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ELEMENTARY LESSONS 
 
 PHYSICS OF AGRICULTURE. 
 
 R H. KING, 
 
 Professor op Agricultural Physics in 
 University op Wisconsin. 
 
 STATE JOURNAL PRINTING COMPANY, 
 
 Printers and Stereotypers. 
 
 1891. 
 
Copyright, 1891, 
 
 BY 
 
 F. H. KING. 
 
 
PREFACE. 
 
 Because of the entire lack of literature relating to the 
 physics of agriculture, in any form available for class instruc- 
 tion, these elementary lessons have been undertaken to meet 
 the immediate needs of our Short Course students. They are 
 intended simply as a temporary expedient to be used until 
 time shall permit the preparation of a suitable text-book on 
 the Physics of Agriculture, this field being at present entirely 
 unoccupied. 
 
 Farm drainage ; the construction, ventilation and warming 
 of farm buildings ; physical principles and care of farm ma- 
 chinery; water supply for farms; and the forecasting of 
 weather changes are among the topics presented to our classes, 
 but which could not be outlined in time to be included here. 
 
INTKODUCTOKY. 
 
 1. Physical and Chemical Changes.— When trees are 
 cut into stove wood or cut into dust with the saw, the pieces 
 which remain are wood still and such changes are phtjsical; 
 but when the wood is placed in the stove and burned changes 
 take place which destroy the wood, as such, and these are 
 chemical changes. When a lump of sugar is dissolved in water 
 the sugar is sugar still and may be recovered as such by 
 evaporating the water, and the change is a physical one ; but 
 when yeast and " mother of vinegar " are added to the sweet- 
 ened water and allowed to stand the sugar is transformed, 
 alcohol and then vinegar appear in its stead, and the changes 
 are chemical ones. The fall of rain and snow to the ground, 
 the flowing of streams to the sea and the evaporation and 
 return of the water to the land again are all physical changes. 
 The operations of tillage, of drainage, the cutting and hand- 
 ling of farm produce and the making of butter are physical 
 processes. The running of farm machinery and the construc- 
 tion of farm buildings involve an understanding oi physical 
 
 .rather than chemical laws. 
 
 Write a Hst of five physical and five chemical changes. 
 
 2. Matter and Force.— The physical universe, so far as 
 we are able to comprehend it at present, appears to be made 
 up of two classes of agencies, one of which is active and called 
 force, while the other is passive, or acted upon, and named 
 matter. Water is matter, and gravity is the unseen force or 
 agency which causes it to flow to the sea or to turn the water- 
 wheel ; air is matter, but gravity is the force which moves it 
 in the wind when it drives the ship or turns the wind-mill. 
 Wood and oxygen are matter, but chemical afiinity is the 
 force which drives their molecules into collision producing the 
 intense heat and light of the fire. 
 
 3. Kinds of Matter.— Chemistry at present distinguishes 
 about seventy kinds of matter which are known as elements 
 or elementary substances ; oxygen, hydrogen, nitrogen, carbon, 
 
iron, sulphur and phosphorus are seven of these. "Water is 
 not one of the elements, for it can be decomposed and shown 
 to consist of oxygen and hydrogen. Sugar is not an element, 
 but is made up of carbon, oxygen and hydrogen. 
 
 4. Constitution of Matter. — Each and every body or 
 mass of elementary substance, is composed of large numbers 
 of minute units or individuals named atoms, which various lines 
 of experiment, observation and reasoning show to be constant 
 in weight and properties, so far as we know them ; and it is in 
 consequence of this constancy of weight and properties that 
 chemistry is able to analyze the various substances and tell us 
 their composition. 
 
 The atoms of which all bodies are composed rarely exist 
 alone; they are bound into tiny clusters called molecules. 
 Some of these molecules are made up of two atoms, like those 
 of common salt containing one of chlorine and one of sodium ; 
 other molecules contain three atoms, like those of water, 'two 
 of hydrogen and one of oxygen ; molecules of cane sugar con- 
 tain forty-five atoms, twelve of carbon, twenty-two of hydro- 
 gen and eleven of oxygen. Commercial analine violet possesses 
 molecules of fifty-seven atoms of five different kinds, and there 
 are other atom clusters or molecules more complex than these. 
 
 5. The Size of Molecules.— The size of molecules is 
 almost inconceivably minute. Sir William Thompson com- 
 putes the number of molecules in a cubic inch of any perfect 
 gas having a temperature of 32° F. and under a pressure of 
 thirty inches of mercur}^, to be lO"^ or ten sextillions. 
 
 We have many strong proofs of the extremely minute size of 
 molecules. If a grain of strychnine be dissolved in one mill- 
 ion grains of water, and if we place one grain of the water 
 containing the strychnine in the mouth, its bitter taste is recog- 
 nizable, and yet the volume of a grain of strychnine is only 
 about -^ of a cubic inch. A cubic inch of analine violet will 
 impart its purple color to more than eight million three hun- 
 dred and eighty-four thousand cubic feet of w^ater. Nobert 
 succeeded in engraving parallel lines on glass at the rate of 
 more than one hundred thousand to the inch, and hence the 
 point of his diamond must have been much thinner than this, 
 and the diameters of the molecules which composed it smaller 
 stiU. 
 
The fact that musk and other perfumes keep the air of large 
 apartments so charged with their molecules that we are able 
 to detect them in spite of the fact that the air is constantly 
 changing and the loss in weight of the perfume is extremely 
 small indeed, is still another striking proof of the minuteness 
 of molecules, and, at the same time, of our ability to recoo-- 
 nize them. 
 
 6. Properties of Atoms.— Atoms, so far as we know 
 them, and are able to deal with them, can neither be created 
 nor destroyed ; they have magnitude and weight, but are in- 
 divisible and impenetrable. 
 
 7. Properties of Molecules.— Molecules possess all the 
 properties of atoms except that they can be both destroyed 
 and created and are divisiljle mto atoms. Whenever a chem- 
 ical change takes place existing molecules are transformed 
 into new ones of a different kind, and chemistry, as a science, 
 deals with these changes, while physics deals with the mole- 
 cules and groups of them. 
 
 8. Structure of Bodies — The bodies or masses of mat- 
 ter with which we are familiar are always composed of mol- 
 ecules, but these molecules are believed to be not in contact 
 with one another. 
 
 If a quantity of salt be placed in a vessel and then water 
 added so that the combined volume before solution fills the 
 vessel, when the salt dissolves the volume wiU be found to be 
 less. 
 
 The fact that bodies change their volume with changes of 
 pressure and of temperature also indicates that the molecules 
 which compose them are not in contact. 
 
 The mercury in a thermometer, for example, tills the bulb 
 at 212° F, and a certain portion of the stem also, but as the 
 temperature falls the mercury in the stem withdraws into the 
 bulb and yet the capacity of the bulb diminishes by contrac- 
 tion at the same time, and this could not take place were there 
 not room in the bulb not occupied by the molecules of mer- 
 cury. 
 
 9. Molecules of Bodies Not at Rest.— Xot only are the 
 molecules which constitute the various bodies around us not 
 in contact with one another, but a large number of facts and 
 observations indicate that they are not relatively at rest. If 
 
6 
 
 a solution of sugar or salt be placed in the bottom of a vessel 
 and covered with water the molecules of sugar and salt travel 
 upward and those of the water downward until, finally, a uni- 
 form mixture of the two liquids has resulted. The same fact 
 is also observed where two gases are brought in contact — diffu- 
 sion takes place. So, if a solid lump of sugar or of salt be 
 placed in water, the molecules travel away and disperse them- 
 selves through the whole mass. The molecules of fragrance 
 from fruits and flowers are constantly traveling away from 
 their respective places of origin. Molecules of camphor leave 
 the solid lump and travel through the surrounding air, and 
 snow disappears into the atmosphere without melting on the 
 coldest of winter days. 
 
 The pressure which steam exerts upon the head of the piston 
 when driving the engine is regarded as due to the collision of 
 the molecules against its face ; and the pressure exerted by all 
 gases is explained in the same way. The temperature of 
 bodies is also an expression of the degree of molecular agita- 
 tion within them. When we place the fingers upon a warm 
 body the motion of its molecules is communicated to the 
 molecules of the cuticle, and this in turn to the nerve end- 
 ings, and onward through the nerves to the nerve centers in 
 the brain, giving rise to the sensations denominated hot, warm, 
 cool or cold. 
 
 The mean distance traveled without collision by a molecule 
 of hydrogen at ordinary temperature and pressure is computed 
 by Crooks at toI-oo mm., or 554^0^-5- i^-? while the velocity is at 
 the rate of about six thousand feet per second. The heavier the 
 molecules are the slower they move, the rates being inversely 
 as the square roots of their weights. Thus the oxygen mole- 
 cule, being sixteen times as heavy as the hydrogen molecule, 
 moves, under like conditions, only one-fourth as rapidly. 
 
 If it is difficult to think of a body like a horse-shoe or a ham- 
 mer maintaining its form when its molecules are neither in 
 contact nor relatively at rest, it may be helpful to turn to the 
 solar system, consisting of the sun, planets, satellites and as- 
 teroids, together with comets and meteors, all of which are 
 in constant and rapid motion, separated by immense distances, 
 and yet as a whole constituting one great body, maintaining 
 a definite form and size as it travels through space. 
 
10. Kinds of Force.— The falling of leaves, of rain-drops 
 and of unsupported bodies generally, is a constant reminder 
 of an influence which the earth, as a whole, exerts upon bodies 
 at its surface. The strength and rigidity of solids as com- 
 pared with fluids ; the union of two boards by means of glue ; 
 the rise of oil in a lamp-wick, and its destruction by burning, 
 with the appearance of heat and light, all convince us of in- 
 fluences of some sort which the molecules of bodies exert upon 
 one another. 
 
 It has been customary to speak of these influences as due 
 to the action of difi'erent kinds of force, and they have re- 
 ceived distinctive names. 
 
 11. Gravitation is the action which any one molecule 
 exerts upon every other molecule, tending to draw them to- 
 gether no matter how great the distance may be between 
 them. The intensity of this attraction is directly propor- 
 tional to the mass and inversely proportional to the distance 
 between the molecules. 
 
 The weight of a load of hay or of a bushel of wheat is the 
 sum of the attractions of every molecule of the earth upon all 
 the molecules of the load of hay or bushel of wheat. 
 
 12. Molecular Forces.— When molecules are brought 
 very close to one another, so that the distances between them 
 become inappreciable, their tendency to come together or 
 their resistance to separation are spoken of as due to molec- 
 ular attraction, and three varieties are designated, viz., cohe- 
 sion, adhesion and chemical afiinity. 
 
 13. Cohesion.— When water is cooled below 32° F. the 
 rate of molecular motion and the mean distance between the 
 molecules so diminishes that the force of cohesion begins to 
 bring them into new relations and to bind them more firmly 
 together, so that a solid body results. The same force comes 
 strongly into action when melted iron, copper or other metal 
 changes from a liquid to a solid state. 
 
 When finely ground graphite is subjected to extreme pres- 
 sure in moulds, after having been first thoroughly cleaned, the 
 molecules of the separate fragments are brought so closely to- 
 gether that they unite into sohd cakes, from which the leads 
 of pencils are sawed. Where molecules of the same kind are 
 thus bound together the acting force is named cohesion. 
 
14. Adhesion. — When the smooth, plane surfaces of two 
 pieces of wood are coated with a paste of glue and brought 
 firmly together they are held very securely when dry. In this 
 case the action between the molecules of glue and the mole- 
 cules of wood on either side serves to make a single body of 
 the three. The action seems to be essentially the same as that 
 of cohesion, but because it occurs between molecules of differ- 
 ent kinds the term adhesion is used to designate this distinc- 
 tion. The coating of walls with white-wash, paint, varnish, 
 and the like are other manifestations of the same force. 
 
 1 5. Chemical Aflanity.— When the temperature of wood 
 is raised to a sufficiently high point m the presence of air an 
 action occurs between the molecules of wood and those of the 
 oxygen of the air, which results in the complete breaking 
 down of both sets of molecules and the formation of new ones 
 of entirely different kinds in their stead. This sort of molec- 
 ular action, as in the case of adhesion and cohesion, takes 
 place only across insensible distances, and the agency which 
 brings it about is named the force of chemical afinity. The 
 rusting of iron, the heating of a manure pile or of a silo, the 
 souring of milk and the processes of digestion are all phenom- 
 ena in which this force is operating to form new molecules 
 from old ones. 
 
 16. States of Substances.— It is common to speak of 
 substances as existing, under different conditions, in a solid, 
 liquid or gaseous state. A critical study of these states, how- 
 ever, shows that no absolute distinction exists between them, 
 and that, by insensible gradations, one state may shade into 
 another. The substance water we know as solid ice, liquid 
 water and gaseous steam. Iron at ordinary temperatures we 
 think of as a solid, but as its temperature is raised it gradually 
 becomes more and more soft until it passes by insensible shades 
 into the condition of a true liquid. 
 
 The ideal solid is a body which, if brought under a force 
 which tends to change its form, responds, if at all, to the 
 force, and then remains unchanged so long as that condition 
 of stress may exist. The steel spring, when loaded, changes 
 its form, and then remains constant until the load is removed, 
 or rather appears to when rough measurements only are ap- 
 
plied as a test ; but if more than a certain load is applied, the 
 form keeps changing so long as the load acts. 
 
 The ideal liquid is the body u^hich constantly changes its 
 form whenever a force is made to act more intensely upon 
 one portion of it than upon another. "We think of water as a 
 perfect fluid, and yet a comparatively heavy load may be 
 placed upon a drop of water resting upon a dusty surface 
 without its changing form, except a definite amount at first. 
 On the other hand, we think of sealing-wax as a solkl, and 
 yet if a bullet be placed upon it, it will, by its own weight, 
 gradually sink through it. But jelly, even when rather soft, 
 will keep its form under the same load which will sink through 
 the sealing-wax ; the sealing-wax conforms to the law of liquids 
 and the jelly to that of solids. 
 
 In the gaseous state the molecules of the substance have at- 
 tained so large a range of motion that the molecular attrac- 
 tions appear entirely overcome, and the molecules continually 
 separate from one another unless some confining surface or 
 wall prevents them. No vessel can be half filled with a gas 
 as it can with a solid or a liquid, for the molecules travel to 
 and fro from side to side or from top to bottom, thus occupy- 
 ing the whole space, no matter whether the number of mole- 
 cules be ten or ten millions. 
 
 17. Work. — ^ When a force, like that exerted by a horse, 
 
 acts upon a quantity of matter and changes its position in any 
 
 direction, work is done, and the amount of it is measured by 
 
 the product of the force and the space through which the mass 
 
 has been moved. 
 
 Work = Force x Space. 
 
 If a horse exerts an average tension of twenty pounds 
 through the whippletree upon the carriage and moves it 
 through ten thousand feet the work done is 
 
 20 lbs. X 10,000 = 200,000 foot-pounds, 
 meaning the equivalent of two hundred thousand pounds lifted 
 one foot in opposition to gravity. 
 
 So if the horse exerts a tension of one hundred pounds in rais- 
 ing a forkful of hay and carries it through a height of forty 
 feet the work done is 
 
 100 lbs. X 40=400 ft.-lbs. 
 
10 
 
 Simple pressure is not work. The load must move before 
 work is done. The man who stands still under a sack of grain 
 does no work on the load he holds. 
 
 The mean rate of doing work is the whole work done di- 
 vided by the time required to do it, and 550 foot-pounds per 
 second is called a Horse-power by Engineers. This, however, 
 is more work than the average horse can do, this being esti- 
 mated by General Morin at 26,150 foot-pounds per minute or 
 435.8 foot-pounds per second. 
 
 A laborer lifting dirt with a spade has been found able to 
 do 470 foot-pounds per minute, and on a tread power, raising 
 his own weight, 4,230 foot-pounds pe? minute ; the first being 
 .018 of an animal horse-power and the latter .16 or a little less 
 than one-sixth. 
 
 18. Energy. — Energy is the ability of a moving body to do 
 work. If a twenty-pound weight, suspended by a cord, be 
 drawn to one side and then allowed to fall, it will rise on the 
 opposite side of the line of rest to a height nearly equal to 
 that from which it fell. This height would exactly equal that 
 from which it falls if the air and the suspending cord offered no 
 resistance. Here the moving weight, on reaching its lowest 
 level, has acquired an amount of energy equal to that which 
 has been expended in raising the weight to the point from 
 which it fell. 
 
 When a hammer is brought to rest on the head of a nail it 
 is the energy of the moving hammer which does the work of 
 forcing the nail into the wood. 
 
 The wind blowing through the wind-mill has its velocity re- 
 duced, and so much of its energy is transformed into motion 
 of revolution in the wheel. The same is true of water in flow- 
 ing through a water-wheel, the water loses energy by impart- 
 ing it to the wheel. 
 
 When the spring of a clock or watch is wound up its mole- 
 cules are drawn out of positions of rest, as with the weight re- 
 ferred to, and in falling back to their positions of rest again 
 their energy is imparted to the train of wheels to which the 
 spring is attached. 
 
 19. Energy and Matter Indestructible.— No discov- 
 ery of modern science is more fundamental and far-reaching 
 
n 
 
 than that of the indestructibility of both matter and energy, 
 and equally fundamental is the other fact that neither of them 
 can be created. 
 
 One form of energy can be transformed into another form 
 and one kind of substance can be decomposed and others made 
 from the components, but in these transformations there is 
 never either annihilation or creation. The few bushels of 
 ashes left from the winter's supply of coal or wood seem to 
 point to a destruction of matter, but their weight added to the 
 weight of the products which escaped through the chimney is 
 actually greater than that of the original fuel, for oxygen from 
 the air has united with it. So when the energy of eight or 
 ten horses is being expended in the threshing of grain it looks 
 as though energy were being annihilated, but it is simply 
 changed into heat, sound and energy of position, not lost. 
 We appear to realize in the waste products of domestic ani- 
 mals and the increase of their bodies a very much smaller 
 weight of matter than they have consumed, but this is because 
 so large a weight passes off in an invisible form through the 
 skin and lungs. Something is never, so far as we know, reduced 
 to nothing; neither is something created from nothing. 
 
 20. Machines Not Generators of Energy.— When, 
 through the aid of a machine, a man or a horse is able to move 
 a load which he could not otherwise handle, the machine is 
 not a source of energy, it is simply a device which enables 
 their energy to be transmitted and used more advantageously ; 
 but there is always some loss in the machine, no matter what 
 that machine may be. Some energy is required to overcome 
 the necessary friction of the moving parts of the machine so 
 that the useful work accomplished never quite equals the en- 
 ergy expended. 
 
 21. Inertia of Matter.— E^ewton's first law of motion 
 may be stated as follows : Every body tends to persevere in its 
 state of rest or of uniform motion in a straight line unless actedr 
 ujpon hy some external force, or briefly, 7natter has inertia. 
 There are many unmistakable illustrations of this law. The 
 sudden starting of a wagon tends to throw a standing person 
 backward because his feet take on the motion first and are car- 
 ried out from under him. In beating a carpet the carpet is 
 driven forward away from the dust. In driving a nail tho 
 
12 
 
 suddenness of the blow forces the wood aside and in front of 
 the nail before the motion can spread to the surrounding wood. 
 When a horse, in rapid motion, suddenly turns a corner, the 
 rider must lean in the direction of turning until his tendency 
 to fall exactly balances his tendency to move on in a straight 
 Hne. It is the principle of inertia which enables the rider to 
 sit securely on the bicycle while it is in motion ; the same prin- 
 ciple explains the standing of the top while in motion, and the 
 constant parallelism of the earth's axis during its revolution 
 about the sun. The rider on the bicycle is moving rapidly in 
 one direction, and for him to fall either to the right or the left 
 would require him to change his direction of motion at a right 
 angle, which is the same thing as trying to turn a corner when 
 at fuU speed — a thing practicably impossible. It is this law of 
 inertia which makes it possible for the penman to make his 
 smooth curves only by rapid movements of the hand. 
 
 22. Centrifugal Force. — Centrifugal force, so called, is 
 another manifestation of the law of inertia. The stone twirled 
 about the head with a string, because of its tendency to move 
 always in a straight line, exerts a constant tension upon the 
 string, and if the rate of motion is great enough the string will 
 be broken. 
 
 It is this manifestation of inertia in circular motion which 
 lies at the foundation of all rotary forms of cream-separators 
 and extractors and of several forms of fat-tests for milk. 
 
 In the Babcock and Beimling "milk-tests" the rapid revolu- 
 tion of the bottles which contain the fat to be separated from 
 the liquid with which it is mixed, throws the heavy liquid to 
 the bottom of the bottles, which reacts upon the fat, forcing 
 it toward the center of the circle, where the velocity is least. 
 The fat, like the heavier liquid, in consequence of its own in- 
 ertia, tends to go to the bottom of the bottles also and is 
 simply prevented from doing so by the greater inertia of the 
 heavier liquid. 
 
 23. The Gravity Method of Creaming.— To under- 
 stand the reason of the more rapid and perfect separation of 
 cream by the centrifugal methods over the simple gravity 
 methods we need to get first the principle of creaming by 
 gravity. 
 
 It is this : If a block whose weight is but one-half that of 
 
13 
 
 an equal volume of water be immersed in water it will be 
 lifted by a force equal to the difference between the weight 
 of the block and that of an equal volume of water, as shown 
 in Fig. 1. 
 
 a s c d e 
 
 Fig. 1. 
 
 Regarding the water of the vessel divided into cubes exactly 
 
 equal in volume to the block of wood, and the block just half 
 
 as heavy as an equal volume of water, then the weight of 
 
 column A equals 
 
 2+2 + 1=5, 
 
 while the weight of column B is 
 
 2 + 2 + 2=6. 
 
 Now, as column B exerts a pressure upward on the column A 
 
 equal to its own weight, the block in column A must be pushed 
 
 upward by a force equal to the difference in the weight of the 
 
 A comparison of columns B and C will show that it makes 
 
 no difference where the block is placed in the liquid, the force 
 
 which tends to lift it to the surface is always the same. If 
 
 the attraction of the earth were just twice as strong as it is 
 
 then the cubes of water and the block in column A would 
 
 weigh 
 
 4 + 4 + 2=10, 
 
 and the cubes of water in column B would weigh 
 
 4 + 4 + 4=12, 
 
 and the lifting force on the block would be 
 
 12-10=2, 
 
 or just twice what it now is ; so if the force of gravity were 
 
 made one hundred times what it now is, the lifting force act- 
 
14 
 
 ing upon tlie immersed block wouM be increased one hundred 
 fold. 
 
 24. Centrifugal Creaming. — The centrifugal methods of 
 creaming are applications of the same principle as the gTavity 
 methods, the only difference being in the substitution of a 
 stronger force in the place of gravity, and by so doing of 
 shortening the time and securing a more complete separation. 
 This is done by transforming the energy of an engine or of 
 some other form of motion into the energy of rapid rotation 
 in the milk, giving rise to a strong outward pressure, which 
 acts exactly as gravity does in the old method of creaming. 
 
 25. To Compute the Centrifugal Force.— The 
 strength of centrifugal force in a milk separator may be com- 
 puted as f oUov/s : 
 
 ^ ... , _, weight of milk x (velocity in feet per sec.)2 
 Centi-ifugal Force=: ^^dUi^^aiS^ 
 
 Suppose the mean diameter of the circle through which one 
 pound of milk is made to revolve is ten inches and that the 
 centrifuge is given seven thousand revolutions per minute. 
 In this case the 
 
 ^. , .^ 10 X 3.1416 x7000_oA^. 
 
 ., ... , „ 1 lb. X (305.4) 2 
 
 then centnfugal force = ^ — - =6950, 
 
 and this means that the creaming force would be six thousand 
 nine hundred and fifty times as great as by the old gravity 
 method. 
 
 26. Strength of the Creaming Force.— Since the mean 
 specific gravity of milk fat at 85° to 90° F. is about .91 and 
 that of milk serum 1.034, the creaming force must be the dif- 
 ference between the two specific gravities, as shown in 23, or 
 
 1.034-.91=.l-24; 
 
 that is, if a ball of butter-fat weighing .91 pounds were placed 
 in milk serum, the lifting force of gravity upon it would be 
 .124 pounds, but if placed in milk serum in the centrifuge 
 under the conditions of 25 the creaming force would be 
 
 6950 X .124=861.8 lbs. 
 
 This enormous creaming force seems unnecessarily large, 
 and so it would be if the fat globules were large enough to 
 
15 
 
 weigh .91 of a pound each, as in the problem assumed, for 
 then creaming by the gravity method would be practically 
 instantaneous, whereas, under existing conditions, it requires 
 about twelve hours. 
 
 The actual diameter of the average fat globule in milk is not 
 far from -^-^jy-Q of an inch, while a sphere of butter-fat weighing 
 one pound would have a diameter of about 3.87 inches. 
 
 Now as the volumes of spheres are to each other as the 
 cubes of their diameters, the pound of fat should contain 
 about seven trillion two hundred and forty-five billion of fat 
 globules. But the surfaces of spheres are to each other as 
 the squares of their diameters, and hence the surface of the 
 pound sphere will contain the surface of the fat globule about 
 three hundred and seventy-four million four hundred and 
 twenty-two thousand five hundred times ; and this being true, 
 the aggregate surface of the seven trillion two hundred and 
 forty-five billion fat globules, whose aggregate volume equals 
 that of the pound sphere, must be 
 
 ^tffHMTf^=19,350 
 
 times the surface of the pound sphere ; and when we remember 
 that the friction increases with the surface, and that more force 
 is required for rapid creaming than for slow, we can see that 
 a much stronger creaming force is really needed. 
 
 27. Storing Energy.— In many forms of machinery 
 where the work to be done, like that of sawing wood with a 
 buzz saw, is not a steady draught upon the source of power, a 
 fly-wheel, or its equivalent, is very useful in allowing the 
 power generator to store energy when work is not being done 
 and give it out again as needed. The wind-mill in pumping 
 water, with most pumps, does work only half the time, and 
 so there is often attached to the pump an air-chamber which 
 acts like a spring in which the mill stores energy by compress- 
 ing air which is given out during the reverse stroke. A con- 
 stant stream is thus maintained and the pump enabled to be 
 worked with lighter winds than would otherwise be possible. 
 
 In the animal mechanism the walls of the arteries are elastic 
 and act like springs. They are stretched by the powerful, 
 quick contractions of the heart, and then, while the heart is 
 resting, the blood is forced on hy the steady return of the 
 
16 
 
 stretched arterial walls, and continuous currents of blood are 
 thus moving through the tissues of tiie body. 
 
 28. Momentum. — When a body weighing ten is moving 
 with a velocity of ten, the quantity of motion is 
 10x10=100, 
 
 and this is called its momentum. If the mass of the body is 
 one thousand and its velocity is five, then 
 
 1,000x5=5,000, 
 
 the quantity of motion, or momentum, of that body. So a 
 body having a mass of five and a velocity of one hundred has 
 the same momentum as a body weighing ten, having a velocity 
 of fifty, for 
 
 5x100=500 and 50x10=500. 
 
 ELEMENTS OF MACHINES. 
 
 29. The Mecliaiiical Powers. — The simple machines 
 known by the names lever^ wheel and axle, inclined ;plane, 
 screio, wedge and hnee find an explanation of their action in 
 the fact that they simply transmit motion with an altered 
 velocity or direction, the quantity remaining always the same, 
 except as it is diminished more or less by the friction and 
 weight of the parts of the machine itself. 
 
 30. The Lever. — The lever may be any bar sufficiently 
 rigid to retain its form when forces are applied to it. The 
 terms used in speaking of the action are the ftdorum, jpower- 
 arm and weight-arm^ these are represented in Fig. 2. 
 
 
 There are three classes of levers, named First, Second and 
 Third, according to the relative positions of the fulcrum to the 
 
17 
 
 points where the power and weight are applied; these are 
 represented in Fig. 3. 
 
 
 The mechanical advantage of the crow-bar, in moving a heavy 
 object, lies in the fact that it enables the muscles to generate 
 energy at their usual relatively rapid rate, and transform it 
 into so slow a velocity in the load to be moved that a heavy 
 weight is required to balance the smaller, more rapidly acting 
 power. Suppose we have a crow-bar sixty inches long, and 
 the fulcrum is placed at two inches from one end when it is 
 being used as a lever of the first class. In this case, as shown 
 in Fig. 4, 
 
 both the power and the weight travel on the circumferences 
 of circles, the power circumference having a radius of fifty- 
 eight inches, and the weight circumference having a radius 
 of two inches. 
 
