus g, by the concave mirror A B,
and thus form an inverted image at g; the diverging rays proceeding from this image are again reflected by the small mirror
C, and received by the eye-glass F, through an aperture in the
middle of the mirror A B. The eye-glass F collects these reflected rays into a new image at I, and this image is seen
magnified through the second eye-glass, G.
What does Fig. 120 represent? Explain the figure. What does Fig.
121 represent? Explain the figure.




18G                 NATURAL PHIILOSOPIIY.
5. In reflecting telescopes, mirrors are used to bring the
image near the eye; and a lens, or eye-glass, is employed to
magnify the image.
6. The advantage of reflecting telescopes is, that they possess greater magnifying power, and do not decompose the
light.*
240. That part of the science of Optics which relates
to colors is called Chromatics.
Colors do not exist in the bodies themselves, but are caused
by the peculiar manner in which the light is reflected from
their surfaces.
241. Light is composed of rays of different colors,
which may be separated by a prism.t
242. A prism is a solid triangular, or three-sided
piece of highly-polished glass, generally six or eight
inches long.;
243. The colors which enter into the composition of
light are seven, namely, red, orange, yellow, green,
* Common telescopes have a defect arising from the convexity of the
object-glass, which, as it is increased, has a tendency to tinge the edges
of the images. To remedy this defect, achromatic lenses were formed by
the union of a convex lens of crown glass with a concave lens of flint
glass. Owing to the difference of the refracting power of these two kinds
of glass, the images become free from color and more distinct, and hence
the glasses which produce them were called achromatic, that is, free from
color.
t This discovery was made by Sir Isaac Newton.
t A prism may be made of three pieces of plate glass, about six or eight
inches long, and two or three broad, joined together at their edges, and
made water-tight by putty. The ends may be fitted to a triangular piece
of wood, in one of which an aperture is made by which to fill it with water,
and thus to give it the appearance and the refractive power of a solid
prism.
Why are mirrors used in reflecting telescopes?  What is the use of the
lens? What is the advantage of the reflecting telescope?
240. What is Chromatics? What causes color?
241. Of what is light composed? How can these rays be separated?
ArNte. By whom was this discovery made?
242. What is a prism? Note. How may a prism be made?
243. How many colors enter into the composition of light? What are
they?




OPTICS.                          187
blue, indigo, and violet. Each of these has a different
degree of refrangibility.
244. When light is made to pass through a prism, the
different colored rays are separated, and form an image
on a screen or wall, in which the colors will be arranged
in the order just mentioned.
1. Fig. 122 represents rays of light passing from the aperture, in a window-shutter, A B, through the prism P. Instead
Fig. 122.
Orange. — L;
Red......
D         E                             B
of continuing in a straight course to E, and there forming an
image, they will be refracted, in their passage through the
prism, and form an image on the screen, C D. But as the
different colored rays have different degrees of refrangibility,
those which are refracted. the least will fall upon the lowest
part of the screen, and those which are refracted the most will
fall upon the highest part.  The red rays, therefore, suffering
the smallest degree of refraction, fall on the lowest part of the
scireen, and the remaining colors are arranged in the order of
their refraction.*
* It is supposed that the red rays are refracted the least, on account of
their greater momentum, and that the blue, indigo, and violet are refracted
the most, because they have the least momentum. The same leason, it
is supposed, will account for the red appearance of the sun, through a fog,
or at rising and setting. The increased quantity of the atmosphere, which
Do these rays all have the same degree of refrangibility?
244. What takes place when light is -made to pass through a prism?
Explain Fig. 122. Why do the red rays fall on the lowest part of the
screen? What is supposed with regard to the red rays? What with
regard to the blue, indigo, and violet rays? Why does the sun appear red
through a fog?




188                NATURAL PHILOSOPHlY.
2. If the colored rays, which have been separated by a
prism, fall upon a convex lens, they will converge to a focus,
and appear white.  Hence, it appears, that white is not a
simple color, but is produced by the union of several colors.
3. The spectrum, formed by a glass prism, being divided into 360 parts, it is found that the red occupies 45 of those
parts, the orange 27, the yellow 48, the green 60, the blue 60,
the indigo 40, and the violet 80.*   By mixing the seven
primitive colors in these proportions, a white is obtained; but,
on account of the impurity of all colors, it will be of a dingy
hue. If the colors were more clearly and accurately defined,
the white, thus obtained, would appear more pure also.  An
experiment to prove what has just been said may be thus performed. Take a circular piece of board, or card, and divide it
into parts, by lines drawn from the centre to the circumference.
Then, having painted the seven colors in the proportions above
named, cause the board to revolve rapidly around a pin or wire
at the centre. The board will then appear of a white color.
From this, it is inferred, that the whiteness of the sun's light
arises from a due mixture of all the primary colors.
4. The colors of all bodies are either the simple colors, as
the oblique rays must traverse, and its being loaded with mists and vapors,
which are usually formed at those times, prevents the other rays from
reaching us.
A similar reason will account for the blue appearance of the sky. As
these rays have less momentum, they cannot traverse the atmosphere so
readily as the other rays, and they are, therefore, reflected back to our
eyes by the atmosphere. If the atmosphere did not reflect any rays, the
skies would appear perfectly black.
* Light is found to possess both heat and chemical action. The prismatic spectrum presents some remarkable phenomena with regard to these
qualities; for while the red rays appear to be the seat of the maximum of
heat, the violet, on the contrary, are the apparent seat of the maximum
of chemical action.
Why does the sky appear of a blue color? What would be the appearance of the sky if the atmosphere did not reflect any rays?  Is white a
simple color?  How is it produced? The spectrum formed by a prism,
being divided into 360 parts, how many of these parts does the red occupy? The orange? The yellow? The green? The blue? The indigo?
The violet?  What are the colors of all bodies?




OPTICS.                         189
refracted by the prism, or such compound colors as arise flroir
a mixture of two or more of them.*
245. The rainbow is produced by the refraction of
the sun's rays in their passage  through a shower of
rain; each drop of which acts as a prism in separating
the colored rays, as they pass through it.
This is proved by the following considerations.  First, A
rainbow is never seen except when rain is falling, and the sun
shining at the same time; and that the sun and the bow are
always in opposite parts of the heavens; and, secondly, that
the same appearance may be produced artificially, by means of
water thrown into the air, when the spectator is placed in a
proper position, with his back to the sun; and, thirdly, that a
similar bow is generally produced by the spray which arises
from large cataracts, or waterfalls.t
246. The color of all bodies depends upon the rays
which they reflect.
1. Some bodies absorb all the rays which they receive except the red rays.  These bodies, therefore, appear of a red
color; some reflect the green, and absorb all the others,these will appear of a green color; and, in general, bodies appear of the color of those rays which they reflect, while they
absorb all the other rays. Sometimes a body reflects a portion
of the rays of several colors.  The body will then appear of a
compound color, composed of the various colors which it reflects.  When a body reflects all the rays, it appears white,when it absorbs all the rays, it appears black. White, then,
* From the experiments of Dr. Wollaston, it appears that the seven
colors formed by the prism may be reduced to four, namely, red, green,
blue, and violet; and that the other colors are produced by combinations
of these.
t The Falls of Niagara afford a beautiful exemplification of the truth
of this observation.
Note. What appears from the experiments of Dr. Wollaston?
245. How is the rainbow produced? How is this proved? First?
Second?  Third?
216. Upon what does the color of all bodies depend? Of what color do
bodies generally appear?  When will a body appear of a compound color'?
Of what color will a body appear that reflects all the rays? When will a
body appear black?




190                 NATURAL PI'ILOSOPHY.
is a mixture of all the primitive colors, and black is the deprnvation of all color.
2. From what has now been said, it appears, that no body
has a permanent or intrinsic color of its own,-but that color,
as well as weight, are accidental, and not essential properties.
All substances appear of the same color, or rather, more
properly speaking, are deprived of all color, in the dark.
3. Light, from whatever source it proceeds, is of the same
nature, composed of the various colored rays; and although
some substances appear differently by candlelight, from what
they appear by day, this result may be supposed to arise from
the weakness or want of purity in artificial light.
4. There can be no light without colors, and there can be no
colors without light.
5. That the above remarks, in relation to the colors of bodies,
are true,-may be proved by the following simple experiment.
Place a colored body in a dark room, in a ray of light that has
been refracted by a prism; the body, of whatever color it
naturally is, will appear of the color of the ray in which it is
placed; for, since it receives no other colored rays, it can reflect
no others.*
* Although bodies, from the arrangement of their particles, have a tendency to absorb some rays, and reflect others, they are not so uniform ill
their arrangement as to reflect only pure rays of one color, and perfectly
absorb all others; it is found, on the contrary, that a body reflects in great
abundance the rays which determine its color, and the others in a greater
or less degree, in proportion as they are nearer or further from its color, in
the order of refrangibility. Thus, the green leaves of a rose will reflect a
few of the red rays, which will give them a brown tinge. Deepness of
color proceeds from a deficiency rather than from an abundance of reflected rays. Thus, if a body reflect only a few of the green rays, it will
appear of a dark green. The brightness and intensity of a color shows
that a great quantity of rays are reflected. That bodies sometimes change
their color, is owing to some chemical change, which takes place in the
Is color an essential property of a body? Of what color do bodies appear in the dark?  Why do some bodies appear differently by candlelight?
What is necessary to produce color?  What experiments are related to
prove the truth of the above? Note. WXhat rays does a body reflect in the
greatest abundance? In what proportion does it reflect the other rays?
Why do the green leaves of a rose appear to have a brown tinge? What
does the brightness and intensity of a color show? Why do some bodies
changce their color?




OPTICS.                        191
247. A multiplying-glass is a convex lens, one side
of which is ground down into several flat surfaces.
When an object is viewed through a multiplying-glass, it
will be multiplied as many times as there are flat surfaces on
the lens. Thus, if one lighted candle be viewed through a lens,
having twelve flat surfaces, twelve candles will be seen through
the lens. The principle of the multiplying-glass is the same
with that of a convex or concave lens.
248. The Kaleidescope* consists of two reflecting
surfaces, or pieces of looking-glass, inclined to each
other at an angle of 60 degrees, and placed between the
eye and the objects intended to form the picture.
The two plates are enclosed in a tin or paper tube, and the
objects, consisting of pieces of colored glass, beads, or other
highly-colored fragments, are loosely confined between two
circular pieces of common glass, the outer one of which is
slightly ground, to make the light uniform. On looking down
the tube through a small aperture, and where the ends of the
glass plates nearly meet, a beautiful figure will be seen, having
six angles, the reflectors being inclined the sixth part of a circle. If inclined the twelfth part, or twentieth part of a circle,
twelve or twenty angles will be seen. By turning the tube so
as to alter the position of the colored fragments within, these
beautiful forms will be changed; and in this manner an almost
infinite variety of patterns may be produced.
OF THE THERMAL, CHEMICAL, AND OTHER NON-OPTICAL
EFFECTS OF LIGHT.
1. The science of Optics treats particularly of light as the
medium of vision. But there are other effects of this agent,
internal arrangement of their parts, whereby they lose their tendency to
reflect certain colors, and acquire the power of reflecting others.
* The word Kaleidescope is derived from the Greek language, and
means, " the sight of a beautiful form."  The instrument was invented by
Dr. Brewster, of Edinburgh, a few years ago.
247. What is a multiplying-glass? How many times will an object,
viewed through a multiplying-glass, be multiplied? What is the principle
of the multiplying-glass?
248. Of what does the kaleidescope consist? Note. From what is the
word kaleidescope derived, and what does it mean? By whom was the
instrument invented? What is here said with regard to the kaleidescope i




192                NATURAL PHILOSOPHY.
which, although more immediately connected with the science
of Chemistry, deserve to be noticed in this connexion.
2. The thermal effects of light, that is, its agency in the
excitation of heat, when it proceeds directly from the sun,
are well known. But it is not generally known that these
effects are extremely unequal in the differently colored rays, as
they are refracted by the prism. It has already been stated
in a note on page 186, that the red rays appear to possess the
thermal properties in the greatest degree, and that in the other
rays in the spectrum there is a decrease of thermal power
towards the violet, where it ceases altogether. But, on the
contrary, that the chemical agency is the most powerful in the
violet, from which it constantly decreases towards the red,
where it ceases altogether.  Whether these thermal and chemical powers exist in all light, from whatever source it is derived,
remains yet to be ascertained. The chromatic intensity of the
colored spectrum is greatest in the yellow, from whence it decreases both ways, terminating almost abruptly in the red, and
decreasing by almost imperceptible shades towards the violet,
where it becomes faint, and then wholly indistinct. Thus it
appears that the greatest heating power resides where the
chemical power is feeblest, and the greatest chemical power
where the heating power is feeblest, and that the optical power
is the strongest between the other two.
3. The chemical properties of light are shown in this, that
the light of the sun, and in an inferior degree that of day
when the sun is hidden from view, is a means of accelerating
chemical combinations and decompositions. The following experiment exhibits the chemical effects of light:
Place a mixture of equal parts (by measure) of chlorine and
hydrogen gas in a glass vessel, and no change will happen so
long as the vessel be kept in the dark and at an ordinary temperature; but on exposing it to the daylight, the elements will
slowly combine and form hydrochloric acid; if the glass be set
In what way does Optics treat of light? What is the thermal property
of light?  Are the thermal properties of the rays the same, or different in
each color of the prism?  How do they differ?  How do the chemical
agencies differ?  Do these powers exist in all light, or in solar light only?
XVhere is the greatest heating power found? Where the greatest chemical power? Where the greatest optical power?
In what are the chemical powers of light shown?  Name the experiments and explain them.




oPTIcs.                      193
in the sun's rays, the union will be accompanied with an instantaneous detonation.  The report may also be produced
by transmitting ordinary daylight through violet or blue glass
to the mixture, but by interposing a red glass between the vessel and the light, all combination of the elements is prevented.
4. The chemical effects of light have recently been employed
to render permanent the images obtained by means of convex
lenses. The art of thus fixing them is termed Photography or
Heliography.* The mode in which the process is performed
is essentially as follows:-The picture, lormed by a camera
obscura, is received on a plate, the surface of which has been
previously prepared, so as to make it as susceptible as possible
of the chemical influence of light. After the lapse of a longer
or shorter time, the light will have so acted on the plate that
the various objects, the images of which were projected upon
it,will appear, with all their gradations of light and shade,
most exactly depicted in black and white, no color being
present. This is the process commonly known by the name of
Daguerreotype, from M. Daguerre, the author of the discovery.
Since his original discovery he has ascertained that by isolating
and electrifying the plate, it acquired such a sensibility to the
chemical influence of light, that one-tenth of a second is a sufficient time to obtain the requisite luminous impression for the
formation of the picture.
5. The chemical effects of light are seen in the varied colors
of the vegetable world. Vegetables grown in dark places are
either white or of a palish yellow. The sunny side of fruits is
of a richer tinge than that which grows in the shade. Persons
whose daily employment keeps them much within doors, are
pale, and more or less sickly in consequence of such confinement.
From what has now been detailed with regard to the nature,
the effects, and the importance of light, we may see with what
reason the great epic poet of our language has apostrophized
it in the words:
"Hail, holy Light! offspring of Heaven, first-born,
Bright effluence of bright essence increate,"
* These words are Greek derivatives; the former meaning " writing oi
drawing by means of light," the latter, " writing or drawing by the aild
of the sun."
Explain the art of Photography, or Heliography. By what name is it
now known? How else are the chemical effects of light seen?
9




194                 NATURAL PHILOSOPHY.
and why the author of the Seasons has in a similar manner
addressed it in the terms:
"Prime cheerer, Light!
Of all material beings first and best!
Efflux divine! Nature's resplendent robe!
Without whose vesting beauty all were wrapt
In unessential gloom; and thou, O Sun!
Soul of surrounding worlds, in whom best seen
Shines out thy Maker! may I sing of thee?"
CHAPTER XI.
ELECTRICITY.
249. The word Electricity* is a term used by phllosophers to signify the operations of a very subtle and
elastic fluid, which pervades the material world. Electricity can be seen only in its effects; which are exhibited in the form of attraction and repulsion.
If a piece of amber, sealing-wax, or smooth glass, perfectly
clean and dry, be briskly rubbed with a dry woollen cloth, and
immediately afterwards held over small and light bodies, such
as pieces of paper, thread, cork, straw, feathers, or fragments
of gold leaf, strewed upon a table, these bodies will be attracted; and fly towards the surface that has been rubbed, and adhere to it for a certain time.  The surfaces that have acquired
this power of attraction are said to be excited; and the substances thus susceptible of being excited are called electrics,
while those which cannot be excited in a similar manner are
called non-electrics.
* This word is derived from a Greek word, which signifies amber, be.
cause this substance was supposed to possess, in a remarkable degree, the
property of producing the fluid, when excited or rubbed. The property itself was first discovered by Thales of Miletus, one of the seven wise men
249. What is electricity?  How call electricity be seen?  How  are
these effects exhibited? What illustration of this is given? What is said
rf the surfaces which have acquired the power of attraction?  What are
electrics?  What are non-electrics?  Note. What is stated with regard
to the word electricity? By whom was this property first discovered'?




ELECTrIC'ir v.                        195
250. The science of electricity, therefore, divides all
substances into two  kinds; namely, Electrics, or those
substances which can be excited, and Non-electrics, or
those substances which cannot be excited.
of Greece. The word is now used to express both the fluid itself, and the
science which treats of it.
The nature of electricity is entirely unknown. Some philosophers consider it a fluid; others consider it as two fluids of opposite qualities; and
others again deny its materiality, and deem it, like attraction, a mere property of matter.  In this volume the opinion of Dr. Franklin is adopted,
who supposed it to be a single fluid, disposed to diffuse itself equally among
all substances, and exhibiting its peculiar effects only when a body by any
means becomes possessed of more or less than its proper share. That when
any substance has more than its natural share, it is said to be positively
electrified, and that when it has less than its natural share, it is said to be
negatively electrified; that positive electricity implies a redundancy, and
negative electricity a deficiency of the fluid. The adoption of this opinion
of Franklin is a matter of mere verbal preference; for whether the effects
described under the name of electricity be the effects of one fluid moving
in a particular direction, or of two fluids moving in opposite directions,
or no motion of a fluid at all, the facts remain the same whatever the
theory may be, and they may be explained as well under one set of
terms as another. Professor Faraday has proposed a nomenclature of electricity, which has been adopted in some scientific treatises. From the
Greek words E:sKrpov, (electricity, or amber, from which it was first produced,) and e;ds, (a way or path,) he formed the word electrodes, that is,
ways or paths of electricity. The course of positive electricity he called
the anode, (from the Greek iavoZos, an ascending or entering way,) and the
course of the negative electricity the cathode, (from the Greek KaOoso;, a
descending way, or path of exit.)  The terms positive and negative are,
however, more frequently employed to designate the extremities of the
channels through which electricity passes.  Positive electricity is sometimes expressed by the term plus, or its character +, and negative electricity by the term minus, or its character -.
Electricity may be excited by several modes-as 1st, by friction, whence
What is stated with regard to the nature of electricity? Whose and
what opinion is adopted in this volume?  When is a substance said to be
positively electrified? When is it said to be negatively electrified? What
does positive electricity imply? What does negative electricity imply?
In how many ways may electricity be excited?  What are the different
kinds of elects:city called?
250. Into how many kinds does the science of electricity divide all sub
stances?  What are thev?




196                 NATURAL PHILOSOPHY.
251. The electric fluid is readily communicated from
one substance to another.  Some substances, however,
will not allow it to pass through or over them, while others
give it a free passage. Those substances through which it
passes without obstruction are called conductors; while
those through which it cannot readily pass are called
non-conductors; and it is found, by experiment, that all
electrics are non-conductors, and all non-electrics are
good conductors of electricity.
1. The following substances are electrics, or non-conductors
of electricity; namely,
Atmospheric air, (when dry,)            Feathers,
Glass,                                 Amber,
Diamond,                                Sulphur,
All precious stones,                    Silk,
All gums and resins,                    Wool,
The oxides of all metals,               Hair,
Beeswax,                                Paper,
Sealing-wax,                            Cotton.
All these substances must be dry, or they will become more
or less conductors.
2. The following substances are non-electrics, or conductors
of electricity; namely,
All metals,                             Living animals,
Charcoal,                               Vapor, or steam.
it is called Frictional Electricity; 2dly, by chemical action, called, from
its discoverers, Galvanic or Voltaic Electricity; 3dly, by the action of
heat, whence it is called Thermo-Electricity; 4thly, by Magnetism.
Frictional Electricity forms the subject of that branch of Electricity usually
treated under the head of Natural Philosophy. Electricity, excited by
chemical action, forms the subject of Galvanism; and Electricity produced
by the agency of heat, or by Magnetism, is usually considered in counexion
with the subject of Electro-Maguetism.  The intimate connexion between
these several subjects shows how close are the links of the chain by which
all the departments of physical science are united
251. What is said with regard to the communication of the electric fluid
from one substance to another? Will all substances allow it to pass throutgh
them? What bodies are called conductors? What bodies are called non
conductors? What has been found, by experiment, with regard to elec
tries and non-electrics? What substances are electrics or non-conductors.?
Why must these substances be dry?  What substances are non-electrice
or conductors?




ELECTRICITY.                        197
3. The following are imperfect conductors, (that is, they
conduct the electric fluid, but not so readily as the substances
above mentioned,) namely,
Water,                                  Common wood,
Green vegetables,                      Dead animals,
Damp air,                              Bone,
Wet wood,                               Horn, &c.
All substances containing moisture,
252. When a conductor is surrounded on all sides by
non-conducting substances, it is said to be insulated.
1. As glass is a non-conducting substance, any conducting
substance surrounded with glass, or standing on a table or
stool, with glass legs, will be insulated.
2. As the air is a non-conductor, when dry, a substance
which rests on any non-conducting substance will be insulated,
unless it communicate with the ground, the floor, a table, &c.
253. When a communication is made between a conductor and an excited surface, the electricity from the
excited surface is immediately conveyed by the conductor to the ground;* but if the conductor be insulated,
its whole surface will become electrified, and it is said
to be charged.
* The earth may be considered as the principal reservoir of electricity;
and when a communication exists, by means of any conducting substance,
between a body containing more than its natural share of the fluid, and
the earth, the body will immediately lose its redundant quantity, and the
fluid will escape to the earth. Thus, when a person holds a metallic tube
to an excited surface, the electricity escapes from the surface to the tube,
and passes from the tube through the person to the floor; and the floor
being connected with the earth by conducting substances, such as the
timbers, &c., which support the building, the electricity will finally pass off
by a regular succession of conducting substances, from the excited surface
to the earth. But if the chain of conducting substances be interrupted,
What substances are mentioned as imperfect conductors?
252. When is a substance said to be insulated?  Give the examples.
253. When a communication is made between a conductor and an excited surface, where is the electricity from the excited substance conveyed?
When is it said to be charged? Note. When a communication exists by
means of ally conducting substance, between a body containing more than
Its natural share of the fluid and the earth, what will become of the redundant quantity which the body possesses?




198                 NATURAL PIIILOSOPHY.
254. The simplest mode of exciting electricity is by
friction.
Thus, if a thick cylinder of sealing-wax, or sulphur, or a
glass tube,* be rubbed with a silk handkerchief, a piece of
clean flannel, or the fur of a quadruped, the electric fluid will
be excited, and may be communicated to other substances from
the electric thus excited.
255. The electricity excited in glass is called the
vitreous or positive electricity; and that obtained from
sealing-wax, or other resinous substances, is called
resinous or negative electricity.
256. The vitreous and  resinous, or, in  other words,
the positive and negative electricities, always accompany each other; for if any surface become positive, the
surface with which it is rubbed will become negative;
and if any surface be made positive, the nearest conducting surface will become negative. And if positive
electricity be communicated to one side of an electric,
(as a pane of glass, or a glass vial,) the opposite side
will become negatively electrified, and the plate or the
glass is then said to be charged.
1. When one side of a metallic, or other conductor, receives
that is, if any non-conducting substance occur between the excited surface
and the course which the fluid takes in its progress to the earth, the conducting substances will be insulated, and become charged with electricity.
Thus, if an excited surface be connected by a long chain to a metallic tube,
and the metallic tube be held by a person who is standing on a stool with
glass legs, or on a cake of sealing-wax, resin, or any other non-conducting
substance, the electricity cannot pass to the ground, and the person, the
chain, and the tube will all become electrified.
* Whatever substance is used, it must be perfectly dry. If, therefore,
a glass tube be used, it should previously be held to the fire, and gently
warmed, in order to remove all moisture from its surface.
What illustration of this is given? What follows if this chain of conducting substances be interrupted?
254. What is the simplest mode of exciting electricity? What illustration of this is given?
255. What is the electricity excited in glass called? What is that obtained from resinous substances called?
256 What is stated with regard to positive and negative electricity?




ELECTRICITY.                      199
the electric fluid, its whole surface is instantly pervaded; but
when an electric is presented to an electrified body, it becomes
electrified in a small spot only.
2. When two surfaces oppositely electrified are united, their
powers are destroyed; and if their union be made through the
human body, it produces an affection of the nerves, called an
electric shock.
257. Similar states of electricity repel each other;
and dissimilar states attract each other.
Thus, if two pith-balls suspended by a silk thread, are both
positively  or both negatively electrified, they will repel each
other; but if one be positively and the other negatively electrified, they will attract each other.
258. The Leyden jar is a glass vessel used for the
purpose of accumulating the electric fluid, procured
from excited surfaces.
1. Fig. 123 represents a Leyden jar. It is a glass jar,
coated both on the inside and the outside with tinfoil, with
a cork or wooden stopper through which a
metallic rod passes, terminating upwards in a    Fig. 123
brass knob, and connected by means of a wire,
at the other end, with the inside coating of the
jar.  The coating extends both on the inside
and outside only to within two or three inches
of the top of the jar.  Thus prepared, when an   l 
excited surface is applied to'the brass knob, or
connected with it by any conducting surface, it
parts with its electricity, the fluid enters the jar,
and the jar is said to be charged.
When the Leyden jar is charged, the fluid is
contained in the inside coating of the vial; and
as this coating is insulated, the fluid will remain
in the jar until a communication be made, by
What follows when one side of a metallic, or other conductor, receives
the electric fluid? What follows when an electric is presented to an electrified body? What follows when two surfaces, oppositely electrified, are
united?
257. How do similar states of electricity act on each other? How do
dissimilar states act on each other? Give the examples.
258. For what is the Leyden jar used? What does Fig; 123 represent?
What is a Leyden jar? When is the Leyden jar said to be charged?




200               NATURAL PHILOSOPHlY.
means of some conducting substance, between the inside and
the outside of the jar.  If then a person apply one hand or
finger to the brass knob, and the other to the outside coating
of the jar, a communication will be formed by means of the
brass knob with the inside and outside of the jar, and the jar
will be discharged.  A  vial or jar that is insulated cannot be
charged.
259. An electrical'battery is composed of a number
of Leyden jars connected together.
The inner coatings of the jars are connected together by
chains or metallic bars attached to the brass knobs of each
jar; and the outer coatings have a similar connexion established by placing the vials on a sheet of tinfoil. The whole battery may then be charged like a single jar. For the sake of
convenience in discharging the battery, a knob, connected with
the tinfoil on which the jars stand, projects from the bottom
of the box which contains the jars.
260. The jointed discharger is an instrument used to
discharge a jar, or battery.
Fig. 124 represents the jointed discharger. It consists of
two rods, generally of brass, terminating at one end in brass
balls, and connected together at the
other end by a joint, like that of a         Fig. 124.
pair of tongs, allowing them to be
opened or closed.  It is furnished
with a glass handle, to secure the
person who holds it from the effects of
a shock.  When opened, one of the
balls is made to touch the outside
coating of the jar, or the knob connected with the bottom of the battery, and the other is applied
to the knob of the jar, or jars. A communication being thus
formed between the inside and the outside of the jar, a discharge of the fluid will be produced.
How can the jar be discharged? Can an insulated jar be charged?
259. Of what is an electrical battery composed? How are the inner
coatings of the jars connected together?  How are the outer coatings
connected? In what way is the battery charged?
260. What is the jointed discharger? What does Fig. 124 represent?
Of what does it consist?




ELECTRICITY.                     201
261. When a charge of electricity is to be sent
through any particular substance, the substance must
form  a part of the circuit of the electricity; that is, it
must be placed in such a manner that the fluid cannot
pass from the inside to the outside surface of the jar, or
attery, without passing through the substance in its
passage.
262. Metallic rods, with sharp points, silently attract
the electric fluid.
1. If the balls be removed from the jointed discharger, and
the two rods terminate in sharp points, the electricity will pass
off silently and produce but little effect.
2. A Leyden jar, or a battery, may be silently discharged by
presenting a metallic point, even that of the finest needle, to
the knob. It is on this principle that lightning-rods are constructed. The electric fluid is silently drawn from the cloud
by the sharp points on the rods, and is thus prevented from
suddenly exploding on high buildings.
3. Electricity, of one kind or the other, is generally induced
in surrounding bodies by the vicinity of a highly-excited electric. This mode of communicating electricity by approach, is
styled induction.
4. A body, on approaching another body powerfully electrified, will be thrown into a contrary state of electricity. Thus,
a feather, brought near to a glass tube excited by friction, will
be attracted by it; and, therefore, previously to its touching
the tube, negative electricity miust have been induced in it.
On the contrary, if a feather be brought near to excited
sealing-wax, it will be attracted, and, consequently, positive
electricity must have been induced in it before contact.
263. When electricity is communicated from one
body to another in contact with it, it is called electricity
by transfer.
264. The electrical machine is a machine constructed
261. What is necessary when a charge of electricity is to be sent through
any particular substance?
262. In what way do metallic rods, with sharp points, attract the electric fluid?  How can the electricity be made to pass off silently?  Upon
what principle are lightning-rods constructed?  When is electricity saud
to be communicated by induction?
263. When is electricity produced by transfer?
9*




')0:2               NATURAL'IIILOSOPIIY.
for the purpose of accumulating or collecting electricity,
and transferring it to other substances.
1. Electrical machines are made in various forms, but all on
the same principle, namely, the attraction of metallic points.
The electricity is excited by the friction of silk on a glass surface, assisted by a mixture or preparation called an amalgam.*
The glass surface is made either in the form of a cylinder or a
circular plate, and the machine is called a cylinder or a plate
machine, according as it is made with a cylinder or with a plate.
2. Fig. 125 represents a plate electrical machine.  A D  is
the stand of the machine, L L L L are the four glass legs, or
Fig. 125.
E
posts which support and insulate the parts of the machine. P
posts which support and insulate the parts of the machine. P
is the glass plate, (which in some machines is a hollow cylinder,) from which the electricity is excited, and H is the handle
by which the plate (or cylinder) is turned. R is a leather
cushion, or rubber, held closely to both sides of the glass plate
* The amalgam is composed of mercury, tin, and zinc. That recommended by Singer, is made by melting together one ounce of tin and two
ounces of zinc, which are to be mixed, while fluid, with six ounces of
mercury, and agitated in an iron, or thick wooden box, until cold. It is
then to be reduced to a very fine powder in a mortar, and mixed with a
sufficient quantity of lard to form it into a paste.
264. For what purpose is the electrical machine constructed? Upon
what principle are all electrical machines constructed? How is the electricity excited?  Of what is the amalgam composed?  In what form is
the glass surface made? When is the machine called a plate machine?
When is it called a cylinder machine? What does Fig. 125 represent?
Explain the figure.




ELECTRICITY.                     203
by a brass clasp, supported by the post G L, which is calledl
the rubber-post.  S is a silk bag,* embraced by the same
clasp that holds the leather cushion or rubber; and it is connected by strings S S S attached to its three other corners, and
to the legs L L and the fork F of the prime conductor. C is
the prime conductor, terminating at one end with a moveable
brass ball, B, and at the other by the fork F, which has one
prong on each side of the glass plate. On each prong of the
fork there are several sharp points projecting towards the
plate, to collect the electricity as it is generated by the friction
of the plate against the rubber. V is a chain, or wire, attached to the brass ball on the rubber-post, and resting on the
table or the floor, designed to convey the fluid from the ground
to the plate. When negative electricity is to be obtained, this
chain is removed from the rubber-post, and attached to the
prime conductor, and the electricity is to be gathered from the
ball on the rubbt-c-post.
OPERATION OF THE MA(CIuNE.-By turning the handle H, the
glass plate is pressed by the rubber.  The friction of the
rubber against the glass plate (or cylinder) produces a transfer
of the electric fluid from the rubber to the plate; that is, the
cushion becomes negatively and the glass positively electrified.
The fluid which thus adheres to the glass, is carried round by
the revolution of the cylinder; and its escape being prevented
by the silk bag, or flap, which covers the plate, (or cylinder,)
until it comes to the immediate vicinity of the metallic points
on the fork F, it is attracted by the points, and carried by
them  to the prime conductor.  Positive electricity is thus
accumulated on the prime conductor, while the conductor on
the rubber-post, being deprived of this electricity, is negatively
electrified. The fluid may then be collected by a Leyden jar
from the prime conductor, or conveyed, by means of a chain
attached to the prime conductor, to any substance which is to
be electrified. If both of the conductors be insulated, but a
small portion of the electric fluid can be excited; for this
reason, the chain must in all cases be attached to the rubber-post,
when positive electricity is required, and to the prime conductor,
when negative electricity is wanted.
* In cylindrical machines this silk bag is called " the flap."
Explain the operation of the machine. To what must the chain be attached when positive electricity is required? To what must it be attached
when negative electricity is wanted?




204                 NATURAL PHILOSOPHY.
EXPERIMENTS WITH THE ELECTRICAL MACHINE.
1. On the prime conductor of the electrical machine is placed
the electrometer,* E.  It consists of a wooden ball mounted
on a metallic stick, or wire, having two pith balls, suspended
by silk, hair, or linen threads.  When the machine is worked,
the pith balls, being both similarly electrified, repel each
other; and this causes them to fly apart, as is represented in
the figure; and they will continue elevated until the electricity
is drawn off. But if an uninsulated conducting substance
touch the prime conductor, the pith balls will fall. The height
to which the balls rise, and the quickness with which they are
elevated, afford some test of the power of the machine. This
simple apparatus may be attached to any body, the electricity
of which we wish to measure.
2. The balls of the electrometer, when elevated, are attracted by any resinous substance, and repelled oy any vitreous
substance that has been previously excited by friction.
3. If an electric, or a non-conductor, be presented to the
prime conductor, when charged, it will produce no effect on
the balls; but if a non-electric, or any conducting substance
be presented to the conductor, the balls of the electrometer
will fall. This shows that the conductor has parted with its
electricity, and that the fluid has passed off to the earth
through the substance, and the hand of the person presenting it.
* The word " electrometer" means " a measurer of electricity."  It is
made in a variety of forms, on the principle that similar states of electricity repel each other. It sometimes consists of a single pith-ball, attached to a light rod, in the manner of a pendulum, before a graduated
arc or circle.
An electroscope is an instrument of more delicate construction, to detect the presence of electricity. The most sensitive of this kind of apparatus, is that called Bennett's Goldleaf Electroscope, improved by Singer.
It consists of two strips of goldleaf suspended under a glass covering which
completely insulates them. Strips of tinfoil are attached to the sides of
the glass, opposite the goldleaf, and when the strips of goldleaf diverge,
they will touch the tinfoil, and be discharged. A pointed wire surmounts
the instrument, by which the electricity of the atmosphere may be ob
served.
What is the first experiment mentioned with the electrical machine?
What does the word electrometer mean?  Of what does it sometimes
consist?  What is an electroscope? What is the second experiment?




ELECTRICITY.                    205
4. When the machine is turned, if a person touch the prime
conductor, the fluid passes off through the person to the floor
without his feeling it.  But if he present his finger, his
knuckle, or any part of the body, near to the conductor, without touching it, a spark will pass from the conductor to the
knuckle, which will produce a sensation similar to the pricking
of a pin, or needle.
5. If a person stand on a stool with glass legs, or any
other non-conductor, he will be insulated.  If in this situation
he touch the prime conductor, or a chain connected with it,
when the machine is worked, sparks may be drawn from any
part of the body in the same manner as from the prime con
ductor.  While the person remains insulated, he experiences
no sensation from being filled with electricity; or, if a metallic
point be presented to any part of his body, the fluid may be
drawn off silently, without being perceived. But if he touch
a blunt piece of metal, or any other conducting substance, or
if he step from the stool to the floor, he will feel the electric
shock; and the shock will vary in force according to the
quantity of fluid with which he is charged.
6. THE TISSUE FIGURE. Fig. 126 is a figure with a dress offancy paper cut into narrow strips. When placed on the prime conductor, or being insulated, is connected  with  it, the  strips  being        Fig. 126.
all electrified will recede and form a
sphere around the head.  On pre-   >               ~
senting a metallic point to the elec-   -
trified strips, very singular combina- 
tions will take place.  If the electrometer be removed from the prime
conductor, and a tuft of feathers, or
hair, fastened to a stick or wire, be
put in its place, on turning the machine the feathers or hair will become
electrified, and the separate hairs will
rise and repel each other. A toy is in this way constructed,
representing a person under excessive fright. On touching
the head with the hand, or any conducting substance, not insulated, the hair will fall.
7. The Leyden jar may be charged by presenting it to the
What is the third? What does this show? What is the fourth? What
is the fifth? What is the sixth?




206               NATURAL PHILOSOPHY.
prime conductor, when the machine is worked. If the ball of
the jar touch the prime conductor, it will receive the fluid
silently; but if the ball of the jar be held at a small distance from the prime conductor, the sparks will be seen darting
from the prime conductor to the jar with considerable noise.
8. The jar may in like manner be filled with negative electricity, by applying it to the ball on the rubber-post, and connecting the chain with the prime conductor.
9. If the Leyden jar be charged from the prime conductor,
(that is, with positive electricity,) and presented to the pith
balls of the electrometer, they will be repelled; but if the jar
be charged from the brass ball of the rubber-post, (that is,
with negative electricity,) they will be attracted.
10. If the electrometer be removed from the prime conductor, and a pointed wire be substituted for it, a wire with
sharp points bent in the form of an S, balanced on it, will be
made to revolve rapidly. In a similar manner the motion of the sun and the earth    Fig. 127
around their common centre of gravity, together with the motion of the earth and the
moon, may be represented.*
11. A chime of small bells, on a stand,
Fig. 127, may also be rung by means of brass
balls suspended from  the revolving wires.
The principle of this revolution is similar to
that mentioned in connexion with the revolving jet, Fig. 85, which is founded on thelaw, that action and reaction are equal and
in opposite directions.
12. If powdered resin be scattered over dry cotton-wool,
loosely wrapped on one end of the jointed discharger, it may
be inflamed by the discharge of the battery or a Leyden jar.
Gunpowder may be substituted for the resin.
265. The universal discharger, is an instrument for
directing a charge of electricity through any substance,
with certainty and precision.
* Such an apparatus is sometimes called an Electrical Tellurium.
It may rest on the prime conductor, or upon an insulated stand.
What is the seventh? How may the jar be filled with negative electricity? What is the eighth? What is the ninth? What is the tenth?
What is the eleventh? What is the twelfth?
265. What is the universal discharger?  What is its use?




ELECTRICITY.                     207
1. It consists of two sliding rods, A B and C D, terminating
at the extremities, A  and D,
with brass balls, and at the A Fig. 128. D
other ends, which rest upon the                  C
ivory table or stand E, having a
fork, to which any small substance may be attached. The
whole is insulated by glass legs
or pillars. The rods slide through
collars, by which means their
distance from one another may be adjusted.
2. In using the universal discharger, one of the rods or
slides must be connected by a chain, or, otherwise, with the
outside, and the other with the inside coating of the jar or
battery. By this means the substance through which the
charge is to be sent is placed within the electric circuit.
3. By means of the universal discharger, any small metallic
substance may be burnt. The substance must be placed in the
forks of the slides, and the slides placed within the electric circuit, in the manner described in the last paragraph. In the
same manner, by bringing the forks of the slides into contact
with a substance placed upon the ivory stand of the discharger,
such as an egg, a piece of a potato, water, &c., it may be
illuminated.
4. Ether, or alcohol, may be inflamed by a spark communicated from a person, in the following manner. The person
standing on the insulating stool, receives the electric fluid from
the prime conductor, by touching the conductor or any conducting substance in contact with it; he then inserts the knuckles
of his hand in a small quantity of sulphuric ether, or alcohol,
held in a shallow metallic cup, by another person, who is not
insulated, and the ether or alcohol immediately inflames. In
this case the fluid passes from the conductor to the person
who. is insulated, and he becomes charged with electricity.
As soon as he touches the liquid in the cup, the electric fluid,
passing from him to the spirit, sets it on fire.
What figure represents it? Of what does it consist? What is necessary in using the universal discharger?  What is effected by this means?
What experiments are shown by means of the universal discharger?
How must the substance be placed? How may ether or alcohol be inflamed?




208               NATURAL PIIlLOSOPHY.
266. The electrical bells are designed to show the
effects of electrical attraction and repulsion.
1. They are thus to be applied.* The ball B of the prime
conductor, with its rod, is to be unscrewed, and the rod on
which the bells are suspended is
to be screwed in its place.  The          Fig. 129.
middle bell is to be connected by
a chain with the table or the
floor.  When  the machine is
turned, the balls suspended be-  
tween the bells will be alternately
attracted and repelled by the          E   C            B
bells, and cause a constant ring-   A
ing.  If the battery be charged!
and connected with the prime
conductor, the bells will continue
to ring until all the fluid from the battery has escaped.
It may be observed, that the fluid from the prime conductor
passes readily from the two outer bells, which are suspended by
chains; they, therefore, attract the two balls towards them.
The balls becoming electrified by contact with the outer bells,
are repelled by them and driven to the middle bell, to which
they communicate their electricity; having parted with their
electricity they are repelled by the middle bell, and again attracted by the outer ones, and thus the constant ringing is
maintained. The fluid which is communicated to the middle
bell, is conducted to the earth by the chain attached to it.
2. SPIRAL TUBE. The passage of the electric fluid from one
Fig. 130.
conducting substance to another, is beautifully exhibited by
means of a glass tube having a brass ball at each end, and
* In some sets of instruments, the bells are insulated on a separate
stand; but the mode described above is a convenient mode of connecting
them with the prime conductor.
266. What are the electrical bells? What figure represents them?
What are they designed to show? How are they to be applied? Explain
the spiral tube.




ELECTRICITY.                        209
coated in the inside with small pieces of tinfoil, placed at small
distances from each other in a spiral direction, as represented
in Fig. 130.
In the same manner various figures, letters, and words may
be represented, by arranging similar pieces of tinfoil between
two  pieces of flat glass.  These experi-             Fig. 131
ments appear more brilliant in a darkened
room.
3. HYDROGEN PISTOL. Fig. 131 represents a hydrogen pistol.* When filled with
hydrogen gas,t if the insulated knob K be
presented to the prime conductor, it will                       K
immediately explode.
4. ELECTRICAL SPORTSMAN. Fig. 133 represents the electrical
sportsman. From the larger ball of a Leyden jar two birds
* This apparatus is made in a variety of forms, sometimes in the exact
form of a pistol, and sometimes in the form of a piece of ordnance. The
form in the figure is a simple and cheap contrivance, and is sufficient to
explain the manner in which the instrument is to be used in any of its
forms.
t A very convenient and economical way of procuring hydrogen gas
for this and other experiments, is by means of the hydrogen gas genes
ator, as represented in Fig. 132. It consists of a glass
vessel, with a brass cover, in the centre of which is a  Fig. 132
stop-cock; from the inside of the cover, another glass
vessel is suspended, with its open end downwards. Within this, a piece of zinc is suspended by a wire.  The
outer vessel contains a mixture of sulphuric acid and
water, about nine parts of water to one of acid. When
the cover, to which the inner glass is firmly fixed, is
placed upon the vessel, the acid acting upon the zinc,
causes the metal to absorb the oxygen of the water, and
the hydrogen, the other constituent part of'the water,
being thus disengaged, rises in the inner glass, from
which it expels the water; and when the stop-cock is
turned the hydrogen gas may be collected in the hydrogen pistol, or any other vessel. In the use of hydrogen
gas for explosion, it will be necessary to dilute the gas with an equal portion of atmospheric air.
Explain the hydrogen pistol.




210               NATURAL PHILOSOPHY.
made of pith,' are suspended by a linen thread,              Fig. 133
silk, or hair.  When the
jar is charged the birds
will rise, as represented      A
in the figure, on account          B
of the repulsion of the
fluid in the jar.
If the jar be hen placed
on the tinfoil of the stand,
and the smaller ball placed within a half inch of    Uliiiil iiiithll i
the end of the gun, a discharge will be produced, and the birds will fall.
FURTHER EXPERIMENTS.
1. If images, made of pith, or small pieces of paper, are
placed under the insulated stool, and a connexion be made
between the prime conductor and the top of the stool, the
images will be alternately attracted and repelled; or, in other
words, they will first rise to the electrified top of the stool, and
thus becoming themselves electrified, will be repelled, and fall
to the ground, the floor, or the table; where, parting with their
electricity, they will again be attracted by
the stool, thus rising and falling with con-   Fg. 134.
siderable rapidity.  In order to conduct
this experiment successfully, the images, &c.,
must be placed within a short distance of
the bottom of the stool.
2. On the same principle light figures may
be made to dance when placed between two
discs, the lower one being placed upon a
sliding stand with a screw to adjust the distance, and the upper one being suspended
from the prime conductor, as in Fig. 134.
3. A hole may be perforated through a
quire of paper, by charging the battery, resting the paper up* This substance is obtained in large quantities from the cornstalk, the
whole of which, with the exception of the outside, is composed of pith.
Explain experiment one. Explain the second experiment.




ELECTRICITY.                       211
on the brass ball of the battery, and making a communication,
by means of the jointed discharger, between the ball of one of
the jars and the brass ball of the box. The paper, in this case,
will be between the ball of the battery and the end of the discharger.
4. The thunder-house, Fig. 135, is designed to show the security afforded by  lightning-rods, when lightning strikes a
building.  This is done by placing
a highly combustible material in              Fig. 135.
the inside of the house, and pass-      P
ing a charge of electricity through
it.  On the floor of the house is a
surface of tinfoil.  The hydrogen                 U U [110
pistol being filled with hydrogen
gas from  the gasometer, must be                  O   1I
placed on the floor of the thunder-  _        _ [     U[ i    L
house, and connected with the wire
on the opposite side. The house being then put together, a chain
must be connected with the wire on the side opposite to the
lightning-rod, and the other end placed in contact either with a
single Leyden jar or with the battery. When the jar, thus situated, is charged, if a connexion be formed between the jar and
the points of the lightning-rod, the fluid will pass off silently, and
produce no effect.  But if a small brass ball be placed on the
points of the rod, and a charge of electricity be sent to it, from
the jar or the battery, the gas in the pistol will explode, and
throw the parts of the house asunder with a loud noise.*
* The success of this experiment depends upon the proper connexion
of the jar with the lightning-rod, and the electrical pistol. On the side of
the house opposite to the lightning-rod there is a wire, passing through the
side, and terminating on the outside in a hook. When the house is put
together, this wire, in the inside, must touch the tinfoil on the floor of the
house. The hydrogen pistol must stand on the tinfoil, and its insulated
knob or wire, projecting from its side, must be connected with the lower
end of the lightning-rod, extending into the inside of the house. A communication must then be made between the hook on the outside of the
house, and the outside of the jar or battery. This is conveniently done by
attaching one end of a chain to the hook, and holding the other end in the
hand against the side of a charged jar. By presenting the knob of the
jar to the points of the lightning-rod no effect is produced, but if a brass
ball be placed on the points at P, and the knob of the jar be presented to
Explain the third experiment. The fourth




212                NAT'URAL PHILOSOPHIIY.
5. If the ball of the prime conductor be removed and a
pointed wire be put in its place, the current of electricity
flowing from  the point, when the machine is turned, may be
perceived by placing a lighted lamp before it; the flame will be
blown from the point; and this will be the case in what part
soever of the machine the point is placed, whether on the prime
conductor or the rubber; or if the point be held in the hand
and the flame placed between it and the machine, thus showing that in all cases the fluid is blown from the point. Delicate
apparatus may be put in motion by the electric fluid when
issuing from  a point.  In this way electrical orreries, mills,
&c., are constructed.
6. Goldleaf may be forced into the pores of glass by
placing it between two slips of window-glass, pressing the
slips of glass firmly together, and sending a shock from a battery through them.
7. If goldleaf be placed between two cards, and a strong
charge be passed through them, it will be completely fused.
8. When electricity enters at a point, it appears in the form
of a star; but when it goes out from a point, it puts on the
appearance of a brush.
GENERAL REMARKS.
1. Lightning is the rapid motion of vast quantities of electric matter. Thunder is the noise produced by the rapid motion of lightning through the air.
2. The aurora borealis (or northern lights) is supposed to be
caused by the electric fluid passing through highly-rarefied
air; and most of the great convulsions of nature, such as
earthquakes, whirlwinds, hurricanes, water-spouts, &c., are
generally accompanied by electricity, and often depend upon it.
3. The electricity which a body manifests by being brought
near to an excited body, without receiving a spark from it, is
the ball, the explosion will take place. If the charged jar be very suddenly presented to the points, the explosion may take place; and the jar
may be silently discharged if it be brought very slowly to the ball. The
thunder-house is sometimes put together with magnets.
Explain the fifth experiment. The sixth. The seventh.
What is lightning?  What is thunder?  How is the aurora borealis
supposed to be caused?




ELECTRICITY.                     213
said to be acquired by induction.  When an insulated but unelectrified conductor is brought near an insulated charged conductor, the end near to the excited conductor assumes a state
of opposite electricity, while the farther end assumes the same
kind of electricity,-that is, if the conductor be electrified
positively, the unelectrified conductor will be negative at the
nearer end and positive at the further end, while the middle
point evinces neither positive nor negative electricity.
4. The experiments which have now been described exemplify all the elementary principles of the science of electricity.
These experiments may be varied, multiplied, and extended in
innumerable forms, by anfingenious practical electrician. Among
other things with which the subject may be made interesting,
may be mentioned the following facts, &c.
5. A number of feathers, suspended by strings from an insulated conducting substance, will rise and present the appearance of a flight of birds. As soon as the substance is discharged the feathers will fall. The experiment may be varied
by placing the sportsman on the prime conductor, without the
use of the Leyden jar, to which the birds are attached.
6. Instead of the Leyden jar a plate of common glass (a
pane of window-glass, for instance) may be coated on both
sides with  tinfoil, leaving the edges bare.  A  bent wire
balanced on the edge of the glass, to the ends of which balls
may be attached, with an image at each end, may be made to
represent two persons tilting, on the same principle by which
the electrical bells are made to ring.
7. A beautiful little sawmill was lately exhibited at a lecture
at the Odeon, in Boston, by Mr. Quimby, its ingenious contriver. The moving power was a wheel, with balls at the ends
of the spokes, situated within the attractive influence of two
larger balls, differently electrified. As the balls on the spokes
were attracted by one of the larger balls, they changed their
electrical state and were attracted by the other, which, in its
return, repelled them, and thus the motion being given to the
wheel was communicated by cranks at the end of the axle to
the saws above.
How is the electricity which a body manifests by being brought near
to an excited body, without receiving a spark from it, said to be acquired? When an insulated, but unelectrified conductor, is brought nulla
an insulated charged conductor, what is said with regard to the end near
the exacited conductor?  What example is given to illustrate this?