 Now the circumferences of these two circles have the same 
 relative lengths as their radii do, and since the lever does not 
 bend, the weight can have a velocity only -^^ or ^^ as great as 
 that of the power, and since the power is ten and its velocity 
 twenty-nine times that of the weight, its momentum must be 
 
 10x29=290; 
 and this being true, the weight, in order to just balance the 
 power, must have mass enough so that, with a velocity of one 
 
18 
 
 the amount of motion shall exactly equal that of the powei', 
 and hence we have 
 
 1x290=290, 
 
 as the load which ten will balance on a lever a'^ting as repre- 
 sented. 
 
 When the crow-bar is used as represented in Fig. 5, it be- 
 comes a lever of the second class, Avith the power-arm sixty 
 inches long, while the weight-arm is still two inches. In this 
 case a power of thirty pounds will balance a load of nine hun- 
 dred pounds. 
 
 Pij.S 14^. A. ^Z 
 
 When the power is applied to the lever between the weight 
 and the fulcrum, as represented in Fig. G, the case becomes a 
 lever of the third class, and a power of nine hundred becomes 
 necessary to move a load of thirty. 
 
 'wkso 
 
 ^0 
 
 
 "/f. 6 
 
 The relation of power to weight in the case of any lever is 
 
 expressed by the equation below, where P. equals power, W. 
 
 equals weight, P. A. equals power-arm and W, A. equals 
 
 weight-arm : 
 
 P. X P. A. = W. X W. A. 
 
 When any three terms in this equation are known the foui'th 
 may readily be found. 
 
19 
 
 How great a load may be moved by a power of thirty 
 pounds acting on a lever having a power-arm of twenty and a 
 weight-arm of three? 
 
 P. xP. A.=:W. xW. A. 
 30 X 20 =W. X 3. 
 600=3 W. 
 W.=2001bs. 
 
 31. The Two-horse Evener.— This is a lever of the sec- 
 ond class where the whippletree clevis-pin acts as the fulcrum 
 for each horse, the weight or load being carried by the center 
 pin. As ordinarily constructed this instrument is designed to 
 divide the work of moving the load equally between the two 
 horses. , This, however, is not done at all times unless the 
 three holes lie in the same straight line. 
 
 When the holes are bored as shown in Fig. Y the load is di- 
 vided equally only when one horse is not behind the other. 
 
 
 f 
 
 
 ^^^ 
 
 I 
 
 \ 
 
 
 tf 
 
 
 
 
 3 
 
 i 
 
 
 Tiy. 
 
 7 
 
 V 
 
 6 
 
 The figure shows that when the near horse falls behind the 
 other the effective length of his lever arm is diminished more 
 than is that of the off horse, and consequently he must pull a 
 larger share of the load. 
 
 When the holes are bored in the same straight line the pos- 
 sibility of this inequality is avoided, as shown in Fig. 8, be- 
 cause the changes in the effective lengths of the lever arms 
 are always equal no matter which horse falls behind. This 
 latter form, although the best so far as dividing the labor 
 
20 
 
 evenly between the two liorses, is rarely adopted in practice, 
 owing chiefly to the possibility of more cheaply constructing 
 the evener the other way. 
 
 Where heavy loads are to be moved, like puilmg stones or 
 stumps, or hauling a load out of a rut or out of the mud, the 
 second type of evener will always allow a matched team to 
 pull a larger load, because the horse which happens to be 
 thrown behind, in attempting to start the load, is placed at 
 a disadvantage and the other horse can only pull enough to 
 hold his end against the one placed at a disadvantage. So, 
 too, in doing heavy work, where one horse is naturally a little 
 freer or stronger than the other, the tendency is always to 
 throw more than half the w^ork upon the slower or w^eaker 
 horse. 
 
 32. "Giving One Horse the Advantage." — The fre- 
 quent practice, where the two horses of a team are not equally 
 strong, of " giving one horse the advantage " is based upon 
 the principle that the amount of work done by each horse is 
 inversely proportional to the length of the lever arm upon 
 which he works. Suppose it is desired to so modify an evener 
 that three-eighths of the work will fall upon one horse and five- 
 eighths upon the other. In this case the horse which is to do 
 five-eighths of the work must have his end of the evener 
 shortened until its length is just three-fifths as long a"> that 
 of the horse which is to do three-eighths of the work. If the 
 distance from 1 to 2 in Fig. 8 is forty-eight inches, then in 
 
21 
 
 order to require the near horse to do five-eighths of the work 
 the power-arm of his lever will be 
 
 2^4 in. -38.4 inches. 
 
 This is given by substituting the numerical values in the 
 general equation of the lever. 
 
 P.xP.A.=W. xW. A. 
 
 By substituting, f x P. A.=l x 24 in. 
 
 Whence, P. A.=^^ in. =38.4 in. 
 
 This length of 38.4 inches will be secured by setting the 
 clevis 9.6 in. nearer the center. 
 
 How far in must the clevis be set to give the other horse 
 an advantage of one-eighth? of one-sixteenth? of one-thirty- 
 second? 
 
 33. Platform Scales. — Levers are often used in combina- 
 tion when it is desired to balance a very heavy load by a small 
 weight, and such combinations are spoken of as compound 
 levers. The various forms of platform scales are examples of 
 such combinations. In the case of hay scales, four thousand 
 to six thousand pounds are balanced or lifted by a few pounds. 
 
 The principle by which such combinations of levers gives 
 these great mechanical advantages will be understood from 
 Fig. 9. 
 
 T,y. *j T'Z 
 
 I ^^ // 
 
 /V ^ ^ \_ >^ /^ "^ 
 
 If F. F. F. F. are fulcrums of the levers I, Ii, III, lY, and 
 their power-arms are each ten while their weight-arms are 
 each one, then a power of two pounds at P. will balance a 
 load of twenty thousand pounds at W. This must be so, for 
 two pounds at P. will cause lever lY to exert a pressure of 
 twenty pounds upon the long arm of lever III; the twenty 
 pounds pressure of lever III will cause a pressure of two hun- 
 dred pounds on lever II ; lever II transmits a pressure of two 
 
thousand pounds to the end of lever I, and this pressure will 
 sustain a load of twenty thousand pounds placed at W. 
 
 For levers in combination the continued product of +he 
 power and power-arms is equal to the w^eight into the contin- 
 ued product of the weight-arms. 
 
 P. X P. Arms=W. x W. Anns, 
 or, 2 X 10 X 10 X 10 X 10=20,000 xlxlxlxl. 
 
 In the platform scales the platform is supported at its four 
 corners by bearings which rest upon four levers, the ends of 
 which are joined by means of a vertical rod to the short end 
 of the graduated scale beam. The accuracy and sensitiveness 
 of such scales depend upon the exactness with which the lever 
 arms are constructed and the delicacy and durability of the 
 bearings and fulcrums which transmit the pressure to the 
 levers. 
 
 34. The Locomotion of Animals.— Most of the higher 
 animals which travel by means of appendages to their bodies 
 propel themselves with a system of levers which are operated 
 by sets of very powerful muscles. 
 
 The mechanism of muscles and their method of contraction 
 make it possible for them to move through only very small 
 distances, and hence w^iere considerable movements are to be 
 executed the results are secured by attaching them to the 
 short arms of levers. In the forearm, for example, the biceps 
 muscle acts upon a lever whose power-arm is only one-sixth 
 as long as the weight-arm, and hence when a weight of fifty 
 pounds is held as represented in Fig. 10 the muscle must exert 
 a tension of three hundred pounds. 
 
23 
 
 The triceps muscle which extends the forearm is a more 
 powerful one than the biceps, and in order to accomplish its 
 much more rapid movements it works upon a relatively much 
 shorter lever arm, the relative lengths of the two arms being 
 about as one to twenty or twenty-four. jSTow it is possible for 
 the triceps muscle to exert a force upon a spring-balance ex- 
 ceeding twenty-four pounds, and hence, since 
 
 RxR A.=W.xA., 
 
 we have P. X 1=34x20, 
 and P. =480; 
 
 which proves that the triceps muscle can exert a tension of 
 four hundred and eighty pounds. It is this powerful muscle 
 acting upon the hammer Avhich enables nails to be so readily 
 driven. 
 
 The great tension which some of the muscles of horses must 
 exert in pulling heavy loads, acting as they do at the short 
 ends of levers, is almost beyond belief. 
 
 35. The Wheel a^d Axle.— With the lever only a small 
 amount of motion can be communicated to a body at once, 
 further movements only being possible after reversing its ac- 
 tion. The wheel and axle, represented in Fig. 11, enables 
 power to be applied continuously in one direction to the load 
 or resistance to be overcome. 
 
 
24 
 
 The relation of power to weight in this element of machines 
 is expressed by the equation 
 
 Power X Power-Radius = Weight x Weiglit-Radius, 
 
 or, briefly, 
 
 P. xP. R.=W. X W. R., 
 
 and by substituting the numerical values given in Fig. 11 we 
 get 
 
 10 X 10=1 X 100. 
 
 The relation of power to weight may also be represented in 
 terms of the diameters or circumferences of the wheel and 
 axle, thus : 
 
 P. xP. R.=W. xW. R 
 
 P. X P. Diam.=W. x W. Diam. 
 
 P. xP. Cir.=W. xW. Cir. 
 
 This mechanical power has by far the most extended use of 
 any in machinery. 
 
 36. Trains of Wheels and Axles.— Wherever a great 
 rotary velocity is desired, as in the case of the wood saw, in 
 the cylinder of a threshing machine, in the fan of a fanning 
 mill, or in the much higher speed of centrifuges, several wheels 
 and axles are joined in a train by means of belts, gears, or 
 friction pulleys; such systems are analogous to compound 
 levers. 
 
 The relation of power to weight both in intensity of action 
 and in relative velocities is expressed by these equations : 
 
 1. For intensity of action : 
 
 Power X Continued product of P. R.= Weight x Continued product of W. R. 
 P. X P. Radii=W. x. W. Radii 
 
 2. For velocity : 
 
 P. X P. Velocity=W. x W. Velocity. 
 
 37. The Sweep Horse-Power.— This machine is an ex- 
 ample of a train of wheels and axles whereby the slow waUi 
 of the horses is converted into the extremely rapid rotation 
 of the cylinder of the thresher, feed-cutter or feed-mill, the 
 sweeps to which the horses are attached constituting radii of 
 the first wheel in the train. Here the small amount of work 
 required of the machines at any one instant makes a high speed 
 of execution desirable. 
 
25 
 
 38. The High Speed of Centrifuges. — This is secured 
 by a combination of wheels and axles connected with belts. 
 Suppose the diameter of the fly-wheel of the engine is twenty- 
 four inches and it makes two hundred and twenty revolutions 
 per minute. If this is belted to a six-inch axle or pulley on 
 the driving-shaft, then the number of revolutions made by the 
 wheel on the driving-shaft will be 
 
 220x^=880. 
 If the shaft-pulley connecting with the axle of the interme- 
 diate pulley has a diameter of ten inches while the axle has a 
 diameter of five inches, then the wheel of the intermediate 
 
 pulley will make 
 
 880xY=1760 
 
 revolutions, and if the wheel of the intermediate pulley has a 
 diameter of twelve inches while the axle of the centrifuge is 
 three inches, then the centrifuge wiU make 
 
 1760 xY= ''040 
 revolutions per minute. 
 
 Change the diameter of a wheel or axle so as to give the 
 centrifuge four thousand revolutions ; six thousand revolutions ; 
 five thousand revolutions. 
 
 39. Exertion of Great Power. — When the exertion of 
 a great lifting force is required at the expense of speed, this 
 may be done by reversing the action of a train of wheels such 
 as is considered in 38. In that case, if the power were ap- 
 plied at the Centrifuge and the work done at the other end 
 of the series, a load Avould be lifted very slowly indeed, but 
 its weight could be very great. 
 
 40. The Inclined Plane.— This mechanical power is a 
 rigid surface inclined to the line of the force or resistance 
 which it is to overcome, and is represented in Fig. 12. 
 
 W^HO 
 
26 
 
 "When the power moves parallel with the length or face of 
 the plane, as in A, the relation of power to weight is given 
 by the equation 
 
 Power X Lengtli of Plane= Weight x Height of Plane, 
 or200xl5r::600x5. 
 
 But when the power moves in a line parallel with the base of 
 the plane, as in B, then the relation of power to weight is 
 given by the equation, 
 
 Power X Length of Base = Weight x Height of Plane, 
 or20xl0==40x5. 
 
 41. The Tread Power. — This method of transferring 
 energy is a practical application of the inclined plane, and the 
 amount which can be transmitted by it depends upon the 
 height of the plane as compared with its length. 
 
 If the length of the tread is eight feet and it is given a 
 slant of one foot in eight feet, then from the equation 
 P. X Length= W x Height 
 
 we get, with two thousand four hundred pounds as the weight 
 
 of two horses, 
 
 P. X 8=2400x1, 
 whence P. =300 lbs., 
 
 as the intensity of the power exerted, diminished, of course, 
 by whatever friction there may be. 
 
 What would be the power if the slant were made one foot 
 in seven feet ? one foot in six feet ? one foot in five feet ? 
 
 42. Traction on Common Roads.— The power required 
 to draw a wagon over common roads varies with the charac- 
 ter and condition of the road. Experiments in England with 
 a four-wheeled wagon have given the following results for 
 level roads as indicated by a dynamometer : 
 
 On cubical block pavement 28 to 44 lbs. per ton. 
 
 On Macadam road 55 to 67 lbs. per ton. 
 
 On gravel road 125 lbs. per ton. 
 
 On plank road 27 to 44 lbs. per ton. 
 
 On common dirt roads 179 to 268 lbs. per ton. 
 
 43. Traction Power of a Horse. — According to the 
 most reliable data available at present, which is certainly far 
 short of what could be desired, a horse in good condition, well 
 fed, and Avcighing not less than one thousand pounds, when 
 actually walking at the rate of two and one-half miles per 
 hour during ten hours per day, can exert a traction of one 
 hundred pounds on a level road or circular horse-path like that 
 
27 
 
 of the sweep-powers. In order that a horse may exert his 
 force most advantageously on a sweep-power the track should 
 have a diameter of thirty to thirty-five feet, — never less than 
 twenty-five. 
 
 44. Increased Speed Diminishes the Traction 
 Power. — If the horse walks more rapidly than two and five- 
 tenths miles per hour, or at a slower pace, the force which he 
 can exert changes also and is less or greater than one hundred 
 pounds. Experience seems to indicate that at speeds between 
 three-quarters of a mile and four miles per hour, and con- 
 tinued ten hours per day, the traction will be given by the 
 following equation : 
 
 2.5 miles x 100=n miles x Traction. 
 
 Thus, at two miles per hour the traction would be : 
 2.5 X 100=2 X Traction; 
 whence, Traction=-|^ or 125 lbs. 
 
 What would be the traction at one mile per hour? at three 
 miles ? at four miles ? 
 
 45. Diminishing the Number of Hours of Work per 
 Day Increases the Traction. — When the speed remains 
 the same, experience has shown that, between five and ten 
 hours per day, diminishing the time increases the possible 
 traction in about the same ratio, or 
 
 10 hours X 100=n hours x Traction, 
 Thus if the horse is to be worked only five hours the trac- 
 tion he may exert will be 
 
 10 X 100=5 X Traction, 
 whence Traction=-L^aa:=200 lbs. 
 
 ' What may the traction be when the horse works six hours ? 
 seven hours ? eight hours ? nine hours ? 
 
 46. Traction Power Diminished by Up-Grades. — 
 When a horse is forced to draw a load up a hill his power of 
 traction is diminished by being forced to lift his own body at 
 the same time. If he is going up a hUl which rises one in ten 
 he must expend a force of one hundred pounds per onie thou- 
 sand pounds to overcome the force of gravity on his own body, 
 and if the load he was drawing weighed one thousand pounds 
 the force of gravity would require another one hundred pounds 
 to overcome the tendency of the load down the hill, leaving 
 all resistance out of consideration. Now if an empty wagon 
 weighs one ton, and the hauling of a ton on a level road of 
 
28 
 
 the same character as the hill requires one hundred and fifty 
 pounds, then the force necessary to carry the load up the hill 
 rising one in ten would be, for a span of horses : 
 
 For two horses. 200 lbs. 
 
 For load 200 " 
 
 For rolling friction 300 " 
 
 Total 700 " 
 
 For one horse 850 " 
 
 The rate at which the horses could move up the hill with 
 this load would be, by 44, 
 
 2.5xl00=ratex350; 
 whence, rate=||S=.''' miles per hour. 
 
 What would be the force required to move the same load 
 up a hill which rises one foot in twelve feet ? one foot in thir- 
 teen feet? one foot in fourteen feet? one foot in fifteen feet? 
 
 47. Good Roads Make High Grades More Objection- 
 able. — It is evident that the better the road-bed is made, 
 thus reducing the traction on the level, the more objectionable 
 a hill becomes, because the force of gravity is just as strong 
 on a good road as on a bad one, and while a much larger load 
 may be hauled on the level, when the hill is reached it cannot 
 be drawn up. It was shown, in 46, that where the traction 
 was one hundred and fifty pounds per ton, a grade of one foot 
 in ten feet added to that traction one hundred pounds per one 
 thousand pounds of load, including the weight of the team. 
 ISTow if the road-bed were improved so as to reduce the trac- 
 tion to seventy-five pounds per ton, double the load could be 
 brought to the hiU, but, unless the grade were also lessened, 
 it could not be moved over it. 
 
 48. Soft and Uneven Roads. — The reason why the 
 traction is so heavy on soft and uneven roads will be readily 
 seen from a study of Fig. 13. 
 
29 
 
 At A, where the wheel is continually cutting into the ground, 
 it is, in effect, constantly tending to rise up a hill which is 
 steadily breaking down, and whose gradient varies with the 
 size of the wheel and the depth to which it sinks into the 
 ground. A wheel four feet in diameter which sinks two 
 inches into the ground is constantly tending to move up a 
 hill which rises about one inch in five and one-third inches. 
 If the wheel has a less diameter than four feet, not only does 
 it sinlv more deeply into the ground with the same load, but, 
 for the same depth, it is forced to tend to rise up a steeper 
 grade. 
 
 So, too, in raising the load over an obstruction, as shown at 
 B, there is, in a measure, the effect of rolling the load up an 
 inclined plane which is steeper in proportion as the height of 
 the obstruction is large and the diameter of the wheel small. 
 This case may, however, be more exactly compared to lifting 
 a load with a bent lever of the first class, where the obstruc- 
 tion is the fulcrum, the distance af the weight-arm and the 
 distance hf the power-arm. The higher the obstruction, and 
 the smaller the wheel, the more nearly equal are the lever 
 arms. It is this fact which explains, in part, why heavy loads 
 may be moved more easily over uneven roads on large wheels. 
 
 49. Wide and Narrow Wagon Tires. — The same fact 
 which makes a large wagon wheel more advantageous on soft 
 ground makes a wide wagon tire better than a narrow one, 
 under the same conditions. It presents more surface to bear 
 the load, and hence does not sink as deeply into the ground 
 as the narrow one does, and, this being true, the load is moved 
 with less traction. So far as lightness of draught is concerned, 
 broad tires are best adapted to field hauling, but, for hard 
 roads, there appears to be but little advantage in this particu- 
 lar. On soft roads the broad tires would be of advantage, 
 provided all wagons using the road were of this character, for 
 then the cutting of the roads would be less and the draught 
 lighter. There is, however, one serious disadvantage of wide 
 tires on an improperly drained road composed of sticky soil : 
 during wet times the wheels so fill with mud between the 
 spokes that the wagon becomes a load in itself. 
 
 50. The Telford System of Road Construction.— The 
 essential features of the system followed by this great Enghsh 
 
30 
 
 road-engineer may be briefly stated to consist in first leveling 
 and thoroughly draining the road-bed, then to lay upon it a 
 solid pavement of large stones, these covered Avith a layer of 
 stones carefully broken, and the whole then covered with a 
 layer of gravel or other fine material. This was the system 
 he followed in the Highlands of Scotland. 
 
 But where much heavier traffic was to be provided for, the 
 middle of the road-bed was made as firm as possible by form- 
 ing a pavement of large stones which were carefully laid by 
 hand on a bed formed to the proper shape of the road and 
 previously well drained. All inequalities were broken off the 
 tops of these stones and the cavities filled in, the size of the 
 stone being 7x3 inches. Over this paving was placed a layer 
 of whinestone — a hard basaltic rock — seven inches in thick- 
 ness, the pieces being broken so that none should exceed six 
 ounces in weight and all be able to pass though a circular 
 opening two and one-half inches in diameter. This layer was 
 again covered with binding gravel sufficient to fill up all the 
 cavities. Great attention was paid to this road until it became 
 thoroughly settled and then it stood the heavy traffic between 
 Carlisle and Glasgow for six years, nothing being required 
 beyond cleaning the dirt off during that time. 
 
 51. The Macadam System of Road Construction. — 
 This differed from the Telford system in that it aimed to se- 
 cure, instead of the hard unyielding surface of that system, a 
 certain amount of elasticity. Macadam, after preparing his 
 road-bed essentially as described in the Telford system, laid 
 upon it several inches of angular fragments broken from the 
 hardest rock he could find, preference being given to granite, 
 greenstone or basalt. This layer was carefully watched by 
 men, and as ruts appeared they wer.e raked full and fresh ma- 
 terial added until a hard, even surface was secured. 
 
 52. Road Drainage. — Perfect drainage is one of the first 
 requisites of a good road, and in some places both surface and 
 under drainage may be required. If the contour of a road is 
 such that the water of rains may stand upon it in places, at 
 all such points the road-bed softens and ruts are cut more or 
 less deeply into it. In the construction of a road, therefore, 
 the aim should be to give the surface such a contour that all 
 rain is shed completely from it, and, at the same time, to de- 
 
31 
 
 part as little from the horizontal section as possible. In Fig. 14 
 is given a profile of the Telford road-bed. 
 
 Surface, 
 
 drass. 
 
 'R^'ad Bed, 
 
 Crhs$ 
 
 Sui/oce 
 
 H H 
 
 r/g, /^ 
 
 The section adopted by Telford is quite flat and more nearly 
 a portion of the side of a flat ellipse than the arc of a circle. 
 It will be seen that in a road-bed thirty feet wide the fall, in 
 the first four feet from the center, is only half an inch, in nine 
 feet two inches, and in fifteen feet six inches. The aim is to 
 have the road-bed as . nearly flat as may be in the central 
 eighteen feet so as not to tilt the load and force the trafiic to 
 follow one line. The tendency is to get the surface too 
 sloping, and when this is done the weight of high loads is 
 thrown more upon the lower set of wheels, which tends to de- 
 velop ruts on that side ; there is also a tendency to slide, so 
 that the wear on the road-bed and upon the wagon-tire is in- 
 creased. The ridge, upon the two sides, is intended to keep 
 stones and dirt from being thrown into the side drainage 
 ditches. The road-bed is often made only eighteen feet wide 
 and the two level strips used, one as a foot-path and the other 
 as storage ground for crushed rock and gravel to be used in 
 repairing the road. 
 
 Where underdrainage is needed, two lines of tile are laid, 
 one on each side just outside of the road-bed but inside of the 
 sided ditches as shown in Fig. 15. 
 
 ^-^^-^T^j>?:^^ 
 
 ^^^^^-~- 3.^- ^-zz^-^ikwv/-: 
 
 rj^./s 
 
32 
 
 The two lines of tile are used to prevent water from run- 
 ning under the road-bed from either side to soften up the 
 ground, the surface, when properly made and kept in repair, 
 keeping water from entering from above. 
 
 53. Results of General Morin's Experiments in 
 France. — General Morin, after a series of experiments car- 
 ried on at the expense of the French government, reached the 
 following general conclusions regarding roads and carriages : 
 
 1. The traction is directly proportional to the load, and in- 
 versely proportional to the diameter of the wheel. 
 
 2. Upon a paved or hard macadamized road the traction is 
 independent of the width of the tire when it exceeds three to 
 four inches. 
 
 3. At a walking pace the traction is the same for carriages 
 with springs as for those without springs, 
 
 4. Upon a macadamized or paved road the traction in- 
 creases with the speed above a velocity of two and one-quar- 
 ter miles per hour. 
 
 5. Upon soft roads of earth or sand the traction is inde- 
 pendent of the velocity. 
 
 6. The destruction of the road is in all cases greater as the 
 diameters of the wheels are less, and it is greater by the use 
 of carriages without springs than of those with them. 
 
 54. The Pulley. — This mechanical power consists of a 
 wheel, having a grooved circumference through which a cord 
 or chain may pass, and so mounted as to revolve freely about 
 an axis. Pulleys are spoken of as either fixed or movable, ac- 
 cording as the axis of revolution is stationary or travels with 
 the load it carries. The two types are represented in Fig. 16. 
 
 Tif./^ ®'-i^ 
 
33 
 
 At A is represented a simple fixed pulley in which the power 
 must be equal to the weight, because, in this case, the pulley 
 may be regarded as a lever of the first class, where the axle of 
 the pulley becomes the fulcrum, and then the two arms are of 
 equal length, each being a radius of the pulley. At B the 
 lower pulley is movable, traveling upward with the load, and 
 here we have the equivalent of a lever of the second class, 
 with the fulcrum at the side of the pulley in contact with 
 rope 2. As the load hangs from the axis of the pulley the 
 power-arm is the diameter of the pulley and the weight-arm 
 is the radius, giving us the equation : 
 
 . P.xP. A.=W.xW. A. 
 or 5x2=10x1. 
 
 At C, D and E are combinations of several movable and 
 
 fixed pulleys. In C we have a system with several separate 
 
 cords, and in this the relation of power to weight is expressed 
 
 by the equation 
 
 P.x2n=W., 
 
 where n equals the number of movable pulleys, or in C, 
 
 P. x.22=W., 
 whence, 4x2x2=16. 
 
 In D and E we have two systems of pulleys where a single 
 continuous cord is used. It makes no difference whether the 
 pulleys are arranged side by side, as in D, or one above the 
 other, as in E, the relation of power to weight is expressed by 
 
 the equation: 
 
 P. X No. cords supporting W. = W., 
 
 whence for D, 4x4=16, 
 
 and forE, 4x6=24 
 
 These equations always suppose no loss due to friction or in 
 bending the ropes. There is, however, always a large and 
 variable loss, so the actual lifting power is less than the theo- 
 retical. 
 
 55. The Horse-fork and Pulley. — The horse-fork and 
 carrier are used in lifting hay, as represented in Fig. 17. The 
 mechanical advantage is that of pulley B, Fig. 16, diminished, 
 of course, by the friction. 
 
 When no puUey is used next to the fork, the traction exerted 
 by the horse must always considerably exceed the weight of 
 
34 
 
 hay lifted, so that a single horse is fully tasked in freeing from 
 the load and raising from two hundred to three hundred 
 pounds of hay. 
 
 ^^g^^%>g^:^-rp 
 
 56. Using the Pulley to Raise Heavy Stone Out of 
 the Ground. — The pulley may frequently be used to ad- 
 vantage in raising heavy stone out of the ground, and in pull- 
 ing stumps, as shown in Fig. 18. 
 
 If a pulley is fixed to the chain in either of the above cases, 
 and the team draws upon a rope passing through it to a fixed 
 attachment, as shown, two horses will exert the traction of 
 four upon the stump or stone, diminished by the friction of 
 the pulley. If the chain is attached to the stone, and so passed 
 over the top as to roll, instead of drag, it from its place, the 
 mechanical advantage will be still greater. 
 
 57. The Screw. — This mechanical power is practically a 
 combination of the inclined plane and the lever. The threads 
 of the screw, and of the nut also, represent inclined planes 
 
35 
 
 free to slide one upon the other. One or the other of these 
 inclined planes is fixed while the other is moved by means of 
 a lever of some form, the movable one carrying the load. 
 
 When the distance between the threads of a screw is one- 
 fourth of an inch and the circumference described by the end 
 of the lever to which the power is applied is three feet, the 
 theoretical load lifted by a power of one hundred pounds is 
 
 100x3x4x12=14,400. 
 But the friction is so variable, and so great with very heavy 
 loads, that it is practically impossible to calculate, from the- 
 ory, the load which may be thus moved. ]S^one of the me- 
 chanical powers can be so compactly constructed as this, and at 
 the same time allow so small a force to exert so great a pres- 
 sure. It is on this account that the screw is so much used in 
 the construction of vices, lifting-jacks and presses. 
 
 58. Friction Between Solids — When one surface rests 
 upon another the roughness or inequalities of the one fit, to a 
 greater or less extent, into those of the other, so that in order 
 that one may be moved upon the other either the two bodies 
 must be, to some extent, separated, or else the interlocking 
 roughness must be broken away. We have seen that mole- 
 cules are not in contact in bodies, and also that they are very 
 small ; from this it follows that no matter how smooth two 
 surfaces may appear there are, always present inequalities of 
 surface and always a resistance which opposes shding, and this 
 is called friction. 
 