214               NATURAL PHILOSOPHY.
8. When the hand is presented to the prime conductor, a
spark is communicated, attended with a slightly painful sensation. But if a pin or a needle be held in the hand with the
point towards the conductor, neither spark nor pain will be
perceived, owing to the attracting (or perhaps, more properly
speaking, the receiving) power of the point.
9. That square rods are better than round ones to conduct
electricity silently to the ground, and thus to protect buildings,
may be proved by causing each kind of rod to approach the
prime conductor when charged. It will thus be perceived,
that while little effect is produced on the pith-balls of the
electrometer by the near approach of the round rod, on the
approach of the square one the balls will immediately fall.
The round rod also, will produce an explosion and a spark,
from the ball of the prime conductor, while the square one will
draw off the fluid silently.
10. The effects of pointed conductors upon clouds charged
with electricity may be familiarly exemplified by suspending a
small fleece of cotton-wool from the prime conductor, and
other smaller fleeces from the upper one, by small filaments.
On presenting a point to them they will be repelled and all
drawn together; but if a blunt conductor approach them they
will be attracted.
11. From a great variety of facts, it has been ascertained,
that lightning-rods afford but little security to any part of a
building beyond twenty feet from them; and that when a rod
is painted it loses its conducting power. The lightning-rods of
the most approved construction, and in strictest accordance
with philosophical principles, are composed of small square
rods, (similar to nail-rods.)  They run over the building, and
down each of the corners, presenting many elevated points in
their course. At each of the corners, and on the chimneys, the
rods should be elevated several feet above the building. Rods
of this description have been placed on all the public schoolhouses and other public buildings of Boston, by order of the
city authorities. They were constructed by Dr. King, who
has introduced an improvement, by twisting the square rods,
Why are square rods better than round ones to conduct electricity silently to the ground, and thus protect buildings from lightning? How
far beyond the rod do lightning-rods afford protection? In wlhat way are
the most approved lightnih-lrods co;astructed'!




ELECTRICITY.                      215
and thus multiplying the sharp surfaces presented to collect
the fluid.
12. The removal of silk and woollen garments, worn during
the day in cold weather, is often accompanied by a slight noise,
resembling that of sparks issuing from  a fire.  A  similar
effect is produced on passing the hand softly over the back of
a cat. These effects are produced by electricity.
13. It may here be remarked, that the terms positive and
negative, are merely relative terms, as applied to the subject
of electricity. Thus, a body which is possessed of its natural
share of electricity, is positive in respect to one that has less,
and negative in respect to one that has more than its natural
share of the fluid. So, also, one that has more than its natural
share is positive with regard to one that has only its natural
share, or less than its natural share,-and negative in respect
to one having a larger share than itself.
14. The experiments with the spiral tube connected with
Fig. 130 may be beautifully varied by having a collection of
such tubes placed on a stand; and a jar coated with small
strips resembling a brick wall, presents, when it is charged, a
beautiful appearance in the dark.
15. The electric fluid occupies no perceptible space of time
in its passage through its circuit. It always seems to prefer
the shortest passage, when the conductors are equally good.
Thus, if two, ten, a hundred, or a thousand or more persons,
join hands and be made part of the circuit of the fluid in passing from the inside to the outside of a Leyden jar, they will
all feel the shock at the same moment of time. But, in its
passage, the fluid always prefers the best conductors. Thus,
if two clouds, differently electrified, approach one another, the
fluid, in its passage from one cloud to the other, will sometimes take the earth in its course, because the air is a bad
conductor.
16. In thunder-storms, the electric fluid sometimes passes
from the clouds to the earth, and sometimes from the earth to
the clouds; and sometimes, as has just been stated, from one
cloud to the earth, and from the earth to another cloud.
What is remarked with regard to the terms negative and positive? How
can this be illustrated? What is said with regard to the time the electric
fluid occupies in its passage through its circuit? What example is given
to show that the fluid prefers the best conductors? In what different ways
does the electric fluid sometimes pass in thunder-storms? 




216f               NATURAL PHILOSOPHY.
17. It is not safe, during a thunder-storm, to take shelter
under a tree, because the tree attracts the fluid, and the
human body being a better conductor than the tree, the fluid
will leave the tree and pass into the body.
18. It is also. unsafe to hold in the hand edge-tools, or any
sharp point which will attract the fluid.
19. The safest position that can be chosen during a thunderstorm, is a recumbent posture on a feather bed; and -in all
situations a recumbent is safer than an erect position. No
danger is to be apprehended from lightning when the interval
between the flash and the noise of the explosion is.as much
as three or four seconds. This space of time may be conveniently measured by the beatings of the pulse, if no timepiece be at hand.
20. Lightning-rods were first proposed by Dr. Franklin, towhom is also ascribed the honor of the discovery that thunder
and lightning are the effects of electricity. He raised a kite,
constructed of a silk handkerchief, adjusted to two light strips
of cedar, with a pointed wire fixed to it; and fastening the
end of the twine to a key, and the key, by means of a piece of
silk lace, to a post, (the silk lace serving to insulate the whole
apparatus,) on the approach of a thunder-cloud, he was able
to collect sparks from the key, to charge Leyden jars, and to
set fire to spirits. This experiment established the identity of
lightning and electricity. The experiment was a dangerous one,
as was proved in the case of Professor Richman, of St. Petersburgh, who fell a sacrifice to his zeal for electrical science, by
a stroke of lightning from his apparatus.
21. Among the most remarkable facts, connected with the
science of Electricity, may be mentioned the power possessed
by certain species of fishes, of giving shocks, similar to those
produced by the Leyden jar. There are three animals possessed of this power, namely, the Torpedo, the Gymnotus Electricus, (or Surinam Eel,) and the Silurus Electricus.  But although it has been ascertained that the Torpedo is capable of
giving shocks to the animal system, similar to those of the
Leyden jar, yet he has never been made to afford a spark, nor
Why is it unsafe, during a thunder-storm, to take shelter under a tree,
or to hold in the hand any edge-tools?  What position is the safest in a
thunder-storm?  When is there no danger to be apprehended from the
lightning? By whom were lightning-rods first proposed? Who first discovered that thunder and lightni;ng are the effects of electricity?




ELECTRICITY.                    217
to produce the least effect upon the most delicate electrometer.
The Gymnotus gives a small but perceptible spark. The electrical powers of the Silurus are inferior to those of the Torpedo
or the Gymnotus, but still sufficient to give a distinct shock to
the human system. This power seems to have been bestowed
upon these animals to enable them to secure their prey; and
to resist the attacks of their enemies.  Small fishes, when put
into the water where the Gymnotus is kept, are generally killed
or stunned by the shock and swallowed by the animal, when
he is hungry. The Gymnotus seems to be possessed of a new
kind of sense, by which he perceives whether the bodies presented to him be conductors or not.*
22. It will be recollected that the phenomena which have
now been described, with the exception of what has just been
stated as belonging to animal electricity, belong to the subject
of frictional electricity.  But there are other forms in which
this subtle agent presents itself, which are yet to be described,
which show that its operations are not confined to beautiful
experiments such as have already been presented, nor to the
terrific and tremendous effects that we witness in the storm
and the thunder-gust. Its powerful agency works unseen on
the intimate relations of the parts and properties of bodies of
every description, effecting changes in their constitution and
character, so wonderfully minute, thorough, and universal, that
it may almost be considered as the chief agent of nature,
the prime minister of omnipotence, the vicegerent of creative
power.
* The consideration of the electricity developed by the organs of these
animals of the aquatic order, belongs to that department called Animal
Electricity.
10




218                 NATURAL PHILOSOPHY.
CHAPTER XII.
GALVANISM, OR VOLTAIC ELECTRICITY.
267. Galvanism, a branch of Electricity, derives its
name from  Galvani,* who first discovered the principles
which form its basis.
1. Electricity is produced by the mechanical action of bodies
on one another; but Galvanism, or Galvanic Electricity, is produced by their chemical action.
2. The motion of the electric fluid excited by galvanic
power, differs from that explained under the head of frictional
electricity, in its duration; for while the latter exhibits itself in
sudden and intermitted shocks and explosions, the former continues in constant and uninterrupted action.
3. The nerves and muscles of animals are most easily
* Dr. Aloysius Galvani was a Professor of Allatomy in Bologna, and
made his discoveries about the year i790. His wife, being consumptive, was advised to take, as a nutritive article of diet, some soup made of
the flesh of frogs. Several of these animals, recently skinned for tlat purpose, were lying on a table in his laboratory, near an electrical machine,
with which a pupil of the professor was amusing himself in trying experiments. While the machine was in action, he chanced to touch the bare
nerve of the leg of one of the frogs with the blade of a knife that he held
in his hand, when suddenly the whole limb was thrown into violent convulsions. Galvani being informed of the fact, repeated the experiment, and
examined minutely all the circumstances connected with it. In this way
he was led to the discovery of the principles which form the basis of this
science. The science was subsequently extended by the discoveries of
Professor Volta, of Pavia, who first constructed the Galvanic or Voltaic
pile, in the beginning of the present century.
To produce electricity mechanically, (as has been stated under the head
of frictional electricity,) it is necessary to excite an electric or non-conducting substance by friction.  But galvanic action is produced by the
contact of different conducting substances with each other.
267. What is Galvanism?  By whom and when was galvanism  discovered'?  What led to the discovery? How is electricity generally produced? How is galvanic electricity produced? How does the motion of
the galvanic fluid, excited by galvanic power, differ from that explained
in the science of Electricity? What bodies are most easily affected by
the galvanic fluid?




GALVANISM.                     219
affected by the galvanic fluid; and the voltaic or galvanic
battery possesses the most surprising powers of chemical
decomposition.
268. The galvanic fluid or influence is excited by tne
contact of pieces of different metal, and sometimes by
different pieces of the same metal.
1. If a living frog, or a fish, having a slip of tinfoil on its
back, be placed upon a piece of zinc, spasms of the muscles
will be excited whenever a communication is made between the
zinc and the tinfoil.
2. If a person place a piece of one metal, as a half-dollar,
above his tongue, and a piece of some other metal, (as zinc,)
below the tongue, he will perceive a peculiar taste; and, in
the dark, will see a flash of light, whenever the outer edges of
the metals are in contact.
3. A faint flash may be made to appear before the eyes by
putting a slip of tinfoil upon the bulb of one of the eyes, a
piece of silver in the mouth, and making a communication between them. In these experiments, no effect is produced so
long as the metals are kept apart; but on bringing them into
contact, the effects above described are produced.
269. The conductors of the galvanic fluid, like those
of frictional electricity, are divided into the perfect and
the imperfect. Metallic substances, plumbago and charcoal, the mineral acids and saline solutions, are perfect
conductors. Water, oxydated fluids, as the acids, and
all the substances that contain these fluids, alcohol,
ether, sulphur, oils, resins, and metallic oxydes, are
imperfect conductors.
270. To produce any galvanic action it is necessary
to form what is called a galvanic circle; that is, a certain order or succession of substances capable of exciting
electricity.
268. How is the galJ-anic fluid or influence excited? What illustrations
of this are given?
269. Into what are the conductors of the galvanic fluid divided? What
substances are perfect conductors? What substances are imperfect con
ductors?
2;70. What is necessary in order to produce galvanic action?




220                 NATURAL PHILOSOPHY.
271. The simplest galvanic circle is composed of
three conductors, one of which must be solid, and one
fluid; the third may be either solid or fluid.
1. The process usually adopted for obtaining galvanic electricity is to place between two plates, of different kinds of
metal, a fluid capable of exerting some chemical action on one
of the plates, while it has no action, or a different action, on
the other.  A communication is then formed between the two
plates.
2. Fig. 136 represents a simple             Fig. 136.
galvanic circle.  It consists of a
vessel containing a portion of diluted
sulphuric acid,* with a plate of zinc
Z and of copper C immersed in it.
The plates are separated at the bottom, and the circle is completed by         /
connecting the two plates on the
outside of the vessel by means of
wires. The same effect will be produced, if, instead of using the wires
the metallic plates come into direct
contact.
3. In the above  arrangement            C,
there are three elements or essential parts;t namely, the zinc, the              l
copper, and the acid. The acid, act-       I
ing chemicallyt upon the zinc, produces an alteration in the electrical
state of the metal. The zinc com* The acid employed in the galvanic circuit must always be one that
has a strong affinity for one of the metals in the circuit. When zinc is
employed, sulphuric acid may form one of the three elements, because
that acid has a strong affinity for zinc.
t It is essential in all cases to have three elements to produce galvamic
action. In the experiments which have already been mentioned, in the
case of the frogs, the fish, the mouth, and the eye, the moisture of the
animal, or of the mouth, supplies the place of the acid, so that the three
constituent parts of the circle are completed.
t A certain quantity of electricity is always developed whenever
271. Of what is the simplest galvanic circle composed? What process
is usually adopted for obtaining galvanio electricity? Illustrate this by
Fig. 136.




GALVANISM.                        221
municating its natural share of the electrical fluid to the acid, becomes negatively* electrified. The copper, attracting the same
fluid from the acid, becomes positively electrified. Any conducting substance, therefore, placed within the line of communication between the positive and negative points, will receive the
charge thus to be obtained. The arrows in Fig. 136 show the
direction of the current of positive electricity, namely, from
the zinc to the fluid,-from the fluid to the copper,-from the
copper back through the wires to the zinc, passing from zinc
to copper in the acid, and from copper to zinc out of the acid.
The substance submitted to the action of the electric current
must be placed in the line of communication between the copper and the zinc. The wire connected with the copper is
called the positive pole, and that connected with the zinc the
negative pole, and in all cases the substance submitted to galvanic
action must be placed between the positive and negative poles.
4. The electrical effects of a simple galvanic circle, such as
has now been described, are, in general, too feeble to be perceived, except by very delicate tests. The muscles of animals,
chemical action takes place between afluid and a solid body. This is a
general law of chemical action; and, indeed, it has been ascertained, that
there is so intimate a connexion between electrical and chemical changes,
that the chemical action can proceed only to a certain extent, unless the
electrical equilibrium, which has been disturbed, be again restored. Hence,
we find that in the simple, as well as in the compound galvanic circle, the
oxydation of the zinc proceeds with activity whenever the galvanic circle
is completed; and that it ceases, or, at least, takes place very slowly,
whenever the circuit is interrupted.
* It appears at first view to be a singular fact, that in a simple galvanic
circle, composed of zinc, acid, and copper, the zinc end will always be
negative and the copper end positive; while in all compound galvanic circles, composed of the same elements, the zinc will be positive, and the
copper negative. This apparent difference arises from the compound circle
bekig usually terminated by two superfluous plates.
What effect will be produced if, instead of allowing the metallic plates
to come into direct contact, the communication between them be effected
by wires? How many parts are there in the above arrangement? What
are they? What effect does the acid produce? What is the electrical
st-te of the zinc? Of the copper? What are the arrows in Fig. 136
designed to show? WVhere must the substance, to be submitted to the
action of the fluid, be placed? What is said of the electrical effects of a
simple galvanic circle?




222                NA'URAL PHILOSOPHY.
especially those of cold-blooded animals, such as frogs, &c.,
the tongue, the eye, and other sensitive parts of the body, being very easily affected, afford examples of the operation of
simple galvanic circles. In these, although the quantity of
electricity set in motion is exceedingly small, it is yet sufficient
to produce very considerable effects; but it produces little or
no effect on the most delicate electrometer.
272. The galvanic effects of a simple circle may be
increased to any degree, by a repetition of the same
simple combination.
Such repetitions constitute compound galvanic circles, and
are called galvanic piles or galvanic batteries, according to the
mode in which they are constructed.
273. The voltaic pile consists of alternate plates of
two different kinds of metal, separated by woollen cloth,
card, or some similar substance.
1. Fig. 137 represents a voltaic pile. A voltaic pile may
be constructed in the following manner: take a number of
plates of silver, and the same number of
zinc, and also of woollen cloth, the cloth     Fig. 137.
having been soaked in a solution of sal           i
ammoniac in water; with these a pile is to    z,
be formed in the following order: namely,   S
a piece of silver, a piece of zinc, a piece of  $i,i,Ii 
cloth, and thus repeated. These are to be         l
supported by three glass rods, placed perpendicularly with pieces of wood at the
top and bottom, and the pile will then be complete; and will
afford a constant current of electric fluid through any conducting substance. Thus, if one hand be applied to the lower
plate, and the other to the upper one, a shock will be felt,
which will be repeated as often as the contact is renewed.
Instead of silver, copper plates, or plates of other metal,
What examples are given illustrating the operation of simple galvanic
circles?
27fi. How may the galvanic effects of the simple circle be increased?
What are compound galvanic circles?
273. Of what does the voltaic pile consist? What does Fig. 137 reprsent?  How may a voltaic pile be constructed? Can any other inet.i
be used'?  What are the arrows in the figure designed to show?




GALVANISM.                        223
may be used in the above arrangement. The arrows in the
figure, show the course of the current of electricity in the arrangement of silver, zinc, &c.
2. Voltaic piles have been constructed of layers of gold and
silver paper. The effect of such piles remains undisturbed for
years. With the assistance of two such piles, a kind of perpetual motion, or self-moving clock, has been invented by an
Italian philosopher. The motion is produced by the attraction
and repulsion of the piles exerted on a pith ball, on the principle of- the electrical bells. The top of one of the piles was
positive, and the bottom negative. The other pile was in an
opposite state; namely, the top negative, and the bottom
positive.
274. The voltaic or galvanic battery is a combination
of metallic plates, immersed by pairs in a fluid which
exerts a chemical action on one of each pair of the
plates, and no action, or, at least, a different action on
the other.*
1. Fig. 138 represents a voltaic battery. It consists of a
trough made of baked wood, wedgewood-ware, or some other
non-conducting substance. It is divided into grooves or partitions, for the reception of the
acid, or a saline solution, and the           Fig. 138.
plates of zinc or copper (or other
metals) are immersed by pairs in
the grooves. These pairs of plates
are united by a slip of metal passing from  the one and soldered to
the other; each pair being placed
so as to enclose a partition between  them, and  each  cell or         il"':        II
groove in the trough containing
* The electricity excited by the battery, proceeds from the solid to the
fluid which acts upon it chemically. Thus, in a battery composed of zinc,
diluted sulphuric acid, and copper, the acid acts upon the zinc, and not on
the copper. The galvanic fluid proceed, therefore, from the zinc to the
acid, front the acid to the copper, &c. Instead of using two different
metals to form the galvanic circuit, one metal, in different states, may be
274. What is the voltaic battery?  What is said in the note with regard to the electricity excited by the battery?  What does Fig. 138 represent?  Of what does the voltaic battery consist?




224                 NATURAL PHILOSOPHY.
a plate of zinc, connected with the copper plate of the succeeding cell, and a copper plate joined with the zinc plate of
the preceding cell. These pairs must commence with copper
and terminate with zinc, or commence with zinc and terminate
with copper. The communication between the first and last
plates is made by wires, which thus complete the galvanic
circuit. The substance to be submitted to galvanic action is
placed between the points of the two wires.
2. A  compound battery of great power is obtained  by
uniting a number of these troughs.  In a similar manner a
battery may be produced by uniting several piles, making a
metallic communication between the last plate of the one and
the first plate of the next, and so on, taking care that the
order of succession of the plates in the circuit be preserved
inviolate.
3. The CJouronne des tasses, repre-         Fig. 139.
sented in Fig. 139, is another form            -a
of the galvanic battery. It consists     zc ze  ze  zo
of a number ofcups, bowls, or glasses,
with the zinc and copper plates irn- c                        z
mersed in them, in the order represented in the figure; Z indicating
the zinc, and C the copper plates;
the arrows denoting the course of
the electric fluid.
4. The electric shock from  the voltaic battery may be received by any number of persons, by joining hands, having
previously wetted them.
employed; the essential principle being, that one of the elements shall be
more powerfully affected by some chemical agent than the other. Thus,
if a galvanic pair be made of the same metal, one part must be softer than
the other, (as is the case with cast and rolled zinc;) or a greater amount
of surface must be exposed to corrosion on one side than on the other; or
a more powerful chemical agent be used on one side, so that a current will
be sent from the part most corroded, through the liquid, to the part least
corroded, whenever the poles are united and the circuit thereby completed.
EIow is the communication between the first and last plates made?
Where must the substance, which is to be submitted to galvanic action,
be placed?  How can a compound battery of great power be obtained?
What does Fig. 139 represent? Of what does this battery consist? How
can the electric shock, firom the voltaic battery, be received by any numlber of persons?




a ALVANISM.                     225
5. SMEE's GALVANIC BATTERY is represented in Fig. 140,
and affords an instance of a battery in its simplest form. It
consists of a glass vessel, (as a tumbler,) on
which rests the frame that supports the ap-     Fig. 140.
paratus within. Two screw-cups rise from
the frame, to which wires may be attached
for the conveyance of the electric current
in any direction.  One of the screw-cups
communicates with a thin strip of platinum,
or platinum foil, which is suspended within the glass vessel between two plates of
zinc, thus presenting each surface of the
platinum to a surface of zinc; and the galvanic action is in proportion to the extent of the opposite surfaces of the two metals, and their nearness to each other. The
other screw-cup is connected with the two zinc plates. The
screw-cup connected with the platinum is insulated' from the
metallic frame which supports it, by rosewood, and a thumbscrew confines the zinc plates, so that they can be renewed
when necessary. The liquid employed for this battery is sulphuric acid, or oil of vitriol, diluted with ten parts of water by
measure. To prevent the action of the acid upon the zinc
plates, their surfaces are commonly amalgamated, or combined
with mercury, which prevents any chemical action of the acid
with the zinc until the galvanic circuit is established, when the
zinc is immediately attacked by the acid.
6. Fig. 141 represents a series of three pairs of this battery,
Fig. 141.
in which it will be observed that the platinum of one is connected
with the zinc of the next, and that the terminal wires proceed,
What figure represents Smee's galvanic battery? Describe it. What
liquid is employed for this battery? Describe Fig. 141.
10*




226                NATURAL PHrILOSOPHY.
consequently, one from a platinum plate, and the other from
a zinc plate, as in a single pair.
7. SULPHATE OF COPPER BATTERY. Fig. 142 represents a
sulphate of copper battery, and Fig. 143 a vertical section of
the same battery. It consists of a double cylinder               Fig 142.
of copper, C C, Fig. 143,
with a bottom of the same
metal, which serves the
double purpose of a galvanic plate, and a vessel
to contain the exciting solution. The solution is
contained  in the space
between the two copper
cylinders.  A  moveable
cylinder of zinc, Z, is let
down into the solution
whenever the battery is
to be used.  It rests on
three arms of wood or                    Fig. 143.
ivory at the top, by means
of which it is insulated.
Thus suspended in the
solution, the surfaces of
zinc and copper respectively, face each other.
A screw-cup, N, is attached to the zinc, and
another, P, to the copper
cylinder, to receive the
wires.  When a communication is made between
the two cups, electricity is excited.  The liquid employed in
this battery is a solution of sulphate of copper (common blue
vitriol) in water. A saturated solution is first made, and to
this solution as much more water is added.*
* A pint of water will dissolve about a quarter of a pound of blue vitriol.
The solution described above will therefore contain about two ounces of
the salt to the pint. The addition of alcohol, in small quantities, increases
Describe the sulphate of copper battery.




GALVANISM.                         227
8. GROVE's BATTERY.  This is the most energetic battery
yet known, and is the one most generally used for the magnetic telegraph. The metals employed are platinum and zinc,
and the solutions are strong nitiic acid, in contact with the
platinum, and sulphuric acid diluted with ten or twelve parts of
water in contact with the zinc. This battery must be used
with great care on account of the strength of the acids used
for the solutions, which send out injurious fumes, and which
are destructive to organic substances.  Fig. 144 represents
Grove's battery. The containing vessel is glass; within this
is a thick cylinder of amalgamated zinc,
standing  on short legs, and divided  by            Fig. 144.
a longitudinal opening on one side, in order
to allow the acid to circulate freely. Inside
of this is a porous cell of unglazed porcelain,
containing the nitric acid, and strip of platinum. The platinum is supported by a strip
of brass fixed by a thumb-screw and an insulating piece of ivory to the arm proceeding from  the  zinc cylinder.   The amalgamated zinc is not acted upon by the
diluted sulphuric acid, until the circuit of the battery is completed. But as the nitric acid will filter through the porous
the permanency of the action of the solution. The zinc cylinder should always be taken out of the solution when the battery is not in use; but the
solution may remain in the battery. The battery will keep in good action
for twenty or thirty minutes at a time.
The sulphate of copper battery, although not so energetic as Smee's, is
found very convenient in a large'class of experiments, and is particularly
recommended to those who are inexpert in the use of acids; because the
sulphate of copper being entirely neutral, will not injure the color nor the
texture of organic substances.
There is another form of the sulphate of copper battery, called the Protected Sulphate of Copper Battery, which differs from the one described
in having a porous cell of earthenware, or leather, interposed between the
zinc and the copper, thus forming two cells, in the outer of which sulphate
of copper may be used, and. in the inner one a solution of sulphate of
soda, (Glauber salt,) Qr chloride of sodium, (common salt,) or even dilute
sulphuric acid. This battery will continue in use for several days, and it
is therefore of great use in the electrotype process.
Describe Grove's battery. Which is the most powerful battery? For
what is it used?




228                 NATURAL PHILOSOPIIY.
cell, and act upon the zinc, it is advisable to remove the zinc
from the acid when the battery is to remain inactive. The
action of Grove's battery may be considered as three times
greater than that of the sulphate of copper battery.
275. The spark from a powerful voltaic battery acfs
upon and inflames gunpowder, charcoal, cotton, and
other inflammable bodies, fuses all metals, burns up or
disperses diamonds and other substances on which heat
in other forms produces little or no effect.*
The wires, by which the circuit of the battery is completed,
are generally covered with glass tubes, in order that they may
be held, or directed to any substance.
276. There are three principal circumstances in which
the electricity produced by the galvanic or voltaic battery, differs from that obtained by the ordinary electrical
machine, namely,1st. The very low degree of intensityt of that pro* The most striking effects of Galvanism on the human frame, after
death, were exhibited at Glasgow, a few years ago. The subject on which
the experiments were made, was the body of the murderer Clydesdale,
who was hanged at that city. He had been suspended an hour, and the
first experiment was made in about ten minutes after he was cut down.
The galvanic battery employed consisted of 270 pairs of four-inch plates.
On the application of the battery to different parts of the body, every
muscle was thrown into violent agitation; the leg was thrown out with
great violence, breathing commenced, the face exhibited extraordinary
grimaces, and the finger seemed to point out the spectators. Many persons were obliged to leave the room from terror or sickness; one gentleman fainted, and some thought that the body had really come to life.
t By intensity is here meant the same that is implied by density, as
applied to matter. The quantity of electricity obtained by galvanic action
is much greater than can be obtained by the machine; but it flows, as it
were, in narrow streams. The action of the electrical machine may be
275 What effects may be produced by a spark from a voltaic battery?
What precaution is taken in regard to the lines of the circuit?
2M6. In how many ways does the electricity produced by the galvanic
or voltaic battery differ from that obtained by the ordinary electrical machine? What is the first? What is here meant by intensity? How does
the quantity of electricity obtained by galvanic action, compare with that
obtained by the machine?




GAI.VANISM.                         2. 29
duced by the galvanic battery, compared with that obtained by the machine.
2d. The very large quantity of electricity which is
set in motion by the voltaic battery; and,
3d. The continuity of the current of voltaic electricity,
and its perpetual reproduction, even while this current
is tending to restore the equilibrium.*
A common electrical battery may be charged from a voltaic
battery of sufficient size; but a battery constructed of a small
number of pairs, even though the plates are large, furnishes no
indication of attraction or repulsion equal to that which is given
compared to a mighty torrent, dashing and exhausting itself in one leap
from a precipitous height. The galvanic action may be compared to a
steady stream, supplied by an inexhaustible fountain. Il other words, the
momentum of the electricity excited by galvanism is less than that from
the electrical machine; but the quantity, as has been stated, is greater.
* Whenever an electrical battery is charged, how great soever may be
the quantity that it contains, the whole of the power is at once expended,
as soon as the circuit is completed. Its action may be sufficiently energetic while it lasts, but it is exerted only for an instant, and like the destructive operation of lightning, can effect, during its momentary passage,
only sudden and violent changes, which it is beyond human power to regulate or control. On the contrary, the voltaic battery continues, for an
indefinite time, to develop and supply vast quantities of electricity, which,
far from being lost by returning to their source, circulate in a perpetual
stream, and with undiminished force. The effects of this continued current on the bodies subjected to its action, will, therefore, be more definite,
and will be constantly accumulating; and their amount, in process of
time, will be incomparably greater than even those of the ordinary electrical explosion. It is therefore found that changes in the composition of
bodies are effected by galvanism, which can be accomplished by no other
means. The science of Galvanism, therefore, has extended the field, and
multiplied the means of investigation in the kindred sciences, especially
that of Chemistry.
To what may the action of the electrical machine be compared? To
what may the galvanic action be compared? What is the second way in
which they differ?  What is the third?  What is said in the note with
regard to the third circumstance in which the electricity obtained by the
ordinary electrical machine differs from  that produced by the galvanic
battery? What is said of the effects of this continued current on the bodies
subjected to its action?




230               NA'TURAL P'IIiLOSOPHY.
by the feeblest degree of excitation to a piece of sealing-wax.
A galvanic battery, consisting of fifty pairs of plates, will affect
a delicate goldleaf electrometer; and, with a series of one
thousand pairs, even pith balls are made to diverge.
277. The effect of the voltaic pile on the animal
body depends chiefly on the number of plates that are
employed; but the intensity of the spark and its chemical agencies increase more with the size of the plates,
than with their number. Galvanism explains many
facts in common life.
1. Porter, ale, or strong beer, is said to have a peculiar taste
when drunk from a pewter vessel. The peculiarity of taste is
caused by the galvanic circle formed by the pewter, the beer,
&c., and the moisture of the under lip.
Works of metals, the parts of which are soldered together,
soon tarnish in the places where the metals are joined.
Ancient coins, composed of a mixture of metal, have crum,
bled to pieces, while those composed of pure metal have been
uninjured.
The nails and the copper in sheathing of ships are soon
corroded about the place of contact. These are all the effects
of galvanism.
There are persons who profess to be able to find out seams
in brass and copper vessels by the tongue, which the eye
cannot discover; and, by the same means, to distinguish the
base mixtures which abound in gold and silver trinkets.
2. From what has now been stated, it will be seen that the
effects of galvanic action depend on two circumstances, namely;
1st, the size of the plates employed in the circuit; and, 2dly,
the number of the pairs constituting a battery. But there is
a remarkable circumstance to be noticed in this connexion;
namely, that there is one class of facts dependent on the extension of the size of the plates, and another on the increase of
their number.  The power to develop heat and magnetism is dependent on the size of the plates, that is, on the extent of the
surfaces acted upon by the chemical agent; while the power to
decompose chemical compounds, and to affect the animal system, is affected in a greater ratio by the increase of the number
277. On what does the effect of the voltaic pile on the body depend'i
What facts in common life does galvanism explain? On what does the
effects of galvanic action depend?




GALVANISM.                       2:31
of the pairs. Batteries constructed of large plates are sometimes called  Calorimotors,* from  their great power of producing heat.  They usually consist of fiom one to eight pairs
of plates.  They are made in various forms; sometimes the
sheets of copper and zinc are coiled in concentric spirals,
sometimes placed side by side; and they may be divided into
a great number of small plates, provided that all the zinc plates
are connected together, and all the copper plates together, and then
that the experiments are performed in a channel of communication,
opened between the SETS OF PLATES, and not between PAIRS as in
the common battery, for it is immaterial whether one large surface be used, or many small ones electrically connected together. The effect of all these arrangements, by which the
metallic surface of a single pair is augmented, is to increase the
quantity produced.
3. The galvanic or voltaic battery is one of the most valuable acquisitions of modern science. It has proved in many instances the key by which science has entered into the innermost
recesses of nature, and discovered the secret of many of her
operations. It has, in great measure, lifted the hitherto impenetrable veil that has concealed the mysterious workings in
the material world, and has opened a field for investigation
and discovery as inviting as it is boundless. It has strengthened the sight, and enlarged the view of the philosopher and
the mman of science, and given a degree of certainty to scientific inquiry hitherto known to be unrneached, and supposed to
be unattainable; and if it has not yet satisfied the hopes of the
Alchymist, nor emulated the gold-converting touch of Midas,
it has shown, almost to demonstration, that science may yet
achieve wonders beyond the stories of Mythology, and realize
the familiar adage that " truth is stranger than fiction."
* The name Calorimotor (that is, the mover of heat) was applied by
Dr. Hare of Philadelphia to a very powerful apparatus which he constrt,,ted, and which he found possessed of a very remarkable power in producing
heat.
What are batteries constructed of large plates sometimes called? Why?
Describe them. Upon what principle is the calorimotor constructed? Are
galvanic batteries of value?




232                 NATURAL PHILOSOPHY
CHAPTER XIII.
MAGNETISM AND ELECTRO-MAGNETISM.
278. Magnetism treats of the properties and effects
of the magnet, or loadstone.*
There are two kinds of magnets, namely, the native
or natural magnet, and the artificial.
The native magnet, or' loadstone,t is an ore of iron,
found in iron mines, and  has the property of attracting
iron and other substances which contain it.
A permanent artificial magnet is a piece of iron to
which permanent magnetic properties have been communicated.
For all purposes of accurate experiment, the artificial
is to be preferred to the native magnet.
If a straight bar of soft iron be held in a vertical position,
(or, still better, in a position slightly inclined to the perpendicular, the lower end deviating to the north,) and struck
several smart blows with a hammer, it will be found to have
* That part of science which relates to the development of magnetism
by means of a current of electricity, will be noticed under the head of
Electro-Magnetism, in which connexion will also be mentioned the development of electricity by magnetism, to which the term Magneto-Electricity has been applied.
t Certain ores of iron are found to be naturally possessed of magnetic
properties, and are therefore called natural or native magnets, or loadstones. Besides iron and some of its compounds, nickel and perhaps cobalt
also possess magnetic properties. But all conductors of electricity are capable of exerting the magnetic properties of attraction and repulsion while
conveying a current of electricity, as will be shown under the head of
Electro-Magnetism.
278. Of what does Magnetism treat? How many kinds of magnets
are there? What are they? What is the native magnet? What property does it possess? What is an artificial magnet? What magnet is
preferred, for all purposes of accurate experiment? How can an artificial
magnet be made?




MAGNETISM.                         233
acquired, by this process, all the properties of a magnet; or,
in other words, it will become an artificial magnet.
279. The properties of a magnet are four; namely,
1. Polarity.
2. Attraction of unmagnetic iron.
3. Attraction and repulsion of magnetic iron.
4. The power of communicating magnetism to other iron.
Besides these properties the magnet has recently been discovered to be possessed of electrical properties. These will be
considered in another connexion.
280. By the polarity of a magnet is meant the property of pointing, or turning to the north and south poles.
The end which points to the north, is called the north
pole of the magnet, and the other the south pole. The
attractive power of a magnet is strongest at the poles.*
When a magnet is supported in such a manner as to
move freely, it will spontaneously assume a position
directed nearly north and south.t
There are several ways of supporting a magnet, so as to
enable it to manifest its polarity. First, by suspending it,
accurately balanced, from  a string.  Secondly, by poising it on
a sharp point. Thirdly, by attaching it to some buoyant substance and allowing it to float freely on water.
* The attractive power of a magnet is generally stated to be greatest
at the poles; but the actual poles, or points of greatest magnetic intensity'
in a steel magnet, are not exactly at the ends, but a little within them.
t The points to which the poles of a magnet turn are the magnetic
poles. These do not exactly coincide with the astronomical poles of the
earth; but although the value of the magnetic needle has been predicated
on the supposition that its polarity is a tendency to point exactly to the
north and south poles of the earth, the recent discovery of the magnetic
poles, as the points of attraction, has not depreciated the value of the compass, because the variation is known and can be corrected.
279. What is the first property of the magnet? Second? Third?
Fourth?
280. What is meant by the polarity of a magnet? Where is the attractive power of a magnet the strongest? When will a magnet assume
a position directed nearly north or south? What is the north pole of the
magnet? WVhat is the south pole? In what ways can a magnet be supported so as to enable it to manifest its polarity'




234                 NATURAL PHILOSOPHY.
281. Different poles of magnets attract, and similar
poles repel each other.*
1. A magnet, whether native or artificial, attracts iron or
steel which has no magnetic properties; but it both attracts
and repels those substances, when they are magnetic; that is,
the north pole of one magnet will attract the south pole of
another, and the south pole of one will attract the north of
another; but the north pole of the one repels the north pole of
the other, and the south pole of one repels the south pole
of another.
2. If either pole of a magnet be brought near any small
piece of soft iron, it will attract it. Iron filings will also adhere
in clusters to either pole.
282. A magnet may communicate its properties to
other unmagnetized bodies. But these properties can
be conveyed to no other substances than iron, nickel, or
cobalt, without the aid of electricity.t
All permanent natural and artificial magnets, as well
as the bodies on which they act, are either iron in its
pure state, or such compounds as contain it.
The powers of a magnet are increased by action, and
are impaired and even lost by long disuse.
* There is here a close analogy between the attractive and repulsive
powers of the positive and the negative forms of electricity, and the northern and southern polarities of the magnet. The same law obtains with
regard to both, namely-between like powers there is repulsion; between
unlike, there is attraction.
t The accuracy of the above statement may, perhaps, be questioned,
since Coulomb has discovered, that " all solid bodies are susceptible oJ
magnetic influence." But the " influence" is perceptible only by the
nicest tests, and under peculiar circumstances. [See Electro-Magnetism.]
281. How do the same and different poles of a magnet affect each other?
What is said with regard to the attraction of magnets, whether native or
artificial?  What analogy is there between the attractive and repulsive
powers of the different kinds of electricity, and the northern and southern
polarities of the magnet?
282. Can a magnet communicate its properties to other bodies? To
what substances, only, can these properties be conveyed? Of what sub
stances are all natural and artificial magnets, as well as the bodies on
which they act, composed?  How can the powers of a magnet be in-reased?




MAGNETISM.                         235
1. When the two poles of a magnet are brought together,
so that the magnet resembles in shape a horse-shoe, or the
capital letter U, it is called a horse-shoe magnet, or a U magnet, and it may be made to sustain a considerable weight by
suspending substances from a small iron bar, extending from
one pole to the other. This bar is called the keeper. A small
addition may be made to the weight every day.
2. Soft iron acquires the magnetic power very readily, and
also loses it as readily,-hardened iron or steel acquires the
property with difficulty, but retains it permanently.
283. When a magnet is broken or divided, each part
becomes a perfect magnet, having both a north and
south pole.
This is a remarkable circumstance, since the central part of
a magnet appears to possess but little of the magnetic power,
-but when a magnet is divided in the centre, this very part
assumes the magnetic power, and becomes possessed, in the
one part, of the north, and in the other, of the south polarity.
284. The magnetic power of iron or steel resides
wholly on the surface, and is independent of its mass.*
* In this respect there is a strong resemblance between magnetism and
electricity. Electricity, as has already been stated, is wholly confined to
the surface of bodies. In a few words, magnetism and.electricity may be
said to resemble each other in the following particulars:
1. Each consists of two species, namely, the vitreous and the resinous
(or, the positive and negative) electricities; and the northern and southern
(sometimes called the Boreal and the Astral) polarity.
2. In both magnetism and electricity, those of the same name repel, and
those of different names attract each other.
3. The laws of induction in both are similar.
4. The influence, in both cases, (as has just been stated,) resides at the
surface, and is'wholly independent of their mass.
What is a horse-shoe magnet? How can it be made to sustain a considerable weight?  What is this bar called?  How does soft iron differ
from hardened iron, with respect to its acquiring and losing the magnetic
power?
283. What effect is produced when a magnet is broken or divided?
Why is this a remarkable circumstance?
284. Where does the magnetic power of iron or steel wholly reside?
Note. In what particulars do magnetism and electricity resemble each
other? What is the first?  What is the second?  What is the tlhird?
What is the fourth?




236                  NATURAL PHILOSOPHY.
285. Heat weakens, and  a great degree of heat destroys the power of a magnet; but the magnetic attraction is undiminished by the interposition of' any bodies,
except iron, steel, &c.
Electricity frequently changes the poles of a magnet; and
the explosion of a small quantity of gunpowder, on one of the
poles, produces the same effect.
Electricity, also, sometimes renders iron and steel magnetic,
which were not so before the charge was received.
286. The effect produced by two magnets, used together, are much more than double that of either one
used alone.
When a magnet is suspended freely from its centre,
the two poles will not lie in the same horizontal direction.   This is called  the  inclination  or the dipping of
the magnet.*
287. The magnet, when suspended, does not invariably point exactly to the north and south points,
but varies a little towards the east or the west. This
variation differs at different places, at different seasons,
and at different times in the day.t
* The tendency of a magnetic needle to dip is corrected in the mari
ner's and surveyor's compasses, by making the south ends of the needles intended for use in northern latitudes somewhat heavier than the north ends.
Compass-needles, intended to be employed on long voyages where great
variations of latitude may be expected, are furnished with a small sliding
weight, by the adjusting of which the tendency to dip may be counteracted. The cause of the dipping of the needle is the superior attraction
caused by the closer proximity of the pole of the magnet to the magnetic
pole of the earth. In north latitude, the north pole of the needle dips; in
south latitude, the south pole.
t The variation of the magnetic needle from what has been supposed
its true polarity, was a phenomenon that for centuries had baffled the
285. What effect has heat on the power of the magnet? By what is
the magnetic attraction diminished? What effect has electricity on the
poles of a magnet? What effect has electricity sometimes on iron a!d steel?
286. What proportion do the effects produced by two magnets, used together, bear to that of either, used alone? What is meant by the inclination or dipping of the magnet?
287. Does the magnet, when suspended, invariably point to the north
and south poillt?




MAGNETISM.                          237
1. The science of magnetism  has rendered immense advant:iges to commerce and navigation, by means of the maliner's
compass.*  The mariner's compass consists of a magnetized bar
of steel, called a needle; having at its centre a cap fitted to it,
science of the philosopher to explain. Recent discoveries have given a
satisfactory explanation of this apparent anomaly. The earth has, in fact,
four magnetic poles, two of which are strong and two are weak. The
strongest north pole is in America, the weakest in Asia. The earth itself
is considered as a magnet, or rather, as composed in part of magnetic substances, so that its action at the surface is irregular. The variation of the
needle fromn the true geographical meridian is therefore subject to changres
more or less irregular.
This subject is very ably treated in "Davis' Manual bf Magnetism,"
(edition of 1847,) to which the student is referred, as probably the best
treatise on the subject that has ever been published. Mr. Davis is one of
those scientific and skilful mechanics, (of whom there are not a few among
us,) who have, as it were, forced their way into the temple of science amid
discouragements and difficulties, but have deposited richer gifts on the
altar than most of those whose contributions were expected. He has
originated many improvements in this department of science; and his devotion to the subject has probably rendered him as familiar with all the
peculiar phenomena relating to it, as any one in or out of the counfry.
His address is, Daniel Davis, Jr., Magnetical Instrument Maker, 428
Washington-street, Boston.
* The invention of the mariner's compass is usually ascribed to Flavio
de Melfi, or Flavio Gioia, a Neapolitan, about the year 1302. Some au-.
tnorities, however, assert that it was brought from China by Marco Paolo,
a Venetian, in 1260. The invention is also claimed both by the French
and English.
The value of this discovery may be estimated from the consideration,
that, before the use of the compass, mariners seldom trusted themselves
out of sight of land; they were unable to make long or distant voyages, as
they had no means to find their way back. This discovery enabled them
to find a way where all is trackless; to conduct their vessels through the
mighty ocean, out of the sight of land; and to prosecute those discoveries,
and perform those gallant deeds, which have immortalized the names of
Cook, of La Perouse, Vancouver, Sir Francis Drake, Nelson, Parry,
Franklin, and others.
What advantage has the science of magnetism rendered to commerce
and navigation? Of what does the mariner's compass consist? Note. To
whom is the invention of the mariner's compass usually ascribed? How
may the value of this discovery be estimated?




238                 NATURAL PHILOSOPHY.
which is supported on a sharp-pointed pivot fixed in the base
of the instrument. A circular plate, or card, the circumference
of which is divided into degrees, is attached to the needle, and
turns with it.  On an inner circle of the card the thirty-two
points of the mariner's compass are inscribed.*
2. The needle is generally placed under the card of a mariner's compass, so that it is out of sight; but small needles,
used on land, are placed above the card, not attached to it,
and the card is permanently fixed to the box.
288. The north pole of a magnet is more powerful in
the northern hemisphere, or north of the equator, and
the south pole in the southern parts of the world.
1. When a piece of iron is brought sufficiently near to a
magnet, it becomes itself a magnet; and bars of iron, that
have stood long in a perpendicular situation, are generally
found to be magnetical.
2. Artificial magnets are made by applying one or more
powerful magnets to pieces of soft iron.t The end which is
touched by the north pole becomes the south pole of the new
magnet, and that touched by the south pole becomes the
north pole. The magnet which is employed in magnetizing a
steel bar loses none of its power by being thus employed; and
as the effect is increased when two or more magnets are used,
with one magnet a number of bars may be magnetized, and
then combined together; by which means their power may be
* The compass is generally fitted by two sets of axes to an outer box,
so that it always retains a horizontal position, even when the vessel rolls.
When the artificial magnet or needle is kept thus freely suspended, so
that it may turn north or south, the pilot, by looking at its position, can
ascertain in what direction his vessel is proceeding; and, although the
needle varies a little from a correct polarity, yet this variation is neither
so great, nor so irregular, as seriously to impair its use as a guide to the
vessel in its course over the pathless deep.
t This mode of making artificial magnets is likely to be wholly superseded by the new mode by electrical aid, which will be noticed in connexion with Electro-Magnetism.
288. Where are the north and south poles of a magnet the most powerfui? What effect has a' magnet on a piece of iron, when it is brought
sufficiently near to it? How are artificial magnets made? Does the magnet which is employed in magnetizing a steel bar lose any of its power by
being thus employed?




MAGNETISM.                         239
indefinitely increased. Such an apparatus is called a magnetic
magfazine. *
3. A magnetic needle is made by fastening the steel on a
piece of board, and drawing magnets over it from the centre
outwards.
4. A horse-shoe magnet should be kept armed, by a small
piece of iron or steel, connecting the two poles.
5. Interesting experiments may be made by a magnet, even
of no great power, with steel or iron filings, small needles,
pieces of ferruginous substances, and black sand, which contains
iron.  Such substances may be made to assume a variety of
amusing forms and positions, by moving the magnet under the
card, paper, or table, on which they are placed. Toys, representing fishes, frogs, &c., which are made to appear to bite
at a hook, birds floating on the water, &c., are constructed on
magnetic principles, and sold in the shops.
* There are many methods of making artificial magnets. One of the
most simple and effectual consists in passing a strong horse-shoe magnet
over bars of soft iron.
In making bar (or straight) magnets, the bars must be laid lengthwise,
on a flat table, with the marked end of one bar against the unmarked end
of the next; and in making horse-shoe magnets, the pieces of steel, previously bent into their proper form, must be laid with their ends in contact, so as to form a figure like two capital U's, with their tops joined together, thus c;, observing that the marked ends come opposite to those
which are not marked; and then, in either case, a strong horse-shoe mag.
net is to be passed, with moderate pressure, over the bars; taking care to
let the marked end of this magnet precede, and its unmarked end follow
it; and to move it constantly over the steel bars, so as to enter or commence the process at a mark, and then to proceed to an unmarked end,
and enter the next bar at its marked end, and so proceed.
After having thus passed over the bars tell or a dozen times on each
side, and in the same direction, as to the marks, they will be converted
into tolerably strong and permanent magnets. But if, after having continued the process for some time, the exciting magnet be moved even once
over the bars in a contrary direction, or if its south pole should be permitted to precede, after the north pole has been first used, all the previously
excited magnetism will disappear, and the bars will be found in their
original state.
What is a magnetic magazine?  How is a magnetic needle made.
What is said with regard to a horse-shoe magnet? How should a horsishoe magnet be kept?




240                 NATURAL PHILOSOPHY.
ELECTRO-MAGNETISM.
289. Electro-Magnetism relates to magnetism which
is induced by the agency of electricity.*
* The passage of the two kinds of electricity (namely, the positive and
the negative) through their circuit, is called the electric currents; and the
science of Electro-Magnetism explains the phenomena attending those
currents. It has already been stated, that, from the connecting wires of
the galvanic circle, or battery, there is a constant current of electricity
passing from the zinc to the copper, and from the copper to the zinc plates.
In the single circle these currents will be negative from the zinc, and positive from the copper; but in the compound circles, or the battery, the
current of positive electricity will flow from the zinc to the copper, and
the current of negative electricity from the copper to the zinc. From the
effect produced by electricity on the magnetic needle, it had been conjectured, by a number of eminent philosophers, that magnetism, or magnetic
attraction is in some manner caused by electricity. In the year 1819,
Professor CErsted of Copenhagen, made the grand discovery of the power
of the electric current to induce magnetism; thus proving the connexion
between magnetism and electricity. In a short time after the discovery
of Professor (Ersted, Mr. Faraday discovered that an electrical spark could
be taken from a magnet; and thus the common source of magnetism and
electricity was fully proved. In a paper published a few years ago, this
distinguished philosopher has very ably maintained the identity of common electricity, voltaic electricity, magnetic electricity, (or electro-magnetism,) thermo-electricity, and animal electricity. The phenomena exhibited in all these five kinds of electricity differ merely in degree and the
state of intensity in the action of the fluid. The discovery of Professor
IErsted has been followed out by Ampere, who, by his mathematical and
experimental researches, has presented a theory of the science less obnoxious to objections than that proposed by the Professor. The discovery
of CErsted was limited to the action of the electric current on needles previously magnetized; it was afterwards ascertained by Sir Humphrey
Davy, and M. Arago, that magnetism may be developed in steel not pre289. Of what does Electro-Magnetism treat? Note. What is the electric current? What does the science of electro-magnetism explain? What
is the difference between the currents in the single and the compound circles?  What is it thought causes magnetic attraction?  What discovery
was made in the year 1819?  By whom?  What further discovery was
made soon after, and by whom?  What does this philosopher maintain?
How many kinds of electricity are there?  How do the phenomena exhibited in these five kinds of electricity differ?




ELECTRO-MAGNETISM.                     24-1
290. The principal facts in connexion with the science
of electro-magnetism are,1. That the electrical current, passing  uninterruptedly*
through a wire, connecting the two ends of a galvanic battery, produces an effect upon the magnetic needle.
2. That electricity will induce magnetism.
3. That a magnet, or a magnetic magazine, will induce
electricity.t
4. That the combined action of electricity and magnetism,
as described in this science, produces a rotatory motion of
certain kinds of bodies, in a direction pointed out by certain
laws.
5. That the periodical variation of the magnetic needle
from the true meridian, or, in other words, the variation of
the compass, is caused by the influence of the electric currents.
6. That the magnetic influence is not confined to iron, steel,
&c., but that most metals, and many other substances, may
be converted into temporary magnets by electrical action.
7. That the magnetic attraction of iron, steel, &c. may be
prodigiously increased by electrical agency.
viously possessing it, if the steel be placed in the electric current. Both
of these philosophers, independently of each other, ascertained that the
uniting wire, becoming a magnet, attracts iron filings, and collects suffcient to acquire the diameter of a common quill; but the moment the
connexion is broken, all the filings drop off, and the attraction diminishes
with the decaying energy of the pile. Filings of brass or copper, or wood
shavings, are not attracted at all.
* All the effects of electricity and galvanism that have hitherto been
described, have been produced on bodies interposed between the extremities of conductors, proceeding from the positive and negative poles. It
was not known, until the discoveries of Professor (Ersted were made, that
any effect could be produced when the electric circuit is uninterrupted.
t The consideration of the subject of electricity, induced by magnetism,
properly belongs to the subject of Magneto-Electricity, in which connexion
it will be particularly noticed.
Note. Can magnetism be developed in steel not previously possessing it' 
Where must the steel be placed? What property has the uniting wire!
290. What are the principal facts in colnexion with the science of electro-magnetism? What is the first? What is the second? What is the
third? What is the fourth? What is the fifth? What is the sixth I
What is the seventh?
11




242                NATURAL PHILOSOPHY.
8. That the direction of the electric current may, in all
cases, be ascertained.*
9. That magnetism is produced whenever concentrated electricity is passed through space.
10. That while in common electrical and magnetic attractions and repulsions, those of the same name are mutually
repulsive, and those of different names attract each other; in
the attractions and repulsions of electric currents, it is precisely
the reverse, the repulsion taking place only when the wires
are so situated that the currents are in opposite direction.
291. A magnet freely suspended tends to assume a
position at right angles to the direction of a current of
electricity passing near it.
1. If a wire, which connects the extremities of a voltaic
battery, be brought over, and parallel with a magnetic needle
at rest, or with its poles properly directed north and south,
that end of the needle next to the negative pole of the battery will move towards the west, whether the wire be on one
side of the needle or the other, provided only, that it be
parallel with it.
Again, If the connecting wire be lowered on either side of
the needle, so as to be in the horizontal plane in which the
needle should move, it will not move in that plane, but will
have a tendency to revolve in a vertical direction; in which,
however, it will be prevented from moving, in consequence of
the attraction of the earth, and the manner in which it is suspended. When the wire is to the east of the needle, the pole
* This is done by means of the magnetic needle. If a sheet of paper
be placed over a horse-shoe magnet, and fine black sand, or steel filings,
be dropped loosely on the paper, the particles will be disposed to arrange
themselves in a regular order, and in the direction of curve lines. This
is, undoubtedly, the effect of some influence, whether that of electricity,
or of magnetism alone, cannot at present be determined.
Where have the boUies been supposed to be placed, in all the effects of
electricity and galvanismn that have hitherto been described? What is the
eighth fact in connexiJn with the science of electro-magnetism? What is
the ninth? What is the tenth? How can the direction of the electric
current be ascertainzed?
291. If a magnet be freely suspended, and a current of electricity be
passed near it, whSat direction will it assume'!? What illustration of this is
given? What second illustration is given?




ELECTRO-MAGNETISM.                      243
nearest to the negative extremity of the battery will be elevated; and when it is on the west side, that pole will be
depressed.
2. If the connecting wire be placed below the plane in
which the needle moves, and parallel with it, the pole of the
needle next to the negative end of the wire will move towards
the east; and the attractions and repulsions will be the reverse of those observed in the former case.*
292. The two sides of an unmagnetized steel needle
will become endued with the north and south polarity,
if the needle be placed parallel with the connecting
wire of a voltaic battery, and nearly or quite in contact
with it. But if the needle be placed at right angles
with the connecting wire, it will become permanently
magnetic; one of its extremities pointing to the north
pole and the other to the south, when it is freely suspended and suffered to vibrate undisturbed.
293. Magnetism may be communicated to iron and
steel by means of electricity from an electrical machine;
but the effect can be more conveniently produced by
means of the  voltaic  battery.   This phenomenon is
called electro-magnetic induction.
* The action of the conducting-wire in these cases exhibits a remarkable peculiarity. All other known forces exerted between two points, act
in the direction of a straight line connecting these points; and such is the
case with electric and magnetic actions, separately considered; but the
electric current exerts its magnetic influence laterally, at right angles to
its own course. Nor does the magnetic pole move either directly towards
or directly from the conducting-wire, but tends to revolve around it without
changing its distance. Hence the force must be considered as acting in
the direction of a tangent to the circle in which the magnetic pole would
move.
In what direction will the pole of the needle next to the negative end
of the wire move, if the connecting wire be placed below the plane in
which the needle moves, and parallel with it?  What is said with regard
to the attractions and repulsions?
292. Huw may the two sides of an unmagnetized steel needle become
endued with the north and south polarity? Under what circumstances will
it become permanently magnetic?
293. How can magnetism be communicated to iron and steel?  How
can the effect be more conveniently produced?




244                NATURAL PHILOSOPHY.
1. If a helix* be formed of wire, and a bar of steel be enclosed within the helix, the bar will immediately become magnetic by applying the conducting wires of the battery to the
extremities of the helix. The electricity from the common
electrical machine, when passed through the helix, will produce the same effect.
2. If such a helix be so placed that it may move freely, as
when made to float on a basin of water, it will be attracted
and repelled by the opposite poles of a common magnet.
294. If a magnetic needle be surrounded by coiled
wire, covered with silk, a very minute portion -of electricity through the wire will cause the needle to deviate
from its proper direction.
A needle thus prepared, is called an electro-magnetic multiplier.  It is, in fact, a very delicate electroscope, or rather
galvanometer,-capable of pointing out the direction of the
electric current in all cases.
295. Among the most remarkable of the facts connected with the science of electro-magnetism, is what
is called  the  electro-magnetic  rotation.   Any  wire,
through which a current of electricity is passing, has a
tendency to revolve around a magnetic pole, in a plane
perpendicular to the current; and that without reference
to the axis of the magnet, the pole of which is used. In
like manner a magnetic pole has a tendency to revolve
around such a wire.
1. Suppose the wire perpendicular, its upper end positive,
or attached to the positive pole of the voltaic battery, and its
* The helix is a spiral line, or a line in the form of a corkscrew. The
wire which forms the helix should be coated with some non-conducting'
substance, such as silk wound around it; as it may then be formed into
close coils, without suffering the electric fluids to pass from surface to surface, which would impair its effect..
What illustration of this is given? Note. What is the helix? Why
should the wire, which forms the helix, be coated with some non-colducting substance? What is said of a helix, if it be placed so that it may
move freely?
294. How can the magnetic needle be made to deviate from its proper
direction?  What is a needle thus prepared called?
295. What is the electro-magnetic rotation? What illustration is given?




ELECTRO-MAGNETISM.                 245
lower end negative; and let the centre of a watch-dial represent the magnetic pole: if it be a north pole, the wire will
rotate round it in the direction that the hands move; if it be
a south pole, the motion will be in the opposite direction.
From these two, the motions which would take place if the
wire were inverted, or the pole changed, or made to move,
may be readily ascertained, since the relation now pointed out
remains constant.
2. Fig. 145 represents the ingenious apparatus, invented by
Mr. Faraday, to illustrate the electro-magnetic rotation. The
central pillar supports
a piece of thick cop-                Fig. 145.
per wire, which, on
the one side, dips into
the mercury contained
in a small glass cup
a. To a pin at the
bottom of this cup, a
small cylindrical magnet is attached by a          b
piece of thread, so that          j
one pole shall rise a
little above the sur-   
face of the mercury,
and be at liberty to
move around the wire. The bottom of the cup is perforated,
and has a copper pin passing through it, which, touching the
mercury on the inside, is also in contact with the wire that
proceeds outwards, on that side of the instrument. On the
other side of the instrument, b, the thick copper wire, soon
after turning down, terminates, but a thinner piece of wire
forms a communication between it and the mercury on the
cup beneath. As freedom of motion is regarded in the wire,
it is made to communicate with the former by a ball and
socket-joint, the ball being held in the socket by a piece of
thread; or else the ends are bent into hooks, and the one is
then hooked to the other. As good metallic contact is required, the parts should be amalgamated, and a small drop
of mercury placed between them; the lower ends of the wire
What does Fig. 145 represent? Explain the figure. How is the freedom of motion, which is required on the wire, obtained? How can the
me i tic contact which is required be obtained?




246               NArURAL PHILOSOPHY.
should also be amalgamated.  Beneath the hanging wire, a
small circular magnet is fixed in the socket of the cup b, so
that one of its poles is a little above the mercury. As in the
former cup, a metallic connexion is made through the bottom,
from the mercury to the external wire.
If now the poles of a battery be connected with the horizontal external wires, c c, the current of electricity will be
through the mercury and the horizontal wire, on the pillar
which connects them, and it will now be found, that the
moveable part of the wire will rotate around the magnetic
pole in the cup b, and the magnetic pole round the fixed wire
in the other cup a, in the direction before mentioned.
By using a very delicate apparatus, the magnetic pole of
the earth may be made to put the wire in motion.
3. Fig. 146 represents another ingenious    Fig. 146.
contrivance, invented by M. Ampere, for illustrating the electro-magnetic rotation; and it
has the advantage of comprising within itself
the voltaic combination which is employed. It
consists of a cylinder of copper about two inches
high, and a little less than two inches internal
diameter, within which is a small cylinder, about
one inch in diameter. The two cylinders are connected together by a bottom, having an aperture
in its centre the size of the smaller cylinder,
leaving a circular cell, which may be filled with
acid. A piece of strong copper wire is fastened
across the top of the inner cylinder, and from the middle of
it rises, at a right angle, a piece of copper wire, supporting a
very small metal cup, containing a few globules of mercury.
A cylinder of zinc, open at each end, and about an inch and
a quarter in diameter, completes the voltaic combination. To
the latter cylinder a wire, bent like an inverted U, is soldered
at opposite sides; and in the bend of this wire a metallic
point is fixed, which, when inserted in the little cup of mercury, suspends the zinc cylinder in the cell, and allows it a
free circular motion. An additional point is directed downIf the poles of a battery be connected with the horizontal external wires,
c c, throughout, what direction will the current of electricity take? Round
what pole will the moveable part of the wire rotate? Round what will
the magnetic pole rotate? What does Fig. 146 represent? Of what does
it consist?




ELECTRO-MAGNETISM.                     247
wards from the central part of the stronger wire, which point
is adapted to a small hole at the top of a powerful bar magnet.  When the apparatus with one point only is charged
with diluted acid, and set on the magnet placed vertically, the
zinc cylinder revolves in a direction determined by the magnetic pole which is uppermost. With two points, the copper
revolves in one direction, and the zinc in a contrary direction.
4. If, instead of a bar magnet, a horse-shoe magnet be employed, with an apparatus on each pole similar to that which
has now been described, the cylinders in each will rev'olve in
opposite directions. The small cups of mercury mentioned in
the preceding description are sometimes omitted, and the
points are inserted in an indentation on the inverted U.*
296. The magnetizing power of the conducting wires
of a battery is very greatly increased by coiling it into
a helix, into which the body to be magnetized may be
inserted. A single circular turn is more efficient than a
straight wire, and each turn adds to the power within a
certain limit, whether the whole forms a single layer, or
whether each successive turn encloses the previous one.
1. When a helix of great power is required, it is composed
of several layers of wire. The wire forming the coil must be
insulated by being wound with cotton, to prevent any lateral
passage of the current.
2. Fig. 147 represents a helix on a stand. A bar of soft
iron, N S, being placed within the helix, is connected with the
* The phenomenon of electro-magnetic rotation is beautifully illustrated
by Mr. Davis, in his treatise on Magnetism, to which reference has already been made. He has invented and prepared a great variety of ingenious contrivances for the illustration of this subject, and his book should
be in the hands of all who desire a thorough acquaintance with all that
has been discovered in the new department of science in which magnetism
and electricity are combined. The author has been indebted to Mr. Davis'
volume for a number of explanations which are incorporated in this work.
How will the cylinders in each revolve, if, instead of a bar-magnet,
a horse-shoe magnet be employed, with an apparatus on each pole similar
to that which has now been described?
296. How may the magnetizing power of the connecting wires be increased?  Is a single circuit preferable to a straight wire? Explain Fig.
147.




248               NATURAL PHILOSOPHY.
battery by means of the screw-cups on the base of the stand.
The two extremities of the bar instantly become strongly
Fig. 147.
magnetic, and keys, or pieces of iron, iron filings, nails, &c.
will be held up so long as the connexion with the battery is
sustained. But so soon as the connexion is broken, the bar
loses its magnetic power, and the suspended articles will fall.
The bar can be made alternately to take up and drop such
magnetizable articles as are brought near it, as the connexion
with the battfery is made or broken.
3. A steel bar placed within the helix acquires the polarity
less readily, but retains it after the connexion is broken.
Small rods or bars of steel, needles, &c. may be made permanent magnets in this way.
297. A bar temporarily magnetized by the electric
current is called an electro-magnet.
To ascertain the poles of an electro-magnet it must be observed that the north pole will be at the farthest end of the helix
when the current circulates in the direction of the hands of a
watch.
298. Magnets of prodigious power have been formed
by means of voltaic electricity.
297. What is a bar called that is temporarily magnetized? How do you
ascertain the poles of an electro-magnet?
298. How have magnets of great.power been formed?




ELECTRO-MAGNETISM.                    249
1. An electro-magnet was constructed by Professor Henry
and Dr. Ten Eyck, which was capable of supporting a weight
of 750 pounds. They have subsequently constructed another,
which will sustain 2063 pounds. It consists of a bar of soft
iron, bent into the form of a horse-shoe, and wound with
twenty-six strands of copper bell-wire, covered with cotton
threads, each thirty-one feet long; about eighteen inches of
the ends are left projecting, so that only twenty-eight feet of
each actually surround the iron. The aggregate length of the
coils is therefore 728 feet. Each strand is wound on a little less
than an inch: in the middle of the horse-shoe it forms three
thicknesses of wire; and on the ends, or near the poles, it is
wound so as to form six thicknesses. Being connected with a
battery consisting of plates, containing a little less than fortyeight square feet of surface, the magnet supported the prodigious weight stated above, namely, 2063 pounds.
2. HELIACAL RING.
Fig. 148  represents a                  Fig. 148.
heliacal ring, or ring of
wire bent in the form of
a helix, with the ends
of the wire left free to
be inserted in the screwcups of a battery. Two
semicircular  pieces  of
soft unmagnetized iron,
furnished with rings —
the upper one for the
hand, the lower one for
weights-are  prepared
to be inserted into the
helix, in the manner of
the links of a chain. As
soon as the ends of the
helix are inserted into
the screw-cups of the
battery, the rings will
be held together with
great force, by magnetic attraction.*
* That the attraction is caused, or that the magnetism is induced by
What weight was the magnet constructed by Professor Henry and Dr
Ten Eyck capable of supporting? Explain the heliacal ring and Fig. 148.
11*




250                NATURAL PHILOSOPHY
COMMUNICATION OF MAGNETISM TO STEEL BY THE
ELECTRO-MAGNET.
299. Bars of the U form are most readily magnetized
by drawing them from the bend to the extremities,
across the poles of the U electro-magnet, in such a
way that both halves of the bar may pass at the same
time over the poles to which they are applied. This
should be repeated several times, recollecting always to
draw the bar in the same direction.
Fig. 149 represents the U electro-magnet, with the bar to
be magnetized. When the bar is thick both surfaces should
Fig. 149
be drawn across the electro-magnet, keeping each half applied
to the same pole. To remove the magnetism it is only necessary to reverse the process by which it was magnetized, that
is, to draw the bar across the electro-magnet in a contrary
direction.
the circulation of electricity around the coils, may be proved by the following interesting experiment. Hold the heliacal ring horizontally over
a plate of small nails, and suspend an unmagnetized bar perpendicularly on the outside of the ring, over the nails, and there will be no attraction. Suspend the bar perpendicularly through the helix, and the
nails will all attach themselves to it in the form of tangents to the circles
formed by the coils of the heliacal ring.
299 How are bars of the U form most readily magnetized?  Explain
Fig. 149.




ELECTRO-MAGNETISM.                   251
THE ELECTRO-MAGNETIC TELEGRAPH.
300. From the description which has now been given
of the electro-magnetic power, it will readily be perceived
that a great force can be made to act, simply by bringing a wire into contact with another conductor, and
that the force can be instantly arrested in its operation
by removing the wire from the contact; in other words,
that by connecting and disconnecting a helix with a
battery a prodigious power can be made successively
to act and cease to act. Advantage has been taken of
this principle in the construction of the American electro-magnetic telegraph, which was matured by Professor
Morse, and first put into operation between the cities of
Baltimore and Washington, in  1844.*   It comes not
within the province of this work to enter into a minute
description of this great invention, but the principles of
its construction may be briefly stated as follows:
1. An electro-magnet is so arranged with its armature, that
when the armature is attracted it communicates its motion to
a lever, to which a blunt point is attached, which marks a
narrow.strip of paper, drawn under it by machinery resem —
bling clock-work, whenever the electro-magnet is in action.
When the electro-magnet ceases to act, the armature falls,
and communicating its motion to the lever, the blunt point is
removed from its contact with the paper. By this means, if
one of the wires from the battery is attached to the screwcup, whenever the other wire is attached to the remaining cup,
the armature is powerfully attracted by the magnet, and the
* For a particular description of this wonderful invention, the student
is referred to Davis' treatise on Magnetism, in which the parts are all described with a minuteness which leaves little more to be desired. The
history, also, of the successive steps by which it was brought to its present
degree of perfection, is also to be found in the same connexion. It will
be sufficient here to state, that it was not until Professor Henry of Princeton, N. J., had discovered the mode of constructing the powerful electromagnets which have been described, that this form of telegraph became
possible.
300. Explain the electro-magnetic telegraph.




252                NATURAL PHILOSOPHY.
point on the lever presses the paper into the groove of a
roller,- thereby making an indentation on the paper corresponding in length to the time during which the contact with
the battery is maintained, the paper being drawn slowly under the roller.
An alphabet of signs or symbols, is formed by indentations
of the paper varying in length, which is easily read by those
connected with the telegraph. Thus, the letter e is represented by one short mark thus -; the letter o, by two marks
thus - -; the letter a, by a short and a long one thus —;
the letter f by a short, a long, and a short one, thus —  -;
the figure 1 by a short, two long, and a short one, thus
-   -.-.  By such an arrangement all the letters and the
numerals are represented by the telegraph.*  A simple contrivance connected with the machinery causes a bell to strike
when the telegraph commences its operations, and thus gives
warning to the attendant.
2. It has already been stated that the motion of electricity
exceeds in velocity even that of light, and that its velocity is
equal to 288,000 miles in a second of time.  The invention
that thus enables man to communicate with his brother man,
with a rapidity that sets time and space at defiance, deservedly ranks as one of the greatest ever achieved by human
ingenuity.t
* The following table presents a view of Morse's Telegraphic Alphabet
A —         B —--       C -- -       D —-        EF —         G —-        H ---        I - -       J —
K —- L-                 M --    N — O-   -
R..... Sq..                          1       ---  TIU --       ~ V.        W —          X -.    -..Z-.            &. --    &c.-...
Numerals....._    2 -. 3...-       4.. 5 —6. —---   7 ----  8 -   9 ----      0
t The invention of the magnetic telegraph has been made since the
first edition of this work was published. The subject of Electro-Magnetism in the earliest editions, was closed by the following remarks. So
early a fulfilment of any portion of them is remarkable and unexpected.
"The science of Electro-Magnetism is yet in its infancy. The discoveries
which,have rewarded the labors of philosophical research are truly wonderful; but man has as yet but lifted the veil, behind which the stupendous operations of nature are carried on. What wonders he will
discover, should he penetrate the recesses of her laboratory, imagination
cannot conceive. It would have excited no little surprise among the




MAGNETO-ELECTRICITY.                    253
3. ELECTRO-MAGNETISM  AS A MOTIVE PowER.-The first
application of electro-magnetism as a motive power was made
by Professor Henry, but he made no attempt to apply it to
practical purposes. Dr. Page has gone further, and suggested improvements which have overcome some of the obstacles to the employment of this power. But thus far it has
been found that the smallest machines present the strongest
probability of this power becoming available.
MAGNETO-ELECTRICITY.
301. Magneto-Electricity treats of the development of
electricity by magnetism.
Electric currents are excited in a conductor of electricity by magnetic changes taking plate in its vicinity.
Thus, the movement of a magnet near a metallic wire,
or near an iron bar enc.osed in a wire coil, occasions
currents in the wire.
1. When an armature, or any piece of soft iron, is brought
into contact with one or both of the poles of a magnet, it
becomes itself magnetic by induction, and by its reaction adds
to -the power of the magnet: on the contrary, when removed
from the contact, it diminishes the power of the magnet, and
these alternate changes in its magnetic state induce a current
of electricity.
2. The most powerful effects are obtained by causing a bar
of soft iron, enclosed in a helix, to revolve by mechanical means
near the poles of a steel magnet. As the iron approaches
the poles, in its revolution, it becomes magnetic; as it recedes
philosophers of the last century, had the opinion been advanced, that electricity and magnetism are identical. Perhaps the future philosopher may
surprise a generation not very distant, by the annunciation of the discovery, that attraction and repulsion of all kinds are to be traced to a common
source; that the same influence which binds the particles of a grain of
sand together, is seen in the vivid flash which causes the'lit lake to
shine;' or heard in the' live thunder,' as it leaps from peak to peak; or
known in the unerring guide which it furnishes the mariner in his course
over the trackless deep; and admired in the music of the spheres, as they
harmoniously roll in grand and majestic course in the immeasurable regions of infinite space."
301. Of what does magneto-electricity treat?  How are electric cur.
rents excited?




254              NATURAL PHILOSOPHY.
from them, its magnetism disappears: and this alternation of
magnetic states causes the flow of a current of electricity,
which may be directed in its couspe to screw-cups, from
which it may be received by means of wires connected with
the cups.
3. THE MAGNETO-ELECTRIC MACHINE.-Fig. 150 represents
the magneto-electric machine, in which an armature, bent
Fig. 150.
w
H                                      -   a
twice at right angles, is made to revolve rapidly in front of
the poles of a compound steel magnet of the U form. The
U magnet, whose north pole is seen at N, is fixed in a horizontal position, with its poles as near the ends of the armature
as will allow the latter to rotate without coming into contact
with them. The armature is mounted on an axis, extending
from the pillar P to a small pillar between the poles of the
magnet. Each of its legs is enclosed in a helix of fine insulated wire. The upper part of the pillar P slides over the
lower part, and can be fastened in any position by a binding
screw. In this way the band connecting the two wheels may
be tightened at pleasure, by increasing the distance between
them. This arrangement also renders the machine more portable. By means of the multiplying-wheel W, which is connected by a band with a small wheel on the axis, the armature
is made to revolve rapidly, so that the magnetism induced in
it by the steel magnet is alternately destroyed and renewed in
Explain the magneto-electric machine, Fig. 150.




THERMO-ELECTRICITY.                     255
a reverse direction to the previous one. When the legs of
the armature are approaching the magnet, the one opposite
the north pole acquires south polarity, and the other north
polarity.  The magnetic power is greatest while the armature
is passing in front of the poles. It gradually diminishes as
the armature leaves this position, and nearly disappears when
it stands at right angles with the magnet. As each leg of
the armature approaches the other pole of the U magnet, by
the continuance of the motion, magnetism is again induced in
it, but in the reverse direction to the previous one.  These
changes in the magnetic state of the armature excite electric
currents in the surrounding helices, powerful in proportion
to the rapidity with which the magnetic changes are produced.
4. Shocks may thus be obtained from the machine, and if
the motion is very rapid, in a powerful machine, the torrent
of shocks becomes insupportable-the muscles of the hands
which grasp the handles are involuntarily contracted, so that
it is impossible to loosen the hold. The shocks, however, are
instantly suspended by bringing the metallic handles into contact *
THERMO-ELECTRICITY.
302. Thermo-electricity expresses a form of electricity developed by the agency of heat.t
* For a further explanation of the magneto-electric machine, the student is referred to Davis' excellent " Manual of Magnetism;" a volume
which ought to be in the hands of all who wish to become acquainted with
the subjects of magnetism and chemical electricity.
t In the year 1822, Professor Seebeck, of Berlin, discovered that currents of electricity might be produced by the partial application of heat to
a circuit composed exclusively of solid conductors. The electrical current, thus excited, has been termed Thermo-electric, (from the Greek
Thermos, which signifies heat,) to distinguish it from the common galvanic
current; which, as it requires the intervention of a.fluid element, was denominated a Hydro-electric current. The term Stereo-electric current
has also been applied to the former, in order to mark its being produced in
systems formed of solid bodies alone. It is evident that if, as is supposed
in the theory of Ampere, magnets owe their peculiar properties to the con302. What is thermo-electricity?




256                NATURAL PHILOSOPHY.
1. If the junction of two dissimilar metals be heated, an
electrical current will flow from the one to the other.
2. Instead of two different metals, one metal in different
conditions can be used to excite the current.
3. Metals differ greatly in their power to excite a current,
when associated in thermo-electric pairs. A current may be
excited with two wires of the same metal, by heating the end
of one, and bringing it into contact with the other. This experiment is most successful when metals are used that have
the lowest conducting power of heat.
4. Thermo-electric batteries have been constructed, with sufficient power to give shocks and sparks, and produce various
magnetic phenomena, indicative of great magnetic power.*
tinual circulation of electric currents in their minute parts, these currents
will come under the description of the stereo-electric currents.
From the views of electricity which have now been given, it appears
that there are, strictly speaking, three states of electricity. That derived
from the common electrical machine is in the highest degree of tension,
and accumulates until it is able to force its way through the air, which is
a perfect non-conductor. In the galvanic apparatus, the currents have a
smaller degree of tension; because, although they pass freely through the
metallic elements, they meet with some impediments in traversing the
fluid conductor. But in the thermo-electric currents, the tension is reduced to nothing; because, throughout the whole course of the circuit, no
impediment exists to its free and uniform circulation.
* The subject of thermo-electricity is more fully treated in Davis'
"Manual of Magnetism," to which reference has already been made, and
to which the author acknowledges he has been indebted for much information in the departments of Electricity and Magnetism.
Note. To what do the magnets owe their peculiar properties? What
follows from this? How many states of electricity are there? What is
said of that derived from the common electrical machine? What is said
of that derived from the galvanic apparatus? What is said of the thermoelectric currents?




ASTRONOMY.                        257
CHAPTER XIII.
ASTRONOMY.*
303. A-tronomy treats of the heavenly bodies, such
as the sun. moon, stars, comets, planets, &c.
The earth on which we live is a large globe, or ball, nearly
eight thousand miles in diameter, and about twenty-five thousand miles in circumference.  It is known to be round, —first,
because it casts a circular shadow, which is seen on the moon,
during an eclipse; secondly, because the upper parts of distant
objects on its surface can be seen at the greatest distance;
thirdly, it has been circumnavigated. It is situated in the
midst of the heavenly bodies, which we see around us at
night, and forms one of the number of those bodies; and it
belongs to that system, which, having the sun for its centre,
and being influenced by its attraction, is called the solart
system.
304. The solar system consists of the sun, which is in
the centre;
Of eight primary planets, named Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune;T
Of four Asteroids, or smaller planets, namely, Ceres,
Pallas, Juno, and Vesta;
Of eighteen secondary planets or moons, of which
* It is proper here to remark, that many of the branches of Natural
Philosophy require in the student an intimate acquaintance with the principles of mathematical science. This is particularly the case with astronomy. As this book is designed for those who have made little progress
in the mathematics, the following treatise on astronomy contains those
facts and principles only of the science which are intelligible, without the
aid of mathematical light.
t The word solar means belonging to the sun.
t This planet was very recently discovered by Verrer, whose name at
first was applied to it. It is now more generally called Neptune.
303. Of what does Astronomy treat? What is said of the earth? How
is the earth known to be round? Where is it situated?
304. Of what does the solar system consist?




258                 NATURAL PHILOSOPHY.
the  Earth  has one, Jupiter four, Saturn  seven, and
Uranus six; and
Of an unknown number of comets.*
The stars, which we see in the night time, are supposed to be
suns, surrounded by systems of planets, too distant to be seen
from the earth. Although they appear so numerous on a
bright night, they become much more so by the aid of glasses.
305. The planetst may be distinguished from the stars
by their steady light; while the stars appear to twinkle.
The  planets, likewise, seem  to  change  their relative
places in the heavens, while those luminous bodies
which are called fixed stars appear to preserve the same
relative position.
1. The sun, the moon, the planets, and the fixed stars,
which appear to us so small, are supposed to be large worlds,
of various sizes, and at different but immense distances from
us. The reason that they appear to us so small is, that on
account of their immense distances they are seen under a small
angle of vision.
2. It has been stated in the early pages of this book, that
every portion of matter is attracted by every other portion,and that the force of the attraction depends upon the quantity
* The planet Neptune, so recently discovered, is undoubtedly attended
by his secondaries or moons, two of which have already been discovered.
Whether more will ever be discovered, is a question which remains for the
future astronomer to answer.
t The meaning of the word planet is properly a wanderer, or a wandering star. These luminaries were so called because they never retain
the same situation, hut are constantly changing their relative positions;
while those stars which appear to retain their places are called fixed stars.
The cause of the motion of the planets will be presently explained.
What are the stars supposed to be? Do we see more by the aid of
glasses than without?
305. How may the planets be distinguished from the stars? How are
the planets distinguished from  the fixed stars? What is the meaning of the word planet? Why are they called planets? What are the
fixed stars?  What are the sun, moon, planets, and fixed stars supposed
to be? Why do they appear so small? What has been stated with regard to the attraction of portions of matter?  Upon what force does this
attraction depend?




ASTRONOMY.                       259
of matter and the distance. As attraction is mutual, we find that
all of the heavenly bodies attract the earth; and the earth, likewise, attracts all of the heavenly bodies. It has been proved, that
a body when actuated by several forces will not obey either one,
but will move in a direction between them. It is so with the
heavenly bodies,-each one of them is attracted by every
other one; and these attractions are so nicely balanced by
creative wisdom, that, instead of rushing together in one mass,
they are caused to move in regular paths, (called orbits,)
around a central body; which being attracted in different directions, by the bodies which revolve around it, will itself revolve around the centre of gravity of the system. Thus, the
sun is the centre of what is called the solar system, and the
planets revolve around it in different times, at different distances,
and with different velocities.
306. The paths or courses in which the planets move
around the sun are called their orbits.
1. In obedience to the universal law of gravitation, the
planets revolve around the sun as the centre of their system;
and the time that each one takes to perform an entire revolution is called its year. Thus, the planet Mercury revolves
around the sun in 87 of our days. Hence, a year on that
planet is equal to 87 days. The planet Venus revolves around
the sun in 224 days. That is, therefore, the length of the
year of that planet. Our earth revolves around the sun in
about 365 days and 6 hours. Our year, therefore, is of that
length.
2. The length of time that each planet takes in performing
its revolution around the sun, or, in other words, the length of
the year on each planet, is as follows. ( The fractional parts of
the day are omitted.)
What follows from attraction being mutual? What direction do bodies
take when actuated by several forces? Is this true with regard to the
heavenly bodies? What is the centre of the solar system? What is said
of the revolution of the planets?
306. What are the paths in which the planets move around the sun
called? Around what do the planets revolve? What is a year on each
planet? How long is the year of the planet Mercury? How long is the
planet Venus performing her revolution around the sun? How long is
tlte earth performing her revolution around the sun? What is the length
of the year on the planet Mercury?  Venus?  Earth?  Mars? Vesta?
Jun-?  Ceres? Pallas? Jupiter? Saturn? Herschel? Neptune?




260                 NArURAL PHILOSOPHY.
Days.              Days.                    Days.
Mercury   87        Vesta  1,325        Jupiter      4,332
Venus   224         Juno    1,592        Saturn    10,759
Earth    365        Ceres  1,681         Herschel 30,686
Mars      686       Pallas  1,686        Neptune unknown.*
3. The mean distanced of each of the planets from the sun
is expressed as follows, in millions of miles.
Millions.           Millions                 Millions
Mercury   36        Vesta    225         Jupiter       495
Venus      68       Juno      254        Saturn        908
Earth      95       Ceres    263         Herschel   1,827
Mars      145       Pallas    264        Neptune
4. While the planets revolve around the sun, each also
turns around upon its own axis, and thus presents each side
successively to the sun.
5. The time in which they turn upon their axes is called their
day, and is thus expressed in hours and minutes of our time.
H.  M.         H.                       H. M.
Mercury 24  5  Vesta   (unknown.)  Jupiter  9 55
Venus  23 20  Juno 27 (probably.)  Saturn  10 16
Earth   23 56  Ceres   (unknown.)  Herschel (unknown.)
Mars   24 39  Pallas   (unknown.)  Neptune
The sun turns on its axis in about 25 days and 10 hours.
* The elements of this planet are not yet sufficiently determined to give
them with any degree of certainty.
t The paths or orbits of the planets are not exactly circular, but elliptical. They are, therefore, sometimes nearer to the sun than at others.
The mean distance is the medium between their greatest and least distance. Those planets which are nearer to the sun than the earth are
called inferior planets, because their orbits are within that of the earth;
and those which are farther from the sun are called superior planets, because their orbits are outside that of the earth.
Note. Of what form are the orbits of the planets? What is meant by
the mean distance?  What planets are called inferior?  Why?  What
planets are called superior?  Why?  What is the distance of the planet
Mercury from the sun? Venus? Earth? Mars? Vesta? Juno? Ceres 
Pallas? Jupiter? Saturn?  Herschel? Neptune? Have the planets
any motion besides that around the sun? What is the time in which they
turn upon their axes called? What is the length of a day on the planet
Mercury? Venus? Earth?  Mars? Vesta? Juno? Ceres? Pallas?
Jupiter?  Saturn?  Herschel? Neptune?




ASTRONOMY.                         2t 1
307. The relative size of the bodies belonging to the
solar system, as expressed by the length of their diameters, is as follows:
Miles.             Miles.                 Miles.
The Sun  877,547        Mars  4,222        Pallas    2,025
Mercury      2,984      Vesta   269        Jupiter  86,255
Venus        7,621       Juno  1,393       Saturn   81,954
Earth         7,924      Ceres 1,582       Herschel 34,363
The Moon   2,180
Fig. 151 is a representation of the comparative size of the
planets.*
Fig. 151.
Jupiter.
ercury Mars  Veus  art.
Sir JFW   H e r                   Herschel
Sir J. F. W. Herschel gives the following illustration of the comparative size and distance of the bodies of the solar system: " On a well-levelled field place a globe, two feet in diameter, to represent the Sun; Mercury will be represented by a grain of mustard-seed on the circumference
of a circle 164 feet in diameter for its orbit; Venus, a pea, on a circle 284
feet in diameter; the Earth, also a pea, on a circle of 430 feet; Mars, a
rather large pin's head, on a circle of 654 feet; Juno, Ceres, Vesta, and
Pallas, grains of sand, in orbits of from 1000 to 1200 feet; Jupiter, a moderate-sized orange, in a circle nearly half a mile in diameter; Saturn, a
small orange, on a circle of four-fifths of a mile in diameter; and Herschel
a full-sized cherry, or small plum, upon the circumference of a circle more
than a mile and a half in diameter.
"To imitate the motions of the planets in the above-mentioned orbits,
307. What is the diameter of the Sun? Mercury? Venus? Earth?
Mars? Vesta? Juno? Ceres? Pallas? Jupiter? Saturn? Herschel?
Tile Moon? What does Fig. 151 represent? What illustration of the
comparative size and distance of the bodies of the solar system is given?
What is necessary in order to imitate the motions of the planets i:l the
above-mentioned orbits?




262                 NATURAL PHILOSOPHY.
308. The ecliptic is the apparent path of the sun, or
the real path of the earth.
It is called the ecliptic, because every eclipse, whether of
the sun or the moon, must be upon it.
309. The zodiac is a space or belt, 16 degrees broad,
8 degrees each side of the ecliptic.
It is called the zodiac, from a Greek word, which signifies
an animal, because all the stars in the twelve parts into which
the ancients divided it, were formed into constellations, and
most of the twelve constellations were called after some animal.*
310. The zodiac is divided into twelve signs,t each
sign containing thirty degrees of the great celestial cirMercury must describe its own diameter in 41 seconds; Venus in 4 minutes and 14 seconds; the Earth in 7 minutes; Mars in 4 minutes and 48
seconds; Jupiter in 2 hours 56 minutes; Saturn in 3 hours 13 minutes;
and Herschel in 12 hours 16.minutes."
* Sir J. F. W. Herschel, in his excellent treatise on Astronomy, says
"Uncouth figures and outlines of men and monsters, are usually scribbled
over celestial globes and maps, and serve, in a rude and barbarous way, to
enable us to talk of groups of stars, or districts in the heavens, by names
which, though absurd or puerile in their origin, have obtained a currency,
from which it would be difficult, and perhaps wrong, to dislodge them. In
so far as they have really (as some have) any slight resemblance to the
figures called up in imagination by a view of the more splendid' constellations,' they have a certain convenience; but as they are otherwise entirely arbitrary, and correspond to no natural subdivisions or groupings of
the stars, astronomers treat them lightly, or altogether disregard them, except for briefly naming remarkable stars; as' Alpha Leonis,'' Beta Scorpii,' &c., by letters of the Greek alphabet attached to them.
"This disregard is neither supercilious nor causeless. The constellations
seem to have been almost purposely named and delineated to cause as
much confusion and inconvenience as possible. Innumerable snakes twine
through long and contorted areas of the heavens, where no memory can
follow them; bears, lions, and fishes, large and small, northern and southern, confuse all nomenclature, &c. A better system of constellations
might have been a material help as an artificial memory."
t The signs of the Zodiac and the various bodies of the Solar system,
are often represented in almanacs and astronomical works, by signs or
308. What is the ecliptic?  Why is it called the ecliptic?
309. What is the zodiac?  Why is it called the zodiac?




ASTRONOMY.                           263
cle.* The names of these signs are sometimes given in
Latin, and sometimes in English. They are as follows:
Latin.      English.          Latin.            English.
I Aries,   The Ram.           7 Libra,         The Balance.
2 Taurus,  The Bull.          8 Scorpio,       The Scorpion.
3 Gemini, The Twins.          9 Sagittarius,  The Archer.
4 Cancer,  The Crab.         10 Capricornus, The Goat.
5 Leo,      The Lion.        11 Aquarius,   The Water-bearer.
6 Virgo,   The Virgin.   12 Pisces,            The Fishes.
311. The orbits of the other planets are inclined to
that of the earth; or, in other words, they are not in the
same plane.
1. Fig. 152 represents an oblique view of the plane of the
characters. In the following list the characters of the planets, &c., are
represented.
O  The Sun.           (D The Earth.           j Ceres.
Q The Moon            & Mars.                Q Pallas.
U Mercury.            t Vesta.               24 Jupiter.
9 Venus.              $ Juno.                1? Saturn.
J Herschel.
The following characters represent the signs of the Zodiac.
r Aries.             fg  Leo.                 t Sagittarius.
} Taurus.            nt Virgo.              Vt  Capricornus.
U. Gemini.             - Libra..       Aquarius.
S  Cancer.            fll Scorpio.           )E Pisces.
From an inspection of Fig. 152 it appears, that when the earth, as seen
from the sun, is in any particular constellation, the sun, as viewed from
the earth, will appear in the opposite one.
* The constellations of the zodiac do not now retain their original
names. Each constellation is about 30 degrees eastward of the sign of
the same name. For example, the constellation Aries is 30 degrees eastward of the sign Aries, and the constellation Taurus 30 degrees eastward
of the sign Taurus, and so on. Thus the sign Aries lies in the constellation Pisces; the sign Taurus in the constellation Aries; the sign Gemini
in the constellation Taurus, and so on. Hence the importance of distinguishing between the signs of theazodiac and the constellations of the zodiac. The cause of the difference is the precession of the equinoxes, a
phenomenon which will be explained in its pronP: connexioll.
310. What are the names of the twelve constellations? How many
decrees does each sign contain?
311. Are the orbits of the other planets in the same plane with that of
the earth?




264               NATURAL PHILOSOPHY.
ecliptic, the orbits of all the primary planets, and of the comet
of 1680. That part of each orbit which is above the plane is
Fig. 152.
Se
0
~j'
4b
s'hown by a white line; that which is below it, by a dark line.
That part of the orbit of each planet where it crosses the
~,C ~,f)+
~I' ~i              -~..l~I~'ro
-O
Ir (II             r~~
m ~ ~ ~ ~ ~ j
~lev:        3  i3    r
$          U  
sh~own by a white line; that which is below it, by a dark line.
Tlmt part of the orbit of each planet where it cros~es the




ASTRONOMY.                       265
ecliptic, or, in other words, where the white and dark lines in
the figure meet, are called the nodes of the planet, from the
Latin nodus, a knot or tie.                        Fig. 153.
2. Fig. 153 represents a section of the plane
of the ecliptic, showing the inclination of the orbits of the planets. As the zodiac extends only
eight degrees on each side of the ecliptic, it appears from the figure that the orbits of some of the
planets are wholly in the zodiac, while those of
others rise above and descend below it.  Thus,          i
the orbits of Juno, Ceres, and Pallas, rise above,
while those of all the other planets are confined
to the zodiac.
312. When a planet or heavenly body
is in that part of its orbit which appears to
be near any particular constellation, it is
said to be in that constellation.
Thus, in Fig. 152, the comet of 1680 appears
to approach the sun from the constellation Leo.  i,
313. The perihelion* and aphelion* of;
a heavenly body express its situation with
regard to the sun. When a body is nearest'           t
to the sun, it is said to be in its perihelion.      <
When farthest from  the sun, it is said to be           v
in its aphelion..
The earth is three millions of miles nearer to
the sun in its perihelion than in its aphelion.  
314. The  apogee* and  perigee* of a 
* The plural of Perihelion is Perihelia, and of Aphe-  A
lion is Aphelia. When a planet is so nearly on a line
with the earth and the sun as to pass between them,'it
is said to be in its inferior conjunction; when behind.
the sun, it is said to be in its superior conjunction; but
when behind the earth, it is said to be in opposition.
What does Fig. 152 represent? What are the nodes of a planet? What
does Fig. 153 represent?
312. When is a planet said to be in any particular constellation?
313. What do the perihelion and aphelion of a heavenly body express?
When is a body said to be in its perihelion? When is a body said to be in
its aphelion? How much nearer is the earth to the sun in its perihelion
than in its aphelion?




266                 NATURAL PHILOSOPHY.
heavenly body express its situation with regard to the
earth. When the body is nearest to the earth, it is said to
be in perigee; when it is farthest from the earth, it is
said to be in apogee.
315. The perihelia of the planets are in the following
signs of the zodiac:-Mercury in Sagittarius, —Venus
in  Aquarius,-the  Earth  in  Capricornus,-Mars in
Virgo, —Vesta  in  Cancer,-Juno  in  Scorpio, —Ceres
in Pisces,-Pallas  in  Aquarius, —Jupiter in Libra,Saturn  in  Capricornus,-and the Georgium   Sidus in
Aries.
316. The axes of the planets in their revolution
around the sun, are not perpendicular to their orbits,
nor to the plane of the ecliptic, but are inclined in
different degrees.
This is one of the most remarkable circumstances in
the science of Astronomy, because it is the cause of the
different seasons, spring, summer, autumn, and winter;
and because it is also the cause of the difference in the
length of the days and nights in the different parts of tile
world, and at the different seasons of the year.
317. The motion of the heavenly bodies is not uni
form. They move with the greatest velocity when
The words perihelion, aphelion, apogee, and perigee, are derived from the
Greek language, and have the following meaning:
Perihelion, near the sun.
Aphelion, from the sun.
Perigee, near the earth.
Apogee, from the earth.
Note. When is a planet said to be in its inferior conjunction? When
is it said to be in its superior conjunction? When is it said to be in opposition?
314. What do the apogee and perigee of a heavenly body express? When
is a body said to be in its perigee? When is it said to be in its apogee?
315. In what sign is the perihelion of the planet Mercury?  Venus?
Earth? Mars?  Vesta? Juno?  Ceres? Pallas? Jupiter?  Saturn?
Georgium Sidus?
316. What is said with regard to the axes of the planets in their revolution around the sun? What does this inclination of their axes cause?
317. What is said with regard to the motion of the heavenly bodies?




ASTRONOMY.'267
they are in perihelion, or in that part of their orbit
which is nearest to the sun; and slowest when zn
aphelion.
318. It has been proved by Kepler, that when a body
moves around a point to which it is attracted, a line*
drawn from the point to the body passes over or describes
equal areas in equal times. This is called Kepler's law.
In Fig. 154, let S represent the sun, and E the earth, and
the ellipse, or oval, be the earth's orbit, or path around the
sun. By lines drawn from the sun at S to the outer edge
of the figure, the orbit is
divided into twelve areas                 D   F
of different shapes, but
each containing the same
quantity of space. Thus,
the spaces E S A, A S B,
D S F, &c., are all supposed to be equal.  Now
if the earth in the space
of one month, will move
in its orbit from E to A,
it will, in another month,
move from A  to B, and
in the third month from        A
B to C, &c., and thus
will describe equal areas
in equal times.           G       —,
The  reason why the                       E
earth (or any other heavenly body) moves with a greater degree of velocity in its
perihelion, than in its aphelion, may likewise be explained by
the same figure.'Thus,The earth, in its progress from F to L, being constantly urged
forward by the sun's attraction, must (as is the case with a
falling body) move with an accelerated motion.  At L, the
* This line is called the radius-vector.
When do they move with the greatest velocity? When is their motion
the slowest?
318. What is Kepler's law? Illustrate this by Fig. 154. Explain. 1,y
Fig 154, the reastOn why the earth, or ally other heavenly body, moves
with a greater degree of velocity ill its perihelion tham in its aphelion.




268                NATURAL PHILOSOPHY.
sun's attraction becomes stronger, on account of the nearness
of the earth; and, consequently, in its motion from L to E, the
earth will move with greater rapidity. At E, which is the
perihelion of the earth, it acquires its greatest velocity.  Let
us now detain it at E, merely to consider the direction of the
forces by which it is urged.  If the sun's attraction could
be destroyed, the force which has carried it from L to E,
would carry it off in the dotted line from E to G, which is a
tangent to its orbit. But while the earth has this tendency to
move towards G, the sun's attraction is continually operating
with a tendency to carry it to S.  Now, when a body is
urged by two forces, it will move between them; but as the
sun's attraction is constantly exerted, the direction of the
earth's motion will not be in a straight line, the diagonal of
one large parallelogram, but through the diagonal of a number
of infinitely small parallelograms; which, being united, form
the curve line E A.
It is thus seen, that, while the earth is moving from L to E,
the attraction of the sun is stronger than in any other part of
its orbit, and will cause the earth to move rapidly. But in its
motion from  E to A, from A to B, from B to C, and from C
to F, the attraction of the sun, operating in an opposite direction, will cause its motion from the sun to be retarded, until, at
F, the direction of its motion is reversed, and it begins again
to approach the sun. Thus, it appears that, in its passage
from the perihelion to the aphelion, the motion of the earth, as
well as that of all the heavenly bodies, must be constantly retarded; while, in moving from their aphelion to perihelion, it is
constantly accelerated; and at their perihelion, the velocity
will be the greatest. The earth, therefore, is about seven days
longer in performing the aphelion part of its orbit, than in
traversing the perihelion part; and the revolution of all the
other planets being the result of the same cause, is affected in
the same manner as that of the earth.
319. The earth is about three millions of miles nearer
to the sun in winter than in summer.
What is said of the motion of the heavenly bodies from perihelion to
aphelion? What is their motion from aphelion to perihelion? When is
their velocity the greatest?  HTow much longer is the earth ill performing
the aphelion part of its orbit than the perihelion part?
319. How much nearer is the earth to the sun in winter than in sum.
rer'!




ASTRONOMY.                       269
The heat of summer, therefore, cannot be caused by
the near approach of the earth to the sun.
Snow and ice never melt on the tops of high mountains; and
they who have ascended in the atmosphere, in balloons, have
found that the cold increases as they rise.
320. On account of the inclination of the earth's axis,
the rays of the sun fall more or less obliquely on different parts of the earth's surface, at different seasons of
the year. The heat is always the greatest when the
sun's rays fall vertically; and the more obliquely they
fall, the less heat they appear to possess.
1. This is the reason why the days are hottest in summer,
although the earth is farther from the sun at that time.
2. Fig. 155 represents the manner in which the rays of the
sun fall upon the earth in summer and in winter.  The north
pole of the earth, at all seasons, constantly points to the north
Fig. 155.
star N; and when the earth is nearest to the sun, the rays
from the sun fall as indicated by W, in the figure; and as
their direction is very oblique, and they have a larger portion
320. What follows froln the inclination of the earth's axis, with regard
to the direction of the sun's rays? When is the heat always the greatest?
What is said of oblique rays? What is the reason that the heat is greater
in summer than in winter? Illustrate this by Fig. 155. How is the earth
situated with regard to its distance from the sun in winter?