 59. The Friction of Rest or Static Friction Between 
 Solids. — When two surfaces have been at rest with reference 
 to each other for a time there is developed the maximum 
 amount of interlocking, and hence the greatest amount of 
 friction. This is analogous to a load standing upon a wagon 
 over night, causing the wheels to become depressed in the 
 surface upon which they rest. The load is started with greater 
 difficulty because the wheels must be rolled out of depressions, 
 and this illustrates the condition of static friction. On the 
 other hand, if the wagon moves rapidly with its load, espe- 
 cially if over soft ground, the wheels do not have time to form 
 deep depressions in the surface, and the resistance to forward 
 progress is smaller, and this is, in a measure, analogous to 
 friction of motion. 
 
36 
 
 60. The Friction of Motion or Kinetic Friction Be- 
 tween Solids. — When two surfaces are sliding rapidly one 
 over the other there is not time to change direction and de- 
 velop the interlocking which is possible with a state of rest, 
 and consequently less power is lost when one solid sUdes rap- 
 idly over another. 
 
 61. Influence of Pressure on the Friction of Solids. 
 When other things remain the same, increasing the pressure 
 increases the friction, and the amount of friction is directly 
 proportional to the pressure. Thus if one hundred pounds 
 produce a friction of two pounds, one thousand pounds avlQ 
 develop a friction of twenty pounds, and this is independent 
 of the amount of surface bearing the load provided the pres- 
 sure is not great enough to crush or tear the surfaces. 
 
 62. Friction Between Liquids and Solids. — In this 
 case the amount of friction follows a different law, for it in- 
 creases with the amount of surface and also with the square 
 of the velocity of sliding motion. It is, however, less than 
 that between solids and solids, and because of this fact the 
 oiling of the bearings of machinery diminishes very much the 
 loss of effective energy through friction. 
 
 Where the velocities of revolution are slow, thick oils, like 
 castor oil, develop but little friction, but as the speed is in- 
 creased the friction increases very rapidly, and this fact makes 
 a thick viscous oil inapplicable as a lubricant where high 
 velocities, like those of the bowls of centrifuges, are required. 
 On the other hand, when a very thin fluid is used as a lubri- 
 cant for slow motions there is time for such freely-flowing 
 fluids to be crowded out of inequalities and thus allow the in- 
 terlocking of solid surfaces to be partially set up and develop 
 a high friction for these low speeds which the thick slow-flow- 
 ing oils prevent ; but for very high speeds the thin fluid is 
 able to maintain the depressions of the solid surfaces full, and 
 the much smaller internal friction of the thin oil gives rise to 
 a relatively lower friction for such speeds. 
 
 It is upon this same principle, in part, that a thick grease 
 serves so well the purpose of a lubricant to lessen friction in 
 the slow sliding which obtains in the axles of a wagon. 
 
 63. Bad Efiects of Dirt in Journals.— When grit of any 
 kind becomes entangled in the lubricants of any journal or 
 
37 
 
 friction surface these particles bridge across or cut the two 
 fihns of oil which closely adhere to the two sliding surfaces, 
 so that friction is set up between solids rather than between 
 liquids as it should be, and there results not only a great loss 
 of energy transmitted by the machine, but also an excessive 
 wear of the bearings, which quickly destroys the fit so es- 
 sential to steady, easy and economical motion. Scrupulous 
 cleanliness of the friction surface of farm machinery should 
 therefore be adhered to as well as ample lubrication. 
 
 64. Belting. — The transmission of power by means of belt- 
 ing is a useful application of the friction between solid sur- 
 faces. In order that power may be economically transmitted 
 by this means the belt must be so tight that little slipj^ing 
 takes place, and for leather belts this is least when the pulley 
 is covered with leather, hair side out, and the belt runs upon 
 this, hair side in. When the belt is running at a high speed 
 the tension may be less in projwrtion to the power transmitted, 
 the actimty of belting being expressed by the equation : 
 
 Activity =Tv, 
 
 where v is the velocity and T the effective tension. When the 
 velocity is very great the tension may evidently be small, and 
 yet the activity or horse-power remain large. It is on this ac- 
 count that small wire cables may be used at very high veloci- 
 ties in transmitting very large amounts of energy. 
 
 It is in consequence of this principle, too, that light ropes are 
 successfully used in transmitting energy to the centrifuge. 
 
 65. Sliding Friction in Machinery is Lost Energy.— 
 The sliding of the inequalities of friction surfaces over one 
 another sets the molecules constituting them into a state of 
 to-and-fro motion, and aU such motions represent energy lost 
 either in the form of heat or of sound ; and it is because no 
 machine can be so constructed as to run absolutely frictionless 
 that they, one and all, fail to transmit all the energy which is 
 imparted to them, and hence it is that perpetual motion is an 
 impossibility. 
 
 66. Friction in the Churn.— In all forms of churns the 
 agitation of the cream results in friction between the mole- 
 cules of milk and between the milk and the parts of the churn, 
 and this causes a transformation of the energy brought to the 
 churn from the source of power largely into heat in the milk, 
 
38 
 
 which causes its temperature either to actually rise or else 
 prevents it from cooling as rapidly as it would otherwise do. 
 Now, if churning is begun with the cream at too high a tem- 
 perature and the surrounding atmosphere is also too high, bad 
 results must necessarily follow. 
 
 STKENGTH OF MATERIALS. 
 
 67. A Stress. — When a post is placed upon a foundation 
 and a load of two thousand pounds is set upon it, the post is 
 undergoing or opposing a stress of two thousand pounds. 
 When a rope is supportmg a load of one thousand pounds in 
 a condition of rest it is subject to a stress of one thousand 
 pounds. The joists under a mow of hay are subjected to a 
 stress measured by the tons of hay which they carry, 
 
 68. Kinds of Stress.— Solid bodies may be subjected to 
 three classes of stresses which tend to break them and will do 
 so if the stress is great enough. These are : 
 
 1. A crushing stress, where the load tends to crowd the 
 molecules closer together, as when kernels of corn are crushed 
 between the teeth of an animal. 
 
 2. A stretching stress, as where a cord is broken by a load 
 hung upon it. 
 
 3. A twisting stress, as where a screw is broken by trying 
 to force it into hard wood with a screw-driver. 
 
 69. Strength of Moderately Seasoned White and 
 Yellow Pine Pillars.— Mr. Chas. Shaler Smith has deduced, 
 from experiments conducted by himself, the following rule 
 for the strength of moderately seasoned white and yellow 
 pine pillars : 
 
 Rule. — Divide the square of the length in inches hy the 
 sqare of the least thickness in inches; multiply the quotient hy 
 .00 Jf, and to this product add 1; then divide 5,000 hy this stem, 
 and the result is the strength in pounds per square inch of area 
 of the end of the post. Iftdtiply this residt hy the area of the 
 end of the post in inches, and the answer is the strength of the 
 post in' pounds. 
 
 In applying this rule in the construction of farm buildings 
 the timbers should not be trusted with more than one-sixth 
 
39 
 
 to one-fourth of the theoretical load they are computed to 
 carry, because the theoretical results are based upon averages, 
 and there is a wide variation in the strength of individual 
 pieces. 
 
 Table of bkeaking load, in tons, of eectangulak pillars of 
 half seasoned white oe yellow tine fiemly fixed and 
 
 - EQUALLY LOADED, COMPUTED FEOM C. S. SmITh's FORMULA : 
 
 HI H 
 
 r 
 
 
 Dimensions 
 
 OF 
 
 Rectangular Pine Pil 
 
 r.ARS 
 
 [N Inches. 
 
 
 4x4 
 
 tons. 
 
 12.1 
 8.7 
 6.5 
 5.0 
 3.9 
 
 4x6 
 
 tons. 
 18.1 
 13.0 
 9.7 
 7.4 
 5.9 
 
 4x8 
 
 tons. 
 24.2 
 17.4 
 12.9 
 9.9 
 7.8 
 
 4x10 
 
 tons. 
 30.2 
 21.7 
 16.1 
 12.4 
 9.8 
 
 4x12 
 
 t07lS. 
 
 36.3 
 26.1 
 19.4 
 14.9 
 11.7 
 
 6.6 
 
 tons. 
 44.5 
 34.6 
 27.2 
 21.7 
 17.7 
 14.6 
 12.2 
 10.3 
 8.8 
 
 6x8 
 
 tons. 
 59.3 
 46.2 
 36.3 
 29.0 
 23.5 
 19.4 
 16.2 
 13.7 
 11.7 
 
 6x10 
 
 tons. 
 74.1 
 
 57.7 
 15.4 
 36.2 
 29.4 
 24.3 
 20.3 
 17.2 
 14.7 
 
 6x12 
 
 tons. 
 88.9 
 69.2 
 54.4 
 43.5 
 35.3 
 29.1 
 24.3 
 20.6 
 17.6 
 
 8x8 
 
 8x10 
 
 tons. 
 126.9 
 105.8 
 87.1 
 72.3 
 60.6 
 51.0 
 43.4 
 37.4 
 32.3 
 
 8x12 
 
 tons. 
 152.3 
 126.3 
 104.5 
 
 86.8 
 72.7 
 61.2 
 52.1 
 44.8 
 38.8 
 
 10x10 
 
 tons. 
 
 182.7 
 
 158.6 
 
 136.7 
 
 117.4 
 
 101.0 
 
 87.2 
 
 75.7 
 
 65.8 
 
 57.9 
 
 10x12 
 
 8 
 10 
 12 
 14 
 16 
 18 
 
 tons. 
 101.7 
 84.2 
 69.7 
 57.9 
 48.4 
 40.8 
 34.8 
 29.9 
 25.9 
 
 tons. 
 219.2 
 190.3 
 164.0 
 140.9 
 121.2 
 102.6 
 
 90 
 
 
 
 
 
 
 90.8 
 
 99 
 
 
 
 
 
 
 79 
 
 94 
 
 
 
 
 
 
 69 4 
 
 
 
 
 
 
 
 
 70. Tensile or Stretching Strength of Timber.— The 
 
 tensile strength of materials is measured by the least weight 
 which will break a vertical rod one inch square firmly and 
 squarely fixed at its upper end, the load hanging from the 
 lower end. Below are given the results of experiments with 
 different varieties of wood, but the strengths vary greatly 
 with the age of the trees, with the portion of the tree from 
 which the pine comes, the degree of seasoning, etc. 
 
 Elm 6,000 lbs. per sq. in. 
 
 Am. Hickory 11,000" " « " 
 
 Maple 10,000 « " " « 
 
 Oak, white and red 10,000 " " " " 
 
 Poplar 7,000 " " " " 
 
 White pine 10,000 " " " " 
 
 71. Tensile or Cohesive Strength of Other Materials. 
 
 Am. cast iron 16,000 to 28,000 lbs. per sq. in. 
 
 Wrought iron wire, annealed 30,000 to 60,000 " " " " 
 
 Wrought iron wire, hard 50,000 to 100,000 « " « « 
 
 Wrought iron wire ropes, per sq. ui. of rope C8,000 " " " " 
 
 Le?,t}ier belts, 1,500 to 5,000, good 8,000 " " " " 
 
 Rope, manUla, best 12,000 " " " " 
 
 Rope, hemp, best , 15,000 " " " " 
 
40 
 
 72. Transverse Strength of Elaterials.— When a board 
 
 is placed upon edge and fixed at one end as represented at A, 
 Fig. 19, a load acting at W puts the upper edge under a 
 crushing stress. 
 
 V 
 
 
 "We know from experience that in case the board breaks 
 under its load when so situated the fracture will occur some- 
 where near 5-6. E^ow in order that this may take place, there 
 must be, with white pine, according to 70, a tensile stress at 
 the upper edge of ten thousand pounds to the square inch, and 
 if the board is one inch thick the upper inch should resist a 
 stress of ten thousand pounds at any point from 5 to 1 ; but 
 we know that no such load will be carried at W. The reason 
 for this, and also for its breaking at 5 rather than at any other 
 point, is found in the fact that the load acts upon a lever arm 
 whose length is the distance from the point of attachment of 
 the load to the breaking point, wherever that may be, and 
 this being true the greatest stress comes necessarily at 5. 
 
 If the board in question is forty-eight inches long and six 
 inches wide, it will, in breaking, tend to revolve about the 
 center of the line 5-6, and the upper three inches will be put 
 under a longitudinal strain, but according to 70, it is capable 
 
 of withstanding 
 
 3x10,000 lbs. =30,000 lbs. 
 
 without breaking; but in carrying the load at the end, as 
 shown, this cohesive power is acting at the short end of a bent 
 lever whose mean length of power-arm is one-half of 4-5 or 
 1.5 inches, while the weight-arm is forty-eight inches in length. 
 It should, therefore, only be able to hold at W, 937.5 pounds; 
 for . 
 
 asP. xP. A.=W. xW. A., 
 we have 3,000 x 1.5=W. x 48, 
 whence VJ'.-^A^=Q3'7.5 lbs. 
 
41 
 
 When a board, in every respect like the one in A, Fig. 19, 
 is placed under the conditions represented in either B or C, 
 Fig. 19, it should require just four times the load to break it, 
 because the board is practically converted into two levers 
 whose power-arms remain the same, but whose weight-arms 
 are only one-half as long each. 
 
 73. The Transverse Strength of Timbers Propor- 
 tional to the Squares of their Vertical Thicknesses.— 
 Common experience demonstrates that a joist resting on edge 
 is able to carry a much greater load than when laying fiat- 
 wise. If we place a 2 x 4 and a 2 x 8, which differ only in 
 thickness, on edge, their relative strengths are to each other as 
 the squares of 4 and 8, or as 16 to 64. That is, the 2x8, con- 
 taining only twice the amount of lumber as the 2x4, will, 
 under the conditions named, sustain four times the load. The 
 reason for this is as follows : In Fig. 20 let A represent a 2 x 4 
 and B a 2 X 8. 
 
 In each of these cases the load draws lengthwise upon the 
 upper half of the joist, acting through a weight-arm F. W. ten 
 inches in length, to overcome the force of cohesion at the fixed 
 ends, whose strength, according to 70, is ten thousand pounds 
 per square inch, or a total 
 
 of 2 X 2 X 10,000 lbs. =40,000 lbs. in the 2 x4 joist, 
 
 and of 2 x4x 10,000 lbs. =80,000 lbs. in the 2x8 joist. 
 
 These two total strengths become powers acting through their 
 
 respective power-arms F. P., whose mean lengths are, in the 
 
 2x4 joist, one inch, and in the 2x8 joist, two inches. 
 
 Now we have, from 30, 
 
 P.xP. A.=W.xW. A,, 
 
42 
 
 and substituting the numerical values, in the 2x4 joist, we 
 get 
 
 4xl0,000xl=W.xl0, 
 
 or 4 X 10,000=10 W., 
 
 and W. =4,000. 
 
 Similarly, by substituting numerical values in the case of the 
 2x8 joist, we get 
 
 8xl0,000x2=W.xl0, 
 
 or 16 X 10,000=10 W.," 
 
 and W. = 16,000. 
 
 It thus appears that the loads the two joists will carry are 
 to each other as four thousand is to sixteen thousand, or as 
 one is to four ; but squaring the vertical thickness of the two 
 joists in question we- get for the 2x4 joist 
 
 4x4=16, 
 
 and for the 2x8 joist 
 
 8x8=64; 
 
 but sixteen is to sixty-four as one is to four, which shows that 
 the transverse strengths of similar timbers are proportional 
 to the squares of their vertical diameters. 
 
 74. The Transverse Strength of Materials Dimin- 
 ishes Directly as the Length Increases. — It will be 
 readily seen, from an inspection of Fig. 20, that lengthening 
 the pieces of joists, while the other proportions remain the 
 same, lengthens the long arm of the lever, while the short arm 
 remains unchanged ; and since the force of cohesion remains un- 
 altered, the load necessary to overcome it must be less in pro- 
 portion as the lever arm upon which it acts is increased. 
 Thus, if the 2 x 8 in Fig. 20 is made twenty inches long, we 
 shall have, from 30, 
 
 P.xP. A.=W.xW. A., 
 
 and by substituting the numerical values we get 
 
 80,000 x2=W.x 20, 
 
 hence 
 
 W. =8,000, 
 
 instead of sixteen thousand, as found in 73. 
 
 75. The Constants of the Transverse Breaking 
 Strength of Wood. — Since the laws given in 72, 73 and 
 74 apply to all kinds of materials, it follows that the actual 
 breaking strength of different kinds of materials will depend 
 upon the cohesive power of the molecules as well as upon the 
 
43 
 
 form and dimensions of the body which they constitute. The 
 breaking strength of a beam of any material is always in pro- 
 portion to its breadth, multiplied by the square of its depth, 
 divided by its length, or, 
 
 Breadth X the square of the depth 
 its length, 
 
 and if the breadth of a piece of white pine in inches is four, 
 its depth in inches ten, and its length in feet ten, we shall 
 have, taking the length in feet, 
 
 4x10x10 ,- 
 —3^— =40. 
 
 Now if we find by actual trial, by gradually adding weights 
 to the center of such a beam, that it breaks at eighteen thou- 
 sand pounds (including half its own weight), the ratio between 
 
 this and forty will be 
 
 18,000 
 -^^=4o0. 
 
 and as this ratio is always found for white pine, when the 
 breadth and depth ar^ taken in inches and the length in feet, 
 no matter what the dimensions of the timbers may be, four 
 hundred and fifty is called its hreaking constant for a center 
 load. 
 
 For other materials this constant is different, and has been 
 determined by experiment and given in tables in various works 
 relating to such subjects. The following are taken from Traut- 
 wine: 
 
 76. Breaking Constants of Transverse Strength of 
 Different Materials.— 
 
 WOODS. 
 
 American White Ash 650 lbs. 
 
 Black Ash 600 " 
 
 Yellow American Birch 850 " 
 
 American Hickory and Bitter-nut 800 " 
 
 Larch and Tamarack 400 " 
 
 Soft Maple 750 " 
 
 American White Pine 450 " 
 
 American Yellow Pine 500 " 
 
 Poplar 550 " 
 
 American White Oak 600 " 
 
 American Red Oak 800? " 
 
 METALS. 
 
 Cast u-on 1,500 to 2,700 lbs. 
 
 Wrought iron bends at 1,900 to 3,600 lbs. 
 
 Brass 850 lbs. 
 
44 
 
 77. To find the Quiescent Center Breaking Load of 
 Materials having Rectangular Cross-sections when 
 Placed Horizontally and Supported at Both Ends 
 
 In placing joists and beams in barns it is important to know 
 the breaking load of the timbers used. This may be deter- 
 mined with the aid of the foUoAvmg rule and the table of con- 
 stants given in 76: 
 
 Rule. — Multiply the square of the depth in inches hy the 
 hreadth in inches and this hy the hreaking constant given in 76 ^ 
 divide the result hy the clear length infeet^ and the result is the 
 load in pounds. 
 
 But in the case of long, heavy timbers and iron beams one- 
 half of the clear weight of the beam must be deducted because 
 they must always carry their own weight. 
 
 Square of \ 
 
 depth j- X Breadth in inches x Constant 
 in inches ) 
 
 Breaking load= 
 
 Length in feet. 
 
 What is the center breaking load of a white pine 2x12 joist 
 twelve feet long? 
 
 Breaking load=^i£ii^l^i^=10,800 lbs. 
 
 What is the breaking load for the same ten feet long? 
 fourteen feet long? sixteen feet long? eighteen feet long? 
 Solve the same problems for other woods. 
 
 78. General Statements Regarding the Quiescent 
 Breaking Loads of Uniform Horizontal Beams.— If 
 the center quiescent breaking load be taken as 1, then, when 
 all dimensions are the same, to find the breaking load : 
 
 (1) When the beam is fixed at both ends and evenly loaded 
 throughout its whole length, multiply the result found by 77 
 by two. 
 
 (2) When fixed at only one end and loaded at the other, di- 
 vide the result obtained by 77 by four. 
 
 (3) When fixed only at one end and the load evenly distrib- 
 uted, divide the result obtained by 77 by two. 
 
 (4) To find the breaking load of a cylindrical beam, first 
 find the breaking load of a square beam having a thickness 
 equal to the diameter of the log and multiply this result by 
 the decimal .589. 
 
45 
 
 79. Breaking Load of Rafters.— In finding the break- 
 ing- load of timl)crs placed in any oblique position as shown in 
 Fig. 21, take the length of the rafter equal to the horizontal 
 S2:)an ac and proceed as in 77 and 78. 
 
 80. Table of Safe Quiescent Center Loads for Hor- 
 izontal Beams of White Pine Supported at Both 
 Ends. — In this table the safe load is taken at one-sixth of 
 the theoretical breaking load. This large reduction is made 
 necessary on account of the cross-grain of timbers and joists 
 and the large knots which weaken very materially the pieces. 
 Where a judicious selection is made in placing the joists, laying 
 the inherently weak pieces in places where little strain can 
 come upon them, much saving of lumber may be made. 
 
 si 
 
 r 
 
 Span 10 feet. 
 
 Span 12 feet. 
 
 Span 14 feet. 
 
 Span 16 feet. 
 
 Breadth. 
 
 Breadth. 
 
 Breadth. 
 
 Breadth. 
 
 s 
 
 2 in. 
 
 4 in. 
 
 Gin. 
 
 2 in. 
 
 4 in. 
 
 (5 in. 
 
 2 in. 
 
 4 in. 
 
 6 in 
 
 2 in. 
 
 4 in. 
 
 6 in. 
 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 IZ6.9. 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 lbs. 
 
 4 
 
 20) 
 
 480 
 
 720 
 
 200 
 
 400 
 
 600 
 
 172 
 
 344 
 
 516 
 
 150 
 
 300 
 
 450 
 
 6 
 
 540 
 
 1080 
 
 1620 
 
 450 
 
 noo 
 
 1350 
 
 386 
 
 772 
 
 1158 
 
 336 
 
 672 
 
 1008 
 
 8 
 
 9()0 
 
 1020 
 
 2880 
 
 800 
 
 KHIO 
 
 2400 
 
 686 
 
 1372 
 
 2058 
 
 600 
 
 1200 
 
 1800 
 
 10 
 
 1500 
 
 8000 
 
 4500 
 
 1250 
 
 2500 
 
 3750 
 
 1072 
 
 2144 
 
 3210 
 
 930 
 
 1872 
 
 2808 
 
 13 
 
 2160 
 
 4320 
 
 6480 
 
 1800 
 
 3600 
 
 5400 
 
 1544 
 
 3088 
 
 4632 
 
 1350 
 
 2700 
 
 4050 
 
 
 Breadth. 
 
 Breadth. 
 
 Breadth. 
 
 Breadth. 
 
 
 8 in. 
 
 lbs. 
 
 lOin. 
 
 lb.<i. 
 
 12 in. 
 
 lbs. 
 
 8 in. 
 
 ib.1. 
 
 10 in. 
 
 lbs. 
 
 12 in. 
 
 lbs. 
 
 8 in. 
 
 10 in. 
 
 2 in. 
 
 lbs. 
 
 8 in. 
 lbs. 
 
 10 in 
 
 lbs. 
 
 12 in. 
 
 
 lbs. 
 
 m. 
 
 lbs. 
 
 4 
 
 mo 
 
 1200 
 
 1440 
 
 800 
 
 1000 
 
 1200 
 
 688 
 
 860 
 
 1032 
 
 600 
 
 750 
 
 900 
 
 <j 
 
 2U!() 
 
 2700 
 
 8210 
 
 1800 
 
 2250 
 
 2700 
 
 1544 
 
 1930 
 
 2316 
 
 1344 
 
 KiSO 
 
 2016 
 
 8 
 
 3H-I(i 
 
 4S()() 
 
 57(i() 
 
 3200 
 
 4000 
 
 4800 
 
 2744 
 
 3430 
 
 4116 
 
 2400 
 
 odilll 
 
 ;;ooo 
 
 10 
 
 00(10 
 
 7500 
 
 0000 
 
 5000 
 
 6250 
 
 7500 
 
 4288 
 
 5360 
 
 6432 
 
 3744 
 
 ■KISO 
 
 5016 
 
 12 
 
 8040 
 
 10800 
 
 12000 
 
 7200 
 
 9000 
 
 10800 
 
 6176 
 
 7720 
 
 9264 
 
 5400 
 
 6750 
 
 8100 
 
• 99 ®*^0 ® ® ® 
 
 • • ® ®^^® . ® ® 9 
 9 O ®'0 @ ® @ o @ 
 
 46 
 
 FLUIDS. 
 
 81. Surface Tension of Liquids.— The molecules of 
 liquids exert an attractive force upon one another, but this is 
 most manifest at their surfaces because the interior molecules, 
 being pulled equally on all sides by surrounding molecules, 
 have their tendency to move balanced in every direction. 
 The surface conditions, however, are different, as will be seen 
 from Fig. 22, where the arrows at 
 A and B show the direction of the 
 action of molecular forces on the in- 
 terior and surface molecules respect- 
 ively. The unbalanced condition of 
 forces between the surface molecules j^- ^4? 
 of liquids causes them to act like a 
 
 thin elastic membrane or skin upon the liquid within. It is 
 the tension of these films which causes rain drops, and the 
 shot from the shot towers to assume the spherical form when 
 falling. The same action gives this form to the fat globules 
 of milk, to dew drops on cabbage leaves and to drops of water 
 on a dusty surface. It is this same surface tension which sus- 
 tains a fine needle on the surface of water and which enables 
 certain insects to walk upon water. 
 
 82. Strength of Surface Tension.— The strength of 
 the tension of fluid surfaces is different for different liquids, 
 and it varies with the surfaces which are in contact. The 
 following table gives the relative surface tensions in certain 
 cases : 
 
 Between clear water and air 82, nearly. 
 
 Between olive oil and air 37, nearly. 
 
 Between chloroform and air 31, nearly. 
 
 Between water and olive oU 21, nearly. 
 
 Between water and chloroform 80, nearly. 
 
 These differences of tension give rise to a great variety of 
 phenomena. When oil is placed on water it tends to spread 
 out indefinitely in a thin sheet. On the other hand, if a little 
 water is placed upon chloroform it tends to draw it into a 
 
47 
 
 sphere or drop. The reason for these facts will be understood 
 from Fio:. 23 : 
 
 ^a^ 
 
 ^^^^^^^^^^^ 
 
 Fig. ^3. 
 
 In A, on the circumference, where the drop of oil, air and 
 water meet, the surface molecules are actuated by three sets 
 of forces represented in direction by the arrows and in inten- 
 sity by the numbers, and it is evident that the molecules so 
 affected must move in the direction of the stronger force, and 
 as the surface tension of the water-air surface is strongest, 
 the oil is drawn out indefinitely until an extremely thin film 
 results. It is on this account that so small a quantity of oil 
 put overboard by a vessel at sea, in times of storm, covers so 
 large an area as often to effectually protect the vessel from the 
 dangers of wave-action. It is in accordance with the same 
 principle that water and other fluids spread out over the sur- 
 faces of solids which they will wet. 
 
 In the case of B, where a drop of water is placed upon chlo- 
 roform, the conditions of A are reversed and the water at first 
 tends to draw up into a sphere. It is in the same manner also 
 that water on a dusty floor or on cabbage leaves is drawn up 
 into drops. 
 
 83. Capillary Action.— When slender glass and other 
 tubes, whose adhesive force for water is greater than the at- 
 traction of the molecules of Avater for one another, are placed 
 vertically in water, the water is seen to rise in them and come 
 to rest above the level of the water in the surrounding vessel. 
 It will also be observed that the hight attained by the water 
 in different tubes varies inversely as their inside diameters. 
 The rise of liquids in slender tubes is in accordance with the 
 principle illustrated in Fig. 23 A, the chief difference being 
 that the movement is in opposition to the force of gravitation 
 and that the rise is checked when the down-pulling forces 
 balance the surface tension. 
 
 The rise of water in soil and of oil in a lamp wick are other 
 
48 
 
 instances appcarently due to a closely allied, if not identical 
 action. 
 
 If, on the other hand, the attraction between the liquid and 
 the walls of the tube is less than the attraction among the 
 molecules themselves, so that the walls are not wet by it, the 
 surface of the liquid in the tube is depressed, the amount being 
 greater as the diameter of the tube is less. This depression 
 is in accordance with the principle explained under B, Fig. 23, 
 
 84. Influence of Surface Tension on Lactometer 
 Readings. — The rise of water on the sides of a tube floating 
 in it, as in the case of the lactometer, tends to draw it more 
 deeply into the liquid and thus gives a higher reading. On 
 the other hand, if the liquid has its surface tension weakened 
 by being overspread with oil, or if the stem of the lactome- 
 ter is made greasy b}^ handling or otherwise, it will then be 
 lifted out of the liquid and too low a reading will be indi- 
 cated. It is important, therefore, in determining the specific 
 gravity of milk by this method to see that the lactometer is 
 thoroughly clean. 
 
 85. Solution of Solids in Liquids.— When salt is placed 
 in water the adhesion between the molecules of water and 
 salt is at first stronger than the cohesion between the mole- 
 cules of salt, and successive layers of salt molecules are sepa- 
 rated and disseminated through the liquid. If the quantity 
 of salt placed in the water be large enough, there will come 
 a stage when the quantity of salt dissolved in the water has 
 so weakened its adhesive power that it ceases to be strong 
 enough to overcome the molecular cohesion of the salt and 
 at this stage further solution is stopped. 
 
 In the majority of cases where solids are being dissolved a 
 rise of temperature so weakens the cohesive force that solu- 
 tion may be carried still further. It is in part the greater 
 solubility of soil ingredients in waiter at high temperatures 
 than at low that makes a warm soil more conducive to plant 
 growth than a cold one. 
 
 86. DifFasion.— When a phial, nearly full of salt or sugar, 
 is placed in a vessel and the vessel carefully filled with water 
 so as to cover the phial, the salt or sugar will in time be dis- 
 parsed through the whole water. The rate at which this dif- 
 fusion takes place is difl"erent for different substances, and in 
 
49 
 
 the table below, the numbers indicate the relative lengths of 
 time required for different substances to travel the same dis- 
 tance in Avater, under like conditions. 
 
 Hydi-ocliloric acid 1 
 
 Salt 2 . 33 
 
 Sugar 7 
 
 Magnesium sulphate 7 
 
 Albumen. 49 
 
 All substances diffuse more rapidly at moderate tempera- 
 tures than at low ones, and here is another reason why a warm 
 soil is more conducive to plant growth than a cold one, for 
 the transfer of food from soil to plant is partly a process of 
 diffusion. 
 
 If two gases are placed in two vessels and an opening be 
 made connecting them, the molecules of each kind of gas will 
 travel from their respective vessels and enter the other until 
 a uniform mixture results. We have seen that the velocities 
 with which molecules travel are inversely proportional to their 
 densities, and it is found that the rate of diffusion of gases 
 obeys the same law, the lighter gas diffusing more rapidly. 
 
 Oxygen enters the aii' cells of our lungs and carbon dioxide 
 leaves them by this process of diffusion, and the same thing is 
 true of the intercellular air passages of leaves into which the 
 stomata lead. 
 
 87. Osmosis. — In case two liquids, which mix, are placed 
 on opposite sides of a porous membrane capable of being wet 
 by one or both of them, currents are established in one or both 
 directions. The membrane first becomes penetrated by the 
 liquid having the strongest attraction for it, and on reaching 
 the other side the liquid diffusing into it causes its attraction 
 for the walls to be lessened, and this allows this portion to be 
 crowded out into the liquid which has been approached and a 
 stream thus established. If the pores in the membrane have a 
 diameter exceeding ^-girV^o- inch, a return current of the sec- 
 ond liquid is established toward the first along the central 
 portion of the pores. It is by this process that the tissues of 
 plants and animals are nourished. Here again a warm tem- 
 perature makes the streams more rapid, and so still another 
 reason is found for having the soil in which the roots grow 
 sufficiently warm. 
 
50 
 
 Osmose of gases as well as of liquids also takes place, and 
 it is by this process that animals get their supply of oxygen 
 and plants their supply of carbon dioxide, 
 
 88. Viscosity, — When the molecules constituting any 
 body are forced to move past one another their mutual molec- 
 ular attraction causes a dragging which sets the disturbed 
 molecules vibrating, and this molecular vibration is at the ex- 
 pense of the energy which produced the movement. This 
 dragging effect of the molecules is called viscosity^ and the 
 amount of energy transformed into heat in consequence of it 
 is a measure of the viscosity. The fat globules in rising 
 through milk serum encounter this viscosity, and a part of the 
 energy of the creaming force is transformed into heat, causing 
 the cream to rise more slowly than it would if there were no 
 viscosity. 
 
 Liquids, in flowing through pipes or other channels, are re- 
 tarded by viscosity so much that in long and slender pipes the 
 amount of water discharged is very much diminished. This 
 fact makes it necessary, in tile draining and in conveying water 
 in pipes to pastures or other points, to use larger pipes than 
 would otherwise be necessary. In all those cases where t' d 
 liquid wets the surface past which it flows the friction is due 
 wiiolly to the viscosity of the fluid, for the layer in immediate 
 contact with the surface remains stationary while the other 
 molecules move past them. This is the case with oils used to 
 diminish friction in machinery. 
 
 When the inner surfaces of pipes are rough and uneven the 
 flow of liquids through them is further diminished by the di- 
 rection of the current being changed at these inequalities and 
 thrown toward the center of the pipes across the course of the 
 central current. It is important, therefore, in selecting tile to 
 avoid those having rough interiors, and also in laying them to 
 avoid making shoulders at the junctions of the many sections. 
 
 The viscosity of air and other gases is due to the promiscu- 
 ous traveling of the molecules, which causes those moving 
 transverse to the stream to be caught in it and thus retard the 
 onward movement, acting much as the eddy-currents set up 
 by inequalities in the surface of water pipes. 
 
 89. Pressure of Fluids. — The great freedom of motion 
 of molecules in masses of liquids and gases causes them to 
 
51 
 
 exert an internal and to transmit an external pressure equal 
 and undiminished in all directions. The proof of this law, for 
 liquids, is found in the fact that when two vessels are so con- 
 nected that water can flow from one to the other the water 
 will have the same hight in both vessels, no matter what form 
 or direction the communicating passage may take. The 
 spherical form of a soap-bubble in mid-air proves the law true 
 forair; for if the pressure from all sides were not equal the 
 form of the bubble would change from that of a sphere. 
 
 90. Pressure of Liquids in Vessels.— The pressure ex- 
 erted by liquids on the walls of vessels which contain them is 
 due to their weight, and, for a given liquid, is always propor- 
 tional to the depth. In the following table the weight of 
 Avater per cubic foot and pressure per square foot are given 
 for different temperatures : 
 
 
 
 Pressure in lbs. pee sq. 
 
 FT. AT DIFFERENT DEPTHS. 
 
 Tem. 
 
 Lbs. per 
 cu. ft. 
 
 
 
 
 
 
 
 Fahr. 
 
 
 
 
 
 
 
 
 62.417 
 
 At 2 ft. 
 
 At 4 ft. 
 
 At 8 ft. 
 
 At 10 ft. 
 
 At 20 ft. 
 
 At 40 ft. 
 
 ^i;. 
 
 124.83 
 
 249.67 
 
 499.34 
 
 624.70 
 
 1248.84 
 
 2476.68 
 
 62.435 
 
 124.85 
 
 249.70 
 
 499.40 
 
 624.25 
 
 1248.50 
 
 2497.00 
 
 40° 
 
 62.423 
 
 124.85 
 
 249.69 
 
 499.38 
 
 624.23 
 
 1248.46 
 
 2496.92 
 
 50° 
 
 62.409 
 
 124.82 
 
 248.64 
 
 499.27 
 
 624.09 
 
 1248.18 
 
 2496.36 
 
 60° 
 
 62.367 
 
 124.73 
 
 249.47 
 
 498.94 
 
 623.67 
 
 1247.34 
 
 2494.68 
 
 70° 
 
 62.302 
 
 124.60 
 
 249.21 
 
 498.42 
 
 623.02 
 
 1246.04 
 
 2492.08 
 
 80° 
 
 62.218 
 
 124.44 
 
 248.87 
 
 497.74 
 
 622.18 
 
 1244.36 
 
 2488.72 
 
 90° 
 
 62.119 
 
 124.24 
 
 248.48 
 
 496.95 
 
 621.19 
 
 1242.38 
 
 2484.76 
 
 ^13° 
 
 59.7 
 
 119.40 
 
 238.80 
 
 477.60 
 
 557.00 
 
 1194.00 
 
 2388.00 
 
 The pressure of the water on the bottom of a vessel can al- 
 ways be found by multiplying the area of the bottom in square 
 feet by the depth of the water, and this product by the weight 
 of a cubic foot of water, which is nearly 62.42. 
 P. on bottom=area x depth x 62,42. 
 
 The lateral or side pressure is proportional to the depth, 
 following exactly the same law as that for the pressure on 
 the bottom. Since the depth at the surface is zero, the lateral 
 pressure is also zero, and since the depth at the bottom of a 
 vessel is the greatest, the lateral pressure must there be at its 
 maximum ; these being true, the mean pressitre on the side of 
 a vessel would be the pressure at one-half the depth of the 
 
52 
 
 liquid, and hence, to fiijd the total pressure on the side of a 
 vessel, we have 
 
 Totel lateral p. ^ Area of sides x depth x 62.42. 
 
 What is the total pressure on the bottom and on the sides 
 of a reservoir 6 x 6 x G feet filled with water at 39.2° F.? at 
 80° F.? 
 
 What is the lateral pressure on the lower six inches of a 
 cvlindrical tank ten feet in diameter filled with water to a 
 depth of ten feet? 
 
 If this pressure is to be sustained by an iron hoop composed 
 of one-eighth inch band iron, how wide should the hoop be? 
 
 91. Pressure of Grain in Bins. — The downward pres- 
 sure of grain in bins follows the same law as that of liquids, 
 but the lateral pressure is always less on account of the fric- 
 tion between the kernels. When grain is heaped up on a level 
 surface it is found impossible to pile beyond a certain hight 
 Avithout increasing the diameter of the pile at the base. A 
 certain angle of slope is maintained, which for wheat is about 
 31°, about 30° for shelled corn, and for oats about 34°. 
 
 The friction of the kernels upon one another is just great 
 enough to maintain this angle, but in filling a bin with wheat, 
 for example, introducing it at the center, after a certain quan- 
 tity has been added the base of the cone is extended until it 
 reaches the sides of the bin, and the addition of any further 
 quantity brings into existence an outward pressure on the 
 walls of the bin tending to spread them. The case is analogous 
 to the retaining walls which are often built to prevent sand or 
 earth from caving or sliding. The amount of this pressure 
 and the method of computing it will be understood from 
 Fie:. 24. 
 
 Fig. U. 
 
53 
 
 CMMC represents a section of a bin sixteen feet square and 
 eight feet deep filled with shelled corn. The cone MOM rep- 
 resents the cone of grain which exerts no pressure on the sides 
 of the bin. The remaining portion MOMCC has its w^eight 
 divided between the sides and the bottom, the sides prevent- 
 ing it from sliding down the inclines OM, OM. The pressure 
 on the side CM, according to the theory of retaining walls, is 
 equal to the weight of tCM, acting as a wedge between the sur- 
 faces tM and CM. As the wedge is a movable inclined plane 
 where the force acts parallel to the base, the pressure may be 
 computed from the equation 
 
 Power X base = weight x liight. 
 
 The height, tC, is 4.5 feet, and the base CM, eight feet. The 
 weight is the weight of corn composing the wedge, and is 
 
 equal to 
 
 4.5x8x16x1728x56 ^^^^33^^ j^^_ 
 3x2150.4 
 
 Substituting the numerical values in the equation of the 
 wedge above we get 
 
 Power X 8=12958.72x4.5, whence, power=7289.28 lbs. 
 as the total pressure on the side of the bin, which is an average 
 of 56.9 pounds per square foot. 
 
 92. Pressure of Silage in Silos.— When silage is cut 
 into a silo the pressure against the sides conforms at first to 
 the same general law as that governing the pressure of grain 
 in bins, as given in 91; but it is always less than w^ould be in- 
 dicated by calculation, because the weight of the silage, com- 
 bined witii the fermenting processes, tends rapidly to compress 
 the cut pieces into fiat disks, which pile up as one brick does 
 upon another, and the mass very soon becomes self-supporting, 
 as has been proven by the silage remaining standing when the 
 walls of large silos have been burned down within a few days 
 after the completion of filling. When in filling a silo the silage 
 is not evenly distributed, so that it settles unevenly, the firmer 
 portions may act as an inclined plane down which the silage 
 resting upon it tends to slide, and in this w^ay give rise to a 
 side pressure wdiich otherwise w^ould not exist, and in my judg- 
 ment most of the disastrous pressures on the waUs of silos 
 have resulted from this unnecessary cause. 
 
54- 
 
 Such experiments as have been conducted to determine the 
 pressure of silage have given results ranging from fifty-three 
 pounds per square foot, at twelve feet deep, at the close of the 
 second day's filling, to fifty-five pounds per square foot at 
 twenty-one feet below the surface four days after the filling 
 commenced. 
 
 93. The Principle of Flotation When a body is im- 
 mersed in a fluid it is pressed or lifted upward with a force 
 exactly equal to the weight of the fluid it displaces, and it is 
 because of this fact that stones can be moved so much more 
 readily under water than out of it. Thus, a stone containing 
 exactly one cubic foot will be lifted up, when in water, with a 
 force of 62.42 pounds, and hence appears that much lighter 
 when moved under those conditions. It is this principle which 
 makes possible water navigation and the ascension above the 
 earth's surface in balloons. 
 
 94. Specific Gravity.— When the specific gravity of cast 
 
 iron is spoken of as 7.2 the meaning is that a cubic inch or 
 
 a cubic foot of that body weighs just 7.2 times as much as 
 
 the same volume of water, and when the specific gravity of 
 
 white pine is given as .4 the meaning is that a given volume 
 
 of that wood weighs only .4 as much as an equal volume of 
 
 water ; hence, for liquids and solids, we have the equation 
 
 cj .„ ., weight of body 
 
 Specific gravity = — . , , „ — ^—. ^ =^— j — 
 
 weight of equal volume of water. 
 
 Air is taken as the standard of specific gravity for gases. 
 
 95. To Find the Specific Gravity of Solids.— The 
 
 principle of flotation affords a very simple means of fending 
 the specific gravity of solids. Since any body immersed in 
 water displaces its volume of water, and since it is also buoyed 
 up by a weight equal to that of the water displaced, it is only 
 necessary to weigh the body whose specific gravity is desired, 
 both in air and in water, the difference in weight giving always 
 the weight of a volume of water the size of the body whose spe- 
 cific gravity is sought. The weight of the body in air di- 
 vided by this difference gives the specific gravity, and hence 
 the rule 
 
 j;^ . „ -i^ _ weight of soUd in air 
 
 1 - g V ty — jQgg ^^ weight in water 
 
 Suppose a body weighs ten in air and when immersed in water 
 
55 
 
 only eight. In this case the weight of a volume of water 
 equal in size to that of the body is 
 
 10-8=2 
 
 and hence, by the rule above, we have 
 
 Specific gravity =-^=5 
 
 a 
 
 Find the specific gravity of a body weighing fifteen in air 
 and fourteen in water. What will be its specific gravity if it 
 
 weighs in water, three? one? four? six? -even? ten? twelve? 
 
 96. Table of Specific Gravities and Weights per 
 Cubic Foot of Different Substances.— 
 
 Sp. gr. Weigh^. 
 
 Ash, Am. wliite, dry. 61 38 lbs. 
 
 Anthracite coal, moderately shaken 58 " 
 
 Brick, common hard .... 125 " 
 
 Carbon dioxide, referred to air 1.5 " 
 
 Charcoal, pines and oaks 22 " 
 
 Clay, dry, in lump, loose 63 " 
 
 Coal, bituminous 1 . 35 84 " 
 
 Coal, bituminous, broken, loose 50 " 
 
 Copper, rolled 8.9 555 " 
 
 Earth, clay loam, dry, nat condition 70 " 
 
 Earth, clay loam, saturated 93 " 
 
 Earth, reddish clay, dry, nat. condition 88 " 
 
 Earth, reddisli clay, saturated 108 " 
 
 Earth, fine sand, nat. condition 106 " 
 
 Earth, fine sand, saturated 124 " 
 
 Elm, dry 56 35 " 
 
 Gypsum, ground, loose 56 " 
 
 Gravel 106 
 
 Hemlock, dry 4 25 " 
 
 Hickory, dry 85 53 " 
 
 Iron, cast 7. 21 450 " 
 
 Ice 92 57.4 « 
 
 Lard 95 59.3 " 
 
 Lead 11.38 709.6 « 
 
 Limestone, broken 1.5 96 " 
 
 Maple, dry 79 49 " 
 
 Oak, white, dry 77 48 " 
 
 Oak, red, black, dry 39 " 
 
 Pine, wliite, dry 40 25 " 
 
 Pine, yellow, northern .55 34.3 " 
 
 Salt, coarse 45 " 
 
 Sandstone 2.41 151 " 
 
 Snow, fresh fallen .' 8.5 " 
 
 Snow, compacted by rain 15.50 " 
 
 Steel 7.85 490 « 
 
 Water, 62'F 1 62.35" 
 
56 
 
 97. To Find the Specific Gravity of Liquids.— The 
 
 principle of flotation stated in 93 also furnishes an easy 
 method of finding the specific gravity of liquids, which is done 
 as follows : Find the difference in weight of any convenient 
 solid in air and in water and then in the liquid whose specific 
 gravity is desired. Suppose the solid selected loses a weight 
 of one in water and a weight of .75 in another liquid, then a 
 volume of water, the size of the body taken, weighs one, and 
 an equal volume of the second liquid weighs .75. Then by the 
 rule we have : 
 
 Sp- gr.=-j-=.7o. 
 
 98. The Lactometer.— The use of the 
 
 lactometer in determining the specific grav- 
 ity of milk is also an application of the 
 principle of flotation, and is simply a mod- 
 ification of the method in 97. In this in- 
 strument, as shown in Fig. 25, the slender 
 and uniform stem is graduated so as to 
 give the specific gravity by direct read- 
 ing. i^^V- ^l 
 
 99. Atmospheric Pressure. — The air which everywhere 
 envelops the earth to a depth probably exceeding five hun- 
 dred miles has weight and exerts a pressure in all directions 
 upon all bodies in it. This pressure, at the level of the sea, is 
 ca})able of sustaining a column of mercury 29.922 inches high 
 on the average when the temperature is at 32° F. and is equal 
 to a pressure of 14.73 pounds to the square inch. The amount 
 of this pressure depends always upon the total quantity of air 
 that exists at the time above the point where the pressure is 
 exerted. This being true, places situated above the level of 
 the sea have a less pressure because they are nearer the u])per 
 limits of the air. 
 
 100. Variations in Atmospheric Pressure.— The 
 pressure exerted by the air at any place is almost con- 
 stantly changing, so that it is rarely the same at any two 
 consecutive moments ; these changes are not as a rule very 
 large or very rapid. A change of one-half a pound to the 
 square inch in twenty-four hours is a large change, and a 
 change of one pound to the square inch never occurs during 
 
57 
 
 short intervals, except when very violent storms are in prog- 
 ress. These changes in pressure are due to the fact that the 
 air is disturbed b}^ currents and waves which owe their origin 
 to various causes. 
 
 101. Soil Breathing. — When the atmospheric pressure 
 is heavy over a given locality air is driven into the air passages 
 in the soil of that place, and then when the pressure changes 
 again, becoming lighter, the compressed air expands and es- 
 capes; thus there is maintained an irregular but constant 
 breathing of the soil in consequence of these changes in atmos- 
 pheric pressure. The soil breathing is further maintained, 
 especially during the growing season, by the daily changes in 
 temperature which occur in the upper two to five inches of 
 soil. During the day the expansion, due to heating, forces air 
 out and then at night the cooling causes the air left in the soil 
 to contract and the reverse action takes place. Just how im- 
 portant this soil breathing is in the operations of tillage we do 
 not know. Its amount will be increased or diminished as we 
 increase or diminish the porosity of the soil and as we modify 
 the conditions which affect the diurnal changes of tempera- 
 ture. 
 
 102. Effect of Atmospheric Pressure on Soil Water. 
 When soil is nearly saturated with water, air can neither 
 enter nor escape from it readily except where large openings 
 or passages exist. In consequence of these facts, when the air 
 pressure over a region becomes less the springs of such regions 
 often discharge more water and the water may stand higher 
 in the wells. The air confined in the soil and unable to escape 
 rapidly, expands when the pressure falls and forces the water 
 toward any openings which may exist. The reverse action 
 also takes place when the air pressure increases, causing the 
 water in the wells to be depressed and the same springs to 
 discharge more slowly. " Blowing wells " owe their character 
 also to the changes in atmospheric pressure. 
 
 103. The Suction Pump.— The common pump is one 
 of the applications of atmospheric pressure. It should be un- 
 derstood, however, that the pressure of the air is in no way a 
 source of power; it originates no part of the energy expended 
 in pumping. Practically the only part the air plays in pump- 
 ing is that of crowding the water up into the cylinder of the 
 
58 
 
 pump after the lifting of the piston has removed the pressure 
 from the water in the suction pipe. The hight to Avhich the 
 atmosphere will sustain a column of water at sea level is 
 thirty-four feet ; but a pump producing a perfect vacuum could 
 not raise water to that hight on account of the downward 
 pressure exerted by the vapor of water and the air contained 
 in water rising into the vacuum formed by the pump and ex- 
 erting a pressure downward upon the column of water raised. 
 Common pumps are necessarily so imperfect in their action 
 that it is found impracticable to have the suction pipe longer 
 than sixteen to twenty feet above the water to be raised. 
 
 104. Size of the Piston.— The amount of water dis- 
 charged by a suction j^ump is determined by the length of the 
 stroke and the area of the piston ; and these in turn are de- 
 termined by the strength of the pumping force and the depth 
 of the well. In working a common pump a man can exert a 
 pressure of only fifteen to twenty pounds comfortably upon 
 the pump handle, and as the power-arm of the lever is only 
 from five to seven times the length of the weight-arm the 
 weight of water which can be lifted at one stroke cannot much 
 exceed seventy-five to one hundred pounds. This being true, 
 it is evident that pumps to be placed in deep wells must have 
 smaller pistons than those placed in shallow ones. It was 
 shown in 89 that the pressure of water is proportional to its 
 depth, and in 90 that water forty feet deep exerts a pressure 
 of two thousand four hundred and ninety-six pounds per 
 square foot when at a temperature of 50° F., or at the rate of 
 seventeen and one-third pounds per square inch, and hence 
 the area of the piston for the pump to lift water forty feet 
 should not exceed 
 
 -— _ = 5.78 square inches, 
 
 and this is given when the diameter of the piston is 2.7 inches. 
 On account of the friction of the piston and of the water in 
 the pipe and of the inertia of the water, a piston of that size 
 would work hard in a well of that depth. In a well where the 
 Avater is to be raised only twenty feet the area of the piston 
 could be twice, and for ten feet four times, as great respect- 
 ively; these would be given by diameters of 3.8 inches and 
 5.4 inches ; but, as in the first case, they are too large for easy 
 action. Three inches would be large for twenty feet. 
 
59 
 
 105. Rate of Pumping.— The rate of discliarge by a 
 pump will be governed by the area of the piston, the length 
 of the stroke and the number of strokes per minute. If the 
 area of the piston is five square inches, the length of stroke 
 five inches and the number of strokes per minute forty, then 
 
 5 X 5 X 40=1,000 cubic inches 
 
 or' 4.3 gallons per minute. 
 
 106. Function of Air Chambers.— In all single-acting 
 pumps the power is able to do useful work on the piston only 
 
 Mg. 26. 
 
60 
 
 when it is moving in one direction. In deep wells, where a 
 long column of water must be quickly set in motion and then 
 allowed to come to rest again, the intermittent action of com- 
 mon pumps is a serious objection, and to avoid this, air cham- 
 bers are sometimes attached. The principle of their action 
 Avill be understood from a study of Fio-. 26. 
 
 Tlie air in the upper portion of the chamber, which cannot 
 escape, is compressed by the rapid action of the piston and 
 then, during the reverse movement, it gradually regains its 
 original volume, forcing the water out in a nearly continuous 
 stream. The water, therefore, is obliged to flow with only 
 one-half the velocity of that which would be required with no 
 air chamber, and consequently a pump having an air chamber 
 properly placed can be worked by a wind-mill in a lighter 
 wind than one without the air chamber. The air chamber 
 attached commonly to pump stalks has no influence on the 
 pumping except when the pump is used to force water above 
 the level of the air chamber. To render the greatest service, 
 an air chamber should be placed at as low a point as practi- 
 cable in the well where there will be but a short column of 
 water between the piston and the air chamber. 
 
 107. The Siphon.— The flow of water through the siphon 
 is maintained by a force ---.t- 
 represented by the differ- 
 ence in pressure in the two 
 arms, the siphon being kept 
 full by atmospheric pres- 
 sure. The action of the 
 siphon is explained as fol- 
 lows : 
 
 When the siphon is filled 
 with water the downward 
 pressure in the short arm 
 is due to the upward pres- 
 sure of the air at d^ Fig. 27, and the downward pressure of 
 the column of water a J, which, using the values in the figure, 
 gives a total of 
 
 2 + 2 + 14.72=18.72. 
 
 The downward pressure in the long arm of the siphon is 
 equal to the downward pressure of the column of water a d 
 
 Fig. ^7. 
 
61 
 
 and the downward pressure of the air on the water in the 
 vessel, or 
 
 (6x2) + 14.72^:26.72. 
 
 As the two air pressures are equal and in opposite directions 
 they balance each other, leaving the force which determines 
 the flow the difference in the pressure of the two columns of 
 water, or 
 
 12-4=8. 
 
 It is evident that the greater the difference in the length of 
 the siphon arms the greater will be the velocity of discharge. 
 
 108. The Flow of Water.— When liquids move in a 
 stream the molecules do not become separated from one an- 
 other to any appreciable extent. The stream moves as a 
 whole, the density of the liquid remaining the same in all its 
 parts. 
 
 The flow of fluids is caused by a difference of pressure 
 within the mass caused either by increasing it at some point 
 or by diminishing it at another. Small velocities are asso- 
 ciated with small differences of pressure and large velocities 
 with large differences. 
 
 109. " Head of Water."— The velocity with which water 
 issues from an orifice in a vessel is due to the pressure of the 
 liquid above the center of figure of the orifice and this dis- 
 tance is called the head. If it were not for the viscosity of 
 the water, and the resistance offered by the orifice itself, the 
 velocity would be equal to that which a body falling in a 
 vacuum would acquire in falhng through the distance equal to 
 the head. This is expressed by the equation 
 
 Velocity = V2pi 
 where H is the head and g is the velocity the force of gravity 
 is able to produce in a falling body during a second of time 
 and is equal to 32.2 feet. If the head is ten feet, then the 
 velocity of discharge, leaving resistance out of consideration, 
 would be 
 
 Velocity = '^/2 x 32.2 x 10=25.3. 
 
 What would be the velocity of discharge with a head of 
 two feet? four feet? six feet? eight feet? twelve feet? 
 
 110. Flow of Water in Pipes.— The quantity of water 
 discliarged by pipes is very much modified by their diameters, 
 
62 
 
 lengths, degree of roughness, and by the presence or absence 
 of curves or angles. Other things being the same, the greater 
 the head the greater the discharge ; the greater the length 
 and the less the diameter the less the discharge ; the greater 
 the number of bends or angles the less the discharge. 
 
 There is no simple rule for computing the amount of water 
 a pijoe of a given length and diameter will discharge under a 
 given head. To compute the discharge exactly the velocity 
 of discharge at the mouth of the pipe and the area of its open- 
 ing are required. Where the pipes range from .75 inch to six 
 inches in diameter and their lengths lie between two hundred 
 and two thousand feet, the equations below give the velocity 
 in feet per second, but with only a rough degree of approxi- 
 mation. 
 
 (1) Velocity in feet per second r=40i/^"'^"^- ^^ I^^P^ ^" ^"^^^ ^ ^^^^^^ ^^ ^'^^^ 
 r length in feet + 54 x diam. in feet. 
 
 This may also be expressed as below, the dimensions all be- 
 ing in feet: 
 
 ,„, ct f 1 x f i 1 1600 X diam. pipe x head 
 
 (2) Square of velocity m feet per seconds length + 54 times diam. 
 
 In case the length of the pipe is twelve hundred to two 
 thousand times the diameter, the factor fifty-four times diam- 
 eter may be omitted without affecting the result very much. 
 In such cases if the diameter and head are expressed in inches 
 the velocities may be more readily determined by the follow- 
 ing: 
 
 „ 2 _ 16^^ ^ diam. x head 
 ^ ' ^ "" 12 X 13 X length. 
 
 If the diameter of a pipe is two inches, its length two hun- 
 dred feet and the head four feet, what is the velocity of dis- 
 charge ? 
 
 By(l),v=40|/-^-5fcl^^=40/,t=3,859. 
 
 whence, v=2.259 ft per second, 
 whence, v=2.309 ft. per second. 
 
 The last formula gives a velocity of .05 feet per second too 
 large. 
 
63 
 
 What is tlie velocity of discliarge when the diameter of the 
 pipe is six inches, length two thousand feet, head four feet? 
 
 To find the discharge of water in cubic feet per second, mul- 
 tiply the velocity in feet by the area of a cross-section of the 
 pipe in feet. 
 
 Discharge = velocity x area of opening. 
 
 HEAT. 
 