270                 NATURAL PHILOSOPIHY.
of the atmosphere to traverse, much of their power is lost.
Hence we have cold weather when the earth is nearest to the
sun. But when the earth is in aphelion, the rays fall almost vertically or perpendicularly, as represented by S, in the figure;
and, although the earth is then nearly three millions of miles
farther from the sun, the heat is greatest, because the rays fall
more directly, and have a less portion of the atmosphere to
traverse.*
3. For a similar reason, we find, even in summer, that early
in the morning, and late in the afternoon, it is much cooler
than at noon, because the sun then shines more obliquely.
The heat is generally the greatest at about three o'clock in
the afternoon; because the earth retains its heat for some
length of time, and the additional heat it is constantly receiving from the sun, causes an elevation of temperature, even
after the rays begin to fall more obliquely.
It is the same cause which occasions the variety of climate
in different parts of the earth. The sun always shines in a
direction nearly perpendicular, or vertical, on the equator;
and with different degrees of obliquity on the other parts of
the earth. For this reason, the greatest degree of heat prevails at the equator during the whole year. The farther any
place is situated from the equator, the more obliquely will the
rays fall, at different seasons of the year; and, consequently,
the greater will be the difference in the temperature.
4. If the axis of the earth were perpendicular to its orbit,
those parts of the earth which lie under the equator would be
constantly opposite to the sun; and as, in that case, the sun
would, at all times of the year, be vertical to those places
equally distant from both poles, so the light and heat of the
sun would be dispersed with perfect uniformity towards each
* This may be more familiarly explained, by comparing summer rays
to a ball or stone thrown directly at an object, so as to strike it with all its
force; and winter rays to the same ball or stone, thrown obliquely, so as
merely to graze the object.
What illustration of oblique and perpendicular rays is given in the note?
Why is it generally cooler early in the morning and late in the afternoon
than at noon?  Why is the heat the greatest at about three o'clock?
What causes the variety of climate in different parts of the earth? Where
does the sun always shine ill a vertical direction? What would follow
were the axis of the earth perpendicular to its orbit? What causes the
variety of the seasons, the different lengths of days and nights, &c?




ASTRONOMY.                        271
pole; we should have no variety of seasons; day and night
would be of the same length; and the heat of the sun would
be of the same intensity every day throughout the year.
5. It is, therefore, as has been stated, owing to the inclination of the earth's axis, that we have the agreeable variety of the
seasons, days and nights of different lengths, and that wiselyordered variety of climate, which causes so great a variety of
productions, and which has afforded so powerful a stimulus to
human industry.*
6. In order to understand the illustration of the causes of
the seasons, &c., it is necessary to have some knowledge of
the circles which are drawn on the artificial representations
of the earth.  It is to be remembered that all of these circles
are wholly imaginary; that is, that there is on the earth itself
no such circles or lines. They are drawn on maps merely for
the purpose of illustration.
7. Fig. 156 represents the earth.  N S is the axis, or imaginary line, around which it daily turns; N is the north pole,
S is the south pole. These poles,
it will be seen, are the extremities            Fig. 156.
of the axis N S.  C D  represents                 v
the equator, which is a circle around
the earth, at an equal distance from      4.
each pole.  The curved lines proceeding from N to S, are meridians.   c
They are all circles surrounding the
earth, and  passing  through  the
poles.  These meridians may  be                  \
multiplied at pleasure.
The lines E F, IK, LM, and                      ~
G H, are designed to represent cir* The wisdom of Providence is frequently displayed in apparent inconsistencies. Thus, the very circumstance which, to the short-sighted
philosopher, appears to have thrown an insurmountable barrier between
the scattered portions of the human race, has been wisely ordered, to establish an interchange of blessings, and to bring the ends of the earth in
communion. Were the same productions found in every region of the
earth, the stimulus to exertion would be weakened, and the wide field of
human labor would be greatly diminished. It is our mutual wants which
bind us together.
What is necessary in order to understand the illustration of the causes
of the seasons? Explain Fig. 156. What are the poles?




272                NATURAL PHILOSOPHY.
cles, all of them parallel to the equator, and for this reason
they are called parallels of latitude.  These also may be
multiplied at pleasure.
But in the figure, these lines, which are parallel to the
equator, and which are at a certain distance from it, have a
different name, derived from the manner in which the sun's
rays fall on the surface of the earth.
Thus the circle I K, 23- degrees from the equator, is called
the tropic of Cancer,* and the circle L M is called the tropic
of Capricorn. The circle E F is called the Arctic Circle. It
represents the limit of perpetual day, when it is summer in
the northern hemisphere, and of perpetual night when it is
winter.
The circle G H is the Antarctic Circle, and represents the
limit of perpetual day and night in the southern hemisphere.
The line L K represents the circle of the ecliptic, which, as
has already been stated, is the apparent path of the sun, or
the real path of the earth. This circle, although it is generally drawn on the terrestrial globe, is, in reality, a circle in
the heavens; and differs from the zodiac only in its width,
-the  zodiac extending eight degrees on each side of the
ecliptic.
8. Fig. 157 represents the manner in which the sun shines
on the earth in different parts of its orbit; or, in other words,
the cause of the change in the seasons. S represents the sun,
and the dotted oval, or ellipse, A B C D, the orbit of the
* Thus on the 21st of March, the rays~ of the sun fall vertically on the
equator, and on each succeeding day on places a little to the north, until
the 21st of June, when they fall vertically on places 231 degrees north of
the equator. Their vertical direction then turns back again towards the
equator, where the rays again fall vertically on the 23d of September, and
on the succeeding days a little to the south, until the 21st of December,
when they fall vertically on the places 233 south of the equator. Their
vertical direction then again turns towards the equator. Hence the circles I K and L M are called the tropics of Cancer and Capricorn. Tile
word tropic is derived from a word which signifies to turn. The tropics,
therefore, are the boundaries of the sun's apparent path north and south of
the equator, or the lines at which the sun turns back.
Why is the circle I K called the tropic of Cancer?  What is the meaning of the word tropic? Why is the circle L M called the tropic of Capricorn? What are the tropics?  What is the circle E F called?  What
does it represent? What is the circle G H called? What does it repre.
sent?




ASTRONOMY.                          273
earth. The outer circle represents the zodiac, with the position of the twelve signs or constellations. On the 21st of
Fig. 157.
Day 4
mmer in             Summer in n
the Northern        the SouthernI 
Hemisphere.   a    Hemwsphere.'
\ C
_eQ"zpa1 Day Nig
June, when the earth is at D, the whole northern polar region
is continually in the light of the sun. As it turns on its axis,
therefore, it will be day to all the parts which are exposed to
the light of the sun.* But, as the whole of the Antarctic
* Day and night are caused by the rotation of the earth on its axis every
24 hours. It is day to that side of the earth which is towards the sun, and
night to the opposite side. The length of the days is in proportion to the
inclination of the axis of the earth towards the sun. It may be seen by
the above figure, that in summer the axis is most inclined towards the
sun, and then the days are the longest. As the north pole becomes less
inclined, the days shorten, till, on the 21st of December, it is inclined
What does Fig. 157 represent? Explain the figure. Explain, by the
figure, the situation of the earth on the 21st of June. What causes day
and night?  To what part of the earth is it day?  To what part is it
night?  To what is the length of the day in proportion?  When are the
days the longest?  Why?  When are they the shortest?  Why?
12*




274                NATURAL PHILOSOrHY.
Circle is within the line of perpetual darkness, the sun can
shine on no part of it. It will, therefore, be constant night to
all places within that circle. As the whole of the Arctic
Circle is within the line of perpetual light, no part of that
circle will be turned from the sun while the earth turns on its
axis. To all places, therefore, within the Arctic Circle, it will
be constant day.
On the 22d of September, when the earth is at C, its axis
is neither inclined to nor from  the sun, but is sidewise; and,
of course, while one half of the earth, from pole to pole, is
enlightened, the other half is in darkness, as would be the
case if its axis were perpendicular to the plane of its orbit;
and it is this which causes the days and nights, of this season
of the year, to be of equal length.
On the 23d of December, the earth has progressed in its
orbit to B, which causes the whole space within the northern
polar circle to be continually in darkness, and more of that
part of the earth north of the equator to be in the shade than
in the light of the sun. Hence, on the 21st of December, at
all places north of the equator, the days are shorter than the
nights, and at all places south of the equator, the days are
longer than the nights. Hence, also, within the Arctic Circle
it is uninterrupted night, the sun not shining at all; and within
the Antarctic Circle it is uninterrupted day, the sun shining
all the time.
On the 20th of March, the earth has advanced still further,
and is at A, which causes its axis, and the length of the days
and nights, to be the same as on the 20th of September.*
231 degrees from the sun, when the days are the shortest. Thus, as the
earth progresses in its orbit, after the days are the shortest, it changes its
inclination towards the sun, till it is again inclined as in the longest days
in the summer
* As the difference in the length of the days and the nights, and the
change of the seasons, &c., on the earth, is caused by the inclination of
the earth's axis, it follows that all the planets, whose axes are inclined,
must experience the same vicissitude; and that it must be in proportion to
Explain, by the figure, the situation of the earth on the 22d of Septemoer. On the 23d of December. On the 20th of March. What follows
from the changes on the earth, caused by the inclination of the earth's
axis? In what proportion are these changes? What is said of the axis of
the planet Jupiter?




ASTRONOMY.                         275
9. From the explanation of figure 157, it appears that
there are two parts of its orbit in which the days and nights
are equal all over the earth. These points are in the sign of
Aries and Libra, which are therefore called the equinoxes.
Aries is the vernal (or spring) equinox, and Libra the autumnal equinox.
10. There are also two other points called solstices, because
the sun appears to stand at the same height in the heavens, in
the middle of the day, for several days. These points are in
the signs Cancer and Capricorn. Cancer is called the summer
solstice, and Capricorn the winter solstice.
321. The sun is a spherical body, situated near the
centre of gravity of the system of planets of which our
earth is one.
1. Its diameter is 877,547 English miles; which is equal to
100 diameters of the earth; and, therefore, his cubic magnitude must exceed that of the earth one million of times.* It
the degree of the inclination of their axes. As the axis of the planet Jupiter is nearly perpendicular to its orbit, it follows that there can be little
variation in the length of the days, and little change in the seasons of that
planet.
There can be little doubt that the sun, the planets, stars, &c., are all of
them inhabited; and although it may be thought that some of them, on
account of their immense distance from the sun, experience a great want
of light and heat, while others are so near, and the heat consequently so
great that water cannot remain on them in a fluid state, yet as we see,
even on our own earth, that creatures of different natures live in different
elements, as, for instance, fishes in water, animals in air, &c., creative
wisdom could, undoubtedly, adapt the being to its situation, and with as
little exertion of power, form a race whose nature should be adapted to the
nearest or the most remote of the heavenly bodies, ai was required to adapt
the fowls to the air, or the fishes to the sea.
* Spheres are to each other as the cubes of their respective diameters.
Note. Is it supposed that the sun, planets, and stars are inhabited?
What is shown by Fig. 157?  Where are these points?  What are they
called? Which is the vernal equinox? Which the autumnal? What
other two points are there? Why are they called solstices? Where are
these points? Which is the summer solstice? Which the winter?
321. What is said of the sun? What is its diameter? How much does
its cubic magnitude exceed that of the earth?




276                NATURAL PIIILOSOPHY.
revolves around its axis in 25 days and 10 hours. This has
been ascertained by means of several dark spots which have
been seen with telescopes on its surface.
2. Dr. Herschel supposed the greater number of spots on
the sun to be mountains; some of which he estimated to be
300 miles in height.
3. It is probable that the sun,* like all the other heavenly
bodies, (excepting, perhaps, comets,) is inhabited by beings
whose nature is adapted to their peculiar circumstances.
4. Although, by some, the sun is supposed to be an immense ball of fire, on account of the effects produced at the
distance of ninety-five millions of miles, yet many facts show
that heat is produced by the sun's rays only when they act
on a suitable medium.  Thus, snow and ice remain during the
year on the tops of the highest mountains, even in climates
where the cold of our winters is never known.
5. The zodiacal light is a singular phenomenon, accompanying the sun. It is a faint light which often appears to stream
up from the sun a little after sunset and before sunrise. It
appears nearly in the form of a cone, its sides being somewhat
curved, and generally but ill defined. It extends often from
50~ to 1000 in the heavens, and always nearly in the direction of the plane of the ecliptic. It is most distinct about
the beginning of March; but is constantly visible in the torrid
zone. The cause of this phenomenon is not known.
322. Mercury is the nearest planet to the sun, and is
seldom seen; because his vicinity to the sun occasions
his being lost in the brilliancy of the sun's rays.
1. The heat of this planet is so great that water cannot
exist there, except in a state of vapor; and metals would be
melted. The intenseness of the sun's heat, which is in the
same proportion as'its light, is seven times greater in Mercury
* In almanacs, the sun is usually represented by a small circle, with
the face of a man in it, thus:.O
How long is it in performing its revolution around its axis? How has
this been ascertained? What did Dr. Herschel suppose these spots to be?
What is the zodiacal light? At what time is it most distinct? Where is
it constantly visible?
322. What planet is nearest to the sun? Why is it seldom seen? What
is said of the heat of this planet? How much greater is the sun's heat in
Mercury than on the earth?




ASTRONOMY.                       27 7
than on the earth; so that water there would be carried off
in the shape of steam: for, by experiments made with a thermometer, it appears that a heat seven times greater than that
of the sun's beams in summer, will make water boil.
2. Mercury, although in appearance only a small star, emits
a bright white light by which it may be recognised when seen.
It appears a little before the sun rises, and again a little after
sunset, but as its angular distance from the sun never exceeds
23 degrees, it is never to be seen longer than one hour and
fifty minutes after sunset; nor longer than that time before
the sun rises.
3. When viewed through a good telescope, Mercury appears with all the various phases, or increase and decrease of
light, with which we view the moon; except that it never appears quite full, because its enlightened side is turned directly
towards the earth only when the planet is so near the sun as
to be lost to our sight in its beams. Like that of the moon,
the crescent or enlightened side of Mercury is always towards
the sun. As no spots are commonly visible on the disk, the
time of its rotation on its axis is unknown.
323. Venus,* the second planet in order from the sun,
is the nearest to the earth, and on that account appears
to be the largest and most beautiful of all the planets.
During a part of the year it rises before the sun, and it
is then called the morning star; during another part of
the year it rises after the sun, and it is then called the
evening star. The heat and,light at Venus are nearly
double what they are at the earth.
* By the ancient poets, Venus was called Phosphor, or Lucifer, when
it appeared to the west of the sun, at which time it is morning star, and
ushers in the light or day; and Hesperus or Vesper, when eastward of
the sun, or evening star.
In what form does water exist in Mercury? How can Mercury be
recognised when seen? At what time does it appear? How does Mercury appear when viewed through a telescope?
323. What planet is nearest to the earth? When is Venus called the
morning star? When is it called the evening star? How much greater
are the light and heat at Venus than that at the earth? What name was
given by the ancient poets to Venus, when morning star? What, when
evening star?




278                NATURAL PHILOSOPHY.
1. Venus, like Mercury, presents to us all the appearances
of increase and decrease of light common to the moon. Spots
are also sometimes seen on its surface, like those on the sun.
By reason of the great brilliancy of this planet, it may sometimes be seen even in the daytime, by the naked eye.* But
it is never seen late at night, because its angular distance from
the sun never exceeds 45 degrees. In the absence of the
moon it will cast a shadow behind an opaque body.
2. Both Mercury and Venus sometimes pass directly between the sun and the earth. As their illuminated surface is
towards the sun, their dark side is presented to the earth, and
they appear like dark spots on the sun's disk. This is called
the transit of these planets.
324. The earth is the next planet, in the solar system,
to Venus. It is not a perfect sphere, but its figure is
that of an oblate spheroid, the equatorial diameter being
about 34 miles longer than its polar diameter.
It is attended by one moon, the diameter of which is about
two thousand miles.  Its mean distance from  the earth is
about 240,000 miles, and it turns on its axis in precisely the
same time that it performs its revolution round the earth;
namely, in twenty-nine days and a half.
325. The earth, when viewed from  the moon, exhibits
precisely the same phases that the moon does to us, but
in opposite order. When the moon is full to us, the
earth will be dark to the inhabitants of the moon; and
when the moon is dark to us, the earth will be full to
* The reason why we cannot see the stars and planets in the daytime,
is, that their light is so faint, compared with the light of the sun reflected
by our atmosphere
What is the greatest distance at which the planets, Mercury and Venus,
ever appear from the sun? What is meant by the transit of these planets?
What is said of the different appearances which Venus presents? Why
can we not see the planets and stars in the daytime?
324. What planet is next to Venus?  What is the form of the earth?
How much larger is its equatorial diameter than its polar? How many
moons has the earth?  What is the diameter of the moon?  What is its
distance from the earth? What is the length of a day at the moon? How
long is it in performing its revolution around the earth?
325. What phases does the earth, when viewed from the moon, exhibit?




ASTRONOMY.                        279
them.  The earth appears to them  about 13 times larger
than the moon does to us.  As the moon, however, always presents nearly the same side to the earth, there is
one-half of the moon which we never see, andfrom which
the earth cannot be seen.
326. Next to the earth is the planet Mars. It is conspicuous for its fiery red appearance; which is supposed
to be caused by a very dense atmosphere.
1. When this planet approaches any of the fixed stars, they
change their color, grow dim, and often become totally invisible. This is supposed to be caused by his atmosphere. The
degree of heat and light at Mars is less than half of that received by the earth.
2. The four small planets, or asteroids, Vesta, Juno, Ceres,
and Pallas, have all been discovered within the present century. Vesta was discovered by Dr. Olbers, of Bremen, in
1807: its light is pure and white.  Juno, by Mr. Harding,
near Bremen, in 1804: its color is red, and its atmosphere
appears cloudy. Pallas was discovered by Dr. Olbers in
1802: it appears to have a dense, cloudy atmosphere. Ceres
was discovered at Palermo, in Sicily, by Piazzi, in 1801: it
is of a ruddy color. All of these small planets undergo various changes in appearance and size, so that their real magnitude is not ascertained with any certainty; and but little is
known of them.*
* It is a remarkable fact, that certain irregularities, observed in the
motions of the old planets, induced some astronomers to suppose that a
planet existed between the orbits of Mars and Jupiter; a supposition that
arose long previous to the discovery of the four new planets just noticed.
The opinion has been advanced, that these four small bodies originally
composed one larger one, which, by some unknown force or convulsion,
burst asunder. This opinion is maintained with much ingenuity and plauHow much larger does the earth appear than the moon?
326. What planet is next to the earth? What renders it conspicuous?
What is supposed to cause this appearance? How much more light and
Aleat does the earth enjoy than Mars? When were the asteroids discovered? By whonl, and in what year was Vesta discovered? What is the
color of its light? By whom and when was Juno discovered?  What is,he color of its light?  When was Pallas discovered? By whom? What
as said of its atmosphere?  When and by whom was Ceres discovered?
What is its color?  What is said in the note with regard to these planets?




280                NATURAL PI11LOSOPHY.
327. Jupiter is the largest planet of the solar system,
and the most brilliant, except Venus. The heat and
light at Jupiter is about 25 times less than that at the
earth. This planet is attended by four moons, or satellites; the shadows of some of which are occasionally
visible upon his surface.
1. The distance of those satellites from the planet are two,
four, six, and twelve hundred thousand miles, nearly.
The nearest revolves around the planet in less than two
days; the next in less than four days; the third in less than
eight days; and the fourth in about sixteen days.
2. These four moons must afford considerable light to the
inhabitants of the planet; for the nearest appears to them
four times the size of our moon; the second about the same
size; the third somewhat less; and the fourth about one-third
the diameter of our moon.
3. As the axis of Jupiter is nearly perpendicular to its
orbit, it has no sensible change of seasons.
4. The satellites of Jupiter often pass behind the body of
the planet, and also into its shadow, and are eclipsed. These
eclipses are of use in ascertaining the longitude of places on
the earth. By these eclipses also, it has been ascertained that
light is about -eight minutes in coming from the sun to the
earth.  For, an eclipse of one of these satellites appears to
us to take place sixteen minutes sooner, when the earth is in
that part of its orbit nearest Jupiter, than when in the part
farthest from that planet. Hence, light is sixteen minutes in
crossing the earth's orbit, and of course half of that time, or
eight minutes, in coming from the sun to the earth.
sibility by Dr. Brewster, in the Edinburgh Encyclopedia, Art. ASTRONOMY.
Dr. Brewster further supposes, that the bursting of this planet may have
occasioned the phenomena of meteoric stones; that is, stones which have
fallen on the earth from the atmosphere
327. Which of the planets is the largest? How much more light and
heat does the earth enjoy than Jupiter? How many moons has this
planet? What is the distance of these moons from the planet?  In what
time do they perform their revolutions around the planet? How does the
size of these moons compare with that of ours? Why has Jupiter no sensible variety of seasons? Of what use are the eclipses of Jupiter's moons?
How long is light in coming from the sun to the earth? How has this
Deen ascertained?




ASTRONOMY.                      281
5. When viewed through a telescope, several belts or bands
are distinctly seen, sometimes extending across his disk, and
sometimes interrupted and broken. They differ in distance,
position, and number. They are generally dark; but white
ones have been seen.
6. On account of the immense distance of Jupiter from the
sun, and also from Mercury, Venus, the Earth, and Mars, observers on Jupiter, with eyes like ours, can never see either of
the above-named planets, because they would always be immersed in the sun's rays.
328. Saturn is the second in size and the last but one
in distance from the sun. The degree of heat and light
at this planet is eighty times less than that at the earth.
1. Saturn is distinguished from the other planets by being
encompassed by two large, luminous rings. They reflect the
sun's light in the same manner as his moons. They are entirely detached from each other and from the body of the
planet. They turn on the same axis with the planet, and in
nearly the same time.* The edge of these rings is constantly
at right angles with the axis of the planet. Stars are sometimes seen between the rings, and also between the inner ring
and the body of the planet. The breadth of the two rings is
about the same as their distance from the planet, namely,
21,000 miles. As they cast shadows on the planet, Dr. Herschel thinks them solid.
2. The surface of Saturn is sometimes diversified, like that
of Jupiter, with spots and belts. Saturn has seven satellites,
or moons, revolving around him at different distances, and in
various times, from less than one to eighty days.
3. Saturn may be known by his ipale and steady light. The
* These rings move together around the planet, but are about thirteen
minutes longer in performing their revolution about him, than Saturn is in
revolving about his axis.
How does Jupiter appear when viewed through a telescope?
328. How does Saturn compare in size with the other planets? How
is Saturn distinguished from the other planets?  What is said of these
rings? How much longer are these rings in performing their revolution
around the planet than the planet is in performing its revolution on its axis?
What is the breadth of these rings? What is said of the surface of Saturn.? How many moons has Saturn? How may Saturn be known?




282                 NATURAL PHILOSOPHY.
seven moons of Saturn, except one, revolve at different distances around the outer edge of his rings. Dr. Herschel saw
them moving along it, like bright beads on a white string.
They do not often suffer eclipse by passing into the shadow of
the planet, because the ring is generally in an oblique direction.
329. Uranus,* the third in size, is the most remote of
all the old planets. It is scarcely visible to the naked eye.
The light and heat at Uranus are about 360 times less
than that at the earth.
1. This planet was formerly considered a small star; but
Dr. Herschel, in 1781, discovered, from its motion, that it is a
planet.
2. Uranus is attended by six moons, or satellites, all of
which were discovered by Dr. Herschel; and all of them revolve in orbits nearly perpendicular to that of the planet.
Their motion is apparently retrograde; but this is probably an
optical illusion, arising from the difficulty of ascertaining which
part of their orbit inclines towards the earth, and which declines from it.t
* This planet was long known by the name of Herschel, the discoverer,
who, in announcing his discovery, named it the " Georgium sidus," in
honor of King George III. The name of Uranus was given to it -by the
continental astronomers.
t It appears to be a general law of satellites, or moons, that they turn
on their axes in the same time in which they revolve around their primaries. On this account, the inhabitants of secondary planets observe
some singular appearances, which the inhabitants of primary planets do
not. Those who dwell on the side of a secondary planet next to the
primary, will always see that primary; while those who live on the opposite side will never see it. Those who always see the primary, will see
it constantly in very nearly the same place. For example, those who
What is said of the moons of Saturn? Why are they not often eclipsed?
329. How does Uranus compare in size with the other planets? How
does the light and heat at Uranus compare with that of the earth? By
whom was this planet discovered?  What name did he give it?  How
many moons has Uranus?  By whom were they discovered?  How are
their orbits situated, with regard to that of the planet? What is said of
their motion? Note. What appears to be a general law of satellites?
What follows from this with regard to the appearances which the inhabitants of the secondary planets must observe?




ASTRONOMY.                         283
3. It is a singular circumstance, that, before the discovery
of Herschel, some disturbances and deviations were observed
by astronomers in the motions of Jupiter and Saturn, which
they could account for only on the supposition that these two
planets were influenced by the attraction of some more remote
and undiscovered planet.   The discovery of Herschel completely verified their opinions, and shows the extreme nicety
with which astronomers observe the motions of planets.
330. The planet Neptune is a recent discovery of
Verrier. But little is as yet known with regard to it.
331. The word comet is derived from a Greek word,
which means hair; and this name is given to a numerous
class of bodies, which occasionally visit, and appear to
belong to the solar system. These bodies seem to consist of a nucleus, attended with a lucid haze, sometimes
resembling flowing hair; from whence the name is derived. Some comets appear to consist wholly of this
hazy or hairy appearance, which is frequently called the
tail of the comet.*.
dwell near the edge of the moon's disk, will always see the earth near the
horizon, and those in or near the centre will always see it directly or
nearly overhead. Those who dwell in the moon's south limb will see the
earth to the northward; those in the north limb will see it to the southward; those in the east limb will see it to the westward; while those in
the west limb will see it to the eastward; and all will see it nearer the horizon in proportion to their own distance from the centre of the moon's
disk. Similar appearances are exhibited to the inhabitants of all secondary planets. These observations are predicated on the supposition that
the moon is inhabited. But it is not generally believed that our moon is
inhabited, or in its present condition fitted for the residence of any class
of beings.
* In ancient times, the appearance of comets was regarded with superstitious fear, in the belief that they were the forerunners of some direful
calamity. These fears have now been banished, and the comet is viewed
as a constituent member of the system, governed by the same harmonious
and unchanging laws which regulate and control all the other heavenly
bodies.
The number of comets that have occasionally appeared within the
330. Who discovered the planet Neptune?
331. What is the meaning of the word comet? To what class of bodies
is this name given? Of what do these bodies appear to consist?




284                NATURAL PHILOSOPHY.
1. Comets, in their revolution, describe long narrow ovals.
They approach very near the sun in one of the ends of these
ovals; and when they are in the opposite end of the orbit,
their distance from the sun is immensely great.
2. The extreme nearness of approach to the sun gives to a
comet, when in perihelion, a swiftness of motion prodigiously
great. Newton calculated the velocity of the comet of 1680
to be 880,000 miles an hour. This comet was remarkable for
its near approach to the sun, being no further than 580,000
miles from it, which is but little more than half the sun's
diameter. Brydone calculated that the velocity of a comet,
which he observed at Palermo, in 1770, was at the rate of
two millions and a half of miles in an hour.
3. The luminous stream, or tail, of a comet, follows it as it
approaches the sun, and goes before it when thle comet recedes
from the sun. Newton, and some other astronomers, considered the tails of comets to be vapors, produced by the excessive
heat of the sun. Of whatever substance they may be, it is
certain that it is very rare, because the stars may be distinctly
seen through it.
limits of the solar system is variously stated, from 350 to 500. The paths
or orbits of about 98 of these have been calculated from observation of the
times at which they most nearly approached the sun; their distance from
it, and from the earth, at those times; the direction of their movements,
whether from east to west, or from west to east; and the places in the
starry sphere at which their orbits crossed that of the earth, and their inclination to it. The result is, that, of these 98, 24 passed between the sun
and Mercury, 33 passed between Mercury and Venus, 21 between Venus
and the earth, 16 between the earth and Mars, 3 between Mars and Ceres,
and 1 between Ceres and Jupiter; that 50 of these comets moved from
east to west; that their orbits were inclined at every possible angle to that
of the earth. The greater part of theIn ascended above the orbit of the
earth, when very near the sun; and some were observed to dash down
from the upper regions of space, and, after turning round the sun, to mount
again.
What is the number of comets that have occasionally appeared? What
discoveries have been made concerning 98 of them?  What is the result?
What is the form of the orbits of comets? What is said of the motion of
comets when in perihelion? What did Newton calculate the velocity of
the comet of 1680 to be in an hour?  For what was this comet remarkable? What is said of the luminous stream of a comet as it approaches
and recedes from the sun? What did Newton, and some other astronomers, consider the tails of comets to be?




ASTRONOMY.                          285
4. The tails of comets differ very greatly in length, and
some are attended apparently by only a small cloudy light,
while the length of the tail of others has been estimated at
from  50 to 80 millions of miles.*
* It has been argued that comets consist of very little solid substance,
because, although they sometimes approach very near to the other heavenly bodies, they appear to exert no sensible attractive force upon those
bodies. It is said that, in 1454, the moon was eclipsed by a comet. The
comet must, therefore, have been very near the earth, (less than 240,000
miles;) yet it produced no sensible effect on the earth or the moon; for
it did not cause them to make any perceptible deviation from their accustomed paths round the sun. It has been ascertained that comets are
disturbed by the gravitating power of the planets; but it does not appear
that the planets are in like manner affected by comets.
Many comets escape observation, because they traverse that part of
the heavens only which is above the horizon in the daytime. They are,
therefore, lost in the brilliancy of the sun, and can be seen only when a
total eclipse of the sun takes place. Seneca, 60 years before the Christian era, states that a large comet was actually observed very near the
sun, during an eclipse.
Dr. Halley and Professor Encke and Biela are the first astronomers that
ever successfully predicted the return of a comet. The periodical time of
Halley's comet is about 76 years. It appeared last in the fall of 1835:
that of Encke is about 1200 days; that of Biela about 64 years. This
last comet appeared in 1832 and in 1838.
The comet of 1758, the return of which was predicted by Dr. Halley,
was regarded with great interest by astronomers, because its return was
predicted. But four revolutions before, in 1456, it was looked upon with
the utmost horror. Its long tail spread consternation over all Europe, already terrified by the rapid success of the Turkislt arms. Pope Callixtus,
on this occasion, ordered a prayer, in which both the comet and the Turks
were included in one anathema. Scarcely a year or a month now elapses without the appearance of a comet in our system. But it is now
known that they are bodies of such extreme rarity, that our clouds are
massive in comparison with them. They have no more density than the
air under an exhausted receiver. Herschel saw stars of the 6th magnitude
through a thickness of 30,000 miles of cometic matter  The number of
comets in existence within the compass of the solar system, is stated by
some astronomers as over seven millions!
WThat is said in the note with regard to comets? Who were the first
astronomers that successfully predicted the return of a cornet?  What is
the periodical time of Halley's comet? Of Encke's? Of Biela's?




286               NATURAL PHILOSOPIHY.
332. The stars are classed into six magnitudes: the
largest are of the first magnitude, and the smallest that
can be seen by the naked eye, are of the sixth. Those
stars which can be seen only by means of telescopes,
are called telescopic stars.
1. The distance of the fixed stars cannot be determined, because we have no means of ascertaining the distance of any
body which exceeds 200 thousand times that of the earth from
the sun. As none of the stars comes within that limit, we cannot determine their real distance. It is generally supposed
that part, if not all, of the difference in their apparent magnitudes is owing to the difference in their distance.*
2. Although the stars generally appear fixed, they all have
motion; but their distance being so immensely great, a rapid
motion would not perceptibly change their relative situation in
two or three thousand years. Some have been noticed alternately to appear and disappear: several that were mentioned
by ancient astronomers, are not now to be seen; and some are
now observed, which were unknown to the ancients.
3. Many stars which appear single to the naked eye, when
viewed through powerful telescopes, appear double, treble, and
even quadruple. Some are subject to variation in their apparent magnitude; at one time being of the second, or third,
and, at another, of the fifth or sixth magnitude.
333. The Galaxy, or Milky Way, is a remarkably
light broad zone, visible in the heavens, passing from
northeast to southwest. It is supposed to consist of
an immense number of stars, which, from their apparent
nearness, cannot be distinguished from each other.
1. Dr. Herschel saw, in the course of a quarter of an hour,
the astonishing number of 116,000 stars pass through the field
of his telescope, while it was directed to the milky way.
* The distance of the stars, according to Sir J. Herschel, cannot be
less than 19,200,000,000,000 miles. How much greater it really is we
know not.
332. Into how many magnitudes are the stars classed? Of what mag.
nitude are the largest? Of what are the smallest? What are telescopic
stars? Why cannot the distance of the fixed stars be determined? To
what is the difference in their apparent magnitudes supposed to be owing?
Have the stars any motion?
333. What is the Galaxy? Of what is it supposed to consist?




ASTRONOMY.                        287
2. The ancients, in reducing astronomy to a science, formed
the stars into clusters, or constellations, to which they gave
particular names.
3. The number of constellations among the ancients was
about fifty. The moderns have added about fifty more.*
4. On a celestial globe, the largest star in each constellation
is usually designated by the first letter of the Greek alphabet;
and the next largest by the second, &c.  When the Greek
alphabet is exhausted, the English alphabet, and then numbers,
are used.
5. The stars, and other heavenly bodies, are never seen in
their true situation, because the motion of light is progressive;
and, during the time that light is coming to the earth, the
earth is constantly in motion. In order, therefore, to see a
star, the telescope must be turned somewhat before the star, and
in the direction in which the earth moves.
Hence, a ray of light passing through the centre of the
telescope, to the observer's eye, does not coincide with a direct
line from his eye to the star, but makes an angle with it; and
this is termed the aberration of light.t
6. The daily rotation of the earth on its axis causes the
whole sphere of the fixed stars, &c., to appear to move round
the earth every twenty-four hours from east to west. To the
inhabitants of the northern hemisphere, the immoveable point,
* Our observations of the stars and nebulma are confined principally to
those of the northern hemisphere. Of the constellations near the south
pole we know but little.
t In determining the true place of any of the celestial bodies, the refractive power of the atmosphere must always be taken into consideration.
This property of the atmosphere adds to the length of the days, by causing
the sun to-appear before it has actually risen, and by detaining its appearance after it has actually set.
How did the ancients divide the stars? What was the number of constellations among the ancients? How many have been added by the moderns? How are the stars designated on the celestial globe? What is the
situation of each constellation now'? Illustrate this. What is the cause
of this difference? Why do we not see the stars, and other heavenly
bodies, in their true situation? How can a star be seen in its true situation? What is meant by the aberration of light?  What is necessary to
be taken into consideration, in determining the true place of the celestial
bodies?  What effect has this property of the atmosphere on the length
of the days?




288               NATURAL PHILOSOPHY.
on which the whole seems to turn, is the Pole Star. To the
inhabitants of the southern hemisphere, there is another, and
a corresponding point in the heavens.
7. Certain of the stars surrounding the north pole, never
set to us. These are included in a circle parallel with the
equator, and in every part equally distant from the north pole
star. This circle is called the circle of perpetual apparition.
Others never rise to us; these are included in a circle equally
distant from the south pole; and this is called the circle of
perpetual occultation. Some of the constellations of the southern hemisphere are represented as inimitably beautiful, particularly the cross.
334. The parallax of a heavenly body is the difference
between the true and the apparent situation of the body.
1. In Fig. 158, AGB represents the earth, and C the
moon. To a spectator at A, the moon would appear at F;
while to another at B, the moon would appear at D; but to a
Fig. 158.                D
-~~A- ~ ~
F
third spectator at G, the centre of the earth, the moon would
appear at E, which is the true situation. The distance from F
to E is the parallax of the moon when viewed from A, and the
distance from E to D is the parallax when viewed from B.
2. From this it appears, that the situation of the heavenly
bodies must always be calculated from the centre of the earth;
and the observer must always know the distance between the
place of his observation and the centre of the earth, in order to
make the necessary calculations, to determine the true situation of the body. Allowance also must be made for the refraction of the atmosphere.
334. What is the parallax of a heavenly body? Explain Fig. 158.
What appears from this? What allowance must also be made?




ASTRONOMY.                         289
335. The moon is a secondary planet, revolving about
the earth in about 29"- days. Its distance from the
earth is about 240,000 miles. It turns on its axis in
precisely the same time that it performs its revolution
about the earth. Consequently it always presents the
same side to the earth.
1. The most obvious fact in relation to the moon,* is that
* The surface of the moon appears to be volcanic and very mountainous. Occasional volcanoes have been seen in action on the dark side.
No heat has been detected in the moon's rays, even when most powerfully
concentrated, that will affect the most delicate thermometer; and hence
some have inferred that the moon extracts the heat from the sun's rays
before it reflects them.
One of the most common errors, with regard to the moon, is that which
ascribes to it an influence over the weather. Tables of the weather have
been compared with the lunar phases for a period of a hundred years, and
over a thousand lunations, during which time about 491 new or full moons
have been attended by a change of the weather, and 509 have not.
The moon is equally innocent of putrefaction, notwithstanding the pop
ular belief that it hastens that process, especially in fish. The same cause
which produces dew, causes moisture on substances exposed to it, and this
moisture is the real cause of putrefaction.
Dr. Olbers of Bremen, by a comparison of a great number of cases, arrived at the conclusion that the moon has no effect on insanity; although
the popular belief is that the fits are aggravated or affected by the lunar
phases.
The density of the moon is estimated at about one-fifth that of the
earth; hence 10 pounds on the earth will be equal to 2 on the moon.
The days and nights on the moon are each equal to 14 of our days. The
axis of the moon is perpendicular to its orbit, and therefore the moon can
have no variety of seasons. The moon likewise has no atmosphere, and
therefore it cannot be inhabited; for there can be no vegetation, no clouds,
no ocean, no liquids, no light in dwellings, no twilight; in short, nothing
that could fit it for the habitation of any order of beings with which we
are acquainted.
In connexion with what has now been stated with regard to the moon
and its volcanic appearances, it will be proper to notice the subject of aerolites, or meteoric stones; because, according to the opinion of some, they
are of lunar origin. Three theories have been broached with regard to
335. Is the moon a primary or secondary planet? How long is it in
performing its revolution about the earth? What is its distance from the
earth?
13




290                 NATURAL PHILOSOPHY.
its disk is constantly changing its appearance, sometimes only
a semicircular edge being illuminated, while the rest is dark;
at another time, the whole surface appearing resplendent.
This is caused by the relative position of the moon with
regard to the sun and the earth. The moon is an opaque
body, and shines only by the light of the sun. When, therefore, the moon is between the earth and the sun, it presents
its dark side to the earth; while the side presented to the
sun, and on which the sun shines, is invisible to the earth.
But when the earth is between the sun and the moon, the illuminated side of the moon is visible at the earth.
2. In Fig. 159, let S be the sun, E the earth, and A B C D
the moon in different parts of her orbit.  When the moon is at
A, its dark side will be towards the earth, its illuminated part
being always towards the sun. Hence the moon will appear
to us as represented at a. But when it has advanced in its
orbit, to B, a small part of its illuminated side coming in sight, it
appears as represented at b, and is said to be horned. When
it arrives at C, one-half its illuminated side is visible, and it
appears as at c. At C, and in the opposite point of its orbit,
the moon is said to be in quadrature. At D its appearance
them: 1st, that they are'formed in the air, from materials existing there
in a sublimated state; 2d, that they are parts of an exploded planet; 3d,
that they are thrown from the volcanoes in the moon.
To the first of these theories there is a material objection, in the fact
that gases, when in contact, must mix, and gases necessary to form these
substances cannot, therefore, remain in the air unmixed.
To the second hypothesis it may be objected, that if they were parts of
a broken planet, they would probably be composed of more heterogeneous
materials. But it is well known that all of them are composed of the same
constituent parts, namely; sulphur, magnesia, manganese, iron, nickel,
chromium, and in one recorded instance only, charcoal.
In favor of the third supposition, which refers them to a lunar origin, it may be remarked, that a body thrown 70 miles from the moon,
would escape from the moon's attraction; and that a velocity six times
greater than that of a cannon-ball, would be sufficient to throw a body
beyond the moon's attraction. As terrestrial volcanoes have thrown bodies
with this velocity, it is not improbable that lunar volcanoes may do the
same.
What is the- most obvious fact in relation to the moon?  How is this
caused? What kind of a body is the moon? By what light does it
shine?




ASTRONOMY.                          291
is as represented at d, and it is said to be gibbous. At E all
illuminated side is towards us, and we have a full moon.
During the other half of its revolution, less and less of its
illuminated side is seen, till it again becomes invisible at A.*
Fig. i59.
C
-    Al
3. The mean difference in the rising of the moon, caused by
its daily motion, is a little less than an hour. But on account
of the different angles formed with the horizon by different
parts of the ecliptic, it happens that for six or eight nights
* The following signs are used in our common almanacs to denote the
different positions and phases of the moon. ) or 9 denotes the moon in
the first quadrature; that is, the quadrature between change and full;
( or ( denotes the moon in the last quadrature; that is, the quadrature
between full and change; O denotes new moon; 0 denotes full moon.
When viewed through a telescope, the surface of the moon appears
wonderfully diversified. Large dark spots, supposed to be excavations or
valleys, are visible to the eye; some parts also appear more lucid than the
general surface. These are ascertained to be mountains, by the shadows
which they cast. Maps of the moon's surface have been drawn, on
which most of these valleys and mountains are delineated, and names are
given to them. Some of these excavations are thought to be 4 miles deep
and 40 wide. A high ridge generally surrounds them, and often a mountain rises in the centre. These immense depressions probably very much
resemble what would be the appearance of the earth at the moon, were all
the seas and lakes dried up. Some of the mountains are supposed to be
volcanic.
How does the moon appear when viewed through a telescope? What
causes the difference in the rising of the moon? What is the mean differ
ence in the rising of the moon?




292                NATURAL PHILOSOPHY.
near the full moons of September and October, the moon rises
nearly as soon as the sun is set. As this is a great convenience
to the husbandman and the hunter, inasmuch as it affords
them light to continue their occupation, and, as it were, lengthens out their day, the first is called the harvest moon, and
the second the hunter's moon.   These moons are always
most beneficial when the moon's ascending node is in or near
Aries.*
OF TIHE TIDES.
336. The tides are the regular rising and falling ol
the water of the ocean twice in about 25 hours. They
are occasioned  by the attraction of the moon upon the
matter of the earth; and they are also affected by that
of the sun.
1. Let M, Fig. 160, be the moon revolving in her orbit; E the
earth covered with water, and S the sun. Now the weight of
Fig. 160.
* The reader who wishes a simple and clear illustration of the causes
which produce the harvest moon, is referred to Wilkins's Astronomy, page
69. To those who wish a fuller treatise on astronomy than is contained
in this volume, Phillip's " Eight Familiar Lectures" are recommended.
They give a clear illustration of those portions of the subject which do not
require the aid of the mathematics.
What is the harvest moon? What is the hunter's moon? When are
the moons always the most beneficial?
336. What are tides? By what are they occasioned?




ASTRONOMY.                     29}3
a particle of matter is its tendency towards the centre of the
earth, and whatever goes to separate the centre from any particle, diminishes that tendency, and consequently lessens the
weight in the same proportion. Now since the water at and
about A, is nearer the moon than that between A and C, and
between A and D, it will be more strongly attracted; and
consequently, such attraction will diminish the weight of the
water more at A than at the points between A and C and
between A and D: hence, the water being made lighter at A,
will by the force of the hydrostatic pressure rise, until the
whole mass comes into equilibrium.
Again, the waters at C and D, and between C and B and D
and B, being nearer the moon than the water at B, will be
more attracted by it, and hence its weight will be increased
more than that of the water at B, which latter weight is also
diminished by the effort of the moon to separate the centre E,
and the particle of matter at B. Therefore, the water being
made lighter at B will rise, the same as at A, and this rise
will also be increased by the motion of the moon and earth
around their common centre of gravity.
2. Thus any particular place, as A, while passing from under
the moon till it comes under the moon again, has two tides.
But the moon is constantly advancing in its orbit, so that the
earth must a little more than complete its rotation, before the
place A comes under the moon. This causes high water
at any place about fifty minutes later each successive day.
3. As the moon's orbit varies but little from the ecliptic,
the moon is never more than 290 from the equator, and is
generally much less.  Hence the waters about the equator
being nearer the moon, are more strongly attracted, and the
tides are higher than towards the poles.
4. The sun attracts the waters as well as the moon. When
the moon is at full or change, being in the same line of direction, (see Fig. 160,) the sun acts with it; that is, the sun and
moon tend to raise the tides at the same place, as seen in the
figure. The tides are then very high, and are called spring
tides.
But when the moon is in its quarters, as in Fig. 161, the
Explain the theory from Fig. 160. What is the greatest distance of the
moon from the equator? Where are the tides highest? Why? Has
the sun any effect on the tides'? What are spring tides? When do they
ceCllr?




294                 NATURAL PHILOSOPIIY.
sun and moon being in lines at right angles tend to raise tides
Fig. 161.
M
at different places; namely, the moon at C and D, and the sun
at A and B.  Tides that are produced when the moon is in its
quarters, are low, and are called neap tides.*
OF ECLIPSES.
337. An eclipse is a total or partial obscuration of one
heavenly body by the intervention of another.
1. The situation of the earth with regard to the moon, or
rather of the moon with regard to the earth, occasions eclipses
both of the sun and moon. Those of the sun take place when
the moon, passing between the sun and, earth, intercepts his
rays. Those of the moon take place when the earth, coming
between the sun and moon, deprives the moon of his light.
Hence, an eclipse of the sun can take place only when the moon
changes, and an eclipse of the moon only when the moon fulls;
There are so many natural difficulties to the free progress of the tides,.lt the theory by which they are accounted for is, in fact, and necessarily,
the most imperfect of all the theories connected with astronomy. It is,
however, indisputable, that the moon has all effect upon the tides, although
it be not equally felt in all places, owing to the indentations of the coast,
the obstructions of islands, continents, &c., which prevent the free motion
of the waters. In narrow rivers, the tides are frequently very high and
sudden, from the resistance afforded by their banks to the free ingress of
the water, whence what would otherwise be a tide, becomes an accumulation. It has been constantly observed, that the spring tides happen at
the new and full moon, and the neap tides at the quarters. This circumstance is sufficient in itself to prove the connexion between the influence
of the moon and the tides.
What are neap tides?  When do they take place?.A37. What is an eclipse? When does an eclipse of the sun take place 1
*Vhau does an eclipse of the moon take place?




ASTRONOMY.                      295
for at the'time of an eclipse, either of the sun or the moon, the
sun, earth, and moon must be in the same straight line.
2. If the moon revolved around the earth in the same plane
in which the earth revolves around the sun, that is, in the
ecliptic, it is plain that the sun would be eclipsed at every new
moon; and the moon would be eclipsed at every full. For at
each of these times, these three bodies would be in the same
straight line. But the moon's orbit does not coincide witlh the
ecliptic, but is inclined to it at an angle of about 50 20'. Hence,
since the apparent diameter of the sun is but about 2 a degree, and
that of the moon about the same, no eclipse will take place at
new or full moon, unless the moon be within a a degree of the
ecliptic, that is, in or near one of its nodes.  It is found that
if the moon be within 16~0 of a node at time of change, it will
be so near the ecliptic, that the sun will be more or less
eclipsed; if within 12~ at time of full, the moon will be more
or less eclipsed.
3. It is obvious that the moon will be oftener within 16-~ of
a node at the time of change, than within 1'20 at the time of
full; consequently there will be more eclipses of the sun than
of the moon in a course of years. As the nodes commonly
come between the sun and earth but twice in a year, and the
moon's orbit contains 3600, of which 161~, the limit of solar
eclipses, and 120, the limit of lunar eclipses, are but small
portions, it is plain there must be many new and full moons
without any eclipses.
4. Altnough there are more eclipses of the sun than of the
moon, yet more eclipses of the moon will be visible at a particular place, as Boston, in a course of years, than of the sun.
Since the sun is very much larger than either the earth or
moon, the shadow of these bodies must always terminate in a
point; that is, it must always be a cone. In Fig. 162, let S
be the sun, m the moon, and E the earth. The sun constantly
illuminates haif the earth's surface, that is, a hemisphere; and
consequently it is visible to all in this hemisphere. But the
What is necessary at the time of an eclipse? How often would there
be an eclipse, if the moon went round the earth in the same plane in which
the earth goes round the sun?  Why?  What is the inclination of the
moon's orbit to the ecliptic? What is the apparent diameter of the sun
and moon? What follows from this? When is the sun eclipsed? When
the moon? Does an eclipse happen every time there is a full or new
moon? What must the shadows of these bodies always be? Why?




296               NATURAL PIIILOSOPHY.
moon's shadow falls upon a part only of this hemisphere; and
hence the sun appears eclipsed to a part only of those to whom
Fig. 162.
-r  C
it is visible. Sometimes, when the moon is at its greatest distance, its shadow, O m, terminates before it reaches the earth.
In eclipses of this kind, to an inhabitant directly under the
point 0, the outermost edge of the sun's disk is seen, forming
a bright ring around the moon; from which circumstance
these eclipses are called annular, from annulus, a Latin word
for ring.
Besides the dark shadow of the moon, m 0, in which all the
light of the sun is intercepted, (in which case the eclipse is
called total,) there is another shadow, r C D S, distinct from
the former, which is called the penumbra. Within this, only a
part of the sun's rays are intercepted, and the eclipse is called
partial. If a person could pass, during an eclipse of the sun,
from O to D, immediately on emerging from the dark shadow,
O m, he would see a small part of the sun; and would continually see more and more till he arrived at D, where all
shadow would cease, and the whole sun's disk be visible. Appearances would be similar.if he went from O to C. Hence
the penumbra is less and less dark, (because a less portion of
the sun is eclipsed,) in proportion as the spectator is more remote from 0, and nearer C or D.  Though the penumbra be
continually increasing in diameter, according to its length, or
the distance of the moon from the earth, still, under the most
favorable circumstances, it falls on but about half of the illuminated hemisphere of the earth. Hence, by half the inhabitants on this hemisphere, no eclipse will be seen.
5. Fig. 163 represents an eclipse of the moon. The instant
the moon enters the earth's shadow at x, it is deprived of the
sun's light, and is eclipsed to all in the unilluminated hemiExplain Fig. 162. When is an eclipse called annular? Explain by Fig163. What is a penumbra?