 111. Nature of Heat. — Heat is a form of molecular en- 
 ergy. When a hot body is brought into contact with a cold 
 one, the molecules comprising the hot body have their veloci- 
 ties slowed down by collision with the slower-moving mole- 
 cules of the cold body and energy is transferred from the hot 
 body to the cold one ; and, if the contact continues, the trans- 
 fer will go on until the molecular energy, per unit of weight, 
 is equal in the two bodies. If a hot ball of iron is allowed to 
 cool in the air, the cooling is the result of the ball doing wo7'l' 
 on the air. The molecules of air which come in contact with 
 the surface of the ball are struck by the molecules of the ball 
 and made to move away with a higher velocity than they 
 had before, just as a ball approaching a bat is struck by it and 
 flies to field leaving the bat motionless, a nearly complete 
 transfer having taken place. When a cold iron is thrust into 
 the forge fire a part of the energy of the molecules of the 
 burning coal and of the products of combustions is transferred, 
 by collisions, to the molecules of iron, and the temperature of 
 the iron rapidly runs up. 
 
 112. Solar Energy.— When the sun rises the tempera- 
 ture of bodies upon which it shines becomes higher as a rule, 
 and when it sets the temperature again falls, and, as a rule, 
 continues to do so until the sun begins to shine on them again. 
 So too, as our days grow longer and longer with the approach 
 of summer, the mean daily temperature becomes higher, and 
 then falls away again as the nights become longer than the 
 days. Such, and many other facts, prove that the sun is a 
 source of energy, and that in some manner this energy is being 
 transferred to the earth. Since the earth travels entirely 
 
64 
 
 around the sun once each year, and yet each day receives en- 
 ergy from it, it follows also that solar energy is leaving the 
 sun continually in all the directions in the plane of the earth's 
 orbit, and is in fact traveling aY\"ay in every other direction. 
 113. How Solar Energy Reaches the Earth.— When 
 one stands on the shore of a small lake and agitates its waters 
 in any manner, waves start out from the place of disturbance, 
 traveling in all directions toward the bottom and the distant 
 shore lines. "When these waves reach the bottom, the shore 
 and the air resting upon the lake, they lose a part of their 
 energy, the lost portion being transferred to whatever foreign 
 medium is struck by them. The energy generated in the 
 muscles of the person agitating the water is thus conveyed 
 away from him in all directions, and, sooner or later, is changed 
 into the energy of molecular motion known as heat. The per- 
 son is therefore a source of energy, which is borne away from 
 him in the form of waves in the water and air, and this wave- 
 energy becomes changed to heat, and thus the person in a 
 small degree warms the pebbles lying on the distant margin 
 of the lake, not by the heat of his body, but by the waves he 
 set up in the water. It was not heat which traveled to the 
 distant shore, but water waves which, striking the sands and 
 pebbles, gave a part of their energy to be transformed into 
 energy of heat in them. 
 
 The sun is wholly immersed in a cold medium called ether 
 and the molecules of the sun's surface beating against this 
 have their energy transformed into waves in it which travel 
 away in all directions just as waves of water spread away 
 from a disturbing body in it, but at a very much more rapid 
 rate, the velocity being one hundred and eighty-six thousand 
 six hundred and eighty miles per second, a speed which brings 
 them to us in about eight minutes after their origin at the 
 sun's surface. Sir Wm. Thompson estimates that the sun is 
 constantly doing work upon the ether at its surface at the 
 rate of one hundred and thirty-three thousand horse power 
 for each square meter of its surface, and the " mechanical 
 value of a cubic mile of sunshine " near the earth is placed at 
 twelve thousand and fifty foot-pounds, and, as this energy is 
 approaching us at the rate of one hundred and eighty-six 
 thousand six hundred and eighty miles per second, the amount 
 
C5 
 
 wliicli falls upon a square mile of the earth's surface in that 
 
 time is 
 
 186680 X 12050 ft. -lbs. =2249494000 ft.-lbs, 
 
 and this is equivalent to about eighty foot-pounds per square 
 foot each second. 
 
 114. Kinds of Ether Waves. — The molecular oscillations 
 or vibrations at the sun's surface are not all of them at the 
 same rate and hence they set up waves of different frequen- 
 cies of vibration in the ether, the slowest known being at the 
 rate of one hundred and seven billions of thrusts upon the 
 ether each second and the most rapid at about the rate of 
 forty thousand billions per second. When the wave frequen- 
 cies lie between three hundred and ninety-two billions and 
 seven hundred and fifty-seven billions per second, such waves, 
 falling in the eye, produce the sensation of light and we speak 
 of them as light waves. Waves slower than three hundred and 
 ninety-two billions per second produce no sensation of light in 
 the eye, but when absorbed by the skin they cause the sensa- 
 tion of warmth and are called dark heat waves. Waves more 
 rapid than seven hundred and fifty-seven billions per second, 
 when they fall upon the molecules of a photographer's plate, or 
 upon a living green leaf, set up such intense vibrations in these 
 molecules as to break them down, producing chemical changes 
 and hence these are called chemical waves. It should, however,* 
 be kept distinctly in mind that there is no lights no heat and no 
 chemical action until the ether waves have dashed against some 
 molecular shore and have been wrecked upon it. When any of 
 these waves fall upon what we call a hlack substance, like a 
 thick layer of lamp-black, they are nearly all absorbed and the 
 body becomes heated. On the other hand, when they fall 
 upon a pure xohite substance, like snow, the waves rebound 
 with nearly their full vigor and there is very little of either 
 heating or chemical effect. When the waves fall upon what 
 we call green substances, like the chlorophyl of growing leaves, 
 most of the chemical waves and a portion of the light waves 
 are wrecked by it and the chemical changes natural to grow- 
 ing leaves are the result. 
 
 115. Work Done on the Earth by the Ether Waves. 
 It was stated in 113 that eighty foot-pounds of energy per 
 square foot reach the earth's surface each second. This seems 
 
66 
 
 like an enormous amount of work when it is figured in horse 
 power for a section of land, the amount being 
 
 2249494000 ,^onnon i, 
 
 — - — -=4089989 horse power, 
 
 and it is diiScult at first to realize that it can be true. To 
 comprehend the situation we need to know that the earth is 
 traveling through a cold region having a temperature of abso- 
 lute zero or — 273° C, with only the thin atmosphere to pro- 
 tect it from that cold. If the mean annual temperature of 
 Wisconsin is 45° F. or 7° C, its temperature is maintained by 
 
 the sun at 
 
 273" + 7''=280°C. 
 
 higher than that of the space which surrounds it. The earth 
 is therefore rapidly sending ether waves back again into space, 
 and thus a large part of the energy which comes to us is lost. 
 The motions of the air, and of the water in the ocean and to 
 and from the land, represent other portions of this energy 
 transformed. Most of the chemical changes occurring in 
 growing vegetation represent other transformations of solar 
 energy, as do the activities of all forms of animal life ; and 
 when to these are added the chemical and physical changes in 
 soils and rocks, due to it, it is plain that the amount needed 
 to maintain the earth in its present state of activity is really 
 very large. 
 
 116. Transfer of Heat.— When one portion of a body is 
 heated, as in the case of a poker thrust into the fire, the heat- 
 energy gradually spreads to the distant end. This sort of 
 transfer is known as conduction, and the rate at which it occurs 
 is very different with different materials. Metals and stone 
 are among the best conductors, while wood, glass, water and 
 woolen fabrics are among the poor conductors. The transfer 
 of heat through air, where currents are prevented, takes place 
 very slowly and it is on this account that several thin gar- 
 ments are warmer than the same weight of the same material 
 as a single garment. It is on this account also that sawdust, 
 in the walls of buildings and about ice, is so serviceable. Hollow 
 walls with dead air spaces utilize the same principle, as does 
 the practice of using one or more thicknesses of building paper 
 in the construction of buildings which are designed to keep 
 heat in or out. 
 
67 
 
 m 
 
 When heat is applied to the lower portion of liquids or 
 gases the conduction of heat to portions of the mass causes it 
 to expand and become relatively lighter than that not afifected, 
 and it is, in consequence, forced to rise, thus establishing up- 
 ward and downward currents. In such cases the heating is 
 by conduction, but the heated mass is then transported, that 
 not heated taking its place. The process is named convection. 
 The third method of transfer of heat is by radiation, and has 
 already been described in 113. 
 
 117. Draught in Chimneys. — The draught in ©himneys 
 is due to two principles, one that of convection, and the other 
 that of aspiration. In all properly constructed chimneys there 
 is a draught, usually, even when there is no 
 difference of temperature of air inside and 
 out, and such draughts are strongest when 
 the wind blows hardest. Why this is so will 
 be readily understood from Fig. 28. The 
 air, in its rapid motion across the top of the 
 chimney, encounters the air molecules in its 
 very top and forces them out and onward 
 with it ; this diminishes the weight of air in 
 the chimney, and the pressure from below 
 forces a new quantity into the moving stream 
 which in turn is driven away. The rapid 
 forward motion of the outer air prevents it 
 from descending into the space left by the 
 air forced forward. When the fire is kindled the air in the 
 chimney is made specifically lighter and is forced out on the 
 principle of flotation (93). When the temperature of the air 
 is raised one degree F. its volume is increased ^y of its orig- 
 inal volume, so that if air enters a stove at 70° F. and has its 
 temperature raised to 234° F. its volume Avould be increased 
 one-third and hence its weight diminished in the same propor- 
 tion, and the relative weights of air per cubic foot inside and 
 outside the chimney would be as two to three. When these 
 conditions exist, it is evident that the higher the chimney is 
 the greater will be the difference in the weight of the two 
 columns of air and the stronger the draught. When the 
 chimney has its top considerably extended above the surface 
 it is placed in a region of more rapidly moving air currents 
 and the draft is made stronger on this account also. 
 
 2 
 
 t 
 
 1 
 
 i' 
 
 1 
 
 3^ 
 
 Fig. 28. 
 
118. Transparency to Ether Waves.— When the hand 
 is placed near a pane of glass, through which the sun is shin- 
 ing, the ether waves falling upon the hand are absorbed and 
 so increase the molecular motion of the skin, raising its tem- 
 perature. The hand, in turn, sends out ether waves toward 
 the sun, but they are of the long sort and cannot pass through 
 the glass, but are reflected back again upon the hand and join 
 with those coming from the sun to raise the temperature to a 
 still higher point. The glass is transparent to the short waves 
 coming from the sun but opaque to the long ones into which 
 they have been transformed in the hand. 
 
 This is the principle upon w^hich the hot-bed is constructed, 
 which is practically an energy trap, allowing it to enter from 
 the sun and then preventing its ready escape. 
 
 On the same principle, too, large windows in the south side 
 of dw^elling-houses, especially if they are double, contribute a 
 very large amount of heat toward warming the room in win- 
 ter, and are really a great saving of fuel, besides contributing 
 so much to healthfulness. The amount of heat which may 
 enter a house in this manner during the winter is much larger 
 than can enter it in summer, because in winter the sun shines 
 more perpendicularly upon the windows, which has the effect 
 of making them larger, as explained in 167. 
 
 Our atmosphere acts practically in the same manner toward 
 the energy received from the sun and that radiated back again 
 by the earth. It is highly transparent to the first. and very 
 opaque to the last. Clouds, fog and smoke are still more 
 opaque to terrestrial radiations, and this is why frosts on a cran- 
 berry marsh or straw^berry bed may sometimes be prevented 
 by producing a cloud of smoke over it. 
 
 119. Temperature. — The temperature of a molecule is 
 an expression of the amount of energy it contains, and all 
 molecules having the same temperatures are assumed to pos- 
 sess the same amounts of energy of motion. When the tem- 
 perature of a given body is doubled its energy of molecular 
 motion is doubled. Could the molecules of a body be brought 
 entirely to rest, its temperature Avould be absolute zero, but 
 this is a condition of things very diificult if not practically im- 
 possible to reach. 
 
 120. Measurement of Temperature.— The common 
 method of measuring temperature is by noting the changes in 
 
69 
 
 volume of a body wlilch are associated \Tith changes in its 
 temperature. The material of a thermometer may be either 
 solid, liquid or gaseous, and all three types are in use. For 
 ordinary jDurposes the mercurial thermometers are the best. 
 The mercury expands more regularly than most other avail- 
 able liquids, thus making the graduation of the stem simple ; 
 it boils at a high and freezes at a low temperature ; it can be 
 readily seen and it responds quickly. 
 
 The sensitiveness of the thermometer depends upon ti.e 
 relative diameters of the bulb and tube ; the finer the bore oi" 
 the tube and the larger the bulb the longer will be each de- 
 gree. Too large a bulb is objectionable because a longer time 
 is required for it to acquire the temperature of the body 
 whose temperature is desired, and too fine a bore has the ob- 
 jection of not being readily seen. The long cylindrical bulbs 
 are better than the spherical ones because they present a 
 larger surface and therefore respond more quickl}", reaching 
 a condition of rest sooner. 
 
 121. Testing a Thermometer. — The bulbs of most 
 thermometers shrink after they are made, and if the gradua- 
 tion has been done before the shrinkage has occurred the 
 reading of the thermometer will be found too high or will 
 ultimately become so. To see whether the thermometer is 
 correct, in this regard, it should be immersed in melting snow 
 or crushed ice, from which the water formed by melting may 
 readily drain away, and allowed to remain until the mercury 
 becomes stationary. 
 
 If tlie thermometer is one of the dairy types or has the bulb 
 exposed, its correctness at blood heat may be determined by 
 placing the bulb under the tongue and keeping the mouth 
 closed over it for about one minute, reading the temperature 
 while the bulb is yet in the mouth. If the person is well the 
 thermometer should indicate about 98.8° F. 
 
 It is rarely true that the diameter of the tube of the ther- 
 mometer stem is uniform throughout, there being a general 
 tendency for the diameter to increase from one end to the 
 other. If the irregularity of the tube is large, it may be cor- 
 rect at the freezing and boiling points and yet incorrect at in- 
 termediate points. If the tube grows larger from the bulb 
 the same amount of expansion in the bulb will cover a shorter 
 
70 
 
 distance on the scale, and vice versa. Large inequalities in 
 the tube may be detected by jarring the thermometer so as to 
 separate a short column of the mercury, say three-fourths of 
 an inch, and carefully measuring its length by divisions of the 
 scale in different portions of the stem ; if there is a large vari- 
 ation the length of the column separated will vary as it is 
 moved from place to place. 
 
 122. Kinds of Thermometer Scales.— There are two 
 scales used in this country, the Fahrenheit and Centigrade. 
 The first places the freezing point of water at 32°, and the 
 boiling point at 212°, the second at 0° and 100° respectively. 
 The Fahrenheit scale, between 32° and 212°, is divided into 
 one hundred and eighty divisions called degrees, while for the 
 Centigrade scale the number of divisions is just one hundred. 
 This being true, 
 
 180° Fahr. = 100° Centigrade 
 
 ,,,^ 100 5° 
 andl F.=jg^--ga 
 
 _„„ 180 9° 
 andl C.=j^=-F. 
 
 To convert the readings of a Fahrenheit scale into Centigrade 
 degrees find the number of Fahrenheit degrees from the freez- 
 ing point and multiply this by |. 
 
 No. of degrees F. from freezing x— =No. degrees C. 
 
 To convert Centigrade degrees into degrees Fahrenheit multi- 
 ply the number of degrees by | and the result will be the 
 number of degrees F. above or below 32° F. 
 
 No. of degrees C. x— =No. of degrees F. abo^e or below 33° F. 
 
 123. The Heat Unit. — It requires sixteen times as much 
 heat to raise the temperature of a pound of hydrogen one de- 
 gree as it does a pound of oxygen, and other ratios are found 
 to exist when other substances are taken. This makes it nec- 
 essary to select a certain substance as a standard when a unit of 
 heat is desired. Water is taken as the standard and one heat 
 unit is the amount necessary to raise a pound of water from 
 32° F. to 33° F. 
 
71 
 
 124. Specific Heat — When the amount of heat which 
 will raise the temperature of a pound of water from 32° F. to 
 33° F. is applied to a pound of dry sand it will have its tem- 
 perature raised through about 10° F. (Oelmer), or the same 
 heat would raise the temperature of ten pounds of sand one 
 degree, and the specific heat of sand is said to be .1, that of 
 water being 1, With the exception of hydrogen, water pos- 
 sesses the highest specific heat known, and this means that it 
 warms more slowly than do other substances ; but the reverse 
 is also true, and when once heated it cools more slowly or gives 
 out a larger amount of heat. This is why large bodies of 
 water make the winters of the lands adjacent to them warmer 
 and the summers cooler. 
 
 125. The Specific Heat of Soils.— Different soils, like 
 other substances, have different specific heats, and hence warm 
 at different rates under the same sunshine, and it is on account 
 of this fact, in part, that one soil is warmer than another. In 
 the following table are given the number of heat units neces- 
 sary to heat one hundred jjounds of water and of varieties of 
 soils from 32° to 33° F,, and the temperature each would have 
 after one hundred heat units had been applied to them at a 
 temperature of 32° F, 
 
 Table of Specific Heat of Dry Soils. 
 
 No. of heat units re- Temperature of 100 
 
 quired to raise 100 lbs. after the appli- 
 
 lbs. from 32° F. to cation of 100 heat 
 
 33" F. units. 
 
 Heat units. Degrees F. 
 
 Water 100.00 33.00 
 
 Moor earth 22.15 36.51 
 
 Humiis 20.86 86.79 
 
 Sandy humus 14.14 - 39.07 
 
 Loam rich in humus 16.62 38.02 
 
 Clayey humus 15.79 38.33 
 
 Loam 14.96 38.68 
 
 Pure clay 13.73 39.28 
 
 Sand 10.08 41.93 
 
 Pure chalk 18.48 37.41 
 
 These figures do not, in themselves, indicate the actual dif- 
 ferences in temperature the several soils named would show 
 under natural conditions because they are not only never per- 
 fectly dry but they have different capacities for holding water, 
 
T2 
 
 and they differ also in their specific gravities, so that one hun- 
 dred pounds of one soil covers more surface, at a given depth, 
 than another one does. "We have not yet the data needed for 
 an exact comparison, by volume, of the specific heat of soils. 
 The higher the per cent, of water any soil contains the more 
 heat will be required to raise its temperature one degree; so, 
 too, the heavier the soil is per cubic foot the more heat will 
 be required to raise its temperature a given number of de- 
 grees. Sand has a less capacity for water than most other 
 soils and is, on this account, naturally warmer, yet its higher 
 specific gravity tends to make it colder than other soils. A 
 cubic foot of dry sand weighs about one hundred and six 
 pounds, while one of clay loam is only about seventy pounds. 
 Saturated sand will contain, in the field, only about eighteen 
 per cent, of water, while the clay loam may have as high as 
 thirty-three per cent. Below are given the number of degrees 
 one hundred heat units will raise the temperature of a cubic 
 foot of sand and of clay loam when each is saturated with 
 water, half saturated and dry. 
 
 Saturated. Half saturated. Dry. 
 
 Sand 3.4° F. 5° F. 9.93° F. 
 
 Clayloam 2.98° F. 4.49° F. 6.02° F. 
 
 Difference 42° F. .51° F. 3.9° F. 
 
 It is thus seen that the greater weight of the sand, per unit 
 volume, tends to offset the greater amount of water held by 
 the clay, giving the two a more nearly equal temperature than 
 they would otherwise possess. It will also be seen that the 
 difference in the per cent, of moisture a soil may contain 
 makes a relatively larger difference in the change in tempera- 
 ture a given amount of heat absorbed will produce. This is 
 one reason why a well-drained soil is warmer than a similar 
 one not so drained. 
 
 126. '• Latent Heat." — When heat is applied to ice at a 
 temperature of 32° F. its temperature does not rise until the 
 melting is completed, the whole energy applied being expended 
 upon the molecules in moving them into new relative positions 
 against the force of cohesion which binds them together in the 
 crystalline arrangement of the ice. The amount of heat re- 
 quired to melt a pound of ice whose temperature is 32° F. is, 
 in round numbers, one hundred and forty-two units, or enough 
 
73 
 
 to raise the temperature of one hundred and forty-two pounds 
 of water from 32° to 33° F. This fact may be demonstrated 
 approximately as follows : 
 
 Take equal weights of water at 32° F. and at 212° F. and 
 mix them. The two weights of water will then be found to 
 possess a temperature nearly equal to 
 
 212- + 32° _ 
 
 If, on the other hand, equal weights of water at 212° F. and 
 dry ice at 32° are placed together and the ice allowed to melt, 
 the resulting water will be found to have a temperature of 
 51° F. The water has had its temperature lowered 
 
 212°-51°=161° F. 
 while the ice has had its temperature raised only 
 
 5r-32==19° R 
 Now if one pound each of ice and water were taken for the 
 experiment it is evident that the number of heat units con- 
 sumed in melting the ice would be 
 
 161-19=143 heat units. 
 
 When water has been raised to the boiling point no further 
 increase of temperature can be effected so long as the pressure 
 upon it remains constant, the whole amount of heat energy 
 being now expended in converting the water into steam at the 
 same temperature. 
 
 If a pound of steam at 212° F. be condensed in 5. 37 pounds 
 of water at 32° F. there will then be 6.37 pounds of water, 
 having a temperature of nearly 212° F. The pound of steam 
 in being converted into water has heated 5.37 pounds of water 
 through 
 
 212°-33°=:180° F. 
 
 without having its temperature appreciably lowered. The 
 molecular energy of the one pound of steam which was ab- 
 sorbed by the 5.37 pounds of water was therefore 
 180 X 5.37=966.6 heat units. 
 This large amount of molecular energy m steam explains 
 why a scald from steam is so much more severe than one from 
 boiling water, and also why so small a quantity of steam, by 
 weight, is requu-ed to boil a barrel of potatoes or to cook a 
 barrel of other feed. 
 
74 
 
 127. Evaporation Cools the Soil.— We have seen that 
 one pound of steam in condensing into water generates 9G6.6 
 heat units, and the reverse statement is also true, namely, to 
 convert a pound of water into the gaseous state, under the 
 mean atmospheric pressure, requires the absorption, by that 
 pound, of 9G0.6 heat units. When one pound of water disa}> 
 pears from a cubic foot of soil by evaporation, it carries with 
 it heat enough to lower its temperature, if saturated sand, 
 32.8° F. ; and if saturated clay loam, 28.8° F. 
 
 To dry saturated sandy soil until it contains one-half of its 
 maximum amount of water requires the evaporation of about 
 9.5 pounds to the square foot of soil surface when this drying 
 extends to a depth of one foot, while the similar drying of 
 clay loam requires the evaporation of 11.5 pounds, and 
 11.5-9.5=2 lbs. 
 
 or the amount of evaporation which must take place in the 
 clay loam to bring it to the same degree of dryness as the 
 sandy soil. But to evaporate two pounds of water requires 
 966.6 X 2=1933.2 heat units, 
 
 and this, if withdrawn directly from a cubic foot of saturated 
 clay loam, would lower its temperature 57.6° F. Here is one 
 of the chief reasons why a wet soil is cold. 
 
 That the evaporation of water from a body does lower its 
 temperature may be easily proved by covering the bulb of a 
 thermometer with a close fitting layer of dry muslin, noting 
 the temperature. If the muslin be now wet, with water having 
 the tem})erature noted, and the thermometer rapidly whirled 
 in a drying atmosphere its temperature will rapidly fall, owing 
 to the withdrawal of heat from the bulb by the evaporation of 
 water from the muslin. 
 
 128. Regulation of Animal Temperatures.— All of 
 our domestic animals require the internal temperature of 
 their bodies to be maintained constantly at a point varying 
 only a little from 100° F., and this necessity requires provisions 
 both for heating the body and cooling it. The coohng of the 
 body is accomplished by the evaporation of perspiration from 
 the skin and the amount of perspiration is under the control 
 of the nervous system. When the temperature becomes too 
 high, because of increased action on the part of the animal, or 
 
in consequence of a high external temperature, the sweat 
 glands are stimulated to greater action and water is poured 
 out upon the evaporating surfaces and the surplus heat is rap- 
 idly carried away ; each pound evaporated by heat from the 
 animal withdrawing about 966.6 heat units. 
 
 129. Bad Effects of Cold Rains and Wet Snows on 
 Domestic Animals.— When cattle, horses and sheep are 
 left out in the cold rains of our climate the evaporation of the 
 large amount of water which lodges upon the bodies, and es- 
 pecially in the long wool of sheep, creates a great demand 
 upon the animals to evaporate this water. The theoretical 
 fuel value of one pound of beef fat is 16,331 heat units, and 
 that of average milk is 1,148 heat units. A pound of beef fat 
 may therefore evaporate 
 
 ^§^=16.8 lbs. of water, 
 9bb.6 
 
 and a pound of average cow's milk 
 
 On this basis, if a cow evaporates from her body four pounds 
 of rain she must expend the equivalent of the solids of 3.39 
 pounds of milk. 
 
 A wet snow-storm is often worse for animals to be out in 
 than a rain-storm, because in this case, the snow requires melt- 
 ing as well as evaporating, and the number of heat units per 
 pound of snow is 
 
 143.65 + 966.6=1109.25 heat units, 
 
 and the heat value of a pound of milk is barely sufBcient to 
 melt and evaporate a pound of snow. 
 
 130. Cooling Milk with Ice and with Cold Water.— 
 If it is desired to cool one hundred pounds of milk from 80° F. 
 down to 40° F. it is practically impossible to do so with water 
 in the summer season m Wisconsin. It is difficult even to 
 cool it as low as 48° F., for most of the well and spring water 
 has a temperature above 45° F. and much of it is above 50° F. 
 If lower temperatures than 48° F. are desired during the warm 
 season some other means must be resorted to. Since it re- 
 
quires one hundred and forty -two beat units to melt a pound 
 of ice, one pound is capable of cooling from 80° F. to 40° F. 
 
 — T-r — =3. To lbs. of milk, 
 40 
 
 supposing the specific beat of milk to be the same as that of 
 water, Avbich is not quite true. To cool one hundred pounds 
 of milk from 80° F. to 40° F. will require, therefore, about 
 
 -g^=26j lbs. of ice, 
 
 supposing it to be used wholly in cooling the milk. 
 
 If the water has a temperature above 40° F., before the milk 
 and ice are placed in it, there will be required enough more 
 ice to cool the water down to the temperature desired for the 
 milk. 
 
 The greatest economy in the use of ice will be secured, there- 
 fore, when the creamer contains just as little water as will 
 cover the cans and give the needed space for the ice, and when 
 the walls of the creamer are made of so poor a conductor of 
 heat as to admit as little as possible from without. 
 
 131. Washing with Snow or Ice. — When ice or snow 
 are used in winter for washing purposes there is a large loss 
 of heat incurred in simply melting the ice and raising the tem- 
 perature of the water from 32° F. up to 45° F., the temper- 
 ature it may have in any well protected cistern. To melt a 
 pound of ice and raise its temperature to 45° F. will require 
 
 143 + 13=155 heat units. 
 
 If three hundred pounds of water are required for a washing 
 then the lost heat will be 
 
 300 X 155=46500 heat units. 
 
 The fuel value of one pound of water-free, non-resinous 
 wood, such as oak or maple, has been found to be 15,873 heat 
 units ; that of ordinarily dry wood, not sheltered, containing 20 
 per cent, of water, is 12,272 heat units. At this latter value 
 it will require, supposing 50 per cent, of the fuel value to be 
 utilized in melting the ice and heating the water, 
 
 2 X 46500 „ KOlu e J 
 __^^-^_^_=7.58 lbs. of wood 
 
7T 
 
 more than would be needed to do the same washing with 
 water at 45° F. ; and if seventeen such washings are done dur- 
 ing- the winter the total cost for fuel would be the value of 
 
 17x7.58=128 lbs. of wood, 
 
 to say nothing of the expense of getting the snow or ice and 
 the unhealthfulness of handling it. 
 
 132. Burning Green or Wet Wood. — Whatever water 
 wood or other fuel may contain when it is placed in the stove, 
 so much of the fuel as is required to evaporate this water 
 must be so expended and is prevented from doing work out- 
 side of the stove. We have seen, 131, that when wood con- 
 tains 20 per cent, of water there is required 
 
 15873-12272=3601 heat units 
 
 per pound of wood to evaporate the water contained, which is 
 22.7 per cent, of the total value. Wood, after being in a rain 
 of several days, contains more water than this, and green 
 wood much more, sometimes as high as 50 per cent., while 
 well-seasoned sheltered wood may contain less than half that 
 amount. 
 