ASTRONOMY.                     297
sphere of the earth. Hence, eclipses of the moon are visible
to at least twice as many inhabitants as those of the sun can
Fig. 163.
A                                                       R
B
G
be; generally the proportion is much greater. Thus, the inhabitants at a particular place, as Boston, see more eclipses of
the moon than of the sun.
6. The reason why a lunar eclipse is visible to all to whom
the moon at the time is visible, and a solar one is not so to all to
whom the sun at the time is visible, may be seen from the nature of these eclipses. We speak of the sun's being eclipsed;
but, properly, it is the earth which is eclipsed. No change
takes place in the sun; if there were, it would be seen by all
to whom the sun is visible. The sun continues to diffuse its
beams as freely and uniformly at such times as at others. But
these beams are intercepted, and the earth is eclipsed only
where the moon's shadow falls, that is, on only a part of a
hemisphere. In eclipses of the moon, that body ceases to receive light from the sun, and, consequently, ceases to reflect
it to the earth. The moon undergoes a change in its appearance; and, consequently, this change is visible at the same
time to all to whom the moon is visible; that is, to a whole
hemisphere of the earth.
7. The earth's shadow (like that of the moon) is encompassed by a penumbra, C RS D, which is faint at the edges
towards R and S, but becomes darker towards F and G.  The
shadow of the earth is but little darker than the region of the
penumbra next to it. Hence it is very difficult to determine
the exact time when the moon passes from the penumbra into
Why are eclipses of the moon visible to more inhabitants than those of
the sun? Why is a lunar eclipse visible to all to whom the moon is visible at the time? What is said of the earth's shadow? Explain by the
figure.                   1 3*




298                NAT'URATL PHIIOSOPHtY.
the shadow, and from the shadow into the penumbra; that is,
when the eclipse begins and ends. But the beginning and
ending of a solar eclipse may be determined almost instantaneously.
8. The diameters of the sun and moon are supposed to be
divided into 12 equal parts, called digits. These bodies are
said to have as many digits eclipsed as there are of those parts
involved in darkness.
9. There must be an eclipse of the sun as often, at least, as
one of the moon's nodes comes between the sun and the earth.
10. The greatest number of both solar and lunar eclipses
that can take place during a year is seven. The usual number
is four-two solar and two lunar.
11. A total eclipse of the sun is a very remarkable phenomenon.
12. June 16, 1806, a very remarkable total eclipse took
place at Boston. The day was clear, and nothing occurred to
prevent accurate observation of this interesting phenomenon.
Several stars were visible; the birds were greatly agitated; a
gloom spread over the landscape, and an indescribable sensation of fear or dread pervaded the breasts of those who gave
themselves up to the simple effects of the phenomenon, without
having their attention diverted by efforts of observation.  The
first gleam of light, contrasted with the previous darkness,
seemed like the usual meridian day, and gave indescribable
life and joy to the whole creation. A total eclipse of the sun
can last but little more than three minutes. An annular eclipse
of the sun is still more rare than a total one.
OF TIME.
338. When time is calculated by the sun, it is called
solar time, and the year a solar year; but when if is
calculated by the stars,* it is called sidereal time, and
* The solar year consists of 365 days, 5 hours, 48 minutes, and 48 secInto what are the diameters of the sun and moon supposed to be divided?  How many digits are these bodies said to have eclipsed? How
often must there be an eclipse of the sun? What is the greatest number,
of both lunar and solar eclipses, that can take place in a year? What is
the usual number? What is said of the eclipse of the sun in 1806?
338. What is time called when calculated by the sun? What is sidereal time?  How much longer is the sidereal year than the solar?




ASTRONOMY.                          299
the  year a  sidereal year.   The  sidereal year is 20
minutes and 24 seconds longer than the solar year.
1. A solar year* is measured from the time the earth
onds; but our common reckoning gives 365 days only to the year. As
the difference amounts to nearly a quarter of a day every year, it is usual,
every fourth year, to add a day. Every fourth year, the Romans reckoned the 6th of the calends of March, and the following day as one day:
which, on that account, they called bissextile, or twice the 6th day; whence
we derive the name of bissextile for the leap year, in which we give to
February, for the same reason, 29 days every fourth year.
* As it may be interesting to those who have access to a celestial globe,
to know how to find any particular star or constellation, the following directions are subjoined:
There is always to be seen, on a clear night, a beautiful cluster of seven
brilliant stars, which belong to the constellation " Ursa Major," or the
2Great Bear. Some have supposed that they will aptly represent a plough;
others say that they are more like a wagon and horses, the four stars representing the body of the wagon, and the other three the horses. Hence
they are called by some the plough, and by others they are called
Charles's wain, or wagon.
Fig. 164 represents these seven stars: a b dg    Fig. 164.
represent the four, and e z B the other three         r
stars. Perhaps they may more properly be             +
called a large dipper, of which e z B represent
the handle. If a line be drawn through the            k
stars b and a, and carried upwards, it will pass
a little to the left, and nearly touch a star rep-      i
resented in the figure by P. This is the polar           I'.,
star, or the north pole star; and the stars b                  +b
and a, which appear to point to it, are called
the pointers, because they appear to point to  B / z  e a
the polar star.                                             g
The polar star shines with a steady and
rather dead kind of light. It always appears  4
in the same position; and the north pole of
the earth always points to it at all seasons of the year. The other stars
seem to move round it as a centre. As this star is always in the north,
the cardinal points may at any time be found by starlight.
By these stars we can also find any other star or constellation.
How is a solar year measured? What is the length of a solar year?
Why is a day added every fourth year, to the year?  How is a sidereal
year measured?




300                 NATURAL PHILOSOPHY.
sets out from a particular point in the ecliptic, as an equinox,
or solstice, until it returns to the same point again. A sidereal
year is measured by the time that the earth takes in making
an entire revolution in its orbit; or, in other words, from the
time that the sun takes to return into conjunction with any
fixed star.
2. Every equinox occurs at a point, 50// of a deg. of the
great circle, preceding the place of the equinox, 12 months before; and this is called the precession of the equinoxes. It is
this circumstance which has caused the change in the situation
of the constellations of the zodiac, of which mention has already been made.
3. The earth's diurnal motion on an inclined axis, together
with its annual revolution in an elliptic orbit, occasions so much
complication in its motion, as to produce many irregularities;
therefore true equal time cannot be measured by the sun. A
clock, which is always perfectly correct, will, in some parts of
the year, be before the sun, and, in other parts, after it. There
are but four periods in which the sun and a perfect clock will
agree: these are the 15th of April, the 16th of June, the 23d
of August, and the 24th of December.
4. The greatest difference between true and apparent time
amounts to between sixteen and seventeen minutes. Tables of
equation are constructed for the purpose of pointing out and
correcting these differences between solar time and equal or
mean time, the denomination given by astronomers to true
time.
Thus, if we conceive a line drawn from the star z, leaving B a little to
the left, it will pass through the very brilliant star A. By looking on a
celestial globe for the star z, and supposing the line drawn on the globe,
as we conceive it done on the heavens, we shall find the star and its name,
which is Arcturus.
Conceiving another line drawn through g and b, and extended some
distance to the right, it will pass just above another very brilliant star. On
referring to the globe, we find it to be Capella, or the goat.
In this manner, the student may become acquainted with the appearance of the whole heavens.
What is the precession of the equinoxes? What change has this circumstance caused, with regard to the situation of the constellations? Can
true equal time be measured by the sun? Why? At what periods of the
year do the sun and a perfect clock agree? Wllat is the greatest difference between true and apparent time?




Many of the foregoing facts, with some others relating to the bodies which compose the Solar System, are arranged in the
following Tables. The numbers in these Tables differ, in some respects, from those contained in the body of the work, as
they were taken from a different authority.
TABLE I. — Of the Sun and Primary Planets.
Dist. in   Time       Time    Diameter        The                    Greatest   Hourly  Eccent.,
-4     -      Mil-   of revolving of turning on    in           Heat &            Dist. from   Motion    Mean
lions. around the Sun. their Axis.   Miles.  Bulk.   Light.  Density. the Ecliptic. in Miles. Dist. 1.
I *                             *       *
R  SUN,       **    *        * 8 124.5  d.  883,217  1,380,000   *
Mercury, i  37      87.98 d.  24.  h.    3,123     -r         6.68     2       70 40'  110,000  0.205
Venus,       68     224.7  "   23.36 "      7,702     A       1.91     1+       30 23'   84,000. 0.007    H
__               I..-._                  _...  
Earth,          93     365.25 "   24.   "      7,916      1      1.00      1       0         68,000  0.017    o
0.,....          -.
+  Mars,       144    687.00 "   24.64 "      4,398.43    _7        10 50'   54,000  0.093
O  Vesta,     223    1313.00 "  unknown unknown unknown.18  unknown   70  9'   45,000  0.097
Juno,        253   1586.00 "   t27.  h.   fi,545       t -.14  unknownj 130         42,000  0.254
- Pallas,     263    1680.00 "  unknown   i2,280        f.13  unknown  340 30'   41,000  0.246
Ceres,     263    1980.00 "  unknown    l,761    j05.13  unknown  100 30'   41,000  0.076
Jupiter,    490   4332.60 "    9.94 h.   89,170    1,400.037         3      1~ 19'   30,000  0.048
Saturn,    900   10759.00 "   10.27 "   79,042    1,000.011         9       20 29'   22,000  0.056
Uranus,   1800  30688.00 "  unknown   3500 0           90.0027       X       0~ 49'   15,000 f 0.047 




TABLE II.-Of Secondary Planets.
OF THE MOON.                                    OF THE SATELLITES OF JUPITER.
Distance from the  Inclination                                             Inclination
Dis arth          of Orbit     RevouEtin around    Distance from Jupiter. of Orbits to the    ReolJutaround
to the Ecliptic.    the Earth.                            Orbit of Jupiter.
Miles.         Deg.  Mill.      Days. Hours. Min.             Miles.     Deg. Min. Sec.    Days. Hours. Min.
240,000           5   50          27   7   43        I        264,490       3  18  38         1   18   27
II       420,815      3  18  00         3   14   58
Moon's diameter, 2159.  Bulk (that of the earth being   III       6713         3  14   58                            Z
1).III  671,234      3  13  58         7      3   42.
Period from change to change, 29 days, 12 hours, 44   IV        1,180,582      2  36  00        16   16   32         >
minutes. 
Distance from Saturn.  of Orbits to the                   Distance from U anus. of Orbits to the
Orbit of Saturn.    around Saturn.                        Orbit of Uranus.    around Uranus.
Miles.      Deg.  Min.      Days. Hours. Min               Miles.     Deg. Min. Sec.    Days.  Hours. Min.
VII      119,627       30              0   22   37        I         224,155      99 43 53  5   21   25
VI       153,496       30              1    8   53                              r 80  167 
I   190,044       30               1   21    18       II       290,821          do.            8    16   57
II       243,449'      30              2   17   44        III       339,052          do.          10   23      4
III      240,005       30              4    12   25       IV        388,718          do.          13   10   56
IV       788,258       30             15   22   41        V         777,481          do.          38     1   48
V      2,297,541       42   45        79     7   54       VI    1,555,872            do.         107    16   42
IT




INDEX,
Aberration of light, 287.                Air, elasticity of the, opposed to its
Accelerated Motion, 32.                        gravitation, 28.
"        "   how produced, 32.    "  essential to animal life, 122.
Accidental Properties of Matter, 19.      "       "    to combustion, 122
Achromatic lenses, 186.                   "       "    to sound, 124.
Acid employed in the galvanic circle,  "  expansion of, 99.
220.                                    "  experiment illustrating the comAcoustics, 17, 18, 124.                        pressibility of, 117.
Acoustic tubes, 130.                      "  experiments showing the elasticity
Acre of ground, quantity of moisture ex-       of, 115. 117.
uded from in twelve hours, 104.         "  experiments showingits fluidity, 119.
Action and Reactibn, 34.                  "  experiments illustrating the gravity
Action and reaction enable birds to fly,       of, 112.
fishes to swim, &c., 36.                 " experiments showing the inertia of,
Action and reaction of elastic and non-        119.
elastic bodies, 36.                     "  experiment illustrating the lightAcute Angles, 38.                              ness of, 117.
Advertisement, 7.                          "  experiment illustrating the mateAeriform Fluids, 19, 97, 98.                   riality of, 116.
"I     "    elasticity of, 99.         "  fluidity of the, effect of, 100.
Aerolites, 289.                            "  heated above the boiling point of
Africa, pestilential winds from, 109.          water, innocuous, proved by exAir, 97, 98.                                   periments of Sir Joseph Banks,
"  a heterogeneous mixture, 98.               136.
"  as a conductor of sound, 127.          "  how affected by the vibrations of a
"  or atmosphere, at what height it           sonorous body, 125.
loses its refractive power, 28.     "  how it becomes a mechanical agent,
"  circulation of the, 108.                   100.
"  compressibility of, 99.               "  impenetrability of the, 104.'"  compression of the,'at the surface   "  inertia of. 100.
of the earth, 28.                    "  motion of the, how caused, 107.
"  condensed, experiments with, 117.    "  necessary to animal life and to com"'     "      how weighed, 118.               bustion, 98.' contained in water and inwood, 115.  "  of what composed, 98.
i decrease of its density, 97.           "  one of the elements of the ancient
"  density of, affected by heat and           philosophy, 18.
cold, 118.                          "  pressure of the, its effects on the' different states of the, at different       liquid form of some bodies, 122.
heights, 98.                         c  pressure of the, retards ebullition,' effect of heat on, 133.                     122.
"  effect of its elasticity on a receiver,  "  principal properties of, 98.
109.                                "resistance of the, to the ascent of
"  effect of its elasticity, 101.             fluids, 94.
effects of the rarity of, in very ele-  "  resistance of the, to falling bodies,
vated situations, 102.                   27.




304                                INDEX.
Air, resistance of the, to the motion of a Angles of reflection, 38.
cannon ball, 44.                      "   of vision, 159.
"  Scales for weighing, 114.              "~ of vision, its connection with the
"  specific gravity of the, 98.                 appearance of the  heavenly
4"     " "          how affected, 101.          bodies, 258.
"  subject to the law of attraction, 28.  Angles, how measured, 38.
weight of the, 99.                     "   of incidence and reflection, 39.
or atmosphere, when heaviest, 102.            128. 141. 157. 165.
why light passing through is partly    "   of incidence and reflection alabsorbed, 158.                              ways equal, 38.
"  when the best conductor of sound,    "   right, obtuse, acute, 38.
130.                              Animal life, air essential to, 122.
"  where unfit for respiration, 102.   Annular eclipses, 296.
"  why invisible, 98.                     "    eclipse of the sun very uncom
"  why it seems heavier in wet than in            mon, 298.
dry weather, 102.                 Anode, 195.
why less dense on an elevated spot, Antarctic Circle, 272.
102.                              Aphelion, 265.
"  why more dense at the surface of Apogee, 266.
the earth, 28.                    Apparatus to illustrate the pressure of
"  Chamber, 117.                         fluids, 83.
"  Gun, force of the, 118.             Apparition, appearance of, presented by
"  principle of the, 118.          concave mirrors, 164.
"  Pump, 109.                          Aquarius, 263.
pumps, how made, 109.               Aqueducts, the Roman, 94.
"  Pump in the steam-engine, 143.      Aqueous humor, 175. 177.
"  Pump, experiments made with the, Arabia, pestilential wind from, 109.
112.                              Arago, M. his discoveries in Electro-'  Pump, operation of the, 110.          Magnetism, 240.
"  Pump,  " Wightman's Companion"  Archer, The, 263.
to the, 112.                      Archimedes, 70.
"  Pump, Wightman's Patent Lever, Archimedes' discovery of the mode of as
111.                                certaining specific gravity of bodies, 88.
"  Pump, the double, 111.              Archimedes, Screw of, 92.
Alcohol, how inflamed by the Electrical  Arctic Circle, 272.
machine, 207.                 Arcturus, 300.
"    how made to appear to boil, 122. Aries, 263.
All's Well, (a watchword,) distance at Arm, bones of the, 58.
which heard, 130.                     Art and Science, connection between, 16.
Alpha Leonis, 262.                          "      "    difference    "    16.
Alphabet of the Magnetic or Electro-  Artesian Wells, 94.
Magnetic Telegraph, 252.              Artificial magnet, 232.
Amalgam, 161. 202.                      Artificial magnets, how made, 238.
American Electro-Magnetic Telegraph,        "       "    new mode of making,
251.                                                       238.
Ampere's contrivance for illustrating  Arts Mechanic, explained, 16.
Electro-Magnetic rotation, 246.       Ash, specific gravity of, 87.
Ampere's theory of Electro-Magnetism,  Asteroids, 257. 279.
240.                                  Astronomy, 17, 18. 257.
Analysis, or decomposition,: 15.        Astronomical Telescope, 184.
Ancient Philosophers, their opinion of  Atmosphere, effect of its refractive
material substances, 18.                              power, 170.
Andes, Volcanoes of, distance at which        "       effect of its refractive
they are heard, 130.                                  power on the length of
Angle, 37.                                              the day, 287.
"   size of, on what it depends, 37.       "        heaviest in dry weather
"   of Incidence, 37.                                and lightest in wet, 102.




INDEX.                                 305
Atmosphere, height of the, 28.           Balloons, cause of their ascending in the
"     instrument for ascertaining    atmosphere, 29.
the weight of the, 101.   Balloon, the Pneumatic, 115.
"     pressure of the, 99.         Balls, velocity of, when propelled by
"      pressure of the, its influ-    gunpowder, 45.
ence on the barometer,  Banks, Sir Joseph, his experiments in a
101.                        heated room, 136.
"      pressure of the, on thd ba-  Barber's (Dr.) Grammar of Elocution,
rometer, on what princi-   131.
ple exerted, 102.          Bar magnets, how made, 239.
"      prevents the sky from ap- Barometer, 101.
pearing black, 188.            "      greatest depression of the
weight of the, 99. 112.                   102.
Atmospheric pressure in the steam-en-         "     highest elevation of the, 102.
gine supplanted  by the  expansive          "     where highest, where lowpower of steam, 143.                                est, 102.
Atmospheric Steam-Engine, 142.                "     why mercury is used in the,
Attraction, 19. 22.                                    102.'    general law of, 15.             Base of a body, 47.
"     capillary, 74.                 Bath, (the Pneumatic Shower,) 120.
"     causes some bodies to rise and  Battery, Electrical. 200.
others to fall, 29.             "    galvanic, electric currents from
"     electrical, 199.                          the, 240.
"     magnetic, 233.                 Beam of light, 154.
"     mutual, 259.                   Beech, specific gravity of, 87.'     of cohesion, 23.               Bell, when loudest in sound, 124.
of gravitation, 23.             "  when heard most distinctly, 130.
"     of gravitation mutual in all   "  metal, 69.
bodies, 26.                  Bellows, The Hydrostatic, 82.
"     of matter, its connection with  Bennett's Gold Leaf Electroscope, unAstronomy, 258.                proved by Singer, 204.
"     two kinds of, 23.              Beta Scorpii, 262.
"    universal, 26.                  Biela's comet, periodical time of, 285.
"     where strongest, 74.             "   prediction  of the return of a
"     and repulsion of electric cur-           comet, 285.
rents, 242.                  Billiards, 39.
Attractions nicely balanced by creative  Birds, muscular power of, 36.
Wisdom, 259.                             "   how they fly, 36.
Aurora Borealis, 212.                    Bissextile, 299.
Autumnal equinox, 275.                   Black, merely a deprivation of color, 190.
Axes, 67.                                  "  lead, why used in machinery, 69.
"  of the planets, not perpendicular  Bladder Glass, 113. 115.
to their orbits, 266.           "' Blowing out" of a steam-engine, 144.
"   of the earth, 271.                 Blue, indigo and violet rays, why least
Axis of a lens, 171.                             refracted, 187.
"  of motion, 41.                        "  vitriol, 226.
Azote, 98.                               Boats, how propelled by oars, 36.
"   &c., why they float in the water,
85.
Bagatelle, 39..  Bodies floating in a fluid, 85.
Baker, The Connecticut, 141.'  to what they owe their sonorous
Balance, 57. 263.                                 properties, 125.
Ball, cause of its rebounding, 37           "   what stand most firmly, 48.
"  Hero's, 118.                        Body of a man, pressure of the atmosthrown from a gun, 39.                phere on, 99.
"  thrown ina horizontal direction, 43. Boiler of a steam-engine, 143.
Ballistic pendulum, 45.                   "'  how secured from bursting, 145.




306                                INDEX.
Bolt Head and Jar, 120.                  Cartesian Devil,  16.
Bones, moveable, of animals, 58.         Castors, why used on tables, &c., 70.
Boreal polarity, 235.                    Cataracts in the eye, mode of removing,
Boston School Philosophical Apparatus,    179.
9. 46.                                 Cat, electricity of the, 215.
Boston School Set, Air Pump of the, 111. Cathode, 195.
Botany, 14.                              Catoptrics, 156.
Bottle, sunk in the sea, pressure on, 79.    "      fundamental law of, 157.
Boxes of a pump, 106.                        "      facts resulting from the fun
Boxwood, specific gravity of, 87                      damental law of, 165.
Bramah's Hydrostatic Press, 84.          Cats, whythey require little light, 176.
Brandy, specific gravity of, 87.         Celsius' scale for the thermometer, 103.
Breadth, 21.                             Central forces, 41.
Breast water-wheel, 97.                  Centrifugal, 41.
Brewster's (Dr.) invention of the Ka                 force, 41.
leidoscope, 191.                       Centripetal, 41.
Brewster, his opinion of the cause of          "     force, 41.
meteoric stones. 280.                  Centre of gravity, 40, 41. 49.
Bright substances, why heated with diI-     "              when the same with
ficulty, 137.                                              the centre of magBrittleness, 19. 24.                                         nitude, 48.
Brydone's calculation of the velocity of    "        "    being supported, the
a comet, 284.                                              whole body is supBuffon's Concave Mirror, 167.                                ported, 46.
Bulk or size, 21.                           "        "    of a system, 259.
Bull, The, 263.                             "        "    of the Solar System,
Buoyancy of salt and of fresh water, 87.                     275.
Burning-glass, effect of one made at        "        "    of two  bodies conLeipsic, 173.                                              nected, 49.
Burning-glasses, why in the form of a  Centre of magnitude, 40, 41.
convex lens, 173.                         "   of motion, 40. 53, 54.
Burning-glasses of concave mirrors. See     "   of the earth the centre of attracDufay and Buffon, 167.         tion, 26.
"      "    of concave mirrors, 167. Centres, three different kinds of, 40
Ceres, 257. 279.
Caloric or heat, 134.                       "  diameter of, 261.
Calorimotors, 231.                          "  distance from the sun, 260.
Camera Obscura, 159                         "  length of day unknown, 260.
"    (portable,) how made,    "  length of the year of, 260.
159.                    "  perihelion of, 266.
~c'    "    the eye a natural, 178.   Chain pump, 92.
Cancer, Tropic of, 272.                  Chalk, specific gravity of, 87.
"    263.                             Chantrey, the celebrated sculptor, 137.
Cannon ball, 43.                         Characters used in Almanacs and Astro-..';  utmost velocity of, 45.          nomical works to represent the planCaoutchouc, or India Rubber, cause of   ets, and also the signs of the Zodiac,
its rebounding, 37.                      263.
Capella, 300.                            Charged, wheln a body is said to be, with
Capillary, 74.                             electricity, 197.'    tubes, 74.                      Charles' Wain, 299.
Capricorn, Tropic of, 272.               Chemical action, general law of, 221.
Caprieornus, 263.                            "     effects of light, 191.
Capstan, 63.                                 "     properties of liquids and flu"    how it differs from a windlass,              ids, 98.
63.                           Chemistry, 14, 15. 17.
carrlrages, cause of their upsetting, 47.     "      and Natural Philosophy, difCarrunrades, 46.                                      ference between, 15.




INDEX.                                 307
Chemistry, science of, 98.               Cold, measure of, 103.
Chisels, 67.                               "  severity of, how we are shielded
Chord, (musical,) how obtained, 126.            from, 102.
o"    f an octave, a fifth, &c., how    "  weather, when the earth is nearobtained, 126.                         est the sun, 270.
Choroid, 175.                            Collision, apparatus for illustrating, 36.
Chromatics, 186.                         Color, an accidental property, 19. 190.
Circle and its parts, 37.                  "   brightness and intensity of, on
"  centre of, 37.                              what dependent, 190.
"  of perpetual occultation, 288.      "'  cannot exist without light, 190.
f"       "      apparition, 288.        "   change of, in bodies, by what
Circuit of electricity, 201.                      caused, 190.
Circular motion, 40.                       "   deepness of, on what it depends,
cc" "    how changed to recti-               199.
linear, 64.             "   of bodies, on what dependent,,,,c   of the revolving jet, by             189.
what caused, 118.     Colors, different refrangibility of the, in
Circumference, 37.                                 a ray of light, 187.
Cistern of a steam-engine, 143.,       "   dispersion of, by lenses, 184.
Clapper of a pair of bellows, 106.          "   of bodies, 188.
Clay, contraction of by heat, 138..     "   the seven reduced to four, 189.
Climate, cause of the variety of, 270.   Combustible substances, 188.
Clock, hour and minute hands of, 53.     Combustion, air essential to, 122.
Clocks, how regulated, i. e. to go faster      "      does not destroy the simple
or slower, 52.                                substances of which a
"'   pendulums of, how kept in mo-                   body is composed, 22.
tion, 51.                      Comet, 283.
" pendulums how lengthened or    "    Brydone's calculation of the veshortened, 52.                           locity of, 284.
"    time of, sometimes before, and     "    Pope Callixtus' anathema of a,
sometimes after that of the              285.
sun, 300.                         "    (Halley's,) periodical time of. 285.
"    why affected by the seasons, 53.    "    return of a, first predicted, 285.
Clothing, warmth of different kinds of,    "    tail of a, 283.
cause of, 135.                         Comets, 257.
Clouds, height of the, 28.                  "    appear to have little attractive
"   of what composed, 28. 104. 133.             force, 285.
Clydesdale, effect of galvanism on, 228.    "    density of, 285.
Coal, specific gravity of, 87.              "    disturbed by the gravitation of
Cobalt possessed of magnetic properties,           the planets, 285.
232.                                      "    immense velocity of when in
Cogs on wheels, 65.                                perihelion, 284.
Cohesion, attraction of, 23.,         "    length of the tails of, 285
Cohesive attraction, aeriforrn fluids des- Comets, motion of tails of, 284.
titute of, 98.         "    Newton's  opinion  respecting
cc"      "      affected by heat,133.          the tails of, 284.
cc"      "      independent of gra-     "    number of, 283.
vita.tion, 26.         "        "      in the Solar System,
cc"      "      not found in fluids,                      285.
99.                    "    orbits of which have been calCoins of alloyed metals, why they per-             culated, 284.
ish, 230.                           "    periodical  time  of Halley's,
" of pure metal, why more imper-                 Encke's, and Biela's, 285.
ishable than those which are        "    revolution of, 284.
alloyed, 230.,             Common Pump, 92.
Cold, 139.132.                               "    air, specific gravity of, 87.
"  and heat, effect of, on density, 140. Compass needles, how corrected, 236.




308                                INDEX.
Compass, why furnished with two sets Conjunction, superior and inferior, 265.
of axes, 238.                 Connecticut Baker, 141.
Complex wheel-work, 64.                  Conservatory of Arts and Trades, walls
Compound battery of great power, 224.      of, how  restored to a perpendicular
"     galvanic circles, 222.          position, 139.
"     Lever, 57.                    Constellation, what is meant by a heav"     motion, 39.                     enly body being in a particular,'265
Compressibility, 19. 24.                 Constellations, 261.
4" of air, experiment illus-" how to find with a globe,
trating the, 117.                       299.
Compression, all substances susceptible         "       in which the sun and the
of, 24.                                   earth relatively appear,
"      heat produced by, 137.                       263.
Conchology, 13, 14.                             "       near the South pole but
Condensation, 97.                                         little known, 287.
4" of air, 111.                                   number of, 287.
Condensed, meaning of the term, 97.             "       of the Zodiac, cause of
"     air, how weighed, 118.                         the change in, 300.
Condenser of a steam-engine, 143.               "       of the Zodiac have lost
"    of the steam-engine, bywhom                     their original names,
invented, 145.                               263.
Condensing syringe, 111.                        "       of the Zodiac, represen
and Exhausting syringe, 117.                  tation of the position'     steam-engines, 145.                            of the, 273.
Conductors of electricity, 196.          Contents, Table of, 11.
Is        it       list of, 195.    Convergency of the scales in a balance,
"     imperfect, of electricity, list   27.
of, 197.                   Converging rays, 153.
Concave glasses, when to be used in.        "       "  effect of on concave
spectacles, 175.                         surfaces, 174.
9" "   why used in specta- " "  reflected from a con
cles, 174.                               cave surface, 166.
"    lens, (single,) 171.                             reflected from  a con
it"    "   (double,) 171.                               vex surface, 166.
lenses diminish objects, 172.        "      "  when unrefracted, 113.
mirror, Dufay's, 167.          Convex glasses, when to be used in
"   objects reflected from,                     spectacles, 174.
164.                     "             why used in spectaConcave mirrors, 162.                                       cles, 174.
i"     "     appearance of appari-       "   lens, (single,) 171.
tions produced by,       "    "    "       focal distance of
164.                                         a, 172.
t      "     effects of, 164.            "    "  (double,) 171.
":   facts with regard to,       "    d'  effect of on the eye, 179.
166.                     "  lenses magnify objects, 172.
"     images reflected by,          "       why they operate asburnwhen  upright and                     ing-glasses, 173.
when inverted, 165.      "     "   rays perpendicular to a,
peculiar property of,                   172.
164.                     "  mirrors, 161.
true focus of a, 163.      "      "    convert parallel to di
"     used as burning-glass-                   vergent rays and make
es, 167.                               divergent still more
surfaces collect sound, 130.                     divergent, 163.
"'"  facts with regard to,        "           facts with  regard  to,
174.                                   165.
Cone, 49.                                    #      "   reflect convergent rays




INDEX.                                  309
either parallel or less  Degrees, 37.
convergent, 163.       Delisle's scale for the thermometer, 103.
C nvex mirrors reflect images smaller  Delphi, Oracle of, 132.
than the objects, 163.  Dense bodies, 23.
"  surfaces disperse sound, 130.     Density, 19, 23.
".    "    facts with regard to, 173.   "    bodies of different, why they fall
Cork, 237.                                          with unequal velocity, 27.
"  specific gravity of, 87.                "    how affected by heat and cold.
"  why it will not sink in water, 29. 85.         140.
Cornea, 175, 176.                        Depth, 21.
Cooper's mode of securing hoops, 138.      "   of a well, estimated by a falling
Copper, specific gravity of, 87.....           body, 33.
Couching (of the eye,) 179.              Descent of falling bodies, how impeded,
Coulomb's discovery that all solid bod-            30.
ies are susceptible of magnetic influ-     "    of projectiles, 43.
ence, 234.                             Devil, the Cartesian, 116.' Couronne des tasses," 224              Dew, how produced, 104.
Crab, The, 263..                Dew and Fog, difference between, 104.
Cranks, 64.                              Diagonal, 38.
Cross, the Southern, 288.                Diameter, 37.
Cross-currents, 91.                          "     of Heavenly bodies, how exCrystalline humor, effect of when too                pressed, 161.
round or too flat, 179.    Diameters of the sun and moon, how
"     humor, convexity of, how        divided with reference to eclipses, 298.
increased or diminished,  Difference between Fog and Dew, 104.
179.                            "     between Natural Philosophy
di    lens, 175. 177.                            and Chemistry, 15.
Current of electricity, 221.             Digits, 298.
"   rapidity of, how ascertained, 91. Dilatation of metallic beams effected by
Currents of electricity, 240.              the weather, 139.
Curvilinear motion, 42.                  Dionysius, the tyrant of Sicily, 128.
Cutting instruments, 67.                     "'    ear of, 128.
Cylinder, 62.                            Dioptrics, 167.
"    how made to roll up a slope, 48.    "    fundamental laws of, 168.
"    of a steam-engine, 143..            "    principles of, 174.
Cyprias, 129.                            Dipping of a magnetic needle, 236.
"   of the magnetic needle, cause of
Daguerre, M., 193.                                  the, 236.
Daguerreotype, 193.                          "   of the magnetic needle, how
Dark, why none but luminous objects can             corrected, 236.
be seen in the, 158.             -     Direction, line of, 47.
Darkness produced by a total eclipse of Discord, (musical,) how produced, 126.
the sun, 155.                          Dispersion of colors by lenses, 184.
Davis, (Daniel, Jun.,) 237.              Distance, mean, of the several planets
Davis, (Daniel. Jun.,) his treatise on mag-    from the sun, 260.
netism, 237. 247. 251. 255.            Distilled water, a standard for computing
Davy, Sir Humphrey, his discoveries in                    specific gravity, 86.
Electro-Magnetism, 240.                   t"      "   why used for computing
Day of a planet, 260.                                     specific gravity of bodDay Glass, 184.                                           ies, 86.
Day and night, cause of, 273.            Distinction between the signs and the
Days, length of the, cause of, 274.        constellations of the zodiac, 263.
Days and nights, cause of the difference  Diverging rays, 153.
of the length of, 266.                     "       "  effectof concave surfaces
Dedication, 4.                                            on, 174.
Deepness of color, on what dependent,        "       "  effect of convex surfaces
190.                                                    on, 173.




310                               INDEX.
Diverging rays, reflected from a concave  Earth, situation of the, 257.
surface, 166.             "   size of, compared with that of
It     it reflected from a convex             falling bodies, 26.
surface, 166.             "   the principal reservoir of electriI)iving Bell, 104.                                city, 197.'"    "   forcing pump attached to       "   when nearest to the sun, 265.
the, and why, 105.        East wind, how produced, 108.'"    "   how supplied with air, 105.   w" inds prevalent at the equator, and
"    "~'its uses, 105.                       why, 108.
r("    "   principle of the, 104.     Ebullition, (meaning of the term,) 122.
Divisibility, 19. 21.                         "     retarded by pressure of the
Double action of the Steam-engine, 143.               air, 122.
"   Air Pump, 111.                    Echo, how produced, 128.
*'  concave lens, 171.                Eclipse, a total of the sun, very uncom-'   convex lens, 171.                            mon, 298.
"   microscope, 180.                     "    of the moon, 294.
Downward pressure of fluids, by what        "    of the sun, 294.
caused, 81.                               "    of the sun, total, duration of,
Drake, Sir Francis, 237.                           298..
Drawing, Perspective, on what depend-       "    (total) of the sun, darkness proent, 160.                                         duced by, 155.
Drop, (the Guinea and Feather,) 119.    Eclipses, 294.
Ducks, guns suitable for, 46.                "    annular, 296.
Ductility, 19. 25.                           "    explanation of the causes of,
Dufav's large concave mirrol, 167.                   295.
Dungeon, called the Ear of Dionysius,        "    how measured, 298.
128.                                       "    length of, 295.
Duration of a total eclipse of thesun, 298.  "    when they take place, 298.
Ecliptic, 262. 272.
Ear of Dionysius, 128.                       "    and Zodiac, difference between,
Earth, 257. 278.                                    272.
"   a magnet, 237.                    Edifice, what kind of, the strongest, 49.
an oblate spheroid, 278.          Eduction Pipe, 143.
apparent size of, when viewed  Effect of the voltaic pile on animal life,
from the moon, 279.               230.
"   attracts the heavenly bodies, 259. Egypt, lands of, how formerly drained,
"   axis of the, 271.                 93.
"   circumference of the, 257.        Elastic fluids, 98.
I'  diameter of the, 257. 261.        Elasticity, 19. 24. 98.
difference of its equatorial and       "     of air and of aeriform fluids,
polar diameters, 278..               99.
distance from the sun, 260.                         experiments illustradistance of moon from the, 278.                     ting, 115. 117.
effect of the inclination of its axis,              shownbymeansofthe
271.                                               Air-pump, 112.
"   how known to be round, 257.            "     the cause  of the sonorous
length of the year of, 260.                   properties of bodies, 125.
"   length of day, 260.               Electric currents. attraction and repulmoon of the, 258.                            sion of. 242.
"   one of the elements of the ancient    "    current, direction of the, may
philosophy, 18.                            be ascertained, 242.
"   perihelion of, 266.                   "    currents, how excited in a con"   phases of the, when viewed from               ductor, 253.
the moon, 278.                      "    shock, 199. 205.
proximity of, to the sun in winter,   "    currents, peculiar action of, 243.
268.                                "   fluid, how communicated, 196.
"   revolution of, 42.                            A ll in thunder-storms, 215.




INDEX.                                 311
Electric fluid, immense velocity of the,  Electrics, 194.
215.                          Electrified positively, 195.
"    shock from  galvanic battery,  Electrodes, 195.
how received, 224.            Electro-Magnet, 248.
Electrical attraction and repulsion, 199.    "      "    formed  by  Professor
"     Battery, 200.                                    Henry and Dr. Ten
"     bells, 206. 208.                                  Eyck, 249.
"     fishes, 216.                       "      "    of Professor Henry, the
"     machine, 201.                                     means of the  con-';        "'   experiments  with                       struction of the Magthe, 204.                             netic Telegraph, 251.
cc"      "      operation  of  the,  Electro-Magnetic induction, 243.
203.                   "       "    Multiplier, 244.
"     machines, on what principle        "       "    rotation, 244. 246.
constructed, 202.     "       6"   rotation, Amp6re's ap"     power of, how test-                      paratus for illustraed, 204.                              ting, 246.
"     Sportsman, 209.                   "        "     rotation,  Faraday's
"     Sawmill, 213.                                      apparatus for illus"     Tellurium, 206.                                    trating, 245.
Electricity, 17, 18. 194.                   "        "     Telegraph, 251.
"     and magnetism, combined  Electro-Magnetism, 17, 18. 196. 232. 240.
action of, 241.                 "        "      as a moving power,
"     by induction, 201. 213.                             253.
"     by transfer, 201.                 "        "      principal facts  in;"    circuit of, 201.                                    connexion  with,
"'    currents of, 240.                                   241.
developed by chemical ac-" " wonders of, 252.
tion, 220.                  Electrometer, 204.
"     developed by heat, 255.       Electroscope, 204. 244.'     different kinds of, always ac-  Electrotype copy of a steel plate, 150.
company each other, 198.   Elm, specific gravity of, 87.'     effects of, 194.              Encke's comet, periodical time of, 285.
"     effect of, on the Magnetic       "    prediction of the return of a
Needle, 241.                           comet, 285.
"     excited in glass, what called,  Engineer, how taught to direct his guns,
198.                         46.
"     experiments in, 210.          Engines, Steam, the Condensing and the
"     Frictional, and Galvanic, or    Non-condensing; or the high and the
Voltaic, 196.                low pressure, 145.
"     General remarks on, 212.      Entomology, 13, 14.
"     how excited, 195.             Ephesus, oracle at, 132
"     how it divides all substances,  Equation of time, tables of, 300.
195.                       Equator, 271.
"     induced by magnetism, 241.  Equatorial regions move fastest in the
"     magnetism induced by, 241.                         daily revolution of''    particular law of, 15.                              the earth, 42.
"      (galvanic,) perfect and im-       "         It   why hottest, 270
perfect conductors of, 219. Equilibrium, how produced in the Steel"     positive and negative, 195.                  yard, 57.
"     positive and negative mere-        I"      of fluids, 76.
ly relative terms, 215.          "       "   "   how applied to the'     principal reservoir of, 197.                          common pumnp,
"     simplest mode of exciting,                            106.
198.                             "       "   "  instrument   for
"     three states of, 256.                                 showing, 95.
velocity of, 252                   #       "   "   result of the, 10n




312                                INDEX.
Equilibrium of fluids, the principle of the  Experiments showing the fluidity of air,
barometer, 102.                                      119.
Equinoxes, 275. 300.                          "      showing the inertia of air,
"     precession of the, 263. 300.                119.
Essential elements of a galvanic circle,      "      to show that heat is pro220.                                        duced by compression.
"    properties of matter, 19.                     &c., 138.
Ether, how inflamed by the electrical         "      with condensed air, 117.
machine, 207.                        "      with full and empty bottles
how made to appear to boil, 122.                sunk in the sea, 79.
Ethics, meaning of, 13.                       "      with  the  electrical ma
Evaporation of water, 22.                              chine, 204.
Exhausting and Condensing  Syringe,           "      with the magnet, 239.
117.                                        "      with the Siphon, 121.
Exhaustion by an air-pump, degree of, Extension, 19, 21.
how shown, 111.                       Eye, a natural Camera Obscura, 178.
Expansibility, 19. 24.                    "  brilliancy of the, by what caused,
Expansion caused by heat different in          176.
all substances except aeri- "  color of the, cause of, 175.
form fluids, 140.            "  remedy for the defects of the, 174.
"     of bodies by heat, instru-    "  explanation of the, 175.
ment to show, 138.          "  imperfections of the, 179.
Expansive power of Steam, substituted    "  of what composed, 175.
for atmospheric pressure in the Steam-   "  parts of, described, 178.
engine, 143.                            "  the human, glasses suitable for,
Experiment illustrating  at once the           when too round or too flat, 175.
materiality, compressi- "  what defects of are remedied by the
bility,  elasticity,  and        use of spectacles, 179.
lightness of air, 116.     "  why affected by sudden changes in
"      illustrating the principal        the degree of light, 176.
truths of Hydrostatics  Eyes, difference produced by using both,
and Pneumatics, 117.             179.
"'    showing that the pressure    "   distinct images formed on each,
of a liquid is exerted in        and why, 179.
all directions, 117.
"      showing that the pressure  Facts explained by galvanism, 230.
of a liquid is in propor-  Fahrenheit's scale for the thermometer,
tion to its depth, 117.    103.
"      showing that the pressure  Falling bodies, law of, 27.
of the air retards evapo-    "  of bodies, by what caused, 25.
ration, 122.             Falls of Niagara, 189.
"      to freeze water artificially,  Faraday's apparatus to illustrate electro123.                                 magnetic rotation, 245.
Experiments (miscellaneous) depending        "    discovery of the  electrical
on two or more proper-               powers of a magnet, 240.
ties of air, 120.             "    nomenclature of electricity,
"      illustrating the gravity of            195.
air, 112.                Figure, 19. 21.
"      illustrating Divisibility, 21. Fir, specific gravity of, 87.
"      illustrating the impenetra-  Fire, one of the four elements of the
bllity of water and of   ancient philosophy, 18.
air, 20.                 Fire-box of a Locomotive Steam-engine,
"      in electricity, 210.         148.
4"     showing mutual action and  Fire-engines, on what principle conreaction, 35.              structed, 107.
#      showing'the elasticity of Firing at a bird on the wing, 50.
air, 115.                  "   at a vessel in motion, 50.




INDEX.                                 313
Fishes, electrical, 216.                 Fluids, surface of, always spherical, 76.
how enabled to rise and to sink in  "  transfer of, from  one vessel to
water, 36.                             another, 121.
how they swim, 36.                  "  upward, downward, and lateral
"   the, 263.                                 pressure of, by what caused, 81.
Fixed Stars, 258.                          "  when discharged with greatest veFlap of an electrical machine, 203.             locity, 90.
Flavio de Melfi, 237.                      "  why they cannot be formed into
Flavio Gioia. See Flavio de Melfi, 237.         figures, 74.
Florence, experiment at, to illustrate the Flying, effected by action and reaction,
impenetrability of water, 20.           36.
Fluid, a, 73.                            Fly-wheels, 60.
"  an aeriform, 97.                    Focal distance of a lens, 172.
"  aeriform explained, 19.             Focus, imaginary, 162.
"  form of bodies, cause of, 74.         "   of a concave mirror, 163.
"  pressure of a, in proportion to its   "   of rays of light, 153.
height, 82.                        "   of sound, 129.
Fluidity of Air, experiments showing  Fog and dew, difference between, 104.
the, 119.                         "  how produced, 104.
"'  of air shown by means of the    "  why it gives a red appearance to
Air-pump, 112.                         the Sun, 187.
"   the consequence of independent  Force, 30.
gravitation, 100.                  "   centrifugal, 41.
Fluids communicate pressure, 83.           "   centripetal, 41.
"  as conductors of sound, 127.         "   of projection, 40.
"discharged more rapidly by means  Forces, central, 41.
of pipes, 91.                       "   operation of equal and unequal,
discharged with greater rapidity             at different angles, 40.
when warm, 91.                      "   which impede a moving body,
"  downward, upward, and lateral                22.
pressure of, how shown, 78.      Forcing Pump, 92. 107.
"  elastic, 98.                          " "         attached to the diving"  floating on other fluids, 85.                       bell, 105.
gravitation of the particles of, 74.  Fountain, Artificial, simple mode of
"  immense pressure they are capa-                making, 94.
ble of exerting, 83.                 "     Hero's, 118.
"  lateral pressure of, to what pro-      "     Glass and Jet, 114.
portioned, 80.                   Fountains, 22.
"  level or equilibrium of, 76.           "     how formed, 94.
"particles of, how arranged, 77.      Fracture of glass chimneys of lamps,
particles of, their peculiar gravita-   how prevented, 139.
tion, 77.                        Franklin, (Dr.,) inventor of Lightning"particles of, when at rest, 78.        rods, 216.
"  preserve their own equilibrium, 76. Franklin, (Sir John,) 237.
"  pressure of, 77.                   Free heat, 134.
"  pressure of, dependent solely on  Friction, 69.
depth, or the height of the          "    allowance made for, in calcucolumn above, 80.                           lating the power of a ma"  pressure of, in what direction ex-            chine, 69.
erted, 78.                           "    by what caused, 69.
pressure of, to what proportioned,     "    kinds of, 69.
78.                                  "    how diminished, 69.
"  prone to expansion, 74.                "    how it increases, 69.
"  subject to the same laws as liquids,   "   the rolling, 69.
98.                                  "   the sliding, 69.
"  subjected to a peculiar kind of at-    "    wheels and rollers, 69.
traction, 74.                    Frictional Electricity, 196.
14




314                                INDEX.
Frog, electrical effect produced on, by  Gases, how  confined in solid bodies,
Galvani, 218.                            122.
Fruit, shrivelled, how restored by the  Gauge of an Air-pump, 111.
Air-pump, 115.                         Gearing, 97.
Fuel, contains heat before it is ignited,  Geese, guns suitable for, 46.
22.                               Gemini, 263.
"simple substances of, not destroyed  General law of chemical action, 221.
by combustion, 22.                Georgium Sidus, or Herschel, perihelion
Fulcrum, 53, 54.                           of, 266.
"    its position to gain power, 55.   Gibraltar, old and new, watchword heard
Fulton, 145.                               between, 130.
Fundamental principle of the laws of  Glass, (Bladder,) 113.
motion, prevails in hydrostatics, 82.    Glass chimneys, how  preserved from
Fusee, 71.                                         fracture, 139.
"   expansion and contraction of, by
heat, 139.
Galaxy, or Milky Way, 286.                 "   (Fountain) and Jet, 114.
"'  of what composed, 286.              "   Hand, 113.
Galileo, 51.                               "   India-rubber, 113.
Galvani, Dr. Aloysius, 218.                "   multiplying, 191.
Galvanic action, effects of, on what de-    "   of what use in mirrors, 161.
pendent, 230.                   "   Plates of " the  Boston  School
"    action, essential principle of,         Set," 76.
224.                             "   purest not free from imperfection,
"    action, substance submitted to,    *    161.
where placed, 224.               "   specific gravity of, 87.' Battery, 223.                        "   why so easily fractured, 139.
"'      "    action of a spark from,  Glauber Salt, 227.
229.                 Globe, how to find constellations with,
6"      "    importance of the, 231.   299.
"'      "   powerful effects ofthe,  Goat, The, 263.
229.                  Gold, specific gravity of, 87.
"    Batteries, 222.                   "  the most malleable of metals, 24.
"    circle, 219.                    Governor, (in mechanics,) 72.
"    circle, essential elements of,      "     of a steam-engine, how ap220.                                       plied, 147.
"    circle, the simplest, 220.          "     of a steam-engine, &c., by
"    effects, how increased, 222.                 whom invented, 145.
"    electricity, 196.               Grammar of Elocution, Dr. Barber's,' electricity, conductors of, 219.    131.
"    electricity, how it differs from   Gravitation, attraction of, 23.
that excited by friction, 218.      "      law of universal, 259.
"    electricity, how produced, 218.       "      of solids and  fluids dif
" electricity, how  usually ob-                      ferent, 76.
tained, 220.                  Gravity, 25. 98.
"    electricity, perfect and imper-     "   causes some bodies to rise,
fect conductors of, 219.                 others to fall, others to remain
"    electricity, how it differs from           stationary, 30.
that obtained by friction, 229.   "   centre of, in the human body, 47.
"    piles, 222.'   force of, unaffected by that of
Galvanism, 17, 18. 196.                             projection, 43.
science  of, its  effect on       "   how all substances are affected
kindred sciences, 229.                by, 28.
"     or Voltaic Electricity, 218.      "   of air, experiments illustrating
Galvanometer, 244.                                  the, 112.
Gas, 98.                                     "   of air, shown by means of the
Gases, elasticity of, 24.                           Air-pump, 112.




INDEX.                                  315
Gravity, specific, 29.                   Heat, electricity developed by, 255.
"   where most powerful, 26.           "  exception to expansive force of,
Great Bear, 299.                                 135.
Grouse, guns suitable for, 46.             "  free and latent, 134.
Grove's Battery, 227.                      "  high degrees of, how measured,
Guinea and Feather Drop, 119.                    138.
Gun, (the Air,) force of the, 118.         "  how propagated, 136.
"         "    principle of the, 118.    "  how reflected from bright or pol"  where it produces little noise, 124.        ished surfaces, 141.
Gunnery, science of, 45.                   "imponderable, 132.
Gunpowder, force of, 45.                   "  its effect on metals, 86.
Guns, long and short, comparative ad-    "  its nature unknown, 132.
vantage of, 46.                    "  its tendency to equal diffusion,
"   for sporting, length of, 46.               135.
Gymnotus Electricus, 216.                  "  laws of, 132.
" measure of, 101. 103.
" jnost obvious effect of, 138.
Hail, 22.                                  "  of summer, cause of, 269.
"  and snow, how  they differ from        "of the sun, effect of the, on the
rain, 86.                                earth's surface, 103.
"  how formed, 86. 104.                   "  operates in opposition to gravity
Hair-spring of a watch, 53.                      and cohesion, 99.
Halley's (Dr.) prediction of the return of    "  or caloric, 134.
a comet, 285.               "               passage of, irresistible, 133.
Hand-glass, 113.                           "  principal effects of, 134.
Handles of tea and coffee pots, why    "  produced by compression, 137.
made of wood, 137.                       "  radiation of, 136.
Harding, Mr., his discovery of the planet    "  reflected, 137.
Juno, 279.                               "  retained longest by those bodies
Hare, (Dr.) inventor of the Calorimo-            which take the longest time to
tor, 231.                                      acquire it, 141.
Harmattan, 109.                            "  sources of, 135.
Harmony, science of, on what founded,    "  when the greatest on the earth's
126.                                           surface, 270.
Harvest Moon, 292.                         "  why apparently unequal in bodies
Hatchets, 67.                                    of the same temperature, 135.
Hearing-trumpets, 129.                   Heated room, Sir Joseph Banks' experiHeat absorbed by dark bodies, 137.         ments in, 136.
at the focus of a convex lens, 173.  Heavenly bodies attract the earth, 259.
"  best conductors of, 136.                    "       "   diameter of, how  ex"  and cold, their effect on density,                     pressed, 161.
140.                                 "       "   motion of the, 266.
a" nd cold, as relative terms, 134.        "       "   motion of, why invisi"  cause of the fluid form of bodies,                     ble, 161.
74.                                  "       "   never seen in their true
"conducted with greater facility by                       position, 169. 287.
some substances than by others,      ","   obedient to the laws of
135.                                                motion, 259.
"  conducting power of, in different       "       "   situation of, calculated
substances, 136.                                    from the centre of the
conduction of, 136.                                   earth, 288.
"  effect of on air, 133.             "            "   when unaffected by re"  effects of on the particles of fluids,                fraction, 169.
91.                              Heavens, appearance of the, how to be*' effect of on the particles of some              come acquainted with, 300.
kinds of substances, 133.            "    why they appear bright in the
"  effect of on water, 133.                        daytime, 170.




316                              INDEX.
Heaviness or weight of bodies, by what Horses, how made to draw unequal porcaused, 25.                             tions of a load, 58.
Heavy bodies, 29.                       Horse-shoe Magnet, 235.
"     4"  how made to float in the        "         "   how to be kept, 239.
air, 28.               Horse-shoe Magnets, how made, 239.
Height, 21.                             Human Voice, Dr. Rush's work on the'    of a building estimated by a                     Philosophy of the, 131.
falling body, 33.                "      "    imitative power of the,
"    of mountains ascertained by the                   131.
barometer, 102.                  "     "    sound of the, how proHeliacal Ring, 249.                                      duced, 130.
Heliography, 193.                       Humor, the aqueous and the vitreousg
Helix, 244. 247.                          175.
"  on a stand, 247, 248.             Humors of the eye, 175.
Hemispheres, the Magdeburgh, 112.            "       "'   shape of the, 177.
it     (Otto Guericke's,) kept to-  Hunter's moon, 292.
gether by the pressure  Hurricanes, how produced, 108.
of the air, against the  Hydraulics, ]7, 18.73. 90.
force of thirty horses,  Hydro-electric current, 255.
113.                     Hydrogen, a constituent part of water,
Henry, (Prof.) his first application of               19.
Electro-magnetism as a moving pow-         "     Gas, convenient mode of proer, 253.                                                curing, 209.
Henry, (Prof.) powerful electro-magnet       "       "  specific gravity of, 87.
formed by, 249.                            "     Pistol, 209.
Hero's Fountain, 118.                        "        "   how  used with the
"   Ball, 118.                                           thunder-house, 211.
Herschel, (or Uranus,) 281, 282.        Hydrometer, 89.
"    diameter of, 261.              Hydrostatic Bellows, 82.; distance from the sun, 260.            "      paradox, 81.
"     length of day unknown, 260.        "      press, 84.
length of the year of, 260.   Hydrostatics, 17, 18. 73.
perihelion of, 266.                 "      principal truths of, by what
"     (Dr.) number of stars seen by                experiment  illustrated,
in a quarter of an hour,                  117.
286,                        Hygrometer, 101. 103.
"     Sir J., his illustration of the
comparative size and distance of the heavenly bo-  Ice, 74.
dies, 261.                   "consequence of the specific gravity
Sir J., his calculation of the       of, 140.
distance of the stars, 286.    "  specific gravity of, 87.
"     Sir John's opinion of  the   "  why it floats in water, 140.
height of the atmosphere,  "  why wrapped in woollen cloths in
28.                               summer, 136.
"     Sir J. W. F.'s treatise on as-   "  and snow more easily melted under
tronomy, 262.                     a dark covering, 137.
Hesperus, 277.                          Ichthyology, 13, 14.
Heterogeneous, 19.                      Identity of common, voltaic, magnetic,
Hiero, King of Syracuse, 70.              thermo and animal electricity, 240.
High-pressure Steam-engine, 145.        Image inverted, how produced, 159.
High tones of musical instruments, how     "   of objects reflected from  convex
caused, 125.                                    mirrors smaller than the obHomogeneous, 19.                                 ject, 163.
Hoops, how secured on casks, &c., 138.     "   seen through telescopes of differ
"Horse Power," meaning of the term as            ent kinds, 183.
applied to the steam-engine, 145.        "  when inverted, 163.




INDEX.                                  317
Images inverted on the retina of the  Juno, distance from the sun, 260.
eye, why seen erect, 179.                i  length of day, 260.
Imaginary focus, 162.                      "  length of the year of, 260.
Impenetrability, 19.                      i"  perihelion of, 266.
"      of the air, 104.           Jupiter, 257.
"      of air and of water, exper-   "    and Saturn, disturbances of their
iments illustrating, 20.           motions, 283.
c      possessed by fluids, 99.      "    diameter of, 261.
Imperfect conductors of electricity, 219.    "    distance and size of his moons,
Incidence and reflection, angles of, 157.           280.
"    angle of, 37. 128.                "    distance from the sun, 260.
Incident and reflected, 36.                 "    eclipses of the satellites of, and
"    motion, 36.                                their use, 280.
Inclination of the earth's axis, important  "    heat and light of, 280.
consequences of, 271.                     "'  length of day, 260.
Inclined Plane, 54. 66.                     "    length of the year of, 260.
" "   application of the, 67.            "    moons of, 258, 280.
it"    "   advantage gained by the       "    perihelion of, 266.
use of the, 66, 67.        "    satellites of. See Table 2d, page
Incombustible substances, 138.                      302.
Indestructibility, 19. 21.                  "    the largest of the planets, 280.
India-rubber, 24.                           "    why no change of the seasons
it    "    Glass, 113. 115.                       in, 280.
Inertia, 19. 22. 30.                     Jupiter's belts, 281.
Inertia of Air, experiments showing, 119.
"'  "    shown by means of the
Air-pump, 112.           Kaleidoscope, 191.
Inferior Conjunction, 265.               Keeper of a magnet, 235.
"    Planets, 260.                    Kepler's law, 267.
Instruments for raising water, 92.       King's (Dr.) Lightning-rods, 214.
4"  on what principle construct-  Knee-joint, (in mechanics,) 73.
ed, 54.                    Knives, 67.
"      wind and stringed, sound of, Knob of an Electrical battery, 200.
how affected, 126.
Insulated, (in electricity,)197. 205.
Intellectual Philosophy, 13.             Ladder, 58.
Intensity of electricity from  a galvanic  Lamps, glass chimneys of, how  pre.
battery, 228.                           served from fracture, 139.
"    of chemical agency in galvan-       i    wicks of should be often reism, how increased, 230.                newed, and why, 75.
Introduction, 13.                            "    why they sometimes fail to give
Invisible, when an object becomes, 160.             light, 75.
Iris, 175, 176.                          Lantern, the magic, 182.
Iron, 70.                                La Perouse, 237.
"  how made magnetic, 238.              Latent heat, 134.
"  specific gravity of, 87.             Lateral pressure of fluids, to what pro"  tenacity of, 25.                       portioned, 80.
Ivory, elasticity of, 24.                 Lathes, 63.
Law of falling bodies, 33.
Laws of Natural Science, general and
Jacket, in the Steam-engine, 146.          particular, foundation of, 15.
Jeffrey, Mr., 145.                        Lead, specific gravity of, 87.
Jet, the straight and revolving, 118.     Leaden hemispheres to show cohesive
Joint, the toggle, or knee, 73.            attraction, 23.
Jointed Discharger, 200.                 Leap-year, 299.
JIuio, 257. 279.                         Leaves of an axle, 65.
"  diameter of, 261.                   Length, 21.




318                                INDEX.
Length of the days and nights, cause of Light, intensity of diminished by reflecthe difference inl the, 266.                   tion, 157.
Lens, 170.                                 "   non-optical effects of, 191.
"   axis of a, 171.                      "   of what composed, 186.
"  double concave and double con-    "   passing from one medium to anvex, 171.                                other, 168.
"  double convex, made by Mr. Park-    "   passing through different medier, effect of, 173.                      ums, 170.
d   focal distance of a, 172.            "   refraction of through a prism, 187.
"  how made both to magnify or to    "   (reflected,) fundamental law of
diminish, 182.                           157.
"  single concave, 17 1.                 "   (reflected,) laws of, 156.
"     "   convex, 171.                   "   Sir Isaac Newton's opinion of,
the Crystalline, 175.                      153.
"c  to estimate the effect of, 172.      "   Thermal effects of, 191
Lenses, achromatic, 186.                   "   time occupied by, in its passage
"    effect of peculiar form of, 171.         from the sun to the earth, 169.
"    single and double convex, focal          280.
distance of, 172.                "   various theories concerning its
"   various kinds of, 171.                    nature, 153.
"   which have the greatest magni-    "   velocity of, 154.
fying power, 180.                "   what portion of useful in vision,
"    which have the shortest focus,           177.
180.                             "   when said to be reflected, 156.
i"   why made small, 180.               "   why some animals require but
Leo, 263.                                        little, 176.
Level or equilibrium of fluids, 76.        "   zodiacal, 276.
Lever, 54.                              Lightning, 212.
"    an appendage to the screw, 68.        "     and electricity, identity of
"   applied to hydrostatic pressure                proved, 216.
in Bramah's Press, 84.              "    rods, 201.
"    different kinds of, 54.               "       "  extent of the security
compound, 57.                                       of, 214.
"    mechanical advantage of the, 54.      "       "  should not be painted,
"    of the first kind, instances of,                    214.
57.                                 "       "  security of shown by
"    of the second kind, 57.                            the  thunder-house,
"    of the third kind, 58.                              211.
Lewis (Dr.) Winslow, 178.                    "       "  square   better  than
Leyden Jar, 199.                                          round ones, 214.
how charged either posi-  Line of direction, 47.
tively or negatively, 205. Linen, why it readily absorbs moisture,
"   jars, charged from the clouds             75.
by Franklin, 216.                 "   garments, why cool, 136.
Libra, 263.                             Lion, The, 263.
Light, aberration of, 287.              Liquid, how it differs from a fluid, 73.
"   absorption of by the atmosphere,    "    form of bodies partly dependent
158.    -                                on the pressure of the air, 122.
"   absorbed by all bodies, 157.      Liquids have little elasticity, 24.
"   always of the same nature, 190.     "    press according to height, appa"   bodies, 29.                                ratus to illustrate, 83.
cannot exist without color, 190.   List of works consulted in the prepara"   chemical effects of, 98. 188. 191.   tion of this volume, 8.
193.,                           Lithology, 13.
"   colors which enter into the com-  Living men, specific gravity of, 87.
position of, 186.               Loadstone, 232.
how projected, 154.               Locomotive Steam-engine, 148.




INDEX.                                319
Locomotive Steam-engine, internal sec-  Magnet, properties of the, 233.
tion of Hinkley and Dru-     "    properties of, how acquired by
ry's, 149.                        soft iron, 232.
"     Steam engine, operation of    "    the U, or Horse-shoe, 235.
the described, 148.          "    variation of the, 236.
Logy, its meaning as an affix, 13.      Magnetic attraction of iron, &c., how
Longitude determined by eclipses of the             prodigiously increased, 241.
satellites of Jupiter, 280.                "    magazine, 239.
Loss of weight in bodies immersed in         "    needle, variation of the, does
water, 88.                                         not impair its utility, 238.
Low-pressure Steam-engine, 145.              "    needle, variation of the, long
Low tones of musical instruments, from              misunderstood, 236.
what they proceed, 125.                    "    needles, how made, 239.
Lucifer, 277.                                "    north pole of, most powerful
Luminous bodies, -152.                              in north, and south pole in
I"      "   how seen, 157.                       south latitude, 238.
Lunar eclipses. 294-297.                     "    pole, revolution of a, 244.
Lungs, state of the, how ascertained by      "    poles, 233.
physicians, 127.                           "       "  the earth has four, 237.
"c "    awhere strongest, 237.
Machine, force of a, communicated to         "    power of the Magneto-Elecanother at a distance, 84.                        tric  machine, when  the
Machines, power of, how limited, 70.                 greatest, 255.
" on what principles construct-          "    power resides on the surface,
ed, 54.                                 and  independent of the
Magazine, magnetic, 239.                            mass, 235,
Magdeburgh cups, or hemispheres, 112.        "    power readily acquired by
Magic Lantern, 182.                                 soft iron, and with difficulty
Magnet, attraction of the, 234..        by hard iron or steel, 235.
attractive power of the, where      "    substances, 234.
strongest, 233.                   "    Telegraph. See Electro-llag"    being broken, each part be-                  xetic Telegraph, 251.
comes a perfect magnet, 235.      "    Telegraph, Alphabet of the,
communication of the proper-                252.
ties of a, 234.                   "    Telegraph, Battery used for
different poles of attract, and             the, 227.
similar repel each other, 234. Magnetism, 17, 18. 232.
"    dipping of a, 236.                   "      and Electricity, combined
"    electrical properties of the, 233.            action of, 241.
"    electricity induced by a, 241.       "      and   Electricity,  resem"    experiments with the, 239.                    blance between, 235.
"    how supported, 233.                  "      by induction, 253.
"    permanent, 232.                      "      communicated by the elec"    powerful one formed by Prof.                  trical machine, or by a
Henry and Dr. Ten Eyck, 249.               voltaic battery, 243.
"    polarity of the, 233.                "      communicated by the Elec-'    polarity of the, analogous to                 tro-Magnet, 250.
the forms of electricity, 234.     "      developed in steel, 240.
"    poles of a, and where situated,      "      how induced by the voltaic
233.                                       battery, 244.
"    position of a, in reference to       "      how removed, 250.
the electric current, 242.         "      induced by electricity, 241.
power of destroyed or weaken-        "      its importance to commerce. ed by heat, 236.                          and navigation, 237.
" power of, increased by action,          "      produced by concentrated
and impaired or lost by dis-               electricity, 242.
use, 234.                     Magnetizing power of the conducting




320                                INDEX.
wires of a battery, how increased, Mars, length of day in, 260.
247.                                    "  length of the year of, 260.
Magneto-Electric machine, mode of ta-    "  perihelion of, 266.
king shocks from,  Materiality of air, experiment illustrating
255.                  the, 116.
"      "    effects, how obtained  Mathematical Science, Principles of,
most   powerfully,   requisite for the study of Natural Phi253.                  losophy, 247.
"     machine, 254.          Matter and its properties, 18.
Magneto-Electricity, 17, 18. 232. 253.     "   attractive, 23.
Magnets, artificial, how made, 238.        "  attraction of, 25.
do not point exactly to  the    "   composed of particles, 23.
north, 236.                     "   organized and unorganized, 14.
"    effects produced by two, more     "  quantity of. in bodies how ascerthan double that of either           tained, 29.
one, 236.                       "  terms used to express the state
formed by voltaic electricity,         in which it exists, 19.
248.                         Measure of heat and cold, 103.
k" inds of, 232.                        "    of the heat of the atmosphere,
"    lose none of their power by               101.
communicating their prop-        "    of the moisture of the atmoerties, 238.                            sphere, 101.
"    most substances may be con-        "    of the weight of the atmosphere,
verted into, 241.                       101.
"    permanent, are of iron, or  Measurer of electricity, 204.
some compounds that con-  Mechanic arts explained, 16.
tain it, 234.                Mechanical science explained, 16.
Magnifying-glass, 180.                       "      Paradox, 48.
Magnifying power of microscopes, how         "      Powers, 53, 54.
computed, 182.                             "          "    their use, 70.
Magnitude, centre of, 41.               Mechanics, or the Laws of Motion, 17. 30.
Mahogany, specific gravity of, 87.           "     fundamental principle of, 55.
Main-spring of a watch, 71.             Media, 167.
Malleability, 19. 24.                   Medium, 70.
Man incapable of flying, and why, 36.       "    dense and rare, 167.
"  of common stature, at what dis-        "    in Optics, 154. 167.
tance invisible, 160.                "    resistance of a, to what proMarble, specific gravity of, 87.                    portioned, 71.
"   why  apparently  colder than  Meniscus, 171.
other substances in the same  Mercurial or water tube, 114.
room, 135.                    Mercury, 257. 276.
Marco Paolo, 237.                           "    appearance of, when viewed
Mariner's Compass, 237.                             through a telescope, 277.
"    description of the,       "    diameter of, 261.
238.                   "    distance from the sun, 260.
te"      "    invention of the, 237.    "    heat of the sun on this planet,'I "    value of the inven-                     276.
tion of, 237.          "    in the barometer, when it will
"    why  balanced  on                ascend and descend, 101.
two sets of axes,      "    length of the year of, 260.
238.                   "    length of day in, 260.
Mars, 257. 279.                             "    perihelion of, 266.'i  cause of the red appearance of,       "    revolution of, 259.
279.                                "    specific gravity of, 87. 95.
"  diameter of, 261.                      "    why seldom seen, 276.
"  distance from the sun, 260.            "    why used in the barometer,
"  heat and light of, 279.                        102.




INDEX.                                321
Meridians, 271.                         Moon, diameter of the, 261. 278.
Metallic points, use of in electricity, 201.    "   difference in the rising of the, 291.
Metals, conducting power of, in their    "  does not affect the changes in the
order, 136.                              weather, 289.
"   specific gravity of, affected by    "   eclipsed by a comet, 285.
heat and cold, 86.               "   entire image of, reflected from
"   the best conductors of heat, 136.         every portion of a sheet of wa
"   why tarnished in parts where             ter on which it shines, 158.
soldered, 230.                   "   has no change of seasons, 289.
Metaphysics, 13, 14.                       "   has no atmosphere, and therefore
Meteoric Stones, 289.                            cannot be inhabited, 289.
Meteorology, 13.                           "  its density, 289.
Microscope, single, double, and solar,    "  its effect on the tides, 292.
180, 181.                   "   its revolution and distance from
"      effect of a, 180.                   the earth, 289.
"      a single, 180.                "   its revolution on its axis and
Microscopes, magnifying power of, how            around the earth, 278.
computed, 182.                           "  its surface volcanic and mounMilk, specific gravity of, 87.                   tainous, 289.
Milky Way, 286.                            "  length of its days and nights, 289.
Mills, 95.                                 "  maps of the surface of, 291.
Mineralogy, 13, 14.                        "  popular errors concerning, reMinus, as applied in electricity, 195.           futed, 289.
Mirror, 161.                               "   signs used in Almanacs, &c., to
"   concave, 162.                             designate its phases. 291.
"'          "    Buffon's, 167.         "  the Harvest and the Hunter's, 292.
is      "'  objects reflected from,    "  uninhabitable, 283.
164.                    "  what supposed to be, 258.
"       "    true focus of, 163.        "   when said to be horned, in quad
Mirrors, concave, effects of, 164.               rature, or gibbous, 290.
"'    "'    and convex, facts with    "   why it always presents the same
regard to, 165, 166.          side to the earth, 289.
convex, 162.                       "  why one half is never seen, 279.
half of the height of an object  Moon's rays have no heat, 289.
will reflect the entire object, Moons, 257.
157.                              "   or satellites, general law of, 282.
"   kinds of, 161.                   Moonlight, objects seen by, why fainter''  of different kinds, their peculiar-   than when seen by daylight, 158.
ities accounted for, 157.      Morning Star, 277.
plane, 161.                      Morse's Electro-Magnetic Telegraph,
"   why made of steel, 161.                      251.
Miscellaneous experiments, depending        "    Telegraphic Alphabet, 252.
on two or more properties of air, 120.  Motion, axis of, 41.
Mist or vapor, how formed into drops of    "   caused by action and reaction,
water, 133.                                     36.
Mobility, 19. 24.                          "   centre of, 53.
Modern science, simple substances of, 18.  "    circular, 40, 41.
Moisture raised from the ground by the      "   compound, 39.
action of the heat of the sun, 103.       "   curvilinear, 42.
Momentum, and how measured, 34.;"   defined, 30.
Moon, 257.                                 "   duration of, unaffected by quan"   See Table II., page 302.                   tity, 34.
"  appearance of the, through a tele-    "    fundamental principle of the
scope, 291.                               laws of, prevails in hydro"   axis of the, perpendicular to its           statics, 82.
orbit, 289.                        "   how  reversed in  the use of
"   changes of the, 290.                        wheels 66.
14*




322                                I N DEX.
Motion, impelled by a single force, 30.   Natural Philosophy, principal branches
"   impelled by two or more forces,            of, 17.
39.                              "    springs, how formed, 94.
"   incident and reflected, 36.      Nautilus, 129.
laws of, 30.                    Naval engagements, distance at which
of animal or mechanical power,   heard, 130.
how impeded, 22.              Neap Tides, 294.
"    of the heavenly bodies not uni-  Near-sighted (or short-sighted) person,
form, 266.                      glasses suitable for, 175.
of the heavenly bodies, why in-  Needle of the Mariner's Compass, where
visible, 161.                   placed, 238.
of the heavenly bodies, why so  Negative electricity, 195.
regular, 27.                      "        "      how obtained from
perpetual, 33.                                         the   electrical
"   produced by wheels, different                           machine, 203.
directions given to the, 66.  Negatively electrified, 195.
"   rapid or slow, how produced by  Nelson, 237.
the same mechanical power,  Neptune, 247. 283.
65.                               "    (the planet,) by whom discov"    rectilinear changed to circular,            ered, 257.
and circular to rectilinear, 64.  "    distance from  the sun un"    resultant, 50.                              known, 260.
"   terms applied to, 32.                "    length of day unknown, 260.
"    uniform, retarded, and accele-      "    length of its year unknown,
rated, 32.                                260.
"    when imperceptible, 161.            "    moons of, 258.
Mountains, height of, ascertained by the  Nerve, the optic, 175.
barometer, 102.                       Newburyport, whispering gallery in a
Mover of heat, 231.                       church of, 129.
Multiplying-glass, 191..       Newcomen, 145.
Music, why it sounds better in bare'    and Savery's steam-engine,
rooms, 130.                                 142.
I"   difference between theory and  Newton, Sir Isaac's, opinion respecting
practice of, 16.                          the tails of comets, 284.
"   its theory a science, its practice   "    Sir Isaac's, discovery of gravan art, 16.                               itation, 51.
Musical chord, how obtained, 126.           "    Sir Isaac's, discovery with re"   instruments, different tones of,            gard to the heterogeneous
how  produced,               quality of light. 186.
126.'e    Sir Isaac, his opinion of the
"' " t           tone of, why af-               porosity of the earth, 24
fected  by  the  Niagara, Falls of, 189.
weather, 127.     Nickel, possessed of magnetic proper-'*       " (     wind and stringed,   ties, 232.
difference   be-  Night and day, cause of, 273.
tween, 126.        "   cause of the length of, 273.
"   strings, to what they owe their  Night-glass. 184.
sonorous properties, 125.     Nitrogen, 98.
Mutual attractions, 26.                 Nodes of a planet, 265.
Non-condensing steam-engines, 145.
Non-conductors of electricity, 196.
Nati e Magnet or Loadstone, 232.               "        or electrics, list of, 196.
Natural History, 14.                    Non-electrics, 194.
"    Philosophy, 14, 15, 17.         North pole, 271.
rr" " and Chemistry, dif-                 "      of a magnet most powerful
ference between                 in north latitudes, 238
15.                 "   wind, how produced, 108.




INDEX.                                323
Northern Lights, 212.                   Oxygen, t8.
Nucleus, 283.                               "    a constituent part of water, 19.
N ut of a screw, 68.
Pails of water, why two can be carried
Oar, why it appears bent when partly in    more easily than one, 49.
the water, 169.                       Pallas, 257. 279.
Oars, 58.                                 "    diameter of, 261.
Objects, how seen, 158.                   "    distance from the sun, 260.
"    further from a concave mirror    "    length of the year of, 260
than its focus, inverted, 163.   "    length of day in, 260.
"    how seen by the eye, 178.         "    perihelion of, 266.
"   reflected fromn a concave mirror,  Parabola, 45.
164.                           Paradox, Mechanical, 48.
"    seen only in the direction of re-   "    the Hydrostatic, 81.
flected rays, 162.                 "    the Pneumatic, 123.
"    when invisible, 160.            Parallax, 288.
Obtuse angles, 38;                      Parallelogram, 38.
Occultation, perpetual circle of, 288.  Parallel motion, appendages for, in the
Octave, chord of an, how obtained, 126.            steam-engine, by whom ap(Ersted, Professor, his discovery  of              plied, 145.
electro-magnietic power, 240, 241.       "    rays, effect of concave surfaces
Oil, specific gravity of, 87.                      on, 174.
" used for smoothing the surface of    "    rays reflected from a concave
water, 92.                                   surface, 166.
"  used for smoothing waves, 92.          "    rays reflected by convex mir"  why it ascends in the wick of a                rors, 165.
lamp, 75.                            "    rays,when made to diverge, 173
"  why it cools more speedily than  Parallels of latitude, 272.
water, 141.                        Parker's Exercises in Rhetorical Read
why used in machinery, 69.            ing, 131.
Olbers, Dr., his discovery of the planets  Parry, 237.
Vesta and Pallas. 279.                Particles, all matter composed of, 23.
Opaque bodies absorb part of the rays       "    of matter, 19.
of light thrown upon them,         "    of a fluid, small, smooth, and
156.                                      globular, 74.
"   bodies, how seen, 157.           Partridges, guns suitable for, 46.'  substances, 153.                  Parts of a body which move with the
Opposition, 265.                          greatest velocity. 42.
Optic Nerve, 175. 178.                  Paxton's Introduction to the Study of
Opticians, their mode of fitting glasses    Anatomy, 178.
to the eye, 175.                      Peculiarities of this volume described, 5.
Optics, 17, 18. 152.                    Pencil of rays, 154.
Oracle of Delphi and at Ephesus, 132.    Pendulum, 51.
Orbits, 259.                                 "      Ballistic, 45.
"    of the planets elliptical, 260.      i"    how and by whom invented
"    of the planets, inclination of,                51.
shown by a section of the           "     length of, to vibrate seconds
plane of the ecliptic, 265.                 in different latitudes, 52.
"    of the planets inclined to that of    "      vibrations of, on what dethe earth, 263.                             pendent, 52.
"    of the planets, oblique view of       "     vibrations of, how caused, 51.
the inclination of the, 263.   Penumbra, 296.
Organized matter, 14.                   Perfect conductors of electricity, 219.
Otto Guericke, the inventor of the air-  Perigee, 266.
pump, 113.                            Perihelia of the planets, 266.
Overshot water-wheel, 96                Perihelion, 265.




3924                              INDEX.
Periscopic lenses or glasses, 171.      Planets, perihelia of the, 266.
Perkins, Jacob, 73.                         "    Primary, table of the size, disPermanency of action in a Sulphate of              tance, bulk, density, diameCopper battery, how increased, 225.              ters, &c., of the, 301.
Permanent artificial magnet, 232.           "   their orbits elliptical, 260.
Perpendicular lines, 38.                    "    the primary, 257.
it      motion upward and down-       "    vary in their distance from  the
ward, time of, 33.                 sun, 260.
"      to a concave or convex         "    what supposed to be, 258.
mirror, 165.            Plate electrical machine, 202.
Perpetual apparition, circle of, 288.   Platinum, specific gravity of, 87.
" Motion, 33.,"    weight of a cubic foot of, 87.
is" " attempted by electri-             "    the most ductile of metals, 25.
cal action, 223.   Plough, 299.
"       is"  impossible, 33.        Plumb-line, 27.
Perspective drawing, to what indebted,  Plus and minus as applied in electricity,
160.                                     195.
Phillip's Lectures on Astronomy, 292.    Pneumatic Balloon, 115.
Philosophy, explanation of the term, 13.      "     Paradox, 123.
"     of the human voice, (Dr.          "     Scales for weighing air, 114.
Rush's,) 1]31.                 "     Shower-bath, 120.
Philosophical knowledge, foundation of,  Pneumatics, 17. 19.
15.                            "     experiment illustrating the
9"      apparatus of the Boston                  principal truths of, 117.
Schools, Schedule of, 9.  Pointers, 299.
t"      apparatus of the Boston  Pointed conductors, effects of, 214.
Schools, 7.              Points, metallic security of, explained
Phosphor, 277.                             by means of the thunder-house, 211.
Photography, 193.                        Poker, 37.
Physics, 13, 14.                         Polar regions move slowest in the daily
Physical science, 14.                           revolution of the earth, 42
Physicians, their mode of ascertaining    f  Star, or Pole-star, 299.
the condition of the lungs, 127.       Polarity of the magnet, 233.
Piazzi, his discovery of the planet Ceres,  Poles of a magnet, 233.
279.                    "                    of a magnet, where situated, 233.
Pigmentum nigrum, 178.                   Pope Callixtus' anathema of a comet,
Pincers, 57.                               285.
Pinion, 65.                              Pores, meaning of, 23.
Pipe, Steam and Eduction, 143.             "   of bodies filled with air, 23.
Pisces, 263.                             Porosity. 19.
Piston-rod of a steam-engine, how made  Porous substances, 75.
to rise, 142.                          Porter and ale, why drunk from pewter
Pith, how obtained, 210.                   cups, 230.
Plain mirrors, 161.                      Positive electricity, 195.
Plane and smooth surfaces reflect sound      "   electricity, how obtained by the
without dispersing it, 130.                      electrical machine, 203.
Planet, meaning of the term, 258.        Positively electrified, 195.
"    supposed destruction of a, 279.   Powder, great charges of, useless and
Planets, 257.                             dangerous, 46.
"   axes of the, not perpendicular  Power, 53.
to the plane of their orbits, 266.  "  gained by loss of time, 82.
"   and moons, nodes of, 265.            "   how gained, 61.
comparative size of the, 261.       "   of a  steam-engine, how  exhow distinguished from stars, 258.         pressed, 145.
"   Inferior and Superior, 260.       Precession of the equinoxes, 263. 300.
invisible on the planet Jupiter,  Preface, 5.
281.                           Press, Bramah's Hydrostatic, 84.




rNDEX.                                 325
Presses, bookbinders', 68.               Pyramid, why a firm structure, 49.
"   cider and wine, 68.             I Pyrometer, 138.
Pressure of fluids, proceeds solely from   Pyronomics, 17, 18. 132.
the height of the column
above, 80.
"    lateral of fluids, from what it  Quality of sound, of all musical instruproceeds, 80.                  ments, by what affected, 126.
" of a liquid exerted in all di-  Questions to this volume, 6.
rections, experiment illus-  Quimby's Electrical Sawmill, 213.
trating, 117.
"    of a fluid in proportion to the
height, 82.                  Radii of a circle, 37.
"    of fluids  dependent on the  Radius, 37.
height of the column above,     "    vector, 267.
and independentofthe length  Railroads, Steam-engine used on, 145.
or breadth of the column, 81. Rain, 22.'"   of the air on the surface of   "  how produced, 86. 104. 133.
liquids, instruments to show,   "  how it differs from snow and hail,
114..                86.
Primary Planets, 257.                      "  water, a standard for computing
Planets, table of the distance,         specific gravity, 86.
bulk, density, &c., of the, 301.    "  water, specific gravity of, 87.
Prime conductor, 203.                    Rainbow, 189.
Printing-presses, 73.                    Ram, the, 263.
Prism, 186.                              Random of a projectile, 46.
Properties, essential of all matter, 19.    Rare bodies, 23.
tt"      "    and accidental, 19.   Rarefaction, 97.
Projectile, 44.                               "      effect of, on the air, 108
"     random of a, 46.              Rarefied, meaning of the term, 97.
Projectiles, descent of, 43, 44.         Rarity, 19. 23, 24.
Projection, force of, 40.                Ray of light, 153.
Protected sulphate of copper battery, Rays, incident and reflected, 157.
227.                                     "   of light entering a small aperture,
Pulleys, power gained by use of, 60.                    158.
"   practical use of, 61.               "           "  "  from different bodies, 154.
Pulley, 34. 59.                  "             " " from terrestrial bodies, di
"   the fixed, 59.                                   verging of, 154.
"   the fixed, on what principle it    ",  "   passing obliquelyfrom one
operates, 59.                                 medium to another, 167.
"   the moveable, 59.                   - "   "   when  perpendicular to
"   the principle of the, 61.                         convex or concave mirPump, the air, 109.                                     rors, 162.
"   height to which water can be,,,    "  when said to diverge, or
raised by the, 106.                           to converge, 153.
"   the chain, 92.            "             of the sun parallel at the earth,
"   the common, 92. 106.                     172.
"   the forcing, 92.                    "  parallel, convergingand diverging,
the lifting, 107.                         reflected from concave or con"   the sucking, 106.                        vex surfaces, 165, 166.
"   boxes, 106.                         "   proceeding from one medium to
Pumps, how water is raised to the boxes         another, 168.
of, 114.                               Reaction, 35.
Pupil, 176.                                 "    caused by elasticity, 37.
"  of the eye, 175.                   Reaumur's scale for the thermometer
"  of the eye, dilatation and contrac-    103.
tion of the, in some animals, Receiver, 109.
176.                            Rectangle, 38.




326                                INDEX.
Rectilinear motion changed to circular, Romans, the ancient, ignorant of the
64.                                      principles of hydraulics, 94.
Red rays, why least refracted, 187.      Rope-dancers, how they perform  their
Reflected and incident, 36.                feats, 47.
Reflected light, fundamental law  of, Ropes, their strength tested by means of
157..    the hydrostatic press, 85.
"   law of, immutable, 157. Rudders, 58.
motion, 36.                    Rum, ingenious mode of stealing, prac"    caused by reaction, 37.   tised by slaves in the West Indies, 85.
"    rays, object seen only in the  Running water, velocity of, how ascerdirection of, 162.             tained, 91.
Reflecting telescopes, advantage of, 186. Rush's (Dr) Philosophy of the Human
" substances, 153.                    Voice, 131.
"'    surface, every portion reflects
an entire image of the luminous body, 158.           Safety-valve, 144.
"'   telescope, 183. 185.           Sagittarius, 263.
Reflection, angle of, 38. 128.           Sahara, pestilential wind from, 109.
Refracting substances, 153.              Sails of vessels, on what principle contelescope, 183.                 structed, 59.
Refraction, effect of, in lengthening the  Saint Paul's Church, London, whisperday, 170.                    ing gallery of, 129.
"      effect of the laws of, 173.    Salt and sugar, particles of, smaller than
"      its effect on the appearance       those of water, 20.
of the heavenly bodies,'water, why heavier than fresh, 87.
169.                       Samiel, or Simoom, 109.
"     of light, 167.                Satellites, or moons, general law of, 282.''    of the atmosphere, effect of  Saturn, 257.
the, 170.                     "    degree of light and heat in, 281.
"     to what proportioned, 169.       "    diameter of, 261.
Refractive power of the atmosphere,    "    distance from the sun, 260.
effect of, on the length of the day,    "    how distinguished from the oth287.                                             er planets, 281.
Refrangibility of light, 167.               "    length of day in, 260.
Relative size of the bodies of the solar    "    length of the year of, 260.
system, 261.                              "    moons of, 258.
Resistance, 30. 53.                         "    perihelia of, 266.
"     of the air to falling bodies, to  "    Satellites of, See Table II., page
what proportioned, 27.               302.
Resultant motion, 50.                       "    spots or belts on, 281.
Retarded motion, 32.                     Savery, 145.
46"     "    how produced, 33.       Savery's (and Newcomen's) Steam-enRetina of the eye, 175.                    gine, 142.
Revolution, time of, called a year, 259.   Sawmill, Quimby's electrical, 213.
Revolving Jet, 118.                      Scales, 57.
Rhetorical Reading, Parker's Exercises      "   for ascertaining specific gravity,
in, 131.                                        87.
Richman, Professor, a martyr to electri- " for weighing air, 114.
cal science, 216.                         "   when suspended are not parallel,
Rifles, how tested, 45.                           27.
Right Angles, 38.. Science and art, difference between, 16.
Rivers, 22.                                 "    defined, 16.
" velocity of, 31.                       "    meaning of, 15.
Rivulets and Springs, how formed, 93.       "    of Harmony, on what founded,
Rocks, how split by means of a wooden              126.
weage, 75.                             Scissors, 57.
Rollers, Friction, 69.                   Sclerotica, 175 178.




INDEX.                                327
Scorpio, 263.                          Simple substances, Metallic and NonScorpion, the, 263.                      metallic, 18.
Screw, 54. 67.                         Singer's amalgam, 202.
"   advantage gained by the use of Single concave lens, 171.
the, 68.                         "  convex lens, 171.
"   of Archimedes, 92.                  "   microscope, 180.
"   of what composed, 68.            Siphon, 92. 95.
"   the, a compound power, 68.         -"  experiments with the, 121.
"   the concave, 68.                    "   how used, 95.
"   the convex, 68.                  Size or Bulk, 21.
"   threads of the, 68.                "  (apparent) of objects, on what deSea, pressure of the water at great depths    pendent, 160.
in the, 80.                            " of the bodies of the solar system
Seams in metals discovered by the             261.
tongue, 230.                          Sky, blue appearance of the, cause of,
Seasons, cause of the, 266.              188.:'   cause of the change of, 272.    Slaves' ingenious mode of stealing rum
Sea-water, why heavier than fresh, 87.   85.
Secondary Planets, 257.                Sliding valve of the Steam-engine, 148
",    "   table ofthe distances,  Smee's Battery, 225.
revolutions, &c., of Smoke and vapor, 22.
the, 302.          Snow, 22.
Seebeck's (Professor) discovery in Ther-    "   and hail, how they differ from
mo-Electricity, 255.                          rain, 86.
Seneca's statement with regard to a    "   and ice more easily melted under
comet, 285.                                   a black covering, 137.
Shadow, 154.                              "   how produced, 86. 104.
" darkness of a, to what propor-  Snuffers, 57.
tioned, 155.                 Soapstone, powdered, why used in ma"    of a spherical body, form of, 155.   chinery, 69.
Shadows, when larger and when smaller  Solar, meaning of the term, 257.
than the bodies which cause    "  Eclipses, 294. 297.
them, 155.                    "'  Microscope, 181.
"    why of different degrees of    "  System, 257.
darkness, 154.                 "  System, bodies of the, how repreShepherds, French, on stilts, 48.               sented in Almanacs, &c., 262.
Ships, why they float on the water, 85.   "  Year, 298.
Shower-bath, the Pneumatic, 120.       Soldered metals, why tarnished, 230.
Showmen, how they produce apparitions, Solid bodies, as conductors of sound, 127.
164.                                 Solids, gravitation of, 77.
Sidereal year, 299.                    Solstices, 275.
4" "' how measured, 300.           Somerset, Marquis of Worcester, an inSigns of the phases of the moon, used in    ventor of the Steam-engine, 145.
almanacs, &c., 291.             Sonorous bodies, 125.
of the Zodiac, 263.                   "      "   all elastic, 125.
"  of the Zodiac, as represented in      "    properties of bodies, to what
Almanacs  and  Astronomical                owing, 125.
works, 262.                     Sound, distance to which it may be con"  of the Zodiac, names of the, 263.           veyed, 130.
of the Zodiac, representation of    "   focus of, 129.
the position of, 273.              "   how   communicated  through
Silver, specific gravity of, 87.                 fluid and solid bodies, 127.
vessels, why used for heating      "   how caused, 124.
liquids, 136.                    "   of a musical string, by what
Silurus Electricus. 216.                         caused, 125.
Simoom, or Samiel, 109.                   "    of the human voice, how proSimple substances, 18.                           duced, 130.




3284                             I'NDEX.
Sound of wind and stringed  instru-  Spherical bodies,whythey roll down a
ments, how respectively pro-    slope, 48.
duced, 126.                    Spiral Tube, 208.
"   produced by strings, quality of,   "   (electrical) tube, 215.
how affected, ]26.             Spirit-level, or water-level, 76.
"    quality of, affected by the furni-  Sponge, 75.
ture in a room, 130.           Spots on the Sun, Dr. Herschel's opinion
"    science of, the nature and laws    of the, 276.
of, 124.                       Spring, hair and main of a watch, 53.
uniform velocity of, available in    "6   height of its rise, 94.
measuring distances, 128.        "    Tides, 293.
"    velocity of, 127.                Springs, 22.
Sounds, how conveyed to great distan-    "    (natural) how formed, 94.
ces, 130.                        "    and rivulets, how formed, 93.
"    when loudest, 124.              Square, 38.
South Pole, 271.                        Standard for computing the  specific
f'    "  of a magnet most powerful   gravity of bodies, 86.
in south latitude, 238.    Stars, 257.
"   wind, how produced, 108.            "   apparent motion of around the
Southern Cross, 288.                             earth, 287.
Southwark bridge, arches of, raised by    "   distance of the, 286
temperature of the air, 139.            "   double, treble, and quadruple, 286.
Space passed over by a moving body,    "   formed into clusters or constellahow estimated, 32.                            tions, 287.
Spaces through which falling bodies    "   how classed, 286.
move, how they increase, 34.            "   how designated on a globe, 287.
Spark from a galvanic battery, action of   "   how distinguished from  planets,
a, 229.                                        258.
Speaking, why difficult in a crowded    "   motion of the, 286.
house, 130.                             "   never seen in their true position,
Speaking-trumpets, 128.                          287.
Specific gravity, 29.                     "   number of, seen by Dr. Herschel
"c      " c of bodies heavier than            in a quarter of an hour, 286.
water, how ascertain-    "   Sir J. Herschel's calculation of
ed, 87.                        the distance of, 286.
"       "   of bodies, instrument for    "   Telescopic, 286.
ascertaining, 89.        "   variation of their apparent mag-'       "   of bodies lighter than            nitude, 286.
water, mode of cornm-   "   what supposed to be, 258.
puting, 88.              "   why not seen in the daytime, 278.
"       "   of bodies, mode of cor-  Stationary Steam-engine, 150.
puting, by whom dis-  Steam, 74. 98.133.
covered, 88.              "   and vapor form clouds, 22.
I,      6, of the atmosphere, by        "   condensation of, ]33.
what affected, 101.       "   effect of, when confined, 141.'L"     "   scales for ascertaining,    "   elastic force of, how increased or
87.                              diminished, 141.
standard for estimating,    "   expansive power of, substituted
86.                             for atmospheric pressure in
"       "   table of, 87.                      the steam-engine, 143.
Spectacles, the principles of, explained,    "   how made to act, 142.
174.                                     "   or vapor, how produced, 141.
Spectrum  formed by a prism, how di-    "   peculiar property of, 141.
vided, 188.                              "   pressure of, on what it depends,
Speed, in machinery, how increased or             141.
diminished, 64.                          "   space occupied by before and afSpheres, proportion of to each other, 275.       ter its conversion to water, 142.




INDEX.                                 329
Steam, supply of to machines, how reg- I Summer solstice, 275.
ulated, 72.                    Sun, 257. 275.
temperature of, 141.               "  an entire image of, reflected from
"   when visible, and when invisible,       every portion of the surface of a
142.                                 sheet of water on which it shines,
withdrawn from the principal cy-        158.
linder of the steam-engine, 142.   "  cause of the red appearance of, 187.
Steam-engine, 142.                        "  cubic magnitude of the, 275.
"      acting parts of described,   "  diameter of the, 261. 275.
143.                       "light of the, time employed in com"     double action of the, 143.         ing to the earth, 169.
"     how constructed, 145.         "  heat of the, 276.
inventors of the, 145.       "  its effect on the tides, 293.
locomotive, Hinkley  and    "  length of day in thle, 260.
Drury's, 149.              "  probably inhabited, 276.
N" Newcomen and Savery's,    "  rays of the, parallel at the earth
142.                             172.
power of, how expressed,        revolution of around its axis, 276.
145.                          spots on the, 276.
"      power of the, as described    "  table of the bulk, density, diameter,
by Mr. Jeffrey, 145.            &c., of the, 301.
"     the atmospheric, 142.         "  the centre of the solar system, 259.
the condensing and non-         time occupied by his light in reachcondensing, or the high         ing the earth, 280.
and the low pressure,   "  total eclipse of in 1806, 298.
145.                       "  what supposed to be, 258.
W watts' double-acting, con-   "  when nearest to the earth. 265.
densing, 143.            Sun's heat on the surface of the earth,
"      Watts' improvements in             effect of the, 103.
the, 142.                      light, why white, 188.
Steam-engines, how  regulated in the    "  rays, how we are shielded from
supply of steam, 72.                          the intensity of the, 102.
Steam-pipe, 143.                        Superior conjunction, 265.
Steel, rendered magnetic, 240.              "    planets, 260.
Steelyard, 55.                          Surinam Eel, 216..'    how managed, 56.              Swimming, the effect of action and reStereo-electric current, 255.                         action, 36.
Stethescope, 127.                            "     how  a person ignorant of
Stilts, how used by shepherds in France,              may float, 87.
48.                                        "      why more difficult in fresh
Straight Jet, 118.                                    than in salt water, 87.
Streams, velocity of, how ascertained,  Synthesis, or combination, 15.
91.                                   Syringe, the condensing and exhausting,
Strings of musical instruments, height    117.
and depth of tone of, on what depen-  Syringes, for procuring fire, 138.
dent, 125.                            System, the Solar, 257.
Strings of musical instruments, to what
they owe their sonorous properties,
125.                                  Table of Diameters, Distances, daily
Sucker, 114.                              and annual revolutions, bulk, density,
Sucking-pump, 106.                        &c., of the Sun and Primary planets,
Sugar and salt, particles of, smaller than    30)1.
those of water, 20.             Tables of equation of time, 300.
"  why it readily absorbs moisture,  Tackle and Fall, 61.
75.                             Tail of a comet, 283.
Sulphate of Copper Battery, 226.        Tangent, 42.
Summer rays, 270.                           "    of a circle, 38.




330                                 INDEX.
Tantalus' Cup, 95.                        Time, (true) cannot be measured by the
Taurus, 263.                                sun, 300.
Telegraph, Electro-Magnetic, 251.         Tin-foil, use of in the Leyden  Jar,
Telescope, 183.                             199.
"     Astronomical, 184.             Tissue figure, 205.
"     Terrestrial, 184.              Toggle-joint, 73.
Telescopes, kinds of, 183.                Tone, height and depth of, on what de"      refracting, two kinds of, 184.        pendent, 125.
Telescopic stars, 286.                      "   of a musical string, how affected
Tellurium, Electrical, 206.                       by tension, 126.
Temperature of steam, 141.                Tones of the human voice, how varied,
Tenacity, 19. 25.                           131.
Tender of a Locomotive Steam-engine,  Tonnage of a vessel, mode of compu148.                                     ting, 85.
Ten Eyckand Henry, powerful electro-  Torpedo, 216.
magnet formed by, 249.                  Torricelli, 110.
Tension of a musical string, how it af-  Torricellian Vacuum, 100. 110.
fects its tone, 126.                    Torrid zone, weather prevalent in the,
Terrestrial Telescope, 184.                 108.
Thales, of Miletus, his discovery in elec-  Total eclipse of the sun, darkness protricity, 194.                                   duced by, 155.
Theology, meaning of, 13.                   "   eclipse of the sun, duration of a,
Theory of Music, 16.                               298.
Thermal effects of light, 191.              "   eclipse of the sun in 1806, 298.
Thermo-electric batteries, 256.           Towel, the end of a, left in a vessel of
Thermo-electricity, 196. 255, 256.          water, will empty it, and why, 75.
" Prof. Seebeck's dis-  Trade-winds, 108.
covery in, 255.     Transfer of fluids from one vessel to anThermometer, 101. 103.                      other, 121.
c" how it differs from a ba-  Transit of a planet, 277.
rometer, 103.              "   of Venus and of Mercury, 278.
"       scales of the, 103.        Translucent substances, 153.
Thickness, 21.                            Transparent bodies, 152.
Threads of a screw, 68.                   Trees uprooted by means of the hydroThunder-cloud, distance of a, how as-    static press, 85.
certained, 128.                         Tropic, meaning of the term, 272.
Thunder-house, 211.                          "    of Cancer, 272.
Thunder-storms, 215.                         "    of Capricorn, 272.
"'     " 4   dangers in, 216.         True focus of a concave mirror, 163.
"      ",  safest position in, 216.  Trumpets, (hearing,) used by the deaf,
Tides, how caused, 292.                                129.
"    irregularities of the, 294.            "      principle on which they are
"    Neap, 294.                                      constructed, 129.
"    Spring, 293.                       Tube, the mercurial or water, 114.
"    theory of the, the most imperfect  Tubes, Acoustic, 130.
of astronomical hypotheses,  Tufts', Otis, stationary  steam-engine,
294.                               150.
Time, 298.                                Tumbler of water, cause of moisture
"   difference between true and ap-    on, in warm weather, 133.
parent, 300.                     Twins, The, 263.
employed in the use of machinery, 170.
"   kept by clocks, when same as that  Undershot water-wheel, 96.
of the sun, 300.                 Undulations of an elastic medium, the
of moving body, how estimated,32.   supposed cause of light, 153
solar and sidereal, difference be-  Undulatory Theory, 153.
tween, 298.                      U magnet, 235.