 It is frequently urged that when some green or wet wood is 
 burned with that which is dry there is a saving of fuel. There 
 is some truth in this, especially in stoves having too strong a 
 draught and too direct a connection with the chimney and if 
 the radiating surface is small or poor. The evaporation of 
 the water prevents so high a temperature from occurring in 
 the stove, which makes the draught less strong, and this gives 
 more time for the heat to escape from the stove before reach- 
 ing the chimney, and hence less is lost in this way. Then as 
 the fire burns more slowly there is not the overheating of the 
 stove, at times, which may occur with lack of care when very 
 dry wood is used, and a considerable saving occurs in this way. 
 These statements apply more particularly to heating stoves 
 than to cooking stoves. Dry wood is best for the kitchen 
 stove under most circumstances, the slower fire being secured 
 when needed by using larger sticks and by controlling the 
 draft. 
 
 133. High Winter Temperatures Associated with 
 Snow Storms. — " It is too cold to snow " is a common say- 
 ing, but the truth is it cannot snow and remain very cold. 
 
Y8 
 
 Speaking in approximate terms, when a pound of water in the 
 form of aqueous vapor in the air is converted into snow there 
 is liberated 
 
 966.6 + 142=1108.6 heat units, 
 
 and, as the specific heat of dry air is only .2375, one heat unit 
 will raise the temperature of one pound of air through 
 
 - 4 21° F 
 
 and 4.21 pounds of air through 1° F. This being true, the 
 freezing of one pound of aqueous vapor will liberate heat 
 enough to warm through 1° F. 
 
 1108.6 X 4.21 pounds =4667.2 pounds of air, 
 
 and as water at 32° F. is Y73.2 times heavier than air at the 
 same temperature, the number of cubic feet of air raised 1° F. 
 must be 
 
 4667.2 
 
 62.417 
 
 57815.6 cu. ft of air, 
 
 which is equivalent to 5781.56 cubic feet raised 10° and to 
 1800 cubic feet raised from 0° F. to 32° F. When a snow fall 
 of four to six inches occurs, over a large area, there is, there- 
 fore, a very large volume of air heated by it. 
 
 PKOTECTION AGAINST LIGHTNI:N"G. 
 
 134. Nature of Electricity. — No very clear statement 
 is yet possible in regard to the real nature of either electricity 
 or magnetism, but the strongest evidence points to the con- 
 clusion that they are manifestations due to some action of the 
 all-pervading ether which we have seen, 113, is the medium 
 through which energy generated at the sun's surface reaches 
 the earth. In the battery, on the telegraph hne, energy is 
 generated by the chemical action there taking place and, by 
 some action not yet clearly seen, the ether pervading the space 
 between and surrounding the molecules of the telegraph wire 
 conveys this energy to the distant stations, where it is ab- 
 sorbed by the receiving instruments and converted into me- 
 
clianical motions which record or indicate the messages sent. 
 In some manner the molecules of a conducting wire prevent 
 the escape of energy to the outside ether as the walls of a 
 speaking tube confine the sound waves developed in them, 
 preventing them from being dissipated in the surrounding air 
 and allowing them to travel to the end only slightly enfeebled. 
 
 When a glass rod is rubbed with a piece of silk or fur the 
 mechanical action develops a state in the ether of the rod 
 which is shown by the ability of the rod, in this condition, to 
 attract hght objects to it. When a person speaks in front of 
 a telephone the sound waves produced by the vibration of his 
 vocal cords set the metal plate, near the end of the telephone 
 magnet, swinging in unison with the vocal cords, and the 
 approaches and recessions of this plate so disturb the ether 
 of the magnet as to cause it to take up a part of the energy of 
 the vibrating plate and then to transmit it to the ether of the 
 wire wrapped about the magnet and leading to the receiving- 
 station, where, by another of those wonderful yet universal 
 transformations of energy, the action is reversed and the me- 
 chanical swing of the plate in the receiving telephone gives 
 back the words which set up the action at the sending station. 
 
 135. Atmospheric Electricity.— What the origin is of 
 the intense electrical manifestations associated with thunder 
 
 Fig. W. 
 
80 
 
 storms as yet lacks positive demonstration, but the close re- 
 semblances of these manifestations to the electrical manifesta- 
 tions developed by friction, when combined with the fact that 
 the strongest atmospheric electrical displays are associated 
 with the most violent air movements where rain or hail is 
 present, has led to a general belief that this electricity owes 
 its origin to the friction of the air currents upon the con- 
 densed moisture they are carrying. Fig. 29 represents the 
 general character of an electrical discharge in the atmosjihere. 
 
 136. Electrical Induction.— When a body, w-hich has 
 become charged with electricity, is brought near another body 
 which has not be3n electrified it exerts an influence upon that 
 body inducing electricity in it, and if the charge is sufficiently 
 intense and the distance is not too great the electricity will 
 break across from one body to the other, and the act may be 
 accompanied by a flash of light and a report. 
 
 137. Positive and Negative Electricity.— It is impos- 
 sible to throw a stone into water, making a depression at any 
 point, without raising a ridge around it which is equal in mag- 
 nitude to the depression, but extending in the opposite direc- 
 tion. When these two opposite phases are developed the}^ 
 tend to come together, and the tendency is stronger in propor- 
 tion as the waves are higher. Something analogous to this 
 state of things seems to occur whenever and wherever elec- 
 tricity is generated. There appears alwa^^s to be engendered 
 two equal and opposite phases which tend to run together and 
 obliterate each other unless prevented from doing so. The 
 one phase is called positive and the other veijatlm electricity. 
 
 138. Conductors and Non-conductors of Elec- 
 tricity. — There is a great difference in the ability of different 
 substances to convey electricity from one place to another ; 
 those which convey electricity readily are called conductors, 
 and those which convey it poorly or not at all are called poor 
 conductors or non-conductors. The metals generally are 
 among the best conductors, and silver and copper are the best 
 of these. Glass, gutta perclia and dry air are among the 
 poorest conductors. 
 
 139. Discharges from a Point.— AVhen a body becomes 
 charged with electricity the charge manifests itself only on the 
 outside surface. If the body is a sphere the intensity of the 
 
81 
 
 cliarge will be uniform at all portions of the surface. If, bow- 
 ever, tbe body is conical or has points upon it the charge will 
 be most intense at the points, and if a discharge takes place it 
 will occur first from the points, and it is this fact which has 
 led to the placing of points on lightning-rods. 
 
 140. When an Object May be Struck by Lightning. 
 When a cloud becomes so heavily charged that the air between 
 it and an adjacent cloud or an object on the ground, in which 
 it has induced the opposite kind of electricity, is no longer 
 able to prevent the electricity from breaking through, a dis- 
 charge or stroke occurs. Usually the nearer the charged 
 cloud approaches an object the more intense will be the charge 
 induced by the cloud in the body approached and the greater 
 will be the chances of a stroke. The intensity of attraction 
 increases as the square of the distance decreases, and this is 
 why, when other conditions are the same, elevated objects, 
 like buildings, are more liable to a stroke than those which are 
 low^er. 
 
 Buildings standmg upon w^et ground are more liable to a stroke 
 than buildings in other respects similar but standing upon dry 
 ground, the greater danger coming from the possibility of a 
 stronger charge being induced upon the house in consequence 
 of the better conduction of the wet soil. Large trees near 
 buildings have a tendency to prevent strokes. 
 
 141. The Function of a Lightning-rod.— Lightning- 
 rods have two functions to perform, the first and chief one 
 being to discharge quietly into the air above, the electricity 
 w^hicli may be induced upon a building as rapidly as it accu- 
 mulates, and thus pi'event a stroke from occurring ; and second, 
 in case a stroke is inevitable, to diminish its intensity and 
 convey to the ground quietly as much of the discharge as pos- 
 sible, thus reducing the damage to a minimum. 
 
 142. Do Lightning-rods Afford Complete Protec- 
 tion? — There is now a general agreement among physicists 
 that properly constructed and mounted lightning rods furnish 
 a large protection to buildings ; they are divided in opinion, 
 however, as to whether complete protection is possible. The 
 rod may be called upon to protect against discharges under 
 two conditions: first, where a heavily-charged cloud comes 
 slowly over the rod, giving it time to discharge the induced 
 
82 
 
 electricity and thus prevent an accumulation ; and second, where 
 an uncharged cloud chances to be over a rod when it instan- 
 taneously becomes charged from some other cloud. When 
 this occurs it is claimed by some that the rod has insufficient 
 time to afford any material protection, and hence that it is 
 hopeless to think of protecting completely against this class of 
 cases. 
 
 143. Essential Features of a Lightning-rod.— For a 
 number of years past there has been a fairly unanimous agree- 
 ment in regard to the essential points of a lightning rod, but 
 some new discoveries in regard to the conduction of rapidly 
 alternating currents, and in regard to electrical inertia, has led 
 to a divergence again upon some points. It may be said that 
 practically all are agreed that : 
 
 1. The rod should be of good conducting material, contin- 
 uous throughout, terminating in several points above, and well 
 connected with permanent moisture below the structure in 
 the ground. 
 
 2. The rod should be in good connection with the building, 
 especially with metal roof and gutters, and should be carried 
 as high as the highest point of the structure to be protected. 
 
 3. The points need not be very fine, but should be coated 
 with some metal which will not rust. 
 
 4. An iron rod, everything considered, is better and cheaper 
 than one of copper, provided it is galvanized and of sufficient 
 size. 
 
 The fundamental point of disagreement at present is in re- 
 gard to the form of the rod ; some claiming that if a sufficient 
 area of cross-section is given the shape is immaterial so far as 
 conducting ability is concerned, the solid round rod being the 
 cheapest and most easily protected from rust ; others maintain 
 that the larger the surface the rod presents the greater will 
 be its conducting power and that the flat ribbon is the cheap- 
 est and best. 
 
 The first view is founded on the fact that, for steady cur- 
 rents, the conducting power is directly proportional to the area 
 of the cross-section. The second view is founded upon what 
 now appears to be the fact that very rapidly alternating cur- 
 rents travel only through an extremely thin layer of the sur- 
 face of the conductor, and what also appears to be the fact, that 
 
83 
 
 lightning discharges are a series of extremely rapid alternat- 
 ing currents. The settling of this point of dispute is likely to 
 require the testimony of actual and extended practical tests 
 with both forms of rods. 
 
 144. Danger to Stock from Wire Fences.— The in- 
 troduction of wire fences has to some extent increased the 
 danger from lightning to stock in pastures, owing to the tend- 
 ency of the wires to become charged, and then give off side 
 sparks to the animals standing near. The danger is less from 
 the barbed wire than from the plain, and the danger from both 
 may be lessened by connecting the several wires with the 
 ground by means of other wires tacked to the sides of the 
 posts, the lower end being turned under the point of the post 
 when set. The staples should be driven astride the two wires 
 so as to hold them in close contact. It is not possible to say 
 just how close together these discharging wires should be 
 placed, but probably not nearer than 15 to 20 rods. 
 
 SOIL PHYSICS. 
 
 145. Nature of Soil. — The basis of all soil consists of the 
 undissolved remnants of the underlying rocks. Associated 
 with these remnants there is always a varying per cent, of 
 organic matter, resulting from the decay of vegetable and 
 animal remains ; a certain amount of dust particles brought 
 from varying distances by the winds, or washed down by rain- 
 drops and snow flakes which have formed about those floating 
 high above the earth's surface ; and a considerable amount of 
 saline substances brought constantly to the surface by the 
 upward movement of capillary water, and left deposited when 
 the water evaporates. 
 
 146. Origin of Soils.— All soils owe their origin to the 
 processes and agencies of rock destruction which have been 
 and still are taking place in three chief ways : 
 
 1. Many rocks have been mechanically broken into larger 
 or smaller fragments. 
 
 2. Other rocks have had their molecules separated by simple 
 solution as salt is dissolved by water, or the molecules have 
 first been changed chemically and then dissolved. 
 
84 
 
 3'. Still other rocks have had some of their mineral constitu- 
 ents dissolved out, leaving the remainder as an incoherent mass 
 of fragments. In Fig. 30 are shown the stages of transition 
 from the underlying rock to the soil above as it occurs onlime- 
 
 Fig. 30. 
 stone hills, while Fig. 31 shows the same facts for a more level 
 limestone surface. On examining the rocks of almost any 
 
 quarry they are found to be divided into blocks of varying 
 sizes by fissures or breaks which owe their origin to a general 
 shrinkage of the rocks and to movements of the earth's sur- 
 face layers. These are the first steps in soil formation, and 
 :ire plainly shown in Figs. 32 and 33. They exert a great in- 
 fluence in rock destruction and soil formation by furnishing 
 easy access for water and the roots of trees to their interior, 
 where the first by freezing and the second by growth expand 
 and break the blocks into smaller fragments. Moving ice, in 
 the form of glaciers, has done a vast amount of rock grinding, 
 the present soil of all except the southwestern portion of our 
 own state being the altered surface of a thick mantle of bould- 
 ers, gravel, sand and clay formed, transported and spread out 
 by glacial action and the waters from the melting ice. Then 
 there are many animals which have contributed largely to this 
 rock grinding and soil formation. Darwin, through a long 
 and careful study, reached the conclusion that in many parts 
 
«5 
 
 of England earthworms pass more than 10 tons of dry earth 
 
 per acre thruno-]i their bodies annually, and that the p-ains of 
 
 Fig. 33. 
 
 Fig. 33. 
 
 Fort Danger, Wis. From a Photograph. 
 After Chamber lin. 
 
 From a Photograph. After 
 Chamberlm. 
 
 sand and bits of flint in these earths are partially worn to fine 
 silt by the muscular action of the gizzards of these animals; 
 
 Fig. 31,. 
 
 A tower-like casting ejected by a species of earthworm, from the Botanic Garden, 
 Calcutta: of natural size, engraved from a photograph. After Darwin. 
 
so 
 
 this same work is going on in 6iir own soils, where the holes 
 bored by angle-worms represent the volume of dirt they have 
 passed through their bodies. All seed-eating birds take into 
 their gizzards and wear out annually large quantities of sand 
 and gravel, after the manner of our domestic foAvls. 
 
 The other two methods of soil formation depend mainly, 
 though not Avholly, upon chemical changes wrought in the rock 
 minerals. Pure water has the power to dissolve, without chem- 
 ical change, greater or less quantities of most rock minerals 
 w^liich are brought to the surface by ca])illary action and be- 
 cjme fine grains in the surface soil; but the larger part of this 
 work is brought about by the action of water in conjunction 
 with oxygen, carbonic, nitric, sulphuric, humic and other acids 
 which it carries down into the rocks, where the work of solu- 
 tion goes on rapidl}^ Mr. T. M. Keade has estimated that the 
 Mississippi alone carries to the sea annually 150,000,000 tons 
 of rock in solution, and yet a large part of the water which 
 enters the soil is brought back again to the surface and evap- 
 orated, leaving the materials it has dissolved as a contribution 
 to agriculture. 
 
 147. Soil-convection.— On the surface of a lake the 
 water which is at the top one moment is at another below the 
 surface, the molecules changmg position continually by con- 
 vection currents due to changes of temperature. There is a 
 movement somewhat analogous to this taking place in every 
 fertile soil, though the movements are less rapid and are due 
 to different causes. Earthworms, ants, crayfish, gophers an 1 
 various other burrowing animals each season bring large 
 amounts of the finer portions of the lower soil and subsoil to 
 the surface, forming systems of galleries with openings lead- 
 ing out to the free air at various places. Each heavy rain, es- 
 ])ecially during the fall and spring, washes the finer surface 
 soil into these galleries, filling them up, and new excavations 
 are again made, thus keeping uj) a slow, but nevertheless a 
 certain circulation, which in some of its effects is like the fall 
 a id spring plowing, but much of it extending to far greater 
 (le^)ths, the angleworms, ants and crayfish often going down 
 f .'om three to five or more feet during dry seasons. Darwin's 
 observations have shown that this rotation of soil, which he 
 attributes largely to the action of earthworms, tends to burv 
 
87 
 
 coarse objects, like flints, lying on the surface, as time passes, 
 and in Fig. 35 is represented one of these cases as cited by 
 him. 
 
 
 ^ 
 
 f> 
 
 o '^■ 
 
 o 
 
 ■'iMMl/Z'/MMmm,/ 
 
 Fig. 35. 
 
 Section reduced to half natural scale, of the vegetable mould m a field drained and reclaimed 
 15 years before. Showing turf, vegetable mould without stones, mould with fragments 
 of burnt marl, coal cinders and quartz pebbles; and subsoil of black peaty sand with 
 quartz pebbles. After Darwin. 
 
 148. Soil Removal.— Pitted against these processes of 
 growth there is a powerful and universal set of agencies con- 
 stantly operating everywhere to transport from higher to 
 lower levels and from the land to the sea the surface soils, 
 and the magnitude of this action has been estimated at not 
 far from one foot each 3,000 years as an average for the whole 
 land surface, and hence the saperficial and exhausted soils are 
 being slowly removed and replaced by new soil originating 
 from the products of rock decay, and brought to the surface 
 by capillary action and that of burrowing animals generally. 
 The absolute amount of soil removal can be appreciated when 
 it is understood that the summits of the bluffs represented in 
 Figs. 36 and 37 show the general level of the surrounding 
 lower land at a former time and that, at times intervening 
 
between the present and that earlier period, vegetation Las 
 grown on soil occupying all the levels between the two shown 
 in the engravings. 
 
 Fig. 36. 
 
 Fig. 37. 
 
 [Giant's Castle, near Camp Douglas, Wi.-- 
 From a Photograph. After ChamberUu. 
 
 Pillar Rock, Wis. From a Photograph. 
 After Chamberlin. 
 
 149. Surface Soil.— Soils proper comprise the surface 
 live to ten inches of fields and woodlands generally. Often- 
 times the depth of the true soil may be less than five inches, 
 and then again it may exceed a depth of ten inches by varying 
 amounts. It is the portion which has been longest and niost 
 com})letely exposed to the disintegrating and solvent action of 
 rock-destroying agencies, and ' as a result of this fact it con- 
 tains a smaller per cent, of the soluble minerals used by plants 
 than the less altered subsoil below. Its chief ino'redients are: 
 
 1. Sand. 
 
 2. Clay. 
 
 3. Humus. 
 
 Composing about 90 to 95',' of the dry weight ; 
 
 which arc commingled in varying proportions, giving rise to 
 diiferent varieties according as one or another of tliese ingredi- 
 ents predominates. The true soil, on account of its more 
 complete aeration and its higher temperature, is tlie chief lab- 
 
89 
 
 oratory in which the nitrogen compounds for plant food are 
 elaborated. 
 
 150. Kinds of Surface Soil.— For practical purposes 
 soils are variously classified. When reference is had to the 
 ease or diMioulty of working the soil it is spoken of as 
 
 1. Light, or 
 
 2. Heavy; 
 
 but these terms have no significance as regards actual weights ; 
 for a sandy soil is spoken of as light, and yet it is the heaviest 
 of all soils, bulk for bulk. The greater weight of the sandy 
 soil is due more to the lack of large cavities which are found 
 in the clayey soils, than to the higher specific gravity of the 
 soil constituents. It is the greater adhesiveness of the clayey 
 soils which causes the plow, hoe or harrow to move with 
 greater difficulty through them. 
 
 "When reference is made to the temperature of soils, at the 
 same season, they are spoken of as 
 
 1. Warm, or 
 
 2. Cold, 
 
 according as the temperature of the soil is relatively high or 
 low. In this case the soils containing the greatest amount of 
 water are, when other conditions are similar, the colder on ac- 
 count of the high specific heat, 125, of the water. 
 
 When the chief ingredients of soil are the basis of distinc- 
 tion they are frequently classified as 
 
 Sand. Clay. Humus. 
 
 Per cent. Per cent. Per cent. 
 
 1. Sandy soil, containing. 80 to 90 8 to 10 1 to 3 
 
 2. Sandy loam, " 60 to 80 10 to 25 3 to 6 
 
 3. Loam, " 25 to 60 60 to 25 3 to 8 
 
 4. Clayey loam, " 10 to 25 60 to 80 3 to 8 
 
 5. Clayey soil, " 8 to 15 70 to 80 3 to 6 
 
 In peaty soil, or those of our low marshes and swamps, 
 there is often as high as 22 to 30 per cent, of humus. It should 
 be kept in mind that the sand, clay and humus of soils are not 
 plant food proper except in a small degree ; they are, except 
 a part of the humus, what is left after the plant food is re- 
 moved. They serve, however, an important purpose in fui'- 
 nishing a proper feeding ground for the roots and a means of 
 supporting plants in their upright attitude. 
 
90 
 
 151. Subsoil. — The subsoil is tlie real source of the nat- 
 ural mineral constituents of plant food, while at the same time 
 it acts as a reservoir for water which is delivered at the sur- 
 face by capillary action or held within its mass until the pene- 
 trating roots remove it. The de])th to which roots i)enetrate 
 the subsoil is really great, and I believe the depth is deter- 
 mined primarily by the water content of the soil, the roots 
 traveling farther when the supply is scanty. Wheat roots are 
 recorded as observed at a depth of seven feet in Rhenish subsoil 
 of a sandy loam. Corn roots with us commonly reach a depth 
 of three feet and often exceed four. It would appear, there- 
 fore, aside from the fact that the subsoil is the parent of the 
 true soil and that it acts as a water reservoir, that the chem- 
 ical composition and physical characters of the subsoil may 
 determine in a large measure the productiveness of land, 
 unless it should be determined by future investigations that 
 the deep-running roots are simply water-gatherers. 
 
 152. Variation in Composition of Subsoils. — There 
 is a marked difference in the composition of those subsoils of 
 Wisconsin which are simply the residuary products of the 
 decay of rocks in place, such as those re])resented in Figs. 30 
 and 31, and those which owe their origin to glacial grinding 
 and mixing. This difference is clearl}^ brought out in the 
 table given below, which is compiled from analyses of typical 
 samples of residuary subsoils from southwest Wisconsin and 
 of glacial subsoils from the vicinity of Milwaukee as given by 
 Chamberlin & Salisbury in the Sixth Annual Report of the 
 United States Geological Survey : 
 
 Residuary (Jlacial Difference. 
 
 Subsoils. Subsoils. 
 
 Per cent. Per cent. Per cent. 
 
 Silica,Si02 55.73 44.53 -11.21 
 
 Alumina, AI2 O3 18.16 8.01 —10.15 
 
 Lime, CaO 99 13.74 -1-13.75 
 
 Magnesia,MgO 1.11 7.43 +6.31 
 
 Potash, K2O 1.34 2.48 +1.24 
 
 Phosphorus, P.,05 03 .09 +.06 
 
 Carbon Dioxide, CO 2 35 17.11 +16.76 
 
 Iron. Fe^Oj 10.57 2.68 -7,89 
 
 Organic matter 9.86 3.33 -7.53 
 
 Other substances 1.37 1.95 +.58 
 
91 
 
 It will be seen that the insoluble sand, clay and iron com- 
 pounds predominate in the residuary subsoils, while the lime, 
 magnesia, potash and jDhosphorus compounds are in excess in 
 the glacial subsoils, and this at first thought seems strange 
 when it is remembered that the resiJ.uary soils are derived 
 directly from magnesium limestones and that two of the four 
 samples giving the average w^ere taken in contact with the 
 limestone itself, but these soils are what is left after the solu- 
 ble carbonates are leached away. 
 
 The photo-engraving of a relief map of Wisconsin, Fig. 38, 
 showing the glaciated and non-glaciated areas of the state, also 
 
 Fig. 38. 
 
 Photo-engraving of a relief map of Wisconsin, showing the glaciated and non-glaciated 
 areas of the state. 
 
 shows, in general, the distribution of the glacial and residuary 
 subsoils. The area of rugged topography in the west and 
 southwest of the state is the region covered by the residuary 
 subsoils. It should not be inferred, however, that the compo- 
 sition of aU of our glacial subsoils is fairly represented by 
 
02 
 
 the samples from the vicinity of ]\Iihvaukee, for in the northern 
 portion of the state there were no hirge areas of hmestono to 
 be ground down by the ice to contribute the large amounts of 
 lime and magnesia found in the locality cited. 
 
 153. Size of Soil Particles.— The size of soil particles 
 has very much to do with the value of a soil, this quality de- 
 termining, in some measure, its water capacity, -its retentive- 
 ness of fertilizers, its drainage, its aeration and the way in 
 which the soil works. In general the relative number of large 
 grains as compared with the smaller ones is greater at the sur- 
 face than at some de])th below ; this difference is due largely 
 to the tendency of rain to pick up and carry away or to carry 
 downward by percolation the finer particles. 
 
 Chamberlin and Salisbury, as a result of their studies bear- 
 ing upon the size of soil particles constituting residuary earths, 
 say: " Out of 158,522 measured particles from several repre- 
 sentative localities, only 929 exceeded .005 mm in diameter, 
 A fairly illustrative example from near the rock surface at 
 Mt. Horeb, Wis., gave, in a single miscroscopic field, the fob, 
 lowing showing: 
 
 Particles less than .00385 mm in diameter 15,153 
 
 Particles between .00385 mm and .005 mm in diameter 308 
 
 Particles more than .005 mm in diameter 54 
 
 I^one of the 54 particles reached so great a diameter as 
 .01 mm," that is, the largest of the 54 large ones had a diam- 
 eter so small that 25,400 of them placed side by side would be 
 required to span a linear inch. 
 
 Many of the soils which tend so strongly to clog the plow 
 are of this extremely fine-grained type, and a partial explana- 
 tion may be found in the minute particles wedging into the 
 microscopic cavities due to the grain or texture of the material 
 of the mold-board. 
 
 154. Needs of Soil Aeration. — The necessity for a con- 
 siderable circulation of air in soil actively supporting veg- 
 etation is generally recognized, and the demand for this 
 circulation is three fold : 
 
 1. To supply free oxygen to be consumed in the soil. 
 
 2. To supply free nitrogen to be consumed in the soil. 
 
 3. To remove carbon dioxide liberated in the soil. 
 
93 
 
 Prominent among the demands for oxygen in the soil may 
 be mentioned : 
 
 1. The respiration of germinating seeds. 
 
 2. The respiration of growing roots. 
 
 3. The respiration of nitric acid germs. 
 
 4. The respiration of free-nitrogen-fixing germs. 
 
 • 5. The respiration of manure fermenting germs. 
 It has been abundantly demonstrated that when oxygen is 
 completely excluded from seeds, placed under otherwise nat- 
 ural conditions for germination, growth does not take place ; 
 if the germination is allowed to commence and then oxygen is 
 withdrawn further devc^.opment will cease. When the air 
 surrounding a sprouting seed contains only ^V of the normal 
 amount of oxygen the germination will go on, but the rate is 
 retarded and a sickly plant is likely to result. Experience 
 abundantly proves that when soil bearing other than swamp 
 vegetation is flooded with water, or even kept in an over- 
 saturated state, the plants soon sicken and die, and this, too, 
 when they may be in full leaf and abundantl}^ supplied with 
 nourishment, sunshine and warmth. The difficulty is the lack 
 of root-breathing. Oxygen in sufficient quantity cannot reach 
 the roots to maintain life. The plants are suffocated. This 
 explanation is apparently disproved by the fact that seeds of 
 various kinds may be germinated in a float of cotton resting 
 on the surface of water, and may even be made to mature 
 seeds if the water in which the roots are immersed is kept 
 supplied with tlie proper foods in solution. The floating gar- 
 dens of the Chinese, consisting of basket-work made strong 
 enough to carry a layer of soil in which crops are matured 
 with their roots immersed constantly in water, is another ap- 
 parent disproof that wet soils kill the plants by depriving 
 them of oxygen. The two classes of cases are, however, very 
 different. In the cases of water culture the free water is sub- 
 ject to strong convection and other currents which rapidly 
 bring the water exhausted of its free oxygen to the surface, 
 where it becomes charged again. In the water-soaked soil, 
 with a relatively much smaller quantity of water, all possibility 
 of convection currents is prevented by the cohesive power of 
 the soil, and the rate of diffusion in such cases must evidently 
 
94 
 
 be extremely slow, so that, viewed in tliis liglit, tlie two sets 
 of cases stand in strong contrast. 
 
 The natural nitrates, so essential to fertile soils, owe their 
 origin to a minute germ closely related to the " mother of 
 vinegar " and called in olden times the " mother of petre." This 
 ferment germ produces the nitric acid of soils which, after 
 uniting with some of the bases contained in the soil, is ab- 
 sorbed by the plants as food. When the production of salt- 
 ])etre was a considerable industry in Europe one of the condi- 
 tions necessary to rapid formation was to keep the rich soil 
 well aerated by frequent stirring and by the introduction of 
 gratings to increase the air spaces. Oxygen is one of the es- 
 scnti.ds to the life of these important germs, and herein lies, 
 in part at least, the advantage of cultivation and of properly 
 drained soils. 
 
 While we have, as 3^et, less positive knowledge in regard to 
 the respiratory needs of the free-nitrogen-fixing germs, now 
 coming rapidly into recognition, there is no reason to doubt 
 the beneficial effects of a properly aerated soil upon them. 
 