INDEX.                                331
Uniform motion, 32.                                of a musket ball, of a cannon
4" "i         how produced, 32.                 ball, of the equatorial reUniversal Discharger, 206.                         gions, of the earth's centre,
Unmagnetized steel, how endued with                31.
polarity, 243.                        Velocity of running water at its surface,
Unorganized matter, 14.                            91.
Upward pressure of fluids, by what          "    of running water, how ascercaused, 81.        -                              tained, 91.
Uranus. See " Herschel," or the ", Geor-    "    of steam-carriages, of sailinggium Sidus," 257. 266. 282.             vessels, of race-horses, &c.,
"     light and heat of, 282.                   31.
"     moons or satellites of, 258.      "    ofwaterin running streams, 90.
282. 302.                       "    to what proportioned, 31.
Ursa Major, 299.                        Ventriloquism, 131.
art of, not unknown to
the ancients, 132.
Vacuo, in, 71.                          Ventriloquist, peculiar power of the, 131.
Vacuum, 100.                            Venus, 257. 277.
"    a perfect, never produced by      "   diameter of, 261.
the air-pump, 109.             "   distance from the sun, 260.
a perfect, pressure of the air   "   heat and light on, 277.
against, 112.                  "   length of day in, 260.
how produced in the common       "   length of the year of, 260.
pump, 105.                      "   perihelion of, 266.
"    in the cylinder of a steam-       "   phases of, 278.
engine, 42.                    "   revolution of, 259.
"    the Torricellian, 100. 110.       "   sometimes seen in the daytime,
Valve, 106.                                      278.
the sliding, of a steam-engine,     "   when called morning, and when
148.                                   evening star, 277.
"    the condensing, of an air-pump, Vernal equinox, 275.
110.                          Verrier, 283.
"    the exhausting, of an air-pump, Verrier's discovery of the planet Nep110.                            tune, 257.
Valves in air pumps, how they open, 111. Vertical line, 27.
"    how made, 106.                       "   lines always perpendicular to
"    of a common pump. 106.                     the surface of the earth, 27.
of the air-pump, 106.            Vesper, 277.
Vancouver, 237.                         Vessels, tonnage of, method of compuVapor, 98.                                ting, 85.
Variation of the magnet, 236.           Vesta, 257. 279.
i    of the magnetic needle, by    "   diameter of, 261.
what caused, 241.              "   distance from the sun, 260.
i'   of the magnetic needle subject   "   length of day unknown, 260.
to changes, 237.              c"  length of the year of, 260.
Velocity, 30.                             "   perihelion of, 266.
"    absolute and relative, 31.     Vibrations of a pendulum, how caused,
"   how measured, 31.                            51.
"    of a body in motion around a        "     of a sonorous body, their.efpoint, on what dependent,                fect on the air, 125.
161.                             "     of sonorous bodies, the foun"    of a heavenly body, increase of             dation of the Science of
explained, 267.                          Harmony, 126.
"    of falling bodies, 27.              "     of strings of musical instru"    of machinery in manufactures,               ments, 125.
how regulated, 72.           Virgin, 263.
"    of rivers, wind, &c., &c., &c.,  Virgo, 263.




332                               INDFX.
Vision, angle of, 159.                  Water, how raised to the boxes or valves'   seat of, 177.                             of a common pump, 114.
Vitreous electricity, 198.                 "   how supplied to water-wheels,
"    humor, 175. 177.                         72.
c"    "    loss of, causes total    "   in motion, by what retarded. 90.
blindness, 179.              in pails or buckets, why heavier
Voice, cultivation of the, 131.                  when lifted from a well, 88.
"  management of the, 131.              "   instruments for raising, 92.
"   sound of the human, how pro-        "   level, (sometimes called a spirit
duced, 130.                              level,) 76.
"   the human, where scarcely heard,    "   mills, supply of water to, how
124.                                    regulated, 72.
Volcanoes of the Andes, distance at    "   one of the elements of the ancient
which they are heard, 130.                     philosophy, 18.
Volta, Professor, inventor of the Gal-     "   or mercurial tube, 114.
vanic pile, 218.                         "   pipes used to communicate presVoltaic Battery, 223.                            sure, 84.
"   Electricity, 218.                   "   remarkable property of, 140.
(("     "       powerful  magnets       "   running, velocity of, how ascerformed by, 248.             tained, 91.
pile, 222.                         "   velocity of, in motion, 90.
"   piles, how constructed, 222.        "   vessels containing, why broken
Vote of the School Committee of Boston           when the water freezes, 140.
adopting this volume, 4.                 "   weight of a cubic foot of, 87.
" wheels, 63.
"c     "   different kinds of, 96.
Walls of the Conservatory of Arts and      "      "   the overshot, undershot,
Trade, how restored to a perpendicu-                   and breast, 96.
tar position, 139.                       "   why it appears more shallow
War, art of, how elevated into a science,         than it really is, 169.
45.                                      "   why pure  in  rain, hail, and
Warmth of different substances used               snow, and impure from rivers,
for clothing, cause of difference in,           springs, and wells, 86.
135.                                  Watson, (Dr.,) experiment of, to ascerWatch, how it differs from a clock, 53.   tain the moisture arising from an acre
"   main-spring of a, 71.              of ground, 103.
Water, a heterogeneous substance, of Watt, (Mr. James,) 72. 145.
what it is composed, 19.    "            "       his improvements in
"   artificially frozen, 123.                              the steam-engine,
"   as a conductor of sound, 127.                          142.
"   Bearer, the, 263.                Watt's double-acting, condensing steam"   compressible, 73.                                       engine, 143.
"   constantly changing, 22.                         "    condensing engine
"   conveyed over hills, &c., in tubes,                     described, 146.
94.                            Waves, how caused, 92.
"   effect of heat and cold on its  Wedge, 54. 67.
density, 140.                    "    advantage gained by the use of
"   effects of heat on, 133.                   the, 67.
", height to which it will rise in a  Wedgewood's Pyrometer, 138.
fountain, &c., 94.             Weight, an accidental property, 19.
height to which it can be raised     "   Lifter, 119.
in the common pump, 106.          "   loss of, in bodies immersed in
"   how made a mechanical agent,                water, 88.
95.                               "   of bodies at different distances
"   how obtained in wells, 94.                  from the earth's centre, 26.
"   how raised in the common pump,'   of substances, unaffected by
106.                                     heat, 132.




INDEX.                               333
Weight of the atmosphere, 112.         Windows, why their refraction affects
"   of the atmosphere, instrument       "      not the appearance of obfor weighing, 101.                       jects seen through them,
"   of the atmosphere, measure of               170.
the weight of the, 101.      Winds, periodical, more regular at sea
"   possessed by aeriform fluids, 98.        than on land, 108.
"   the effect of gravitation, 25.    "   how  sometimes rendered pesWells, artesian, 94.                             tilential, 109.
"   how water is obtained in, 94.      "   quality of, by what affected, 109.
West wind, how produced, 108.             "   the Trade, 108.
Wheel and axle, 54. 62.                   "   why so variable in the temperate
c"       "  advantage  gained by              zones, 108.
use of the, 62.         "   why they subside on the going
c"       "  how  its power is in-             down of the sun, 108.
creased, 63.         Winter rays, 270.
cc"      "  on what principle con-    "   solstice, 275
structed, 63.        Wollaston, (Dr.,) experiments of, in reWheels, friction, 69.                    lation to colors, 189.
"   how turned, 65.                 Wood, an imperfect conductor of heat,
"    mode of securing tires on, 138.    137.
o"    f clocks and watches, 58.     Woodcocks, guns suitable for, 46.
"   why sometimes locked in de- Woollen garments, why warm, 136.
scending steep places, 70.          "     "    why warmer than
"   why used on vehicles. 70.                            others, 135.
Wheelwrights' mode of securing the  Worcester, Marquis of, an inventor of
iron tires on wheels, 138.             the steam-engine, 145.
Whirlwinds, how produced, 108.
Whisper, the velocity of a, 127.
Whispering-galleries, 129.             Year, a measure of the revolution of a
White, 189.                                    heavenly body, 259.
"   how obtained, 188.                 "  length of the, on the several,'   not a simple color, 188.                planets, 259.
Whitfield, 129.                           "   sidereal, 299.
Wick, of a lamp, should be often re-    "   solar, 298.
newed, 75.
Width, 21.
Wightman, (Joseph M.,) 9.              Zero, of the thermometer, where fixed
Wightman's apparatus for illustrating    in different scales, 103.
inertia, 22.                         Zodiac, 262.
"Wightman's Companion to the Air-    "   and Ecliptic, difference between,
pump," 112.                                    272.
Wilcox's (Mr. A.,) electrotype copy of a  "   characters representing the signs
steel plate, 150.                              of the, 263.
Wilkins' Astronomy, 292.                  "   constellations of the, cause of
Wind, 107.                                      the change in, 300.
instruments, sound of, how af-      "   extent of the, 265. 272.
fected, 126.                      "   how divided, 262.
"   the north, how produced, 108.       "   names of the  signs of the,
Windlass, 63.                                    263.
how it differs from a capstan,    "   representation of the, 273.
63.                           "   signs of the, as represented in
Windmills, 63.                                   almanacs  and  astronomical
Winaow, how most easily raised, 58.             works, 262.
Windows, cause of moisture on, in cold Zodiacal light 270.
weather, 133.                        Zoology, 14.




QUESTIONS FOR REVIEW AND EXAMINATION.
DIVISIONS OF THE SUBJECT.
ART.. —What is Natural Philosophy?
ART. 2.-What are the principal branches of Natural Philosophy? What
is Mechanics? Of what does Pneumatics treat?  Of what does
Hydrostatics treat? Hydraulics? Acoustics? Pyronomics? Optics? Astronomy? Electricity? Of what is Galvanism a branch?.
Of what does Magnetism treat? Electro-Magnetism? MagnetoElectricity 7
OF MATTER AND ITS PROPERTIES.
ART. 3.-What is Matter?
ART. 4.-How many essential properties of matter are there? What are
they?  Why are they called essential properties?  What other
properties exist in different bodies?  Why are they called accidental properties?  Are color and Weight essential or accidental
properties? Why? What terms are used in Philosophy to express
the state in which matter exists?  What follows from this?
Fig. 1. Explain the principles which Fig 1 illustrates. What example
can you give to prove the impenetrability of water?  What of the
air? What of solids?
ART. 5.-What is meant by Impenetrability?  Does impenetrability belong to fluids? Why do fluids appear less impenetrable than solid
bodies? What is supposed to be the form of the particles of fluids?
ART. 6.-What is meant by Extension?  What terms are used to express the size of a body?  What is length?  Breadth?  Height,
depth, or thickness? What is the difference between height and
depth?
ART. 7.-What is meant by Figure?  May bodies be of the same shape
or figure and of different dimensions? Give an example. What
constitutes figure?
ART. 8.-What is meant by Divisibility?  Is there any known limit to
the divisibility of matter? Mention some examples of the extreme
divisibility of matter.
ARi 9.-What is meant by the Indestructibility of matter?  May it be
changed in form and in external appearance? Give examples of
such changes.




QUESTIONS FOR EXAMINATION.                     335
kRT 10.-What is meant by Inertia?  Can a body at rest put itself in
motion? Can a body in motion stop itself? When a stone or ball
is thrown from the hand, how many forces continually operate
to stop it. What are they?  How could a body in motion be
made to move forever?
Fig. 1.                               Fig. 2.
FPg. 2.  Explain the apparatus for illustrating inertia, as shown in
Fig. 2.
ART. 1 l.-What is attraction?  Is every portion of matter attracted by
every other portion of matter? How does this attraction increase and diminish?
ART. 1 2.-How  many kinds of attraction are there belonging to all
matter? What is the attraction of gravitation, or gravity?
What is the attraction of cohesion, or cohesive attraction?
What causes a stone to fall to the ground?  By what are the
particles which compose the stone held together? Of what is
matter composed?  Is the cohesive power which unites them
the same in all bodies? How may cohesive attraction be illustrated? Do the particles of matter in bodies absolutely touch
each other? What are the spaces between them called? What
do you understand by density?  What by rarity?
ART. 13.-What is compressibility?  Are all substances susceptible
of it?
ART. 14.-What is expansibility?
ART. 15.-What is mobility?
ART. 16.-What is elasticity? What substance possesses this property
in a remarkable degree?
ART. 17.-What is malleability? Does this property belong to all the
metals? What metal possesses it in the highest degree?
ART. 1 8.-What is brittleness?  What bodies are most brittle?
ART. I 9. —What is ductility? Which is the most ductile of the metals?
ART. 20.-What is tenacity? Which is the most tenacious of the metals?




336            QUESTIONS FOR  EXAMINATION.
OF GRAVITY AND WEIGHT.
ART 21.-What do you understand by the term gravity? Do all bodies
possess this attraction?  To what is its forceproportional?  If a
body be unsupported, will it remain stationary? Why will it
fall?
ART. 22.-What causes weight?  When you say that a body weighs an
ounce, what do you mean by it?  What, therefore, is weight?
ART. 23.-Where is the force of gravity greatest? How does it change?
How does it decrease above the surface of the earth? How
below?
ART. 24.-In what direction will a falling body approach the surface of
the- earth? Will the lines of suspension of different bodies evei
be parallel?  Where will they meet, if sufficiently produced?
Fig. 3. Explain Fig. 3..
ART. 25.-Will all bodies at equal distances from the earth, fall to it in
the same time? Why not? What bodies fall fastest? To
what is the resistance of the air proportional?
ART. 26. —In what proportion are all substances influenced by gravity?
Is air affected by it? How far does the air extend above the
surface of the earth?  What causes the air to be more dense at
the surface of the earth?  What causes this pressure?  Why
does not the air fall to the earth like other bodies? Where is
the air heaviest? What effect have gravity and elasticity upon
the air?
ART. 27.-What is specific gravity? Illustrate this Does the attraction of the earth cause all bodies to fall? What bodies will fall?
What rise? How does the air cause them to rise? Why do
not cork and other light bodies sink in water? Explain the
principle upon which balloons rise. What effect has gravity on
bodies lighter than the air? What effect on bodies of equal
weight?  What effect on those that are heavier?  What affects
the rapidity of their descent? To what is the resistance of the
air proportioned?
MECHANICS-LAWS OF MOTION.
ART. 28,-What is Mechanics?
ART. 29.-What is motion?  Why cannot a body put itself in motion?
Why cannot a body stop itself when in motion?
ART. 30. -What is force? What is resistance?
ART. 3 1.-When is the motion of a body in a straight line?  In what
direction will it move?
Asr 3 2.-What is meant by velocity?




QUESTIONS FOR EXAMINATION.                   337
ART.3 33.-To what is the velocity of a moving body proportional?
ART. 34.-How  is the velocity of a moving body determined? If one
body go through six miles in an hour, and another twelve, how
does the velocity of the latter compare with that of the former?
What is meant by absolute velocity? Give an example. When
is the velocity of a body termed relative? Give an example.
ART. 3 5.-How is the velocity of a body measured? Illustrate this.
ART. 36.-How do you ascertain the time employed by a body in motion? Illustrate this.
ART. 3 7.-How can you ascertain the space? Illustrate this.
ART. 38.-How many terms are applied to motion to express its kind?
What are they? What is uniform  motion? Accelerated?
Retarded?
ART. 39.-How is uniform motion produced? Why is not a ball struck
by a bat, or a stone thrown from the hand, an instance of uniform motion? How can it be made an instance?
ART. 40.-How  is accelerated motion produced? Give an instance of
accelerated motion. How far does a stone fall the first second
of time? The second? Third? Fourth?  How can you measure the height of a building, or the depth of a well?
ART. 41.-How is retarded motion produced?  Give an example. How
does the time of the ascent of a body thrown perpendicularly
upwards, compare with that of its descent? Why cannot perpetual motion be produced?
ART. 42.-What is the momentum of a body?  How can the momentum of a body be ascertained? Does the quantity of motion
communicated to a body affect the duration of the motion? If
but little motion is communicated, how will the body move? If
a great degree? How long will the motion continue? How
can a light body be made to have a greater momentum than a
heavy one? Give an instance of this.
ART. 43.-What is meant by action? Reaction? Illustrate this.
ART. 44.-How do action and reaction compare?
Fig. 4.  Explain Fig. 4.
Fig. 3.                    Fig. 4.
15




338            QUESTIONS FOR EXAMINATION.
Fig. 5.  Explain the experiment of Fig. 5.
Fig. 6.  Explain the experiment of Fig. 6. Upon what principle do birds
fly?  Explain how. Upon what principle do fishes swim?
Upon what principle do boats move upon the water? Explain
how.
ART. 45.-How may motion be caused? When caused by action, what
is it called? When caused by reaction, what is it termed?
ART. 46.-What is the angle of incidence? What is an angle?  Upon
what does the size of an angle depend? What is a circle?
What are radii? What lines in Fig. 8 are radii? What are
diameters? In Fig. 8, what line is the diameter? How is the
circumference of all circles divided? Into how many parts does
the diameter of a circle divide it? How are all angles measured?
Fig. 8.  Illustrate this by Fig. 8. How many degrees do right angles
contain?  Acute?  Obtuse?  Illustrate these angles by Fig. 8.
What is a perpendicular line? What line is perpendicular in
Fig. 8? What is a tangent? What line is a tangent in Fig. 8?
What is a square? What is a parallelogram?  A rectangle?
What is a diagonal? What lines are diagonals in Figs. 9, 10,
and 11?
Fig. 7.  Explain the angle of incidence by Fig. 7.
ART. 47.-What is the angle of reflection? Illustrate this by Fig. 7.
ART. 48.-How do the angles of incidence and reflection compare with
each other? Illustrate this by Fig. 7. What follows from what
has been stated with regard to the angles of incidence and
reflection?
ART. 49.-What is compound motion?
ART. 50.-In what direction will a body, struck by two equal forces in
opposite directions, move?
ART. 5 I.-When struck by two forces inclined to each other, how will
it move? What is this line called?
Fig. 9.  Illustrate these, first, by Fig. 9, which represents a ball struck
by two equal forces in different directions.
Fig. 10. Second, by Fig. 10, which represents a ball struck by two unequal forces, acting at right angles.
Fig. 11. Third, by Fig. 11, where the forces operate in the direction of
an acute angle. Fourth, by Fig. 11, where the forces operate
in the direction of an obtuse angle.
ART. 52.-What is circular motion? How is it caused? Illustrate this.
ART. 53.-How many different centres are there which require to be
noticed? Define each of them.




QUESTIONS FOR EXAMINATION.                    339
ART. 54.-Is the centre or axis of motion supposed to be at rest, or does
it move? To what do the terms centre of motion and axis of
motion relate?
ART. 55.-What are the two forces called which produce circular motion? What is the name of each? What do the words centripetal and centrifugal mean?
ART. 56.-Define a centripetal force. Also a centrifugal force.
ART. 57.-If the centrifugal force be destroyed, to what point will the
body tend? What would be its direction if the centripetal force
were destroyed? Give an example.
ART. 5 8.-What parts of a body move with the greatest velocity? In
what proportion does the velocity of all the parts diminish?
Fig. 12. What does Fig. 12 represent?
ART. 59.-What follows, with regard to the motion of the earth, from
the illustration of Fig. 12?
ART. 60.-Of what is curvilinear motion always the result?  Why?
Fig. 5.                                     Fig. 7.
A  BCDE F                                R               C
Fig. 8.                Fig. 9.              Fig. 10.
- X                                                X
T     D          ic 1                   C
D                A
Fig. 11.                        Fig. 12.




340            QUESTIONS FOR EXAMINATION.
ART. 6 I.-How many forces act upon a ball thrown in a horizontal direction? What are they? Why do bodies fall to the ground?
ART. 62.-Does the force of gravity either increase or decrease the force
of projection? Give an illustration.
Fig. 13. Explain Fig. 13.
ART. 63.-What is a projectile? What lines do projectiles describe?
From what cause? Give the illustration.
Fig. 14. Explain Fig. 14. How great is the resistance of the air calculated to be to a cannon-ball of two pounds weight, with the
velocity of 2000 feet in a second?
ART. 64.-When a body is thrown horizontally, or upwards or downwards obliquely, in what curve will it move? In what line will
it move when thrown upwards or downwards perpendicularly?
Fig. 15. Explain Fig. 15.
ART. 65.-What is the random of a projectile?  At what angle does
the greatest random take place?
Fig. 16. Explain Fig. 16.
ART. 66.-When the centre of gravity of a body is supported, will the
body stand or fall? What if the centre be unsupported? What
is a line of direction?
ART. 67.-What is the base of a body?
Fig. 17. Explain the base in Fig. 17.
Fig. 18. Explain what is meant by the base in Fig. 18.
ART. 68.-If the line of direction falls within the base, will the body
stand or fall? Give an illustration.
ART. 69.-What shaped bodies are easily overturned?  What bodies
must stand more firmly than others? Why?  Why do bodies
which have a narrow base overturn more easily than those which
have broad bases? Why can a person carry two pails of water
more easily than one? Why is a pyramid the firmest of all
structures?
ART. 70.-If two bodies of equal weight are fastened together, where is
the centre of gravity?  If one be heavier than the other?
Fig. 19. What does Fig. 19 represent?
Fig. 20. What does Fig. 20 represent?
Fig. 21. What does Fig. 21 represent?  Explain each one separately.
RESULTANT MOTION AND THE PENDULUM.
ART. 71.-What is resultant motion?  Give the examples of this kind
of motion.
Fig. 22. Explain Fig. 22.




QUESTIONS FOR EXAMINATION.                    341
ART. 72.-Of what does a pendulum consist?  By whom was the pendulum invented?  What led him to the discovery?  By whom
was the principle of gravitation discovered? What led him to
the discovery?
ART. 73.-What are the movements of the pendulum called?  What is
meant by its arc? What causes its vibrations?
ART. 74.-How do the vibrations of pendulums of equal length compare?
Fig. 23. Explain this by Fig. 23.
ART. 75.-Upon what does the time of the vibrations of a pendulum
depend?
ART. 76.-What is the length of a pendulum which vibrates sixty times
in a minute? Do different situations affect the vibrations? How
can a pendulum which vibrates seconds at the equator be made
to vibrate seconds at the poles?
Fig. 13.                Fig 14.             Fig. 15.
C                     D                A
"'%, 6    B                 A
E        C
a
Fig. 16.              Fig. 17               Fig. 18.
90
D            E
C          F
Fig. 19.                Fig. 21.              Fig. 23.
Wm. A WA
Fig. 20.                                D D
Fig. 22...           _ G F
A   --      B
CE
E H




342            QUESTIONS FOR EXAMINATION.
ART. 7 7.-How is a clock regulated? What effect has the lengthening
of the pendulum? The shortening? What is a clock? Of
what use is the weight?  What do the wheels show?  Why do
clocks go slower in summer than in winter? How does a watch
differ from a clock?
OF THE MECHANICAL POWERS.
ART. 78.-What are the mechanical powers?  How many things are
to be considered in order to understand the power of a machine?
What is the first? Second? Third? Fourth? Fifth? What
is the power that acts? What is the resistance to be overcome?
What is the fulcrum? What is the velocity?
ART. 79.-How many mechanical powers are there? What are they?
ART. 80.-What is a lever? How many kinds of levers are there?
How do they differ?
ART. 81. —What is a lever of the first kind? What figure illustrates
this?
Fig. 24. Explain it by Fig. 24. To what is the advantage gained by
this lever proportional  What follows from this? What is
meant by an inflexible bar? What is a fundamental principle
in Mechanics?
Fig 25. Illustrate this by Fig. 25. Does this principle apply to all the
mechanical powers?
Fig. 26. Explain the common steelyard in Fig. 26.
Fig. 27. Also in Fig. 27. What is a balance, or pair of scales?  Give
some examples of levers of the first kind.
Fig. 28. Explain Fig. 28.
ART. 82.-What is a lever of the second kind? What figure illustrates
this?
Fig. 29. Explain Fig. 29. To what is the advantage gained by this lever
proportional? Give some examples of levers of the second kind.
ART. 83.-What is a lever of the third kind? In what proportion must
the power exceed the weight in this lever?
Fig. 30. Explain Fig. 30. Give some examples of levers of the third
kind.
ART. 84.-What is a pulley? How many kinds of pulleys are there?
What are they? What is a fixed pulley?
Fig. 31. Explain Fig. 31. What advantage is gained by this pulley?
What is the use of this pulley? Upon what principle does the
fixed pulley operate?
ART. 85. —How does the moveable pulley differ from the fixed pulley?
Fig. 32. Explain Fig. 32.




Fig. 24.
a,,                    Fig. 26.  B
Fig. 25.
Fig. 28.
Fig. 29.               Fig. 30.          Fig. 31.            Fig. 32.
WL                                                   B
~W~~
W




344            QUESTIONS FOR EXAMINATION.
ART. 86.-How can the power gained by the use of the moveable pulley
be ascertained?  What illustration of this is given?
Fig. 33. What does Fig. 33 represent? Explain it.
ART. 87.-Upon what principle do pulleys act? What advantage is
gained by the use of pulleys and other mechanical powers?
What are some of the practical uses of the pulley? What is a
tackle and fall? Is there any time or velocity gained by the
power in the mechanical powers? To what is the product of
the weight, multiplied by its velocity, always equal? What rule
is given?
ART. 88.-Of what does the wheel and axle consist? What is a cylinder?  What figures illustrate the wheel and axle?  Explain.
To what is the advantage gained in proporeion?
Fig. 34. What does Fig. 34 represent? Explain it.
Fig. 35. What does Fig. 35 represent? Explain xi
ART. 89.-Upon what principle are the wheel and axle constructed?
Explain how. Upon what principle is the capstan on board of
vessels constructed? Of what does it consist? What other
things are mentioned as constructed upon this principle? Are
wheels aul essential part to most machines? Are they applied
in more than one way? When they are affixed to the axle,
in what proportion is the power increased? What are flywheels, and for what are they used? How are they made to
revolve? When once set in motion, what causes them to move
on for some time?  Of what service are they in a machine?
For what are cranks sometimes connected with the axle of a
wheel? How are they made?
Fig. 36. What does Fig. 36 represent? Explain it. For what are cranks
often used? How does the velocity of the wheel compare with
that of the axle? To what is this velocity in proportion? Is
any advantage taken of this in driving machinery where the
speed is to be increased or diminished?  How would rapid motion be produced? Slow motion?
Fig. 37. Explain Fig. 37. What is the usual way of transmitting the
action of the axles to the adjoining wheels? What are the cogs
on the surface of the wheel called? Those on the axle? What
is a pinion?
Fig. 38. Explain Fig. 38. By what are wheels sometimes turned?
What figure represents one?  In what way can the motion be
made direct or reversed?
Fig. 39. What does Fig. 39 represent? Explain it.
Fig. 40. What does Fig. 40 represent?  Explain it. In what way can
different directions be given to the motion produced by wheels?




Fig. 33.                                       Fig. 34
H
F       tp
w
Fig. 35.                                  Fig. 36.
Fig. 37.
w       \                       d   c   b   a
F                S
Fig. 38.                                   Fig. 39
Fig. 40.
P                                                 IM1
15*




346            QUESTIONS FOR EXAMINATION.
Fig. 41. What does Fig. 41 represent?  Explain it.
Fig. 42. What does Fig. 42 represent? Explain it.
ART. 90.-What is an inclined plane? What figure represents an inclined plane?
Fig. 43. Explain Fig. 43. To what is the advantage gained by the use
of the inclined plane in proportion? What follows from the
greater or less inclination of the plane? Give some instances
of the application of the inclined plane. Is any time gained by
the use of the inclined plane? Upon what principle are chisels
and other cutting instruments, which are sloped only on one
side, constructed?
ART. 91.-Of what does the wedge consist?
Fig. 44. What does Fig. 44 represent? Explain it. To what is the advantage gained by the wedge in proportion? Of what use is
the wedge? Give some examples of the wedge.
ART. 92.-Of what does the screw consist?  Of how many parts is it
generally composed? What are they?
Fig. 45. What figure represents the screw and the nut? Explain the
figure.
Fig. 46. How does Fig. 45 differ from Fig. 46? Is the screw a simple or
compound power? How is the power of the screw estimated?
How does the closeness of the thread affect the power? What
is the use of the screw? How can its power be increased?
To what is the screw applied? What is meant by friction in
machinery? How many kinds of friction are there? What
are they?  How is the rolling friction produced? The sliding?
Which is overcome with the less difficulty, the rolling or sliding?
What allowance must always be made, in calculating the power
of a machine? What proportion of the power is usually computed to be destroyed by friction? Between which is friction
the less, rolling bodies or those that slide? What causes friction? In what proportion is it diminished? In what manner
can it be lessened? What is the use of wheels? In what proportion do they overcome the obstacles, such as stones, &c., in
the road? Why, in descending a steep hill, are the wheels of a
carriage often locked? How do castors, which are put upon
furniture, facilitate the moving of it? How is the motion of all
bodies influenced?
ART. 93.-What is meant by a medium? Give examples.
ART. 94.-To what is the resistance of a medium in proportion? What
illustration is given? When would a machine be perfect?
ART. 95.-Of what does the main-spring of a watch consist? What is
its use? Does the spring exert a stronger force when closely




QUESTIONS FOR EXAMINATION.                    347
coiled, or when partly loosened? What is done in order to correct this inequality?
Fig. 47. What does Fig. 47 represent? Explain it.
ART. 96.-What is a governor?
Fig. 48. Explain Fig. 48. What is said of the use of the governor?
ART. 97.-Of what does the knee-joint, or toggle-joint, consist?  In
what proportion does it increase in power?
Fig. 41.                       Fig. 42.
Fig. 43.         Fig. 44.      Fig. 45.          Fig. 46.
b
Fig. 47.                             Fig. 48
A                         A
M        l    D




348           QUESTIONS FOR EXAMINATION.
Fig 49 What does Fig. 49 represent? Explain the figure. Give an
instance of the operation of the toggle-joint. What is its use
in printing-presses?
HYDROSTATICS.
ART. 98.-Of what does Hydrostatics treat?
ART. 99.-What is a fluid? Does the attraction of cohesion have much
influence on the particles of fluids? What follows from this?
ART. 100OO-How  do fluids differ from liquids?  Can water be compressed? What is supposed to be the primary cause of the fluid
form of bodies?  What effect has heat upon bodies?  What
illustration is given? Why do fluids gravitate in a more perfect manner than solids?  What is inferred from the slight
degree of cohesion in the particles of fluids?  Why smooth?
Why globular? Why cannot fluids be formed into figures, or
preserved in heaps? What do you understand by capillary
attraction? Explain the reason of it. Explain why the same
takes place in all porous substances. Explain all the circumstances attending the burning of a lamp. Explain the experiment with the glass plates.
ART. 101.-What is meant by the level or equilibrium of fluids? Have
all fluids a tendency to preserve this equilibrium?  What follows from this? Of what is this level or equilibrium of fluidt,,
the natural result? How does the gravitation of solid bodies
differ from that of fluids?
ART. 102.-Do fluids of different densities all preserve their own equilibrium?  What illustration is given to prove this?
ART. 103.-Upon what principle is a water-level constructed? Of what
does it consist? For what is it used?
Fig. 50.  What figure represents a water-level? Explain the figure.
ART. 104.-In what manner do solid bodies gravitate?  What is the
centre of gravity? What effect has gravity on the particles of
fluids?
ART. 105.-How is the pressure of fluids exerted? How long will the
particles of fluids remain at rest?
Fig. 51.  Explain Fig. 51.
Fig. 52.  What does Fig. 52 represent? Explain it. If the equality of
the pressure be undisturbed, what will follow? If the fluid be
agitated, when will it again come to a state of rest? How is
the downward pressure of fluids shown? The lateral pressure?
The upward pressure?
ART. 106.-To what is the pressure of a fluid in proportion?  In what
direction is this pressure exerted? What illustrations are given




QUESTIONS FOR EXAMINATION.                   349
to prove this? Why can a bottle, filled with water or any other
liquid, be let down to any depth without injury?  What experiment is mentioned in the note?  What opinion have some
philosophers expressed?
ART. 107.-What causes the lateral pressure? What follows from this?
Fig. 53. Explain Fig. 53.
ART. 108.-Does the length or the width of the vessel in which a fluid
is contained have any effect upon the lateral pressure? By
what is it affected?
ART. 109.-How does the lateral pressure on one side of a cubical vessel compare with the pressure downwards? How would you
explain this?
ART. 1 10.-What causes the upward and downward pressure?
Fig. 54. Illustrate this by Fig. 54.
ART. 1 11.-Upon what does the force of pressure depend?  What is
meant by the hydrostatic paradox? What is the use of the
hydrostatic bellows?
Fig. 55.  What figure represents the hydrostatic bellows?  Explain the
figure. What is the fundamental principle of Mechanics?  Is
this the principle of the hydrostatic bellows?
Fig. 49.              Fig. 50.             Fig. 51.
A           A  C  B                            Fig. 49.
Fig. 53.                               Fig. 55.
AX                        A!A
Fig. 54.




350             QUESTIONS FOR EXAMINATION.
Fig. 56. What does Fig. 56 represent? Explain why fluids press according to the heights, and not according to the quantity.
ART. 1 1 2.-What fact is mentioned in this number in regard to the pressure of water? Upon what principle is Bramah's hydrostatic
press constructed?
Fig. 57.  What figure represents this?  Explain the figure. To what
uses is this press applied?
ART. 113.-When will one fluid float upon another?
ART. I 14.-What is stated with regard to a body specifically lighter
than a fluid? What illustration is given of this? How do the
specific gravities of water and cork compare with each other?
Upon what principle is it that boats, ships, &c., are made to
float upon the water? What rules have been formed from the
knowledge of the specific gravity of water and the materials of
which vessels are composed?
ART. 1  5. —What standard has been adopted to estimate the specific
gravity of substances in general? Why could not metals have
been adopted? Why is distilled water used? What bodies will
sink when immersed in water?  What will float?  What is
the weight of a cubic foot of water?  What is the use of the
table? How does the specific gravity of living men compare
with that of water? Which is the greater, the specific gravity of sea water, or of lakes and rivers? Why?
ART. 1 16.-How is the specific gravity of bodies that will sink in water
ascertained?  What illustration is given?
Fig. 58.  Explain Fig. 58. Why will gold weigh less in the water than
out of it? How does this upward pressure of the particles compare with the downward pressure of a quantity of water of the
same dimensions? What follows from this? What rule is given
with regard to all bodies heavier than water that are immersed
in it?  What is the specific gravity of a body?  What is the
reason that a bucket of water, drawn from a well, is heavier
when it rises above the surface of the water, than while it is
below it?
ART. 11 17.-How can the specific gravity of a body that will not sink in
water be ascertained? What illustration is given? By whom
was the method of ascertaining the specific gravities of bodies
discovered? In what manner did he ascertain it?
ART. 118. —What is an hydrometer? Upon what principle is it constructed? Explain its construction. In what proportion does
the scale sink?




QUESTIONS FOR EXAMINATION.                   351
HYDRAULICS.
ART. 119.-Of what does Hydraulics treat? What retards the motion
of water? Why does the surface of a canal or river have a
greater velocity than any other part?
ART. 1 20. —Does the fulness of a vessel, from the orifice of which a fluid
is running, have any effect upon its velocity?
Fig. 56.
A       B   c
Fig. 58.
Fig. 57..:,  00.         /. \




352            QUESTIONS FOR EXAMINATION.
ART. 121.-When a fluid spouts from several orifices in the side of a
vessel, from which is it thrown to the greatest distance?
ART. 122.-What effect will a pipe, fitted to an orifice, have with regard
to the quantity discharged? What will be the effect if the pipe
project into the vessel? How can the quantity discharged
through a pipe or orifice, be increased? Why will heat increase it?
ART. 1 23.-How can the velocity of a current of water be ascertained?
Fig. 59.  What does Fig. 59 represent,? Explain it. How is the rapidity
of the current estimated? What is the use of the instrument?
ART. 124.-What causes waves? What is sometimes done to remove
this friction?
ART. 1 25.-What instruments are used for raising liquids?
ART. 1 26.-Where is the chain-pump used? What figure represents it?
Fig. 60. Explain the figure.
ART. 1 27.-What is said of the screw of Archimedes?
Fig. 61. Explain the use of the screw by Fig. 61.
ART. 1 28.-How are springs and rivulets formed?
Fig. 62. Explain Fig. 62.
ART. 1 29.-How high will a spring rise?
ART. 130.-How are fountains formed? How high will the water spout
through the ducts? What prevents the fluid from rising to the
same height with the reservoir?
Fig. 63.  Explain the method of making artificial reservoirs by Fig. 63.
ART. 13.-What is the siphon?
Fig. 64.  What figure represents a siphon? Explain it. In what manner is the siphon used? How can the siphon be used to show
the equilibrium of fluids? How high will the liquid rise in each
side of the siphon? What is Tantalus' cup?
Fig. 65.  What does Fig. 65 represent? Explain it.
ART. 132.-How, and for what purposes is water used as a mechanical
agent?  How many kinds of water-wheels are there?  What
are they?
ART. 133.-What is the overshot wheel? Where does it receive its
motion?
Fig. 66.  Explain Fig. 66. What causes the wheel to turn? How does
this wheel compare in power with the other water-wheels?
ARMT 134.-What is the undershot wheel? Where does it receive its
motion?




-                            Filg. 60.
Fig. 62
Fig. 63.         Fig. 64.      Fig. 65.            Fig. 66.
1/(2          ct p




354            QUESTIONS FOR EXAMINATION.
Fig. 67.  What does Fig. 67 represent? How does this wheel differ fromn
the overshot?
ART. 135.-What is the breast-wheel?  How is it set in motion?
Fig. 68. What figure represents the breast-wheel? Explain it. To
what is the motion given to the wheels which have been described, communicated?
PNEUMATICS.
ART. 136.-Of what does Pneumatics treat?
ART. 1 3 7.-What is the air which we breathe? How far does it extend
above the surface of the earth? Does it possess properties common to liquids in general?  How does its specific gravity compare with that of water? Of what two principal ingredients
does the air consist? What is the proportion of these parts to
each other?
ART. 138. —What other fluids are named belonging to the class of elastic fluids?
ART. 139.-Have the air and other similar fluids weight? With what
power alone has heat to contend in a6riform fluids?
ART. 140. —What two principal properties has the air?
ART. 141. —What is the weight of a column of air one inch square at
the base, and reaching to the top of the atmosphere? Is the
pressure exerted equally in all directions?
ART. 142.-What is meant by the elasticity of the air?  How do the
a/riform fluids differ from liquids? When is the air said to be
rarefied? When condensed?  Is the air near the surface of
the earth rare or dense?
ART. 143.-How does the air become a mechanical agent?
ART. 1144.-What is a vacuum?
ART. 145.-What is a barometer?  What does the word barometer
mean? What is a thermometer?  What does the word thermometer mean? What is a hygrometer? What does the
word hygrometer mean?
Fig. 69.  What figure represents a barometer?  Explain its construction.
What height of mercury is the pressure of the atmosphere capable of sustaining?  What efiect has the pressure of the atmosphere on the mercury in the tube?  In what proportion
does the mercury rise and fall? In what way can barometers
be made of other fluids?  Why is mercury used in preference
to any other fluid? Is the air heaviest in wet or dry weather'?
On what principle is the pressure of the atmosphere on the mer



QUESTIONS FOR  EXAMINATION.                    355
cury, in the cup of a barometer, exerted? What follows from
this? For what other purpose, besides measuring the pressure
of the atmosphere, and foretelling the variations of the weather,
is the barometer used? Is the air the more dense at the surface of the earth or upon a hill? What is a thermometer?
Fig. 70   What figure represents a thermometer?  Explain its construction. What effect have heat and cold on most substances?
What follows from this? Whose scale is generally used in this
country? For what is the hygrometer used? Of what kind
of substances may it be constructed? What experiment is
given in the note to show the quantity of moisture raised from
the ground by the heat of the sun?
Fig. 67.                              Fig. 69.
i30
I..29
7 O                    L~~~~~~~~27
Fig. 68.                             Fig. 70
ml                                   i




356           QUESTIONS FOR EXAMINATION.
ART. 146.-Is air impenetrable, like other substances? How is this
shown? Upon what principle is the diving-bell constructed?
Fig. 71.  What figure represents the diving-bell?  Why does not the
water rise in the bell? Explain the figure.
ART. 1 47.-By what means is water raised in the common pump? How
is the pressure removed?
Fig. 72.  What figure represents the common pump? Explain it. Which
of the mechanical powers is the handle of the pump? How
high can water be raised by the common pump? Why? Why
is the common pump sometimes called the lifting-pump?
ART. 148. —How does the forcing-pump differ from the common pump?
Fig. 73. What figure represents the forcing-pump? Explain it.
ART. 149.-What is wind?  In what two ways may the motion of the
air be explained? Explain the manner in which the air is put
in motion. How are the north, south, east, and west winds
produced
ART. 150.-What is an air-pump?  What is the vessel called from
which the air is exhausted? On what principle are all airpumps constructed?  Can a perfect vacuum ever be obtained?
Fig. 74.  Describe the air-pump represented by Fig. 74.
Fig. 75.  Describe Wightman's patent lever air-pump, represented by
Fig. 75.
ART. 151.-Experiments with the air-pump.
Fig. 76.  What does Fig. 76 represent?  Explain the experiments that
are made with them.
Fig. 77.  What does Fig. 77 represent?  Explain the experiments nlade
with it.




Fig. 71.           Fig. 72.                  A         Fig. 73.
Fig. 73.
a                       VT
/ 13                             Fig. 74. 
Fig. 75.                                  Fig. 76.
iiX ~ ~~~~                       ms               Fig 77.




358            QUESTIONS FOR EXAMINATION.
Fig. 78.  What does Fig. 78 represent?  Explain the experiments made
with it.
Fig. 79.  What does Fig. 79 represent?  Explain the experiments made
with it.
Fig. 80.  What does Fig. 80 represent? Explain its use.
Fig. 81.  What does Fig. 81 represent, and what experiments are made
with it?
Fig. 82.  What does Fig. 82 represent?  Explain its use.
Fig. 83.  What does Fig. 83 represent? Explain the experiment made
with it.
Fig. 84.  What does Fig. 84 represent? Explain its use.
Fig. 85, 86. Explain the uses of Figures 85 and 86.
Fig. 87. What does Fig. 87 represent? Explain its use.
Fig. 88, 89. Explain the Figures 88 and 89, and the experiments which
are made with them.
Fig. 90.  What does Fig. 90 represent? Explain the experiments which
may be made with it.
Fig. 91.  What does Fig. 91 represent?  Explain the experiments to be
made with it.
ACOUSTICS.
ART 15 2.-What is that science called which treats of the nature and
laws of sound? What does it include?
ART. 153.-What causes sound? What illustrations are given to prove
this?
ART. 154.-In what proportion are sounds loud or faint? Why does a
bell sound louder in cold than in warm weather? Why is
sound fainter on the top of a mountain than near the surface
of the earth?
ART. 155.-What are sonorous bodies?
ART. 156.-To what do sonorous bodies owe their sonorous property?
Are all elastic bodies sonorous?
ART. 157. —What causes the sound produced by a musical string?
Upon what does the height and depth of the tone depend?
Which strings, in, a musical instrument, produce the low
tones? Why?




Fig. 80          Fig. 81.                  Fig. 82.
Fig. 79.
Fig  84.                         Fig. 85.
Fig. 83.
Fig. 86       Fig. 87     Fig. 88.
Fig. 89.
Fig. 90.                            Fig. 91.
\\                   Ullltf lil ll Ii
PW




360            QUESTIONS FOR EXAMINATION.
Fig. 92. Explain Fig. 92.
ART. 158.-Upon what is the science of harmony founded?  How is
the chord of an octave produced? How is the chord of a
fifth produced? How is a musical chord produced? A discord?
ART. 15 9.-Upon what does the quality of the sound produced by strings
depend? Upon what does that produced by wind instruments
depend?  What strings produce the lowest tones?  How may
different tones be produced from the same string? How may
different tones be produced from the same wind instrument?
ART. 160.-What, in some degree, affects the quality of the sound of
all musical instruments? What effect have heat and cold on
the materials of which the instrument is made? What follows
from this?  Why are most musical instruments higher in tone,
or sharper, in cold weather?
ART. 161.-Through which is sound communicated more rapidly, and
with greater power, through solid bodies, or the air? How fast
is it conducted by water? How fast by solids? What examples are given to show that sound is communicated more rapidly through solid bodies than the air or fluids?
ART. 162.-What is a stethescope?  Of what does it consist?  For
what is it used?
ART. 163.-How fast does sound move? Does the force or direction of
the wind make any difference in its velocity? What advantage results from this uniform velocity of sound? How can
the distance of a thunder-cloud be ascertained?
ART 164.-How is an echo produced?  Why cannot an echo be heard
at sea, or on an extensive plain? How must a person stand in
order to hear an echo? By what law is sound communicated
and reflected? What anecdote is related of Dionysius? Upon
what principle are speaking-trumpets constructed? Explain
the manner in which the vibrations of the air are reflected.
Upon what principle are hearing-trumpets constructed? How
far does the musical instrument, called the trumpet, act upon
the principle of the speaking-trumpet? How can the continued
sound, given by some shells, when held near the ear, be explained?
ART. 165-.-Upon what principle may whispering-galleries be constructed?
ART. 166. —In what way can sounds be conveyed to a much greater
distance than through the air? What are the tubes, used to
convey sounds, called? Why do the softer kinds of furniture




QUESTIONS FOR EXAMINATION.                    361
in a room affect the quality of the sound? What general rule
is given with regard to the reflection of sound? Is the air a
better conductor when it is humid, or when it is dry? Why
can a sound be heard better in the night than in the day?
ART. 167.-How is the sound of the human voice produced? How
are the tones varied and regulated?  Upon what does the
management of the voice depend? What is ventriloquism?
Was this art known to the ancients? What is supposed, by
some authors, concerning the responses at Delphi, Ephesus,
&c.? Is ventriloquism a natural gift, or an acquired one?
PYRONOMICS.
ART. 168.-What is Pyronomics?  What is said in regard to the nature of heat? Is it ponderable or imponderable? What effect has heat upon bodies? What two forces continually act
in opposition to each other?' In what can the effect of heat
be seen?  How does it separate the particles?  What would
be the effect were the heat removed?  Upon what has heat
the most remarkable effect?  How does it affect it?  What
effect has heat upon air? How is rain produced? What is
stated with regard to heat?  Can the progress of heat be arrested? What is caloric? In what two states does heat exist?
What is free heat? Give some examples of free heat. What
is latent heat?  Give some examples of latent heat. How are
the terms heat and cold generally used?  What illustration of
this is given?
ART. 169.-What are the three principal effects of heat on bodies to
which it is applied? Give an example of each. What are the
sources of heat?
ART. 1 7 O —In what way does heat tend to diffuse itself? Why do bodies
of the same absolute temperature appear to possess different degrees of heat?  What illustration of this is given? What appears from this?
Fig 92.
-e    - -                                       2 —--— F —l




362            QUESTIONS FOR EXAMINATION.
ATR. 1'71.-What causes the difference in the warmth of substances
used for clothing?
ART. 17 2.-In what two ways is heat propagated?  When is it propagated by conduction?  When is it propagated by radiation?
ART. 1 773.-Do all bodies conduct heat with the same degree of facility?
What bodies are the best conductors? In what order do the
metals stand with respect to their conducting power? Is wood
a good conductor of heat? Why are wool, fur, &c., so efficacious in preserving the warmth of the body? What is related in the note with regard to the conducting power of
heat?
ART. 174.-What bodies reflect the heat?  What bodies absorb the
heat? Why do bright bodies, when placed near the fire, seldom become heated? Will snow melt most readily under white
or black cloth?
ART. 17;5.-What effect is produced on all bodies when violently compressed or extended? What experiments are here related to
illustrate this? What is said of the air when strongly compressed?
ART. 17 6.-Into what classes are all substances, as affected by heat,
divided? What substances are combustible? What substances
are incombustible?
ART. 17 7.-What is a pyrometer?  Of what does it consist? How does
Wedgewood's pyrometer measure high temperatures?
ART. 178.-What is the most obvious and direct effect of heat on a
body? What application of this principle is related in the
note? What is said of the effect of heat and cold on glass?
When hot water is suddenly poured into a cold glass, why will
the glass crack? When cold water is applied to a heated
glass, why will the glass crack?
ART. 179. —Is the expansion caused by heat in solid and liquid bodies
the same in all substances?  How do a6riform fluids differ, in
this respect, from solid and liquid bodies? Upon what does
the expansion of solid bodies in some degree depend? Why
has heat more power over gases and vapors?
A 11T. 180.-What effect has heat and cold upon the density of all substances?  What exception is there to this remark?  Why are
the vessels, containing water and other similar fluids, so often
broken when the liquid freezes in theml?  Why does ice float
upon the water, instead of sinking in it? What is stated in the
note with regard to this property of water?




QUESrIONS FOR EXAMINATION.                    363
ART. 1 81.-Can all bodies be raised to the same temperature by the
same quantities of heat? What bodies retain their heat the
longest?
ART. 1 82.-What becomes of the heat which is thrown upon a bright
or polished surface? How do the angles of incidence and reflection compare with each other?
ART. 183.-When is water converted into steam or vapor?
ART. 1 84.-How does the temperature of the steam compare with that
of the liquid fromn which it is formed while it remains in contact
with that liquid?
ART. 185.-By what is the elasticity of steam increased and diminished?
Upon what does the amount of pressure, which steam exerts,
depend?
ART. 186.-What is the great and peculiar property of steam, on which
its mechanical agencies depend?
ART. 187.-What is the steam-engine?
ART. 188. —How much larger space does steam occupy than water? By
what mode is steam made to act? By what impulse does the
piston rise? What causes the piston to descend? What improvement did Mr. Watt introduce into the steam-engine?
Fig. 93.  What does Fig. 93 represent?  What are the principal parts?
What does A represent? What does C represent? What does
B represent? What does K represent? By what is the steam
led off from the cylinder? What does L represent? What
does O represent? What does M represent? What does N
represent? What does R represent? When the valves are all
open, what becomes of the steam? When the valves F and Q
are closed, and G and P open, upon what does the steam press?
What does the cylinder draw with it in its descent? Which of
the mechanical powers is this working-beam? What are attached to this working-beam? What is their use? What becomes of the steam when the stop-cocks G and P are closed, and
Fig. 93.