 In regard to the manure fermenting germs we have abun- 
 dant proof of the need of ventilation from their action in the 
 strong heating of the well ventilated coarse horse manure 
 when contrasted with the absence of heating in close cow 
 dung free from coarse litter. 
 
 Xot only must oxygen and nitrogen be introduced into the 
 soil, but the large amounts of carbon dioxide liberated by the 
 fermenting processes and by the decomposition of the bicar- 
 bonates contained in soil-waters must be passed out in order 
 to make room for the other gases to enter in a sufficiently 
 concentrated form to answer the conditions of life going on 
 there. 
 
 155. Methods of Soil Aeration.— Most field soils, when 
 in their natural undisturbed condition and nearly saturated 
 with water, are impervious to such air currents as the greatest 
 differences of atmospheric pressure and temperature in a given 
 locality can produce. It is on this account, in part, that earth- 
 worms come to the surface in such great numbers during and 
 after heavy rains. The many perforations made by earth worms 
 constitute so many chimneys in and out of which the air moves 
 
95 
 
 with every change of atmospheric pressure and temperature. 
 Cultivation as soon as possible after rains aerates the soil at 
 the time when it contains an abundance of moisture at the 
 surface and is in the best possible condition for the rapid ac- 
 tion of the niter germs, which need plenty of air, moisture and 
 warmth. 
 
 . Harrowing winter grain in the spring tends to make the 
 aeration of the soil more perfect by breaking up the crust 
 formed by the deposit of saline substances brought up by cap- 
 illary action. 
 
 Drainage, by carrying off the water more rapidly and to a 
 greater depth, opens the pores of the soil, making its breath- 
 ing more perfect. 
 
 Strong-rooted crops, like the red clover, which send their 
 roots deeply into the subsoil, leave it so channeled by the de- 
 cay of those roots that a more perfect circulation of air is thus 
 secured. 
 
 156. Soil Moisture.— The moisture contained in soils is 
 of the utmost importance agriculturally, for without it all 
 growth is impossible. Some of its chief functions may be 
 stated as follows : 
 
 1. By its solvent power it facilitates and promotes chemical 
 changes in the soil. 
 
 2. By its expansive power when freezing it mechanically 
 divides the coarser soil particles into finer ones. 
 
 3. By its capillary movements it conveys food to the roots 
 of plants. 
 
 4. By its osmotic power it transports plant food to the 
 leaves for assimilation. 
 
 5. By the same power it conveys the assimilated food to the 
 tissues for growth. 
 
 6. By its osmotic power it swells the seed and ruptures the 
 seed coats preparatory to germination. 
 
 7. By the pressure it is under in the plant it gives succulent 
 tissues much of their rigidity. 
 
 8. By its high specific heat it prevents the soil temperatures 
 from becoming too high by day and too low during the night. 
 
 157. Amount of Water Consumed by Plants.— Hell- 
 riegel found, by experiments conducted in Prussia, that the 
 
96 
 
 amounts of water drawn from the soil and given to the air by 
 various plants under good condition of growth, for each pound 
 of dry matter produced by the crop in coming to maturity, 
 were as stated in the table below : 
 
 Number of Pounds of Water Transpired by Plants in Producing 
 One Pound of Dry Matter. 
 
 
 Water. 
 
 
 Water. 
 
 
 Lbs. 
 
 
 Lbs. 
 
 Barley 
 
 Summer rye 
 
 Oats ... 
 
 310 
 
 Horse beans . 
 
 282 
 
 353 
 
 Peas 
 
 .. .273 
 
 376 
 
 Red clover 
 
 310 
 
 Summer wheat , 
 
 338 
 
 Buckwheat , 
 
 363 
 
 This, it will be seen, is at an average rate of more than 325 
 tons of water for each ton of dry matter when growing under 
 the chmatic conditions of Prussia. This amount seems enor- 
 mous and may perhaps be too high, but there can be no ques- 
 tion but that the quantity is very large, and necessarily so, 
 because practically all of the dry matter of the plant requires 
 to be in solution when in transit to the place where it is finally 
 deposited as a part of the structure. 
 
 The chemical analyses of nineteen natural spring and well 
 waters from different localities in Wisconsin show the saline 
 ingredients to constitute .0475 per cent, of their Aveight, or 
 .95 pounds of solids in solution per ton. The ash in a ton of 
 the dry matter contained in corn ensilage is about 152 pounds, 
 and for the average spring water to yield this would require 
 160 tons, supposing the total saline ingredients to be used by. 
 the plant. While it is probable that soil water as it comes in 
 contact with the roots of plants is much more highly charged 
 than the average spring water, it is also true that not all salts 
 held in solution contribute to the ash of plants, so that it still 
 seems imperative to assume a large consumption of water per 
 pound of dry matter in plants. 
 
 If we take the average of Hellriegel's results, given in the 
 table, as applying to corn in Wisconsin, and 8,400 pounds as 
 the average yield of dry matter per acre, about 12 inches of 
 rain must be drawn from our soils by this crop each year, and 
 yet this is a full third of our total mean annual rainfall. Not 
 all the water which evaporates from a corn field during the 
 
97 
 
 growing season can pass through the corn, and there are eight 
 months each year when the corn is not in the ground but 
 evaporation goes on during all that time. To these losses 
 must be added the water carried out of the state by rivers. 
 Such facts should teach, in an emphatic manner, the need of 
 adopting such methods of tillage as will tend to conserve the 
 soil moisture. 
 
 158. Position and Attitude of the Water-Table. — 
 The water-table is the surface of standing water in the soil. 
 The distance the water-table lias below the surface exerts a 
 marked influence upon the yield of crops per acre. If the 
 water lies too close to the surface, drainage is required to se- 
 cure the best yields ; when the w^ater-table lies too low, none 
 
 Fig. W. 
 
 Fig. U, 
 
 Figs. 39, 40 and 41, showing the relations o. the water-table to the surface of the ground 
 on the Experiment Farm. Figs. 39 and 41 represent north and south profiles, and 
 Fig. 40 one extending across the other two. The vertical parallel Unas represent wells. 
 Vertical scale,— 1 in. =40 ft. ; horizontal scale,— 1 in.=about 380 ft. 
 
98 
 
 of tliat water is available for ])lant growth. Permanent ponds 
 and lakes are continnations of the water-table above the sur- 
 face of the ground, and tlieir levels lie at varying distances 
 below the level of Jthe water in the ground, the water-table 
 rising usually as the distance from these bodies of water in- 
 creases and as the ground rises. 
 
 In Figs. 39 to 42 the position and attitude of the water-table 
 is shown as it occurs on the Experiment Farm. In these cases 
 
 yK 
 
 
 Showing the relation of the water-table to the surface on Picnic Point. Vertical scale, for 
 the water-table 1 in. =4 ft., for the surface, 1 in. =8 ft. 
 
 the water stands highest under the highest ground, which ap- 
 pears to act simply as a reservoir in which the rains accumu- 
 late, the fi'iction of the soil retarding the flow toward the 
 lake. 
 
 The common belief that wells are supplied with water from 
 adjoining lakes or rivers, the water simply Altering through 
 the soil into them, is not generally true though it may be 
 in some exceptional cases. Neither is it usually necessary to 
 dig to a depth of the level of adjoining lakes before water is 
 found. Here at Madison water is obtained in wells, in some 
 cases, 20 feet above the level of the lakes, and the wells may " 
 not be more than 40 rods from the lake shore and sunk simply 
 in a ridge of glacial sand and gravel lying between the two 
 lakes. 
 
 159. Fluctuations in the Level of the Water-Table. 
 The level of the water in the ground is not constant, but stands 
 higher after a series of wet years and faUs again with a suc- 
 cession of dry seasons. There is also an annual rise and faU 
 of the water-table, the water standing lowest toAvard the lat- 
 ter part of fall or early winter and highest in the spring. In 
 those cases where the water-table lies near the surface it is 
 frequently raised by single heavy rains. Even changes in at- 
 
99 
 
 mospheric pressure affect slightly the level of water in wells, 
 causing it to rise with a falling barometer and fall with a ris- 
 ing barometer. 
 
 The growth of crops appears also to affect the hight of the 
 Avater-table when it lies near enough the surface to come within 
 range of root action. This effect is shown in Fig. 43. The 
 same figure also shows to what extent the water-table fell 
 during a growing season. 
 
 Fig. h3. 
 
 Showing changes in the surface of the water-table under alternate fallow plats and plats 
 of growing corn. The straight lines connect the water-levels of wells 1 and 7 on the 
 dates specified at the right, and the broken Une joins the water surfaces of wells 2, 
 3, 4, 5 and 6 on the same dates. 
 
 160. Best Hight of the Water-Table.— It is a matter 
 of great importance, as bearing upon all questions of land 
 drainage, to know at just Avhat distance below the surface of 
 the ground the water-table should lie to interfere least, and at 
 the same time to contribute most, to plant growth. In Eu- 
 ropean cultivation it is held that the tillage of moors and bogs 
 can only be successful when the water-table is maintained at 
 least 3 feet below the surface in summer and 2 feet in winter. 
 For light and gravelly soils in good condition a depth of 4 to 
 8 feet is held to be best for the majority of crops. The prob- 
 lem is manifestly a complex one which cannot be simply stated. 
 The case must vary with the character of the soil, with the 
 season, and with the habit of the cultivated crop, as to whether 
 it is naturally a shallow or a deep-rooted one. 
 
100 
 
 161. The Vertical Extent of Root-Feeding. — Just 
 how deeply root-feeding may extend below the general Ihnit 
 of root growth must depend upon the vertical distance through 
 which capillary action is able to pass water upward into the 
 root zone. In the fall of 1889 it was found that clover and 
 timothy, growing upon a rise of ground some 28 to 30 feet 
 above the water-table, had reduced the water content of sand, 
 at a depth of 5 feet, to 4.92 per cent, of the dry weight, when 
 its normal capacity was about 18 per cent,, and this seems to 
 be a case of strong root-feeding to a depth of more than 5 
 feet. 
 
 In the table below are given the percentages of water in the 
 soils of closely contiguous localities bearing different crops ; 
 the distance between the two most distant localities not ex- 
 ceeding twelve rods and the ground nearly level : 
 
 Showing Depth of Root-Feeding as Indicated by the Water Content 
 OF the Soil August 24, 1889. 
 
 Clover in Timothy and Corn. Fallow 
 
 Depth of Sample. Pasture. Blue Grass. Ground. 
 
 Per cent. Per cent. Per cent. Per cent. 
 
 0-6 in 8.39 6.55 6.97 16.28 
 
 6-12 in 8.48 7.63 7.80 17.74 
 
 12-18in 12.42 11.49 11.60 19.88 
 
 18-24in 13.27 13.58 11.98 19.84 
 
 24-30 in 13.52 13.26 10.84 18.56 
 
 40^3in 9.53 18.51 4.17 15.90 
 
 Distance of lower sample 
 above water-table 2.36 ft. 1.97 ft. 2.12 ft 2.22 ft. 
 
 This table shows clearly that root-feeding, in the case of 
 both clover and corn, extended to a depth of at least four feet," 
 and that the corn had fed deeper than the clover. It also 
 shows that the timothy and blue grass had exhausted the soil 
 moisture near the surface more than either of the other crops, 
 but that the depth of feeding was less. 
 
 The strong difference which is shown to exist between the 
 amount of water in the fallow ground and the ground bearing 
 crops shows in a marked manner the strong drying influence 
 of growing vegetation upon the soil. 
 
101 
 
 162. Capacity of Soil to Store Water.— The rainfall 
 of our state during the summer season is rarely enough to 
 meet the demands of vegetation during the growing period, 
 but the soil acts as a reservoir, retaining considerable quanti- 
 ties of that which falls at other times. All soils, however, 
 have not the same storage capacities, and hence on fields re- 
 ceiving the same rainfall the water supply for crops may be 
 very unequal. 
 
 Klenze makes the following general statements in regard to 
 the water capacity of different soils : 
 
 1. The saturation capacity of a given kind of soil increases 
 as the size of the smallest particles decreases. 
 
 2. The capillary capacity of a given soil containing only 
 capillary spaces decreases as it is made more close and firm. 
 
 3. The saturation capacity of soils is decreased by increasing 
 the number of cavities which are larger than the capillary 
 spaces. 
 
 4. The saturation capacity of soil decreases as the tempera- 
 ture increases. 
 
 In the following table are given the percentage and abso- 
 lute capillary capacity of a section of soil 5 feet deep, as found 
 by experiment, the soil being in its natural condition : 
 
 Per cent Founds Inches 
 
 of Water, of Water, of Water. 
 
 Surface ft. of clay loam contained. 32.3 23.9 4.59 
 
 Second ft of reddish clay contained 23.8 22.2 4.26 
 
 Third ft. of reddish clay contained 24.5 22.7 437 
 
 Fourth ft. of clay and sand contained. . . . 22.6 22.1 4.25 
 
 Fifth ft of fine sand contained 17.5 19.6 3,77 
 
 Total 110.5 . 21.24 
 
 These figures show that the actual storage capacity of 5 feet 
 of soil is really very large, in the case in question, aggregating 
 
 g„"^.^ — ^=2406.69 tons per acre, 
 
 and this, at the rate of 325 tons of water per ton of dry mat- 
 ter, is sufficient, were it aU available, to give a yield of 
 
 2406.69 „ ,^, 
 
 — 0;^=— =7.405 tons of dry matter. 
 
102 
 
 yclter 
 
 ylft. 
 
 F/rst /t 
 
 ScU. 
 
 Fig. 42 represents the proportions by vol- 
 ume, of soil, air and water in the above sec- 
 tion. 
 
 163. Proportion of Soil-Water Avail- 
 able to Plants. — Kot all the water which 
 soils contain is available to plants, and con- 
 siderable must remain unused if large yields 
 are expected; we have also seen that soil 
 fully saturated is not in a suitable condition 
 to produce crops. Hellriegel concludes fi-om 
 observations of his own that soils give the 
 best results when they contain from 50 to 00 
 per. cent, of their saturation amounts, but 
 this, I think, should be understood as apply- 
 ing strictly only to the upper 12 to 24 inches of 
 soil because, as the season advances and the 
 roots develop downward, the water of the 
 subsoil is drawn upon gradually as it is 
 needed, and the per cent, of saturation is re- 
 duced to the proper amount. 
 
 During the season of 1890 Litch Dent and 
 White Australian Flint corn grew side by 
 side at the Experiment Farm in a light clay 
 loam underlaid with sand, the soil contain- 
 ing at the time of planting 22.41 per cent, 
 of water, and at the time of cutting 15.45 
 per cent., the mean saturation capacity being 
 about 25 per cent. The Dent gave a yield 
 of 9,875 pounds of dry matter per acre and 
 the Fhnt 6,000 pounds. The amount of 
 water lost by transpiration, evaporation and 
 drainage was at the rate of 456 pounds of 
 water per pound of dry matter for the Dent 
 corn, and of 610 pounds for the Flint. 
 
 An examination of the figures in 160 will show how com- 
 pletely crops may reduce the water-content of soil during dry 
 seasons ; those given there, for corn, being from the same local- 
 ity as the above for the year 1889. 
 
 164. Kinds of Soils which Yield Their Moisture to 
 Plants Most Completely.— The sandy soils yield their 
 moisture to plants much more completely than do the clayey 
 
 Walter. 
 Ait. 
 
 Soli. 
 
 fVa^ter 
 
 Fig. U. 
 
 Showing the relative 
 volumes of water, 
 air and soil in the 
 upper five feet of 
 cultivated ground. 
 
103 
 
 and other soils having a greater water capacity. This is clearly 
 shown in 160, where sand, at the bottom under the corn, 
 contains only 4.17 per cent, while the clay with sand mixed, 
 in the second foot of the same section, contains an average of 
 11.79 per cent. The saturation capacity of the first is about 
 18 per cent., while that of the latter is about 26 per cent. The 
 sand had given up more than three-fourths of its water while 
 the clay still retained nearl}^ one-half. 
 
 If we compare the absolute amounts of water given up l)y 
 each of the two soils in question we shall find that the sand 
 had yielded 13.83 pounds per cubic foot, while the clay had 
 yielded only 12.5 pounds. It thus becomes evident that while 
 the percentage capacity of the sand is much below that of clay 
 its greater weight per cubic foot and the greater freedom w^th 
 which it yields water to plants makes its practical storage ca- 
 pacity for water, so far as crops are concerned, nearly as great 
 as the loamy clays. It is thus very clear that a sandy soil 
 kept well fertilized has many advantages over the colder, less 
 perfectly aerated and more obstinate clayey ones, which crack 
 badly in excessively dry weather and become supersaturated 
 in wet seasons. 
 
 165. Movements of Soil Water.— The water in the 
 ground is subject to at least three classes of movements : 
 
 1. Those due to gravitation. 
 
 2. Those due to capillarity. 
 
 3. Those due to gaseous tension. 
 
 The direction of movement in each of these cases may be 
 either : 
 
 1. Downward. 
 
 2. Lateral. 
 
 3. Upward. 
 
 The gravitational movements are the most rapid, most ex- 
 tended and belong to two types : 
 
 1. Percolation movements. 
 
 2. Drainage or current movements. 
 
 The percolation movements are, as a rule, slower than the 
 drainage movements and are usually downward, being only 
 occasionally and locally upward ; they consist of the slow fil- 
 tering of water through the smaller soil pores. It is chiefly 
 by percolation that all water finds its way into the ground. 
 
104 
 
 The drainage currents consist of those portions of the per- 
 colation waters which could not be retained in the surface 
 soil b}^ capillary action. They move like streams of water on 
 the surface or like currents through pipes, giving rise to springs 
 and flowing wells. 
 
 The capillary movements, 81 to 83, constitute the slow 
 creeping of water over the surface of soil particles and those 
 of root-hairs. In direction the}^ are chiefly toward the sur- 
 face of the ground and toward the root-hairs, during the time 
 when these are in action ; but after showers there may be 
 capillary movement downward provided there is unsaturated 
 soil below, but even under these conditions it will not always 
 occur. 
 
 The gaseous tension movements originate in the changes in 
 volume of the confined au* due to changes of temperature and 
 of atmospheric pressure referj-ed to in 101 and 158. 
 
 166. Rate of Percolation. — The rate at which water 
 percolates through soils varies with the character and phys- 
 ical condition of the soil. As a general rule the percolation is 
 more rapid through the coarse-gr-ained soils than it is through 
 those of a flner texture, and it is on this account that sandy 
 soils leach so badly. Clayey subsoils, especially if they are 
 underlaid with sand, very often shrink and break into great 
 numbers of small cuboidal blocks leaving numerous fissures 
 between them which open down to the sand below; through 
 these a large amount of percolation may take place ; and this 
 effect is greatly intensified when the surface of the ground 
 becomes cracked, as it often does when not prevented by cul- 
 tivation. When in this condition such soils may leach even 
 worse than sandy soil. The perforations made by earth- 
 worms and other burrowing animals also exert a considerable 
 effect upon the percolation of water and the leaching of soils. 
 
 In case a winter sets in with fall rains insufficient to sat- 
 urate the soil and close up tlie shrinkage cracks and the chan- 
 nels formed by burrowing animals, considerable water finds its 
 way into the ground after it has been deeply fi'ozen. During 
 the winter rains and thaws which occurred in 1889, 1890 and 
 1891, there was a large amount of percolation on the Experi- 
 ment Farm made evident by the alternate starting and stop- 
 ping of the discharge of water in the tile drains. These facts 
 
105 
 
 have a significance in their bearing upon the practice of winter 
 hauling and spreading of manure. 
 
 167. Rate of Capillary Movement.— The rate of cap- 
 illary movement in soils varies with the kind of soil, with 
 the physical conditions, and also with the amount of water it 
 contains. It appears to be more rapid in sand than it is in 
 clay, and more rapid, in clay containing humus than in that 
 without. It is more rapid in a well firmed soil than in one 
 possessing large pores. The degree of closeness may, how- 
 ever, be so great as to impede the rate of movement. 
 
 I have found that water may rise through 4 feet of fine . 
 quartz sand at a rate exceeding 1,75 pounds per square foot in 
 24 hours, and in a light clay loam at a rate greater than 1.27 
 pounds per square foot. In these cases, however, the soil was 
 devoid of all spaces except those produced by the form and 
 size of the particles, and the rate w^as measured by the amount 
 of evaporation ; but as the soil remained wet at the surface 
 throughout the experiment the possible capillary rates must 
 exceed those stated by undetermined amounts. I have found 
 changes in the water-content of the soils of fields which in- 
 dicate that, under these conditions, the rate of capillary move- 
 ment, when the soil is wet, may exceed 1.66 pounds per square 
 foot. 
 
 When the soil is perfectly dry the rate at which water 
 moves through it is relatively very slow, so slow that five 
 cylinders of soil, each 6 inches in diameter and 12 inches high, 
 standing in water one inch deep, and in a saturated atmos- 
 phere, required the intervals stated below for water to reach 
 the surface in sufficient quantity to make it appear wet. 
 
 In clay loam, time required to travel 11 inches 6 days. 
 
 In reddish clay, time required to travel 11 inches 22 days. 
 
 In reddish claj', time required to travel 11 inches 18 days. 
 
 In clay with sand, time required to travel 11 inches 6 days. 
 
 In very fine sand, time required to travel 11 inches 2 days. 
 
 These are very funflamental facts in their bearing on the 
 control of evaporation by surface tillage. 
 
 168. Translocation of Soil- Water.— It frequently hap- 
 pens, in certain soils after rains and in most if not all soils after 
 rolling or firming, that water is brought up into the surface 
 stratum from the deeper layers; this change of position is 
 
106 
 
 named translocation and has important bearings upon ques- 
 tions of tillage. 
 
 The translocation caused by rolling or otherwise firming the 
 soil is due to the fact that reducing the non-capillary pores in 
 soil increases its capacity for water and the rate at which 
 water will move into it by capillarity, and this influence is 
 sometimes felt to a depth of tbree to four feet. The deeper 
 soil-waters may in this way, therefore, be brought to the sur- 
 face or within the zone of root growth. 
 
 The translocation caused by wetting the surface depends 
 upon the i)rinciple that when the per cent, of water in a soil has 
 fallen below a certain limit its ability to take water from another 
 soil is decreased, and that when it has risen above a certain 
 limit this ability is then diminished, that is, for each soil there 
 is a certain water-content at which the water enters it at the 
 most rapid rate. It therefore frequently happens that the 
 water-content of the surface soil is below that at which water 
 enters it most rapidly, and when a rain comes which restores 
 its strongest action again, water is also taken into it from the 
 soil below so that the surface stratum may, in consequence of 
 a rain, receive more water than actually fell, while the soil be- 
 low^ is, by translocation, rendered actually drier than before 
 the rain. This fact has an important bearing upon surface 
 tillage immediately after shoAvers, upon the transplanting and 
 watering of trees and upon questions of irrigation. If the sur- 
 face, after a rain, is allowed to remain undisturbed, the rapid 
 evaporation which occurs in such cases may take away in a 
 short time not only that which had fallen but also that which 
 was brought up by capillarity fi'om below, whereas simply 
 stirring the surface, destroying the capillary connection below, 
 would allow the surface only to dry and act as a mulch, retain- 
 ing tlie balance in the ground for the use of the cro]). 
 
 169. Influence of Topography on Percolation.— The 
 slope of the surface influences, sometimes in a marked man- 
 ner, the percolation of rain-water and the water-content of 
 the soil. Whenever rains occur which are sufficiently heavy 
 to ca;use water to flow along the surface, from the hill-tops 
 toward the lower and flatter areas, less water is left to perco- 
 late on the liighest sloping ground, while the more nearly level 
 areas may have not only the Avater which falls as rain upon 
 
107 
 
 them but a portion of that which has fallen upon other ground. 
 ]N'or is this all; as the water-table is generally higher under the 
 high ground, 157, there is a constant tendency for the water 
 in the soil itself to percolate from the high lands toward the 
 low lands, and so, when the water-table here lies within reach 
 of root action, to increase the water supply for the season, 
 sometimes to a disadvantageous extent, making drainage nec- 
 essary where in the absence of the high land it would not be 
 needed. 
 
 In those cases where the water-table under the high land 
 is below the level of the surface of the low lands, and the low 
 lands remain long over-saturated, there is a tendency for the 
 water to percolate toward the higher ground, but of course to 
 return again at a later season. 
 
 170. Influence of Topography upon Evaporation. 
 It is a matter of common observation that the south and 
 southwest slopes of steep hills are often simply grass-covered, 
 while the north and northeast slopes may be heavily wooded. 
 This difference of verdure is due largely to a difference in soil 
 moisture on the opposite slopes, which is determined chiefly 
 by the difference in the rate of evaporation upon the two 
 slopes. 
 
 Other things being the same, the rate of evaporation, in our 
 latitude, is greatest on hill-sides sloping to the southwest and 
 least on those sloping to the northeast. Several conditions 
 work in conjunction to produce this effect : 
 
 1. More air comes in contact with windward than with lee- 
 ward slopes, and as rapid changes of air over a moist surface 
 increase the amount of water taken up, the evaporation is 
 greater on the windward slope. 
 
 2. Our prevaihng winds, during the growing season, are 
 southwesterly, and hence more air comes in contact with 
 southwest slopes. 
 
 3. Westerly and northerly winds are, with us, almost al- 
 ways drier than easterly and southerly winds, and as evapora- 
 tion is more rapid under dry than under moist air the westerly 
 slopes are drier than easterly ones. 
 
 4. Other things being the same, surfaces which are nearest 
 vertical to the sun's rays receive most heat, and for this reason 
 southward slopes, in the northern hemisphere, become most 
 
108 
 
 heated, and as evapoi\ation takes place more rapidly at high 
 than at low temperatures, southerly and southwesterly slopes 
 lose most moisture from this cause. Fig. 45 shows how a surface 
 
 Flfj. Jf5. 
 
 inclined toward the south must receive more heat per square 
 foot than either the level surface or the one inclined north- 
 ward. If A65B is a section of a cylinder of sunshine falling 
 upon the hill AEB, it is evident that A64E, the portion falling 
 on the south slope, is greater than E45B, the portion falling on 
 the north slope. It will also be evident that the 20-degree 
 slope receives more heat than does the 5-degree slope, and this 
 more than the level surface. 
 
 The effect of the wind upon the evaporation from the soil is 
 at its maximum at the summit of a hill, because at this place 
 the wind velocity is greatest, no matter from what direction 
 it may be blowing. 
 
 171. Effect of Woodlands on Evaporation.— A piece 
 of woodland which lies to the southwest and west of a held 
 exerts a considerable effect upon the humidity of the air which 
 traverses that field, the tendency being to make the air more 
 moist. Taking a specific illustration, the air on the leeward 
 side of a second growth black-oak grove was found, on one 
 occasion, to contain 3.3 per cent, more moisture than did that 
 on the windward side at the same time; and again, when the 
 wind was in the opposite direction, observations in the same 
 locahties showed 3.8 per cent, more moisture on the leeward 
 side, the observations in the four cases being taken about 10 
 rods from the margin of the grove. There was observed at 
 
109 
 
 the same time a difference of air temperature of 1.5° F., the 
 leeward air being this much cooler in the field 10 rods from 
 the grove, the width of the grove being about 30 rods and 
 the trees from 20 to 30 feet high. 
 
 TILLAGE. 
 
 172. The Objects of Tillage.— The chief objects of till- 
 age may be briefly stated as follows : 
 
 1. To destroy undesired vegetation. 
 
 2. To place organic matter of various kinds beneath the sur- 
 face where it will more readily ferment and decay and be 
 brought within reach of root action. 
 
 3. To develop a loose, mellow and uniform texture in certain 
 soils. 
 
 4. To control the water-content of soil. 
 
 5. To control the aeration of soil. 154 and 155. 
 
 6. To control the temperature of soil. 
 
 173. The Destruction of Undesired Vegetation.— In 
 securing this object of tillage we have two classes of vegeta- 
 tion to destroy, one, like the prairie grasses of a virgin soil or 
 like the cultivated meadow grasses, which must be destroyed 
 before there is root room for the desired crop, and the other 
 which is designated by the general term of weeds. 
 
 Plants spread out two broad surfaces, one in the air to ob- 
 tain carbon dioxide, oxygen and sunshine, and the other in the 
 soil to obtain water, nitrates and other food constituents. It 
 requires but little study to reveal the fact that plants usually 
 spread out their leaf surfaces in such a manner that each leaf 
 shall be forced as little as possible to breathe the air of an- 
 other leaf, and that one shall shade another as httle as pos- 
 sible. In a dense forest or thicket no fact stands out more 
 prominently than the race each plant makes to outreach its 
 neighbor and get into bright sunshine and free air. A study 
 of root development shows that the same law is followed be- 
 neath the surface. There are times of scarcity of food, and 
 each root and rootlet tends to develop away from its neighbor 
 into an unoccupied territory. Such facts teach, with abundant 
 
110 
 
 evidence, that there is no room for weeds in any soil wliere an- 
 other crop is expected. 
 