364            QUESTIONS FOR EXAMINATION.
F and Q are open? How is the regular and continued motion
produced?  To what is this motion of the piston communicated? What is the use of the air-pump M? For what is the
safety-valve R used?
ART. 1 89.-How is the power of a steam-engine expressed? What is
an engine of 100 horse power?
ART. 190.-What are the principal forms in which the steam-engine is
constructed?  How do they differ from each other? What becomes of the' steam after having moved the piston in the noncondensing engines?  What kind of engines is generally used
on railroads? What becomes of the steam after having moved
the piston in the condensing engines?
Fig. 94.  What does Fig. 94 represent? What does A represent? What
does B represent? What does C represent? What does D
represent? What does E represent? What does F represent?
What does G G represent? What does I represent? What
does K represent? What does LL represent? What does
M M represent? What does N N represent? What does 0 0
represent? What is said of the governor?
Fig. 94.
Tc    1  D      L
u  Cr..                                       I




ART. 19 l.-Describ e the locomotive steam-engine.        \     c
Fig. 95.  In Fig. 95, what do F and D represent?  What \
do the following references respectively represent,
namely, SSS?  BBB?  pppp?  CC?  X? \
L? PT NN? RGKK?
Q 111                             Fig. 95.
0         PI
B[0 7
F   in,~,,,-, —-----— 7                                           C  C
> ti   S   8   > >                           -  n~~~~~~~~~~~~~~~~~S -
__= —_ —- 
d  il




366            QIUESTIONS FOR EXAMINATION.
OPTICS.
ART. 1920-Of what does Optics treat?  Into what classes does the
science of Optics divide all substances? What are luminous
bodies?  Give an example of a luminous body. What are
transparent bodies? Give an example of a transparent body.
What are translucent bodies? Give an example of a translucent body. What are reflecting substances?  Give an example of a reflecting body. What are refracting substances?
What are opaque substances? What is light? What did Sir
Isaac Newton suppose it to be?  What other opinions have
been formed concerning it?
ART. 193. —What is a ray of light?
ART. 194. —When are rays of light said to diverge?
Fig. 96. What does Fig. 96 represent?
ART. 195. —When are rays of light called converging? What is the
point, at which converging rays meet, called?
Fig. 97. What does Fig. 97 represent?
ART. 196. —What is a beam of light?
Fig. 98. What does Fig. 98 represent?
ART. 1 97.-What is a pencil of light?
ART. 198.-What is a medium
ART. 199.-In what manner do the rays of light proceed from terrestrial
bodies? In what kind of lines do the rays of light proceed from
the sun?
ART. 200 —In what way is light projected forward from any luminous
body? With what rapidity does it move?
ART. 201l.-From  what point, in a luminous body, does light radiate?
ART. 202.-How is a shadow produced? Why are shadows of different
degrees of darkness? To what is the darkness of a shadow
proportioned, when the shadow is produced by the interruption
of the rays from a single luminous body?
ART. 203.-What is said of the shadow of the opaque body when the
luminous body is the larger?
Fig. 99. Explain Fig. 99.
ART. 204.-What is said of the shadow of the opaque body when the
luminous body is the smaller?
Fig. 100. Explain Fig. 100.
ART. 205.-How many shadows are produced when several luminous
bodies shine upon the same object?
Fig. 101. Explain Fig. 101.




QUESTIONS FOR EXAMINATION.                      367
ART. 206.-What is the consequence when rays of light fall upon an
opaque body which they cannot pass? What is meant by the
reflection of light?
ART. 207.-What is Catoptrics?
ART. 208.-By what laws is light governed? How is light reflected
when it falls perpendicularly on an opaque body? How is it
reflected when it falls obliquely?  How do the angles of incidence and reflection compare with each other? By what light
are opaque objects seen? By what light are luminous objects
seen?
Fig 102. Explain the angles of incidence and reflection. Is the same
principle applicable to all kinds of surfaces?  Explain its application to mirrors.
Fig. 96.                               Fig. 97.
Fig. 98.                             Fig. 99.
A      B
Fig. 100.
D
Fig. 101.                            Fig. 102
M              i
A             B
b    ~1D
MN              R




3(8           Q(UESTIONS FOR EXAMINATION.
ART. 209.-Why is the intensity of light diminished every time it is reflected?
ART. 210.-Does every portion of a reflecting surface reflect an entire
image of the luminous body shining upon it? When the sun or
moon shines upon a sheet of water, why do we not see an image reflected from every portion of the surface? Why do objects, seen by moonlight, appear fainter than when seen by
daylight? By what light does the moon shine? What absorbs
some of the rays of light in traversing the atmosphere?
ART. 21 1.-How are all objects seen?  Why can none but luminous
bodies be seen in the dark?
ART. 21 2.-What kind of an image is formed when rays of light, proceeding from an object, enter a small aperture?
Fig. 103. Illustrate this by Fig. 103. What is a camera obscura?  How
can a portable camera obscura be made?
ART. 213.-How is the angle of vision formed?
Fig. 104. Explain Fig. 104.
Fig. 105. What does Fig. 105 represent? What effect has the nearness
of the object to the eye, on the angle? Illustrate this by the
figure. Upon what does the apparent size of all object depend?
Why do objects appear so large? To what is the art of perspective drawing indebted for its accuracy?
ART. 21 4.-How large an angle must a body subtend to be visible?
ART. 21 5.-When is the motion of a body invisible? Why is the motion of the heavenly bodies invisible? Upon what does the real
velocity of a body, in motion round a point, depend?
Fig. 106. Explain Fig. 106. Why does the velocity of both, to an eye at
E, appear to be the same?
ART. 216.-How many kinds of mirrors are there?  What are plain
mirrors?  Do they magnify or diminish the object? What are
convex mirrors? What part of a sphere is a convex mirror?
What are concave mirrors'? What part of a sphere is a concave mirror?
Fig. 107. In Fig. 107, which part of the sphere represents a convex mirror? Which part a concave mirror?  Explain the figure.
ART. 2 1 7.-How does the image of an object reflected from a convex
mirror compare with the object?
Fig. 108. Give the illustration.
ART. 21 S.-What is the focus of a concave mirror?
ART 219.-When an object is further from the concave mirror than the
focus, how will the image appear? When the object is between




QUESTIONS FOR  EXAMINATION.                   369
the mirror and the focus? How must the object be placed that
the image may appear upright?  And as the object is removed
towards the mirror?
ART. 220.-If the object be placed between the mirror and the focus,
how does the image compare with the object?
Fig. 103.                                  Fig. 104.        A
B
Flg. 105.                              Fig. 106.
F   D    A.
G    E    B                           _,.~ B
Fig 107.
G
Fig. 108.
16*




370            QUESTIONS FOR EXAMINATION.
Fig. 109. Explain Fig. 109. What peculiar properties have concave
mirrors? What facts are stated with regard to convex mirrors,
as resulting from the fundamental law of Catoptrics? What is
said of parallel rays? What is said of diverging rays? What
is said of converging rays, when they tend towards the focus of
parallel rays? What is said of converging rays, when they
tend to a point nearer the surface than the focus? What is
said of converging rays, when they tend to a point between the
focus and the centre? What is said of converging rays, when
they tend to a point beyond the centre? What is said of converging rays, when they tend to the centre?  What is said
with regard to parallel rays, when reflected from a concave
surface? What is said of converging rays? What is said of
diverging rays, if they diverge from a focus of parallel rays?
What, if from a point nearer to the surface than that focus?
What, if from a point between that focus and the centre? If
from a point beyond the centre? If from the centre?
ART. 221.-What is Dioptrics?
ART. 222.-What is meant by the refraction of light? When does this
take place?
ART. 223.-What is a medium, in Optics?  Give some examples of
media. In what proportion is a medium dense or rare?
ART. 221.-What are the three fundamental laws of Dioptrics?
Fig. 110. Illustrate the first law by the line A B, in Fig. 110. Illustrate
the second law by the line C B. Illustrate the third law by
the line F B. In what proportion does the refraction increase
or diminish? Why does an oar or a stick, when partly immersed in water, appear bent? Why does the part which is in
the water appear higher than it really is? Why does a body
of water, when viewed obliquely, appear more shallow than it
really is? In what direction can we look so as to cause no
refraction? What experiment is here related?
ART. 225.-Why do we not see the heavenly bodies in their real situation? In what direction do we see them? What causes twilight? Upon what does the duration of twilight depend?
What other reason is given why we do not see the heavenly
bodies in their true situation? When does the refraction of
light not affect the appearance of the heavenly bodies? Why
do the heavens appear bright in the daytime?
ART. 226.-What effect is produced when a ray of light passes from one
medium to another, and through that into the first again?
Why does the refractive power of flat window-glass produce
no effect on objects seen through it?




QUESTIONS FOR  EXAMINATION.                    371
ART. 227.-What is a lens?  How  are all lenses to be considered?
What is a single convex lens?
Fig. 111. What part of Fig. 111 represents a single convex lens?
What is a single concave lens? What part of Fig. 111 represents a single concave lens? What is a double convex lens?
What part of Fig. 111 represents a double convex lens? What
is a double concave lens? What part of Fig. 111 represents a
double concave lens? What is a meniscus?  What part of
Fig. 111 represents a meniscus? What is the axis of a lens?
What line in Fig. 111 represents the axis of all the five
lenses?
ART. 228.-What is stated with regard to the form of the lenses? How
is light refracted in passing from a rarer to a denser medium?
How, in passing from a denser to a rarer? What must be
considered in estimating the effect of lenses? Through what
must a perpendicular, to any convex or concave surface, always, when prolonged, pass? What is stated with regard to
convex lenses?  What, with regard to concave lenses?
ART. 229.-What is the focal distance of a lens? To what is this equal
in a single convex lens? To what is it equal in a double convex lens?
Fig. 109.
Fig. 110.                          Fig. 111.
0                      A       B        C        D    E
E    F )




372            CIUESTIONS FOR EXAMINATION.
ART. 230. —When parallel rays fall on a convex lens, which one is perpendicular to its surface? How are the other rays, falling
obliquely, refracted? What property of a convex lens gives it
its power as a burning-glass? Where are all the parallel rays
of the sun, which pass through the glass, collected? How
does the heat at the focus compare with the common heat of
the sun? What is related with regard to the effects of lenses
produced by burning-glasses?
ART. 23 1.-What is the first effect related as resulting from the laws of
refraction with regard to convex surfaces? What is said of
diverging rays?  What is said of rays converging towards the
centre of convexity? What of rays converging to a point beyond the centre of convexity? What of rays converging to a
point nearer the surface than the centre of convexity? When
the rays proceed out of a denser into a rarer medium, what
occurs? What is stated of parallel rays, proceeding from a
rarer into a denser medium, through a concave surface? What
is said of diverging rays?  What is said of converging rays?
Of what are the above eight principles the necessary consequence? What is the reason that so many different principles
-are produced by the operation of these laws?
ART. 232.-For what are double convex and concave glasses, or lenses,
used in spectacles? What glasses are used when the eye is
too flat? What are used when the eye is too round?
ART. 233.-Of what is the eye composed?  What are the different
parts of the eye? First?  Second?  Third?  Fourth?  Fifth?
Sixth? Seventh?  Eighth?  Ninth?  Tenth?
Fig. 112. What does Fig. 112 represent? Explain the figure.
Fig. 113. What does Fig. 113 represent? Explain the figure. What
part of the eye does the cornea form? Is its degree of convexity the same in all persons and all periods of life? What
is its principal office? From what does the iris take its name?
What is the use of the iris?  What is the pupil?  What is its
form in the human eye? How much more light is the pupil
capable of admitting, when expanded to its utmost extent, than
when most contracted? What is said of those animals which
are said to see in the dark?  What light, only, is of use in
vision?  What becomes of the light which falls on the iris?
What is the aqueous humor? What is its form? Of what use
is it? What is the crystalline lens? What is its office? What
is the vitreous humor? Why do persons sometimes experience
pain when passing from a dark place into strong light? What
is the shape of the vitreous humor?




QUESTIONS FOR  EXAMINATION.                    373
Fig. 114. Explain Fig. 114. What is the retina? What is the choroid?
By what is its outer and inner surface covered? What is its
office?  What is the opinion of some philosophers with regard
to the choroid?  What is the sclerotica?  From what does it
derive its name?  What is its office?  What are attached to
the sclerotica? What is the optic nerve?
ART. 234.-What philosophical instrument does the eye resemble in its
construction?
Fig. 115. Explain Fig. 115. Why do the objects appear erect when the
images are inverted? Why do we see only one image when
an image is formed on both eyes? What are the defects which
are remedied by the use of concave and convex lenses? In
what other way is the eye subject to imperfection? Is there
any remedy for this? By what is the convexity of the crystalline humor increased or diminished? What is effected by
this means?
Fig. 112.                                 Fig. 113.
Fig. 114.
lfr   a!




374            QUESTIONS FOR  EXAMINATION.
ART. 235. —What is a single microscope?  What is the use of this microscope?  What figure represents a microscope?
Fig. 116. Explain Fig. 116. What lenses have the greatest magnifying
power?  What lenses have the shortest focus?
ART. 236.-Of what does a double microscope consist?  What is the
use of these two lenses?
Fig. 117. What does Fig. 117 represent? Explain the figure.
ART. 237.-What is the solar microscope?  Of what does it consist?
By what, in this microscope, are the sun's rays reflected, and
upon what? For viewing what objects, only, is the microscope,
above described, used? How do those microscopes, used for
viewing opaque objects, differ from these? How is the image
then formed? How is the magnifying power of a single microscope ascertained? Illustrate this. How is the magnifying
of the compound microscope ascertained?  In what proportion
is the magnifying power of the solar microscope? Illustrate
this. How may a lens be made to magnify or diminish an
object?
ART. 238.-What is the magic lantern? How are objects, viewed by
the magic lantern, generally represented? What figure represents a magic lantern?
Fig. 118. Explain Fig. 118. In what proportion will the size of the
image increase or diminish?
ART. 239.-What is a telescope?  How many kind- of telescopes are
there? What are they? What is a refracting telescope?
What is a reflecting telescope? Why is the image of an object, seen through a refracting telescope, less clear and perfect
than when seen through a reflecting telescope? How many
kinds of refracting telescopes are there? What are they'!
How do they differ the one from the other?
Fig. 119. What does Fig. 119 represent? Explain the figure.




Fig. 116.
Fig. 117.
Fig. 118.
Fig. 119.
A               M
O 1Ci jIui 3zzcc7I7   N




376          QUESTIONS FOR EXAMINATION.
Fig. 120. What does Fig. 120 represent? Explain the figure.
Fig. 121. What does Fig. 121 represent? Explain the figure. Why are
mirrors used in reflecting telescopes?  What is the use of the
lens?  What is the advantage of the reflecting telescope?
ART. 240.-What is Chromatics? What causes color?
ART. 241.-Of what is light composed?  How can these rays be separated? By whom was this discovery made?
ART. 242.-What is a prism?  How may a prism be made?
ART. 243.-How many colors enter into the composition of light? What
are they? Do these rays all have the same degree of refrangibility?
ART. 244L.-What takes place when light is made to pass through a
prism?
Fig 122. Explain Fig. 122. Why do the red rays fall on the lowest
part of the screen? What is supposed with regard to the red
rays?  What with regard to the blue, indigo, and violet rays?
Why does the sun appear red through a fog?  Why does the
sky appear of a blue color? What would be the appearance
of the sky if the atmosphere did not reflect any rays? Is
white a simple color? How is it produced? The spectrum
formed by a prism being divided into 360 parts, how many of
these parts does the red occupy? The orange? The yellow?
The green? The blue? The indigo? The violet? What
are the colors of all bodies? What appears from the experiments of Dr. Wollaston?
ART. 245.-How is the rainbow produced? How is this proved? First?
Second? Third?
ART. 246.-Upon what does the color of all bodies depend? Of what
color do bodies generally appear? When will a body appear
of a compound color? Of what color will a body appear that
reflects all the rays? When will a body appear black? Is
color an essential property of a body? Of what color do bodies
appear in the dark? Why do some bodies appear differently
by candlelight? What is necessary to produce color?  What
experiments are related to prove the truth of the above? What
rays does a body reflect in the greatest abundance? In what
proportion does it reflect the other rays? Why do the green
leaves of a rose appear to have a brown tinge? What does
the brightness and intensity of a color show? Why do some
bodies change their color?
ART 247.-What is a multiplying-glass?  How many times will art
object, viewed through a multiplying-glass, be multiplied?
What is the principle of the multiplying-glass?




QUESTIONS FOR EXAMINATION.                    377
ART. 21S.-Of what does the kaleidescope consist?  From what is the
word kaleidoscope derived, and what does it mean? I)y whom
was the instrument invented? What is here said with regard
to the kaleidescope? In what way does Optics treat of light?
What is the thermal property of light? Are the thermal properties of the rays the same, or different in each color of the
prism? How do they differ?  How do the chemical agencies
differ? Do these powers exist in all light, or in solar light only?
Where is the greatest heating power found?  Where the
greatest chemical power?  Where the greatest optical power?
In what are the chemical powers of light shown? Name the
experiments and explain them. Explain the art of Photography,
or Heliography. By what name is it now known?  How else
are the chemical effects of light seen?
Fig. 120.
A
B
Fig. 121.
Fig. 122.
Violet........
IndigPo.. -- - 
Blue....  -    o I
Green.._-  
Orange -,, _ w 




378             QUESTIONS FOR EXAMINATION.
ELECTRICITY.
ART. 219.-What is electricity?  How can electricity be seen?  How
are these effects exhibited? What illustration of this is given?
What is said of the surfaces which have acquired the power of
attraction?  What are electrics?  What are non-electrics?
What is stated with regard to the word electricity? By whom
was this property first discovered? What is stated with regard
to the nature of electricity?  Whose and what opinion is
adopted in this volume? When is a substance said to be
positively electrified? When is it said to be negatively electrified? What does positive electricity imply? What does negative electricity imply? In how many ways may electricity be
excited? What are the different kinds of electricity called?
ART. 250.-Into how many kinds does the science of electricity divide
all substances?  What are they?
ART. 25 I.-What is said with regard to the communication of the electric fluid from one substance to another? Will all substances
allow it to pass through them? What bodies are called conductors? What bodies are called non-conductors? What has
been found, by experiment, with regard to electrics and nonelectrics?  What substances are electrics or non-conductors?
Why must these substances be dry? What substances are
non-electrics or conductors? What substances are mentioned
as imperfect conductors?
ART. 252. —When is a substance said to be insulated?  Give the
examples.
ART. 253.-When a communication is made between a conductor and
an excited surface, where is the electricity from the excited
substance conveyed? When is it said to be charged? When
a communication exists by means of any conducting substance,
between a body containing more than its natural share of the
fluid and the earth, what will become of the redundant quantity
which the body possesses? What illustration of this is given?
What follows if this chain of conducting substances be interrupted?
ART. 254. —What is the simplest mode of exciting electricity? What
illustration of this is given?
ART. 255.-What is the electricity excited in glass called?  What is
that obtained from resinous substances called?
ART. 256f.-What is stated with regard to positive and negative electricity? What follows when one side of a metallic, or other




QUESTIONS FOR EXAMINATION.                    379
conductor, receives the electric fluid? What follows when an
electric is presented to an electrified body?  What follows
when two surfaces, oppositely electrified, are united?
ART. 25 7.-How do similar states of electricity act on each other? How
do dissimilar states act on each other? Give the examples.
ART. 258.-For what is the Leyden jar used?
Fig. 123. What does Fig. 123 represent? What is a Leyden jar? When
is the Leyden jar said to be charged? How can the jar be discharged? Canll an insulated jar be charged?
ART. 259.-Of what is an electrical battery composed?  How are the
inner coatings of the jars connected together?  How are the
outer coatings connected? In what way is the battery charged?
ART. 260.-What is the jointed discharger?
Fig. 124. What does Fig. 124 represent? Of what does it consist?
ART. 261.-What is necessary when a charge of electricity is to be sent
through any particular substance?
ART. 262.-In what way do metallic rods, with sharp points, attract the
electric fluid?  How can the electricity be made to pass off
silently? Upon what principle are lightning-rods constructed?
When is electricity said to be communicated by induction?
ART. 263.-When is electricity produced by transfer?
ART. 264.-For what purpose is the electrical machine constructed?
Upon what principle are all electrical machines constructed?
Fig. 123.                              Fig. 124.
~. —




380            QUESTIONS FOR EXAMINATION.
How is the electricity excited? Of what is the amalgam composed?  In what form is the glass surface made?  When is
the machine called a plate machine? When is it called a cylinder machine?
Fig. 125. What does Fig. 125 represent? Explain the figure. Explain
the operation of the machine. To what must the chain be attached when positive electricity is required? To what must it
be attached when negative electricity is wanted? What is the
first experiment mentioned with the electrical machine? What
does the word electrometer mean? Of what does it sometimes
consist?  What is an electroscope?  What is the second experiment? What is the third? What does this show? What
is the fourth? What is the fifth?
Fig. 126. What is the sixth? What is the seventh? How may the jar
be filled with negative electricity? What is the eighth? What
is the ninth? What is the tenth?
Fig. 127. What is the eleventh?  What is the twelfth?
ART. 265.-What is the universal discharger?  What is its use?
Fig. 128. What figure represents it? Of what does it consist? What
is necessary in using the universal discharger? What is effected by this means?  What experiments are shown by means
of the universal discharger? How must the substance be placed?
How may ether or alcohol be inflamed?
ART. 266. —What are the electrical bells?
Fig. 129 What figure represents them? What are they designed to
show?  How are they to be applied?
Fig. 130. Explain the spiral tube.




Fig. 125.
E
Fig. 126.                                       Fig. 127.
Fig. 128.                                   Fig. 129.
W 4E.130Bi
Fig. 130.
o    f  And~~~blO _a~0   Oe09 N 0~  




382            QUESTIONS FOR EXAMINATION.
Fig. 131. Explain the hydrogen pistol.
Fig. 132. What does Fig. 132 represent?
Fig. 133. Explain Fig. 133.
Explain experiment one.
Fig. 134. Explain the second experiment. The third.
Fig. 135. Explain the fourth experiment. The fifth. The sixth. The
seventh. What is lightning? What is thunder?  How  is
the aurora borealis supposed to be caused? How is the electricity which a body manifests by being brought near to an
excited body, without receiving a spark from it, said to be
acquired?  When an insulated, but unelectrified conductor,
is brought near an insulated charged conductor, what is said
with regard to the end near the excited conductor? What
example is given to illustrate this?  Why are square rods
better than round ones to conduct electricity silently to the
ground, and thus protect buildings from lightning?  How far
beyond the rod do lightning-rods afford protection?  In what
way are the most approved lightning-rods constructed? What
is remarked with regard to the terms negative and positive?
How can this be illustrated? What is said with regard to
the time the electric fluid occupies in its passage through its
circuit? What example is given to show that the fluid prefers
the best conductors? In what different ways does the electric
fluid sometimes pass in thunder-storms?  Why is it unsafe,
during a thunder-storm, to take shelter under a tree, or to hold
in the hand any edge-tools? What position is the safest in a
thunder-storm? When is there no danger to be apprehended
from the lightning'? By whom were lightning-rods first proposed? Who first discovered that thunder and lightning are
the efbects of electricity?
GALVANISM, OR VOLTAIC ELECTRICITY.
ART. 267. —What is Galvanism? By whom and when was galvanism
discovered?  What led to the discovery?  How is electricity
generally produced? How is galvanic electricity produced?
How does the motion of the galvanic fluid, excited by galvanic
power, differ from that explained in the science of Electricity?
What bodies are most easily affected by the galvanic fluid?
ART. 268.-How  is the galvanic flnid or influence excited?  What illustrations of this are given,?




QUESTIONS  FOR EXAMINATION.                      st63
ART. 269.-Into what are the conductors of the galvanic fluid divided?
What substances are perfect conductors? What substances
are imperfect conductors?
ART. 270.-What is necessary in order to produce galvanic action?
ART. 27 1.-Of what is the simplest galvanic circle composed?  What
process is usually adopted for obtaining galvanic electricity?
Fig. 136. Illustrate this by Fig. 136. What effect will be produced if,
Fig. 131.                              Fig. 133.
Fig. 132.
Fig. 134.
Cn:                          Fig. F36.
Fig. 135.
I ] 1        E E u I
I_   iA           l                        i I   LI




384            QUESTIONS FOR EXAMINATION.
instead of allowing the metallic plates to come into direct contact, the communication between them be effected by wires?
How many parts are there in the above arrangement?  What
are they? What effect does the acid produce?  What is the
electrical state of the zinc?  Of the copper?  What are the
arrows in Fig. 136 designed to show? Where must the substance, to be submitted to the action of the fluid, be placed?
What is said of the electrical effects of a simple galvanic circle?
What examples are given illustrating the operation of simple
galvanic circles?
ART. 272.-How may the galvanic effects of the simple circle be increased? What are compound galvanic circles?
ART. 273.-Of what does the voltaic pile consist?
Fig. 137. What does Fig. 137 represent? How may a voltaic pile be
constrpcted? Can any other metal be used?  What are the
arrows in the figure designed to show?
ART. 274. —What is the voltaic battery? What is said with regard to
the electricity excited by the battery?
Fig. 138. What does Fig. 138 represent? Of what does the voltaic battery consist? How is the communication between the first and
last plates made? Where must the substance, which is to be
submitted to galvanic action, be placed? How can a compound
battery of great power be obtained?
Fig. 139. What does Fig. 139 represent? Of what does this battery consist? How can the electric shock from the voltaic battery be
received by any number of persons?
Fig. 140. What figure represents Smee's galvanic battery? Describe it
What liquid is employed for this.battery?
Fig. 141. Describe Fig. 141.
Figs. 142, 143. Describe the sulphate of copper battery.




Fig. 137.                                    Fig. 138.
Fig. 139.                                     Fig. 140.
c zCe z 20
FPig. 141.
Fig. 142.                            Fig. 143.
17




386            QUESTIONS FOR EXAMINATION.
Fig. 144.'Describe Grove's battery. Which is the most powerful battery?
For what is it used?
ART. 275.-What effects may be produced by a spark from a voltaic
battery? What precaution is, taken in regard to the lines of
the circuit?
ART. 276.-In how many ways does the electricity produced by the galvanic or voltaic battery differ from that obtained by the ordinary
electrical machine? What is the first?  What is here meant
by intensity? How does the' quantity of electricity obtained by
galvanic action compare with that obtained by the machine?
To what may the action of the electrical machine be compared?
To what may the galvanic action be compared? What is the
second way in which they differ?  What is the third?  What
is said in the note with regard to the third circumstance in
which the electricity obtained by the ordinary electrical machine differs from that produced by the galvanic battery? What
is said of the effects of this continued current on the bodies
subjected to its action?
ART. 277. —0n what does the effect of the voltaic pile on the body depend?  What facts in common life does galvanism  explain?
On what does the effects of galvanic action depend? What are
batteries constructed of large plates sometimes called? Why?
Describe them. Upon what principle is the calorimotor constructed? Are galvanic batteries of value?
MAGNETISM AND ELECTRO-MAGNETISM.
ART. 278.-Of what does Magnetism treat? How many kinds of magnets are there? What are they? What is the native magnet?
What property does it possess? What is an artificial magnet?
What magnet is preferred, for all purposes of accurate experiment?  How can an artificial magnet be made?
ART. 279.-What is the first property of the magnet? Second? Third?
Fourth?
ART. 280.-What is meant by the polarity of a magnet? Where is the
attractive power of a magnet the strongest? When will a
magnet assume a position directed nearly north or south? What
is the north pole of the magnet? What is the south pole? In
what ways can a magnet be supported so as to eliable it to
manifest its polarity?
ART. 281.-How  do the same and different poles of a magnet affect
each other? What is said with regard to the attraction of
magnets, whether native or artificial? What analogy is there




QUESTIONS FOR EXAMINATION.                  387
between the attractive and repulsive powers of the different
kinds of electricity, and the northern and southern polarities of
the magnet?
ART. 282.-Can a magnet communicate its properties to other bodies?
To what substances, only, can these properties be conveyed?
Of what substances are all natural and artificial magnets, as
well as the bodies on which they act, composed?  How call
the powers of a magnet be increased? What is a horse-shoe
magnet? How can it be made to sustain a considerable
weight?  What is this bar called? How does soft iron differ
from hardened iron, with respect to its acquiring and losing the
magnetic power?
ART. 283.-What effect is produced when a magnet is broken or divided? Why is this a remarkable circumstance?
ART. 284.-Where does the magnetic power of iron or steel wholly
reside 7 In what particulars do magnetism and electricity resemble each other? What is the first? What is the second?
What is the third? What is the fourth?
ART. 28S.-What effect has heat on the power of the magnet?  By
what is the magnetic attraction diminished? What effect has
electricity on the poles of a magnet? What effect has electricity sometimes on iron and steel?
ART. 286.-What proportion do the effects produced by two magnets,
used together, bear to that of either, used alone? What is
meant by the inclination or dipping of the magnet?
ART. 287.-Does the magnet, when suspended, invariably point to the
north and south points? What advantage has the science of
magnetism rendered to commerce and navigation? Of what
does the mariner's compass consist? To whom is the invention
of the mariner's compass usually ascribed? How may the
value of this discovery be estimated?
Fig. 144.




388            QUESTIONS FOR EXAMINATION.
ART. 288.-Where are the north and south poles of a magnet the most
powerful? What effect has a magnet on a piece of iron, when
it is brought sufficiently near to it? How are artificial magnets
made? Does the magnet which is employed in magnetizing a
steel bar lose any of its power by being thus employed? What
is a magnetic magazine? How is a magnetic needle made?
What is said with regard to a horse-shoe magnet? How
should a horse-shoe magnet be kept?,
ART. 289.-Of what does Electro-Magnetism treat.? What is the electric current? What does the science of electro-magnetism
explain? What is the difference between the currents in the
single and the compound circles? What is it thought causes
magnetic attraction? What discovery was made in the year
1819?  By whom? What further discovery was made soon
after, and by whom? What does this philosopher maintain?
How many kinds of electricity are there? How do the phenomena exhibited in these five kinds of electricity differ? Can
magnetism be developed in steel not previously possessing it?
Where must the steel be placed?* What property has the
uniting wire?
ART. 290. —What are the principal facts in connexion with the science
of electro-magnetism?  What is the first?  What is the second? What is the third? What is the fourth?  What is the
fifth?  What is the sixth?  What is the seventh?  Where
have the bodies been supposed to be placed, in all the effects of
electricity and galvanism that have hitherto been described?
What is the eighth fact in connexion with the science of electromagnetism?  What is the ninth?  What is the tenth?  How
can the direction of the electric current be ascertained?
ART. 291.-If a magnet be freely suspended, and a current of electricity
be passed near it, what direction will it assume? What illustration of this is given?  What second illustration is given?
In what direction will thhe pole of the needle next to the negative end of the wire move, if the connecting wire be placed
below the plane in which the needle moves, and parallel with
it? What is said with regard to the attractions and repulsions?
ART. 292.-How may the two sides of an unmagnetized steel needle
become endued with the north and south polarity?  Under
what circumstances will it become permanently magnetic?




QUESTIONS FOR EXAMINATION.                   389
ART. 293.-I-ow can magnetism be communicated to iron and steel?
How can the effect be more conveniently produced?  What
illustration of this is given?  What is the helix?  Why should
the wire, which forms the helix, be coated with some non-couducting substance? What is said of a helix, if it be placed
so that it may move freely?
ART. 294.-How can the magnetic needle be made to deviate from its
proper direction? What is a needle thus prepared called?
ART. 295. —What is the electro-magnetic rotation? What illustration
is given?
Fig. 145. What does Fig. 145 represent?  Explain the figure. How is
the freedom of motion, which is required on the wire, obtained?
How can the metallic contact which is required be obtained?
If the poles of a battery be connected with the horizontal external wires, c c, throughout, what direction will the current of
electricity take?  Round what pole will the moveable part of
the wire rotate? Round what will the magnetic pole rotate?
Fig. 146. What does Fig. 146 represent?  Of what does it consist?
How will the cylinders in each revolve, if, instead of a barmagnet, a horse-shoe magnet be employed, with an apparatus
on each pole similar to that which has now been described?
Fig. 145.                           Fig. 146.
b_     -11(1a                   




390          OQUESTIONS FOR EXAMINATION.
ART. 296. —How may the magnetizing power of the connecting wires
be increased? Is a single circuit preferable to a straight
wire?
Fig. 147. Explain Fig. 147.
ART. 297.-What is a bar called that is temporarily magnetized? How
do you ascertain the poles of an electro-magnet?
ART. 298.-How  have magnets of great power been formed?  What
weight was the magnet constructed by Professor Henry and
Dr. Ten Eyck capable of supporting?
Fig. 148. Explain the heliacal ring and Fig. 148
ART. 299.-How are bars of the U form most readily magnetized?




Fig. 147.
Fig. 148.




392             QUESTIONS  FOR  EXAMINATION.
Fig. 149. Explain Fig. 149.
ART. 300.-Explain the electro-magnetic telegraph.
ART. 301.-Of what does magnleto-electricity treat? How are electric
currents excited?
Fig. 150. Explain the magneto-electric machine, Fig. 150.
ART. 302.-What is thermo-electricity?  To what do the magnets owe
their peculiar properties? What follows from this? How many
states of electricity are there?  What is said of that derived
from the common electrical machine? What is said of that
derived from the galvanic apparatus? What is said of the
thermo-electric currents?
Fig. 149.
~~~~~~I _ _ _ _ _ _ _ __ _     _   _ _ __Fig.    150.
ililll'!ftlfl!~
U




QUESTIONS FOR EXAMINATION.                    393
ASTRONOMY.
ART. 303.-Of what does Astronomy treat? What is said of the earth?
How is the earth known to be round? Where is it situated?
ART. 304.-Of what does the solar system consist? What are the stars
supposed to be? Do we see more by the aid of glasses than
without?
ART. 305.-How may the planets be distinguished from the stars? How
are the planets distinguished from the fixed stars? What is
the meaning of the word planet? Why are they called planets? What are the fixed stars? What are the sun, moon,
planets, and fixed stars supposed to be? Why do they appear
so small? What has been stated with regard to the attraction
of portions of matter? Upon what force does this attraction
depend? What follows from attraction being mutual? What
direction do bodies take when actuated by several forces? Is
this true with regard to the heavenly bodies? What is the
centre of the solar system? What is said of the revolution of
the planets?
ART. 306.-What are the paths in which the planets move around the
sun called?  Around what do the planets revolve?  What is
a year on each planet? How long is the year of the planet
Mercury? How long is the planet Venus performing her revolution around the sun? How long is the earth performing her
revolution around the sun?  What is the length of the year on
the planet Mercury? Venus? Earth? Mars? Vesta? Juno?
Ceres? Pallas? Jupiter? Saturn?  Herschel?  Neptune?
Of what form are the orbits of the planets? What is meant
by the mean distance?  What planets are called inferior?
Why?  What planets are called superior?  Why?  What is
the distance of the planet Mercury from the sun?  Venus?
Earth? Mars? Vesta? Juno? Ceres? Pallas? Jupiter?
Saturn?  Herschel?  Neptune?  Have the planets any motion besides that around the sun? What is the time in which
they turn upon their axes called? What is the length of a
day on the planet Mercury? Venus? Earth?  Mars? Vesta?
Juno? Ceres? Pallas? Jupiter? Saturn? Herschel? Neptune?
ART. 307.-What is the diameter of the Sun?  Mercury?  Venus?
Earth? Mars? Vesta? Juno? Ceres? Pallas? Jupiter?
Saturn? Herschel? The Moon?
17*




394            QUESTIONS FOR EXAMINATION.
Fig. 151. What does Fig. 151 represent? What illustration of the comparative size and distance of the bodies of the solar system is
given? What is necessary in order to imitate the motions of
the planets in the above-mentioned orbits?
ART. 308.-What is the ecliptic?  Why is it called the ecliptic?
ART. 309.-What is the zodiac?  Why is it called the zodiac?
ART. 3 10.-What are the names of the twelve constellations? How
many degrees does each sign contain?
ART. 31 1.-Are the orbits of the other planets in the same plane with
that of the earth?
Fig. 152. What does Fig. 152 represent?  What are the nodes of a
planet?
Fig. 153. What does Fig. 153 represent?
ART. 3 1 2.-When is a planet said to be in any particular constellation?
ART. 313.-What do the perihelion and aphelion of a heavenly body
express? When is a body said to be in its perihelion?  When
is a body said to be in its aphelion? How much nearer is the
earth to the sun in its perihelion than in its aphelion? When
is a planet said to be in its inferior conjunction?  When is it
said to be in its superior conjunction? When is it said to be in
opposition?
ART. 31 4.-What do the apogee and perigee of a heavenly body express?
When is a body said to be in its perigee? When is it said to
be in its apogee?
Fig. 151.
Jupiter.
rercury Mars Vlenus  Karth.
O           QO   O   O    1 0) Herschel
0 ~ rce




Fig. 152.                     Fig. 153
~~~~:..f
b+      I  OI~~~~~~~~~?
(Ii~~~g     j     ~        ~!




396            QUESTIONS FOR EXAMINATION.
ART. 315o.-In what sign is the perihelion of the planet Mercury? Venus?' Earth? Mars? Vesta? Juno? Ceres? Pallas? Jupiter? Saturn? Georgium Sidus?
ART 316.-What is said with regard to the axes of the planets in their
revolution around the sun? What does this inclination of their
axes cause?
ART. 317. —What is said with regard to the motion of the heavenly
bodies?  When do they move with the greatest velocity?
When is their motion the slowest?
ART. 318.-What is Kepler's law?
Fig. 154. Illustrate this by Fig. 154. Explain, by Fig. 154, the reason
why the earth, or any other heavenly body, moves with a
greater degree of velocity in its perihelion than in its aphelion.
What is said of the motion of the heavenly bodies from perihelion to aphelion? What is their motion from aphelion to
perihelion?  When is their velocity the greatest?  How much
longer is the earth in performing the aphelion part of its orbit
than the perihelion part?
ART. 3 1 9.-How much nearer is the earth to tne sun in winter than in
summer'?
ART. 320.-What follows fromn the inclination of the earth's axis, with
regard to the direction of the sun's rays? When is the heat always the greatest? What is said of oblique rays? What is
the reason that.the heat is greate in summer than in winter?
Fig. 155. Illustrate this by Fig. 155. How is the earth situated with regard to its distance from the sun in winter? What illustration
of oblique and perpendicular rays is given in the note? Why
is it generally cooler early in the morning and late in the afternoon than at noon? Why is the heat the greatest at about
three o'clock?  What causes the variety of climate in different
Darts of the earth? Where does the sun always shine in a vertical direction? What would follow were the axis of the earth
perpendicular to its orbit? What causes the variety of the seasons, the different lengths of days and nights, &c.? What is
necessary in order to understand the illustration of the causes
of the seasons?
Fig. 156. Explain Fig. 156. What are the poles? Why is the circle I K
called the tropic of Cancer? What is the meaning of the word
tropic? Why is the circle L M called the tropic of Capricorn?
What are the tropics? What is the circle E F called? What
does it represent? What is the circle G H called? What does
it represent?




Fig. 154.
D   P
B
E
Fig. 155.
Fig. 156.
Ot           _//~t




398            QUESTIONS FOR EXAMINATION.
Fig. 157. What does Fig. 157 represent?  Explain the figure.  Explain,
by the figure, the situation of the earth on the 21st of June.
What causes day and night? To what part of the earth is
it day? To what part is it night?  To what is the length of
the day in proportion'? When are the days the longest? Why?
When are they the shortest?  Why?  Explain, by the figure,
the situation of the earth on the 22d of September. On the 23d
of December. On the 20th of March. What follows from
the changes on the earth, caused by the inclination of the
earth's axis?  In what proportion are these changes?  What
is said of the axis of the planet Jupiter? Is it supposed that
the sun, planets, and stars are inhabited? What is shown by
Fig. 157? Where are these points? What are they called?
Which is the vernal equinox'? Which the autumnal?  What
other two points are there? Why are they called solstices?
Where are these points?  Which is the summer solstice?
Which the winter?
ART. 321.-What is said of the sun? What is its diameter?  How
much does its cubic magnitude exceed that of the earth? How
long is it in performing its revolution around its axis?  How
has this been ascertained?  What did Dr. Herschel suppose
these spots to be?  What is the zodiacal light?  At what time
is it most distinct? Where is it constantly visible?
ART. 3 22.-What planet is nearest to the sun? Why is it seldom seen?
What is said of the heat of this planet? How much greater is
the sun's heat in Mercury than on the earth?  In what form
does water exist in Mercury?  How can Mercury be recognised when seen? At what time does it appear?  How does
Mercury appear when viewed through a telescope?
ART. 3 23.-What planet is nearest to the earth? When is Venus called
the morning star?  When is it called the evening star?  How
much greater are the light and heat at Venus than that at the
earth? What name was given by the ancient poets to Venus,
when morning star?  What, when evening star?  What is the
greatest distance at which the planets, Mercury and Venus, ever
appear from the sun? What is meant by the transit of these
planets? What is said of the different appearances which
Venus presents?  Why can we not see the planets and stars in
the daytime?
ART 324Am.-What planet is next to Venus?  What is the form of the
earth?  How much larger is its equatorial diameter than its
polar? How many moons has the earth? What is the diameter of the moon? What is its distance from the earth? What is




QUESTIONS FOR EXAMINATION.                    399
the length of a day at the moon? How long is it in performing
its revolution around the earth?
ART. 3 25.-What phases does the earth, when viewed from the moon,
exhibit? How much larger does the earth appear than the
moon?
ART. 326.-What planet is next to the earth? What renders it conspicuous?  What is supposed to cause this appearance? How
much more light and heat does the earth enjoy than Mars?
When were the asteroids discovered? By whom, and in what
year was Vesta discovered? What is the color of its light?
By whom and when was Juno discovered?  What is the color
of its light? When was Pallas discovered? By whom? What
is said of its atmosphere? When and by whom was Ceres
discovered? What is its color? What is said in the note with
regard to these planets?
ART. 32T.-Which of the planets is the largest? How much more light
and heat does the earth enjoy than Jupiter? How many
moons has this planet? What is the distance of these moons
from the planet? In what time do they perform their revoluFig. 157.
s the Northern,\  the Southern
Hemisphere.         Hemsphere.
D," I  Cn.%al Day mzd z;




400            QUESTIONS FOR EXAMINATION.
tions around the planet? How does the size of these moons
compare with that of ours? Why has Jupiter no sensible variety of seasons? Of what use are the eclipses of Jupiter's moons?
How long is light in coming from the sun to the earth? How
has this been ascertained? How does Jupiter appear when
viewed through a telescope?
ART. 328.-How does Saturn compare in size with the other planets?
How is Saturn distinguished from the other planets? What
is said of these rings? How much longer are these rings in
performing their revolution around the planet than the planet is
in performing its revolution on its axis? What is the breadth of
these rings? What is said of the surface of Saturn? How
many moons has Saturn? How  may Saturn be known?
What is said of the moons of Saturn? Why are they not often
eclipsed?
ART. 329.-How does Uranus compare in size with the other planets?
How does the light and heat at Uranus compare with that of
the earth? By whom was this planet discovered? What
name did he give it? How many moons has Uranus? By
whom were they discovered? How are their orbits situated,
with regard to that of the planet? What is said of their motion? What appears to be a general law of satellites? What
follows from this with regard to the appearances which the inhabitants of the secondary planets must observe?
ART. 330. —Who discovered the planet Neptune?
ART. 331.-What is the meaning of the word comet? To what class of
bodies is this name given? Of what do these bodies appear to
consist? What is the number of comets that have occasionally
appeared? What discoveries have been made concerning 98 of
them? What is the result? What is the form of the orbits of
comets? What is said of the motion of comets when in perihelion? What did Newton calculate the velocity of the comet
of 1680 to be in an hour?  For what was this comet remarkable? What is said of the luminous stream of a comet as it
approaches and recedes from the sun? What did Newton,
and some other astronomers, consider the tails of comets to be?
What is said in the note with regard to comets? Who were
the first astronomers that successfully predicted the return of a
comet?  What is the periodical time of Halley's comet? Of
Encke's? Of Biela's?
ART. 33 2.-Into how many magnitudes are the stars classed? Of what
magnitude are the largest? Of what are the smallest? What
are telescopic stars? Why cannot the distance of the fixed




QUESTIONS FOR EXAMINATION.                    401
stars be determined? To what is the difference in their apparent magnitudes supposed to be owing? Have the stars any
motion?
ART. 333.-What is the Galaxy?  Of what is it supposed to consist?
How did the ancients divide the stars? What was the number
of constellations among the ancients? How many have been
added by the moderns? How are the stars designated on the
celestial globe? What is the situation of each constellation
now? Illustrate this. What is the cause of this difference?
Why do we not see the stars, and other heavenly bodies, in
their true situation?  How can a star be seen in its true situation?  What is meant by the aberration of light?  What is
necessary to be taken into consideration, in determining the
true place of the celestial bodies? What effect has this property of the atmosphere on the length of the days?
ART. 334.-What is the parallax of a heavenly body?
Fig. 158. Explain Fig. 158. What appears from this? What allowance
must also be made 7
ART. 335.-Is the moon a primary or secondary planet?  How long
is it in performing its revolution about the earth? What is its
distance from the earth?  What is the most obvious fact in
relation to the moon?  How is this caused? What kind of a
body is the moon 7 By what light does it shine?
Fig. 158.
A                                                   -




402            QUESTIONS FOR EXAMINATION.
Fig. 159. -Explain Fig. 159. How does the moon appear when viewed
through a telescope? What causes the difference in the rising
of the moon? What is the mean difference in the rising of the
moon? What is the harvest moon? What is the hunter's
moon?  When are the moons always the most beneficial?
ART. 336.-What are tides?  By what are they occasioned?
Fig. 160. Explain the theory from Fig. 160. What is the greatest distance of the moon from the equator? Where are the tides
highest? Why? Has the sun any effect on the tides? What
are spring tides? When do they occur?
Fig. 161. What are neap tides? When do they take place?
ART. 337.-What is an eclipse?  When does an eclipse of the sun take
place? When does an eclipse of the moon take place? What
is necessary at the time of an eclipse? How often would there
be an eclipse, if the moon went round the earth in the same
plane in which the earth goes round the sun?  Why? What
is the inclination of the moon's orbit to the ecliptic? What
is the apparent diameter of the sun and moon? What follows
from  this? When is the sun eclipsed?  When the moon?
Does an eclipse happen every time there is a full or new
moon? What must the shadows of these bodies always be?
Why?
Fig. I 6(. Explain Fig. 16M. When is all eclipse called annular?




Fig. 159.
ig. 16o
Fig. 162.'_4 1;i~~~~~~~~~~S" ~ ~ ~   ~       I
0        W9,                                          0~ E
&a'.~~~~~~S




404             QUESTIONS FOR EXAMINATION.
Fig. 163. Explain by Fig. 163. What is a penumbra? Why are eclipses
of the moon visible to more inhabitants than those of the sun?
Why is a lunar eclipse visible to all to whom the moon is visible at the time? What is said of the earth's shadow? Explain by the figure. Into what are the diameters of the sun
and moon supposed to be divided? How many digits are these
bodies said to have eclipsed? How often must there be an
eclipse of the sun? What is the greatest number, of both lunar
and solar eclipses, that can take place in a year?  What is
the msual number?  What is said of the eclipse of the sun in
1806?
ART. 33 8.-What is time called when calculated by the sun?  What
is sidereal time? How much longer is the sidereal year than
the solar? How is a solar year measured? What is the length
of a solar year? Why is a day added every fourth year, to
the year?  How is a sidereal year measured?  What is the
precession of the equinoxes? What change has this circumstance caused, with regard to the situation of the constellations?  Can true equal timne be measured by the sun?  Why?
At what periods of the year do the sun and a perfect clock
agree? What is the greatest difference between true and apparent time?
Fig. 164. What does Fig. 164 represent l.j




rig. 163.
S                                          I
o
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Fig. 164.
P
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Willard's Series of School Histories and Charts.
MRS. EMMA WILLARD'S
SERIES  OF SCHOOL HISTORIES AND  CHARTS.
I. WILLARD'S HISTORY OF THE UNITED STATES, OR REPUBLIC OF AMERICA. 8vo. _Price $1.50.
II. WILLARD'S SCHOOL HISTORY OF THE UNITED STATES.
Ill. WILLARD'S AMERICAN CHRONOGRAPHER, $1.00.
A CHART OF AMERICAN HISTORY.
1. WILLARD'S UNIVERSAL HISTORY IN PERSPECTIVE. $1.50.
II. WILLARD9S TEMPLE OF TIME, $1.25.
A CHART OF UNIVERSAL HISTORY
W I L L A R D'S
HISTORY OF THE UNITED STATES.
The large work is designed as a Text-Book for Academies and
Female Seminaries: and also for District School and Family
Libraries. The small work being an Abridgment of the same, is
designed as a Text-Book for Common Schools. The originality
of the plan consists in dividing the time into periods, of which
the beginnings and terminations are marked by important events;
and constructing a series of maps illustrating the progress of the
settlement of the country, and the regular advances of civilization.
The Chronographic Chart, gives by simple inspection, a view of
the divisions of the work, and the events which mark the beginning and termination of each period into which it is divided.
A full chronological table will be found, in which all the events ot
the History are arranged in the order of time. There is appended
to the work the Constitution of the United States, and a series ot
questions adapted to each chapter, so that the work may be used
in schools and for private instruction.
The Hon. Daniel Webster says, of an early edition of the above work, in a letler
to the author, " I keep it near me, as a Book of Reference, accurate in facts and dates "
(18~




Gillespie's Manual of Road-Making.
ROADS AND RAILROADS.
A MANUAL OF ROAD-MAKING:
Comprising the principles and practice of the Location, Construc.
tion, and Improvement of ROADS, (common, macadam, paved
plank, &c.,) and RAILROADS. By W. M. GILLESPIE, A. M.,
Professor of Civil Engineering in Union College. Price $1.60.
Recommendation from Professor MIahan.
I have very carefully looked over Professor Gillespie's Manual of RoadMaking. It is, in all respects, the best work on this subject with which I am acquainted; being, from its arrangement, comprehensiveness and clearness, equally
adapted to the wants of Students of Civil Engineering, and the purposes of per
sons in any way engaged in the construction or supervision of roads. The appearance of such a work, twenty'years earlier, would have been a truly national
benefit, and it is to be hoped that its introduction into our seminariesmay be so
general as to make a knowledge of the principles and practice of this branch of
engineering, as popular as is its importance to all classes of the community.
(Signed,)
D. H. MAHAN,
Professor of Civil Engineering in the Military
Academy of the United States.
From a Report of a Committee of the American Institute.
This work contains in a condensed form, all the principles, both ancient and
modern, of this most important art; and almost every thing useful in the great
mass of writers on this subject...... Such a work as this performs a great
service for those who are destined to construct roads-by showing not only what
ought to be done, but what ought not to be done; thus saving immense outlay of
money, and loss of time in experiments..... The committee therefore, recom
mend it to the public.
From the American Railroad Journal.
The views of the author are sound and practical, and should be read by the
people throughout the entire length and breadth of the land....  We recommend this Manual to the perusal of every tax-payer for road-making, and to the
young men of the country, as they will find useful information in relation to each
department of road-making, which will surely be useful to them in after-life.
From Silliman's American Journal of Science.
If the well-established principles of Road-Making, which are so plainly set
forth in Prof. Gillespie's valuable work, and so well illustrated, could be once
put into general use in this country, every traveller would bear testimony to the
fact that the author is a great public benefactor.
From the Journal of the Franklin Institute.
This small volume contains much valuable matter, derived from the best
authorities, and set forth in a clear and simple style. For the want of information which Is contained in this Manual, serious mistakes are frequently made,
and roads are badly located and badly constructed by persons ignorant of the true
(Ii6)