 When we remember that each ])onnd of dry matter requires 
 more than 300 pounds of water taken from the soil, and that 
 in most soils there is usually a scant sup})ly of moisture at 
 best, the importance of a weedless surface should be appre- 
 ciated. 
 
 The following definite case will serve to show how rapidly 
 weeds may consume the water of soil. 
 
 On May 13, ISSl), the water-content in the soil, on adjoin- 
 ing margins of a field just planted to corn and one of clover 
 and timothy, was determined on the Experiment Farm, with 
 the results below : 
 
 Corn ground. Clover ground. 
 
 Per coif, of ivatcr. Per cent, of water. 
 
 Surface to 6 in. contaiueii 23.33 9.59 
 
 12 to 18 in. contained 19.13 14.79 
 
 18 to 24 in. contained 16.85 13.75 
 
 These figures illustrate in a very forcible manner the great 
 power vegetation has of withdrawing water from the soil, 
 how naked tillage conserves it, and the importance, in all ex- 
 cept the wettest seasons, of not allowing weeds to occupy 
 cultivated fields. 
 
 174. Plowing in Organic Matter.— The decomposition 
 of nwst animal and vegetable tissues is the result of a growth 
 in and upon them of micro-organisms which, like all other liv- 
 ing thing's, require a bountiful supply of moisture. Moisture 
 is usually found in abundance at the surface in the shade of 
 dense forests, but in open cultivated fields the stems of plants 
 and coarse manures are too dry, most of the time, to maintain 
 the life of micro-organisms unless they are buried a little dis- 
 tance below the surface where the rate of evaporation will be 
 checked and where there is a better capillary connection be- 
 tween them and the water of the soil. In this condition, if 
 the soil is sulficiently aerated so that the respiration of the 
 life going on there is ample, the organic tissues are rapidly 
 broken down and quickly become available as food for crops. 
 
 175. Circumstances which Modify the Time and 
 Depth of Plowing in of Manure.— We are yet a long way 
 from being in possession of the rigid knowledge which is 
 
Ill 
 
 needed to make specific and exact statements regarding mat- 
 ters like these. There are some general statements, however, 
 which may be helpful in practice if not followed too implicitly 
 and without judgment. 
 
 Coarse manures, when plowed in, tend at first, to cut off the 
 capillary connection with the soil-water below, and where the 
 plowing occurs in the spring, certain crops are liable to suffer 
 from drought because of a lack of moisture in the surface soil ; 
 this is especially liable to be the case if the spring is dry. If 
 heavy, soaking rains follow the plowing in of such manure, 
 the soil particles are washed m between the straws and other 
 litter and a good connection established between the surface 
 and the soil below. This is wimt does happen usually in the 
 case of fall plowing, and explains why on many, if not most, 
 soils, the fall plowing in of such manures is preferable. It is 
 evident that on soils naturally too wet, and especially in wet 
 seasons, the spring plowing, in such cases, might be prefer- 
 able. 
 
 If manure is plowed in too deeply, and especially if the soil 
 is close and fine, there is danger of too little air to permit of 
 rapid decay, and the effects of manure under such conditions 
 will be only partially felt the first season. 
 
 If the soil is a leachy one, plowing the manure in deeply 
 tends to increase the loss by underdrainage. 
 
 176. Effect of Manures on the Water Capacity of 
 Soils. — Humus stands foremost among the ingredients of soil 
 in its power to retain capillary water. The barnyard manures, 
 besides containing large quantities of sahne fertilizers, contain 
 much undigested vegetable fiber, which, when plowed into the 
 soil, tends to decay into ordinary soil humus and thus to in- 
 crease the water capacity of the lands to which they are 
 applied ; in this respect they have a superior value, when com- 
 pared with most commercial fertilizers, especially if it shall be 
 established that organic matter, in contact with dry earth, does 
 oxidize with a loss of free nitrogen. 
 
 177. The Importance of Good Tilth.— It is a gener- 
 ally recognized fact that one of the chief objects of tillage is 
 to produce a mellow seed-bed of uniform texture, and there are 
 several desirable ends which are met, wholly or in part, by 
 good tilth. 
 
112 
 
 One of the strong recommendations of a ricli sandy soil is 
 found in the evenness of its texture and the lack of adhesion 
 between its grains which permit of ahnost perfect symmetry in 
 the development of roots and allows the root hairs to occupy 
 most completely the soil interspaces. When this is true, not 
 only is ah. the soil laid under tribute, but each and every root- 
 let, with its numerous root hairs, is doing full duty. If, on the 
 other hand, the soil is uneven and filled with hard lumps, a 
 large portion of it is not only unavailable but it stands as a 
 positive hindrance to root development, checking rapid root- 
 growth and making a much greater actual length of roots nec- 
 essary in order to come in contact with a sufficient amount of 
 soU. ]^or is this aU; during the process of cultivation the 
 lumps tend to work to the surface and become very dry ; in 
 this condition they absorb a large percentage of the summer 
 rains, and, as they are almost completely surrounded by free 
 air, they give back this moisture to the atmosphere and thus 
 prevent it from rendering any service. 
 
 On the principle of oxidation of nitrogenous compounds 
 with the liberation of free nitrogen the lumpy condition of 
 soil should be expected to be a large source of loss of that im- 
 portant element of plant food. 
 
 Mellow soU favors root-development in being easily crowded 
 aside by the expanding roots, and this is a matter of some im- 
 portance in all the succulent root crops, like beets, parsnips, 
 turnips and carrots, for the actual soil displacement in an acre 
 of these crops is very great, and the conclusion seems irresist- 
 ible that a hard soil must mechanically impede root-growth in 
 such crops to a large extent. 
 
 A mellow, even-textured soil is likely to be much better 
 aerated than one not in this condition and better supphed with 
 moisture also. 
 
 178. Control of the Water-Content of Soils.— The 
 operations of tillage aiming to control the water-content of 
 soils proceed along one of three lines of action : 
 
 1. To conserve the water contained in the soil. 
 
 (a) By surface tillage. 
 
 (b) By flat culture. 
 
 (c) By mulching. 
 
113 
 
 2. To reduce the quantity of ^Yater in the soil. 
 
 (a) By deep tillage. 
 
 (b) By decreasing the water capacity. 
 
 (c) By ridge culture. 
 
 (d) By surface drainage. 
 
 (e) By underdrainage. 
 
 (f) By tree planting. 
 
 3. To increase the quantity of water in the soil. 
 
 (a) By increasing the w^ater capacity. 
 
 (b) By irrigation. 
 
 (c) By firming the surface soil. 
 
 179. Conservation of Soil Water.— On the great ma- 
 jority of cultivated lands there is, as a rule, an insufncient 
 supply of moisture to give the largest possible j^ield when 
 other things are favorable, and hence it becomes a matter of 
 importance to check the evaporation from the soil surface and 
 divert the water currents through the growing crop. 
 
 180. Surface Tillage to Check Evaporation.— In one 
 of my experiments, where the rate of evaporation from the 
 undisturbed surface of clay loam had been going on at the 
 rate of .9 pounds per square foot in 24 hours, simply removing 
 the crust of salts brought to the surface and deposited there 
 by evaporation, increased the rate of evaporation to 1.27 
 pounds per square foot in the same time, and I found the same 
 fact true for fine sand. These facts have a bearing upon the 
 practice of harrowing winter grain in the spring, suggesting 
 that the practice may, in some cases, cause a waste of water. 
 
 In the case of the fine sand referred to, the evaporation had 
 been taking place at the rate of .91 pounds per square foot in 
 24 hours, just before the crust was removed ; after its removal 
 the surface was cut in small squares with the blade of a sharp 
 knife held vertical to the surface, and then the rate of evapora- 
 tion rose from .91 pounds to 1.T5 pounds per square foot per 
 day. On removing a thin laj^er of the sand, and replacing it 
 immediately, the rate of evaporation fell to less than .5 pounds 
 per square foot daily. It is thus shown that one form of sur- 
 face tillage may increase the rate of evaporation while another 
 form may check it in a very decided manner. 
 
 A tool working like the disc harrow when the discs are run- 
 ning at a smaU angle, simply slicing the surface as the knife 
 
114 
 
 did, increases the surface exposed to the air without destroy- 
 ing the capillary connection with the soil below, and tends to 
 hasten rather than retard evaporation; but if the tool com- 
 pletely removes a surface layer, leaving the ground covered 
 with a layer of loose soil, a mulch is provided which excludes 
 the air, in a measure, and greatly retards evaporation. 
 
 181. Plat Cultivation When the surface of the ground 
 
 is thrown into ridges, as in hilling potatoes or corn, the 
 amount of surface exposed to the air is increased, and this, 
 other things being the same, tends to increase the rate of 
 evaporation from the surface and diminish the supply of 
 moisture for the crop. When three-foot rows are ridged to a 
 hight of six inches the surface is increased more than 5 per 
 cent., and when ridged to the hight of eight inches more 
 than 9 per cent. 
 
 182. Deep Tillage to Increase Evaporation.— When 
 the ground is stirred to a considerable depth repeatedly there 
 is a large and rapid evaporation from the soil stu'red, and this 
 is one of the chief objects of discing and harrowing lands 
 that are to be planted early in the spring. The ground is cold 
 from the low temperature of winter and from the large vol- 
 ume of contained water which requires a great amoimt of heat 
 to warm it. Getting rid of this moisture by deep iHIage pro- 
 vides a warm and mellow seed-bed, well aerated, which also 
 acts as a mulch to conserve the deeper water of the soil until 
 a time when it is needed. '^ 
 
 183. Firming ilie Ground to Control Moisture.— 
 Rolling or other ^^lse firming land, after it has been tilled, may 
 have two distinct -'Ejects as regards the control of soil water- 
 These are : 
 
 1. To dry the soil as a whole. 
 
 2. To increase the moisture of the seed-bed. 
 
 We have shown by two distinct lines of investigation con- 
 ducted in the fields of the Experiment Farm that rollmg tilled 
 land tends to dry the soil, as a whole, the effect being meas- 
 urable at a depth of at least four feet. This drying effect is 
 brought about — 
 
 1. By increasing the capillary power of the surface. 
 
 2. By increasing the surface temperature. 
 
 3. By increasing the wind velocity at the surface. 
 
115 
 
 These three hnportant effects tending to dry the soil may 
 be employed to secure the most rapid evaporation when re- 
 peated deep tillage and rolling follow each other at short in- 
 tervals. Stirring the soil deeply, exposes a large surface of 
 moist earth to the air which dries quickly, and if this is rolled 
 as soon as dry enough, the soil again becomes wet at the ex- 
 pense of the deeper soil moisture, and this is soon lost if deep 
 tillage follows. Repetitions of these processes are an excel- 
 lent treatment for a seed-bed in too damp cold soil. 
 
 When the soil of the seed-bed is too dry for the proper ger- 
 mination of seeds, then firming the ground tends to increase 
 the moisture by bringing it from below to the place where it 
 is most needed, and the press-wheels used on various forms of 
 drills and planters have this to recommend them. They con- 
 centrate the moisture at the points where it is -most needed, 
 leaving the remaining portion of the field covered with a loose 
 protecting mulch. In the case of broadcast seeding, rolling is 
 generally required, if the seed-bed is too dry, and if this roll- 
 ing is followed, in one or two days, with a light harrow to 
 develop a thin mulch, it will check the surface evaporation with- 
 out destroying the good capillary connections produced by 
 the rolling. 
 
 184. Puddled Soils. — All soils when completely or nearly 
 saturated with moisture become very plastic, and when they 
 are worked under these conditions the water and air are 
 crowded out of the larger interspaces and the soil becomes 
 much more compact. This is especially true of the adhesive 
 clayey soils whose particles, after such treatment, become so 
 firmly united as to develop into obstinate clods so injurious to 
 good tilth. Great care should always be taken not to work 
 soils when they are too wet. The roller should never be used 
 when the soil will adhere to its surface. 
 
 185. Advantages of a Warm Soil.— The advantages of 
 a warm soil are several, and may be briefly stated as follows : 
 
 1. Soil ingredients are more soluble in warm than in cold 
 water. 
 
 2. Root absorption is more rapid at warm than at cold tem- 
 peratures. 
 
 3. Germination is more rapid at moderately high than at 
 low temperatures. 
 
 4. Nitrification takes place most rapidly at about 90° F. 
 
116 
 
 It is a general law with all living beings that their vital 
 processes can go on normally only within certain limits of tem- 
 perature, and the range is usually a comparatively narrow one. 
 
 In our own case a change of a few degrees above or below 
 98° F. in the body, as a whole, produces very serious disturb- 
 ances ; and while these ranges are larger with plants, yet they 
 are not so wide but that the bounds may frequently be 
 crossed. 
 
 186. Best Soil Temperature in Certain Cases.— 
 llalx^rlandt found that the germination of wheat, rye, oats and 
 flax is best at IT to 87.8° F., and that corn and pumpkins 
 germinate best between 92° and 101° F. He found, for ex- 
 ample, that when corn germinated in three days at a soil tem- 
 perature of G5.3° F., it required 11 days to germinate at 51° F., 
 and while oats germinated in two daj^s at a temperature of 
 65.3° F., 7 days were required w^hen the temperature was 
 41° F. 
 
 Sachs found that tobacco and pumpkin plants wilted when 
 the soil temperature fell much below 55° F. on account of a 
 too slow root absorption. It is found that the " mother of 
 petre " develops niter at an appreciable rate only above a tem- 
 perature of 54° F., that its maximum power is manifested at 
 98° F., and that at 113° F. its power is less strong than at 
 59° F. 
 
 187. Control of Soil Temperature. — The temperature 
 of soils may be increased in several ways as follows : 
 
 1. By diminishing the water capacity. 
 
 2. By diminishing the water content. 
 
 3. By diminishing the surface evaporation. 127. 
 
 4. B}^ smoothing the surface. 
 
 5. By means of fermenting manures. 
 
 6. By increasing percolation. 
 
 It has been shown, 124 and 127, that diminishing the 
 water in soil and lessening the surface evaporation favors, in 
 a marked degree, the production of high soil temperatures, 
 while the reverse conditions tend in the opposite direction. 
 
 Smoothing the surface, as in the case of rolling, has a very 
 appreciable effect in raising the soil temperature. The results 
 observed in a special case are given in Fig. 46. It will be ob- 
 served that the air temperature over the unrolled ground is 
 higher than it is over the roUed, which shows that this soil 
 
Ill 
 
 must be losing heat faster ; and since both surfaces must have 
 been receiving the same amounts from the sun, it is plain that 
 if the air is warmed more over the unrolled ground the soil 
 itself must be warmed less. 
 
 Fig. J,6. 
 
 differences of temperature of rolled and unrolled soil and associated air tempei t. - 
 tares. 
 
 The air receives more heat from the unrolled ground for two 
 reasons. 
 
 1. Its many lumps present a much greater contact surface. 
 
 2. The lumps being dry become warmer at the surface than 
 the more moist rolled soil. 
 
 Further than this, the lumps, being in poor connection with 
 the soil below, conduct their heat slowly downward while at 
 the saT'.e time they shade the lower soil; and by exposing a 
 very large surface to the sky they cool rapidly by radiation. 
 
 The measured differences of soil temperature due to this 
 cause have been as great as 6.5° to 10° F., the lower figure 
 having been observed at a depth of three inches and the higher 
 at 1.5 inches. 
 
 The heating effect of fermenting manures in the soil has 
 been observed to produce a rise in temperature of nearly 1° F. 
 
 In the case of well drained soil the percolation of warm 
 summer rains often carries rapidly and deeply into the soil 
 considerable heat and thus raises the temperature directly, and 
 as this water must evaporate more slowly from the drained 
 soil, if at all, than from the undrained, it is not cooled as much 
 as it might have been had percolation not occurred, thus leav- 
 ing all the water to evaporate in a short time. 
 
118 
 
 IMPLEMENTS OF TILLAGE. 
 
 188. The Plow. — Foremost among the implements of 
 tillage unquestionably must be placed the plow. Historically, 
 it is probably one of the oldest of farm tools, and when viewed 
 from the standpoint of evolution no instrument has advanced 
 more slowly or has been changed more profoundly. It has 
 grown from a natural fork formed by the branches of a tree, 
 as depicted on an ancient monument in Asia Minor, with the 
 shorter limb simply sharpened and laboriously guided and 
 awkwardly drawn through the soil by the longer arm, to our 
 present almost self -guiding twisted wedge of hardened steel 
 susceptible of an extreme polish. 
 
 189. The Work Done by a Plow.— The mechanical 
 principles which do or should dictate the construction of a 
 plow can be most easily comprehended when a clear notion of 
 the work a ploAv is expected to perform is first in mind. 
 Speaking simply of the sod and stubble plows, the first has 
 two functions : 
 
 1, A cutting function. 
 
 2. An inverting function. 
 
 The stubble plow has three functions : 
 
 1. A cutting function. 
 
 2. A pulverizing function. 
 
 3. An inverting function. 
 
 With both plows the cutting is required in two planes, one 
 vertical and the other horizontal, to separate a furrow-slice of 
 the desired width and depth. The inversion of the furrow- 
 slice, required in both cases, necessitates first a lifting of the 
 slice and then a rolling of it to one side, bottom up. The 
 pulverizing of the furrow-slice is most simply done by bend- 
 ing the slice u])on itself more or less abruptly and then drop- 
 ping it suddenly upon the ground. 
 
 190. The Mechanical Principles of Plows.— The 
 plows under consideration are sliding three-sided wedges hav- 
 ing one horizontal plane face, called the sole; one vertical 
 plane face, called the land-side, and a third twisted and oblique 
 face, one portion of which is called the share and the other the 
 mold-hoard. The two lines formed by the meeting of the 
 
119 
 
 twisted oblique face with the land-side and w^tli the sole are 
 cutting- edges. This wedge is simply shoved through the ground 
 by a force applied to the standard through the plow-beam, 
 and is guided in its course by a pair of levers in the form of 
 handles. 
 
 A study of Figs. 47 to 52 will show that, in these types of 
 plows, the cutting edges are very oblique to the directions in 
 which they move, and that the obliquity is greatest in the 
 hreaking type. It will algo be seen that the strong difference 
 between the elevating and inverting surfaces or mold-boards, 
 in these plows, consists in the steepness of the inclined surface 
 and the abruptness of the twist in them, these being least 
 abrupt in the breaking plow, Fig. 52, and most abrupt in the 
 full stubble, Fig. -17. 
 
 191. Advantage of Oblique Cutting Edges.— There 
 are several conditions which have led to placing the cutting 
 edges of plows oblique to the direction in which they are 
 drawn. 
 
 1. The shin, coulter and share free themselves from roots, 
 stubble and grass more perfectly. 
 
 2. The shin, coulter and share require less 'power to cut 
 roots. 
 
 3. The plow enters the ground more easily and runs more 
 steadily. 
 
 4. There is less friction of the furrow slice on the inverting 
 surface. 
 
 When the coulter is placed with its cutting edge in a nearly 
 vertical attitude straw and roots tend to double around the 
 edge and clog under the beam, increasing the draft and tend- 
 ing to draw the plow out of the ground. If the coulter is dull 
 and the roots are long and tough, they fold over the edge 
 and thus increase the draft by making- the edge in the soil 
 thicker. When the cutting edge is made to incline backward 
 the roots tend to slide upward and are severed by a partially 
 drmoing cut, and this requires a less intense power than the 
 straight chisel thrust. 
 
 The obliquity of the share, particularly in the sod plow 
 where a large part of its work consists in cutting roots, ma- 
 terially lessens the draught by bringing a drawing cut upon 
 the roots by forcing them sidewise in its wedging action and 
 
120 
 
 Fig. J,7. 
 
 Fig. Jt8, 
 
 Fig. h^. 
 
121 
 
 MOLINE.ILL. 
 
 Fi^. 50. 
 
 MOLINE.ILL. 
 
 Msf. SI. 
 
 ^^fM^^ 
 
 Fiij. 62. 
 
122 
 
 drawing the cutting edge across them Avhilc they are under 
 tension. 
 
 When hard spots in the furrow-slice are to be cut througli 
 the more oblique the share is the greater distance will the 
 horses travel before it is cut off, and as the resistance is over- 
 come in a longer time less power is required per second. Of 
 course so much work must be done in plowing a given length 
 of furrow, but the oblique share tends to develop an even, 
 steady pull all the time, while the less oblique form allows the 
 inecpialities of the soil to develop an irregular draft which is 
 moi'e Avasteful. It is, in effect, like the triangular sections in 
 a mowing machine, which allow the horses to be cutting all 
 the time. 
 
 192. Function of the Land-side. — The land-side is made 
 necessary by the inequalities of the soil and the tendency of 
 the horses to vary their course from a straight line. When 
 the oblique share is brought against a more resisting spot of 
 soil, a root or a small pebble, were it not for the land-side the 
 plow would run too far to land and the furrow would become 
 crooked. This side pressure developed by the share produces 
 friction between the land-side and the edge of the furrow and 
 the land-side should, therefore, be of such a character as to 
 move most easily under this friction. 
 
 ■ 193. The Line of Draft. — There is a certain point, A, 
 Fig. 53, in the mold-board of the plow, to which if the horses 
 could be attached the plow would '' swim free " in the soil ; 
 and the attachment of the team to the bridle, B, of the plow 
 should be in such a position that the point of attachment, 1"), 
 
123 
 
 of the traces to the harness, shall lie in the same plane with 
 A, as represented by the line ABD. If the attachment to the 
 bridle is made at C the draft of the team will draw the plow 
 more deepl}'" into the ground ; and should it be at some point 
 below B, or, what would amount to the same thing, should 
 the horses be hitched shorter, the draft would tend to run the 
 plow out of the ground. ISTot only is it important to adjust 
 the plow so that it will " swim free " vertically, but it should 
 likewise be adjusted to " swim free " from right to left. When 
 this is done, a properly constructed plow will almost hold itself 
 and will then move with the least possible draft. 
 
 If the plow requires any considerable power to be applied 
 to the handles in guiding it, no matter in what direction, not 
 only is the work harder for the man, but the draft is harder on 
 the team and at the same time the plow is wearing out more rap- 
 idly. So, too, the man who carelessly holds his plow, allowing 
 it to waver from side to side and run shallow and deep, is mak- 
 ing not only more work for himself and for his team, but is un- 
 necessarily wearing out his plow and at the same time produc- 
 ing a seed-bed which w^ill necessarily yield a smaller crop. 
 
 194. Draft of the Plow. — The records we have, thus 
 far, bearing upon the draft of plows are, in many respects, 
 very unsatisfactory, ow4ng partly to inherent difficulties in 
 making measurements which represent the actual resistance 
 of the soil to the plow, partially because of unreliable methods 
 of measurements, and again because the varying percentage 
 of water in soil greatly modifies its plasticity and its weight. 
 
 Mr. Pusey, in 1840, in England, made some extended trials 
 of the draft of plows in soils of different kinds, and the fig- 
 ures below show the average results of trials with ten plows, 
 the total mean draft being given and also the draft in pounds 
 per square inch of a cross-section of the furrows plowed : 
 
 No. of Size of Draft. Draft per 
 
 Loamy sand 
 
 Sandy loam 
 
 Moor soil. 
 
 Strong loam 
 
 Blue clay 
 
 Sandy loam (J. C. Morton) 
 
 Stiff clay loam (N. Y. 1850) '. . 14 7x10 407 " 5.81 " 
 
 Plows. 
 
 furroio. 
 
 
 sq. in. 
 
 10 
 
 5x9 
 
 227 lbs. 
 
 5.04 lbs. 
 
 10 
 
 5x9 
 
 250 " 
 
 5.55 " 
 
 10 
 
 5x9 
 
 280 " 
 
 6.22 " 
 
 10 
 
 5x9 
 
 440 « 
 
 9.78 " 
 
 10 
 
 5x9 
 
 661 " 
 
 14.69 " 
 
 5 
 
 6x9 
 
 566 " 
 
 10.48 " 
 
124 
 
 Prof . J. W. Siinborn has made extended trials of plows re- 
 cently in Missouri and Utah. The average of all his trials, 
 reported in Bulletin No. 2 of Utah Experiment Station, is 5.98 
 pounds per square inch of furrow turned. If we separate 
 these trials historically we get, by leaving the clay out of the 
 English trials : 
 
 English trials, 1840, draft per sq. in. 7.41 lbs. 
 American trials, 1850, draft per sq. in. 5.81 lbs. 
 American trials, 1890, draft per sq. in. 5.98 lbs. 
 
 Both English and American experiments agree in showing a 
 decrease of power per square inch with increase of width of. 
 furrow when the depth remains the same; but this statement 
 should not be construed as saying that a wide furrow can be 
 plowed with less total draft than a narrow one. 
 
 The effect of depth on the draft is not so clearly shown by 
 the experiments on record, but they appear to indicate an in- 
 crease of powder, per square inch, required with increase of 
 depth. 
 
 195. Effect of the Beam-wheel on the Draft of the 
 Plow.— If the wheel under the beam of the plow is so ad- 
 justed in hight as not to bring the attachment of the horses 
 to the plow-bridle above the line of draft there is found a ma- 
 terial lessening of the draft of the plow with its use. The re- 
 duction of the draft is occasioned by the more even running 
 of the plow, making it unnecessary for the plowman to be al- 
 ternately pressing down upon the handles, or raising them, 
 in order to maintain the desired depth of furrow. If the 
 wheel is so high as to bring the line of draft in the condition 
 represented by the line ABD, Fig. 53, a part of the power 
 of the team is expended in producing pressure downward upon 
 the wheel while the full resistance of the plow still remains to 
 be overcome. The proper adjustment of this wheel is secured 
 when it simply rolls on even ground without carrying weight ; 
 when m this condition it will prevent the plow from entering 
 too deeply into the less resisting soils, and will act to force it 
 deeper into the harder portions. 
 
 196. Draft of Sulky Plows.— It is generally claimed 
 by plow manufacturers that sulky plows are of lighter draft, 
 relatively, than the free-swinging types, the claim being based 
 upon the assumption that the friction of the sole and land- 
 
125 
 
 side are transferred to the well oiled axles of the wheels and 
 a rolling resistance secured instead of a sliding one, which 
 ordinarily, on bare ground, is much less. The few records of 
 trials, we have seen, do not appear to show a material differ- 
 
 ence in the draft. There seems to be no good reason, how- 
 ever, why a sulky plow, idien xji'operly hung and with the line 
 of draft so adjusted that the power of the horses is not con- 
 verted into a downward pressure upon the wheels, should not 
 lessen the draft, and especially in the gang types. If a jjIow 
 of the requisite strength could be made so light that the up- 
 
 Fig. 65. 
 
126 
 
 ward draft against the furrow-slice were siiiRoient to take the 
 Aveight entirely from the ground, and if the adjustment for 
 landing were perfect, there would remain only the friction of 
 the furrow-slice itself. In such a case the only work left for 
 wheels would be such as has been described for the beam- 
 wheel of the walking plow, but such a condition appears prac- 
 tically impossible. 
 
 197. Effect of Coulters on the Draft of Plows.— The 
 use of the coulter is chiefly confined to sod plowing, and in 
 this work it is simply indispensable in securing a proper fur- 
 row-slice where there is any considerable turf. The early 
 English trials, and those of Gould, in New York, indicate a 
 saving of power by their use, but Professor Sanborn, through 
 his Missouri and Utah experiments, comes to the conclusion 
 that they increase the draft from 10 to 15 per cent, and ad- 
 vises farmers to dispense with them. This position is surpris- 
 ing, in the face of general practice, and I believe untenable. 
 When the coulter is very thick, dull and set in an improper 
 place or attitude it will necessarily increase the draft. 
 
 If the coulter is thick and set ahead of the lifting action of 
 the plow-point, and especially if it is dull, it offers a large re- 
 sistance by being forced to compress the soil and cut the roots 
 at the greiitest disadvantage ; but if it is so placed, in the rear 
 of the point, as to do its cutting and side-wedging above the 
 place where the point and share are lifting and cutting, the 
 two wedging and cutting bodies mutually assist each other ; 
 the roots in both cases are then severed while under strain 
 and to a greater extent, with a drawing cut and, I believe, 
 w^ith an appreciable saving of poAver. So, too, when the wheel 
 coulter is dull and set far forward, it becomes necessary to 
 hitch to the plow-bridle at so high a point, in order to force 
 the coulter into the ground, that there may be loss of power as 
 there may be with a beam-wheel ; but when this form of coul- 
 ter is sharp and set well back where the beam of the plow acts 
 with leverage to force the coulter through the sod and where 
 the cutting occurs under the lifting strain of the point and 
 mold-board, there can but be a lessening of draft in tough sod. 
 
 198. The Scouring of Plows. — There are certain soils 
 whose texture and composition are such that the most perfect 
 plow surfaces fail to shed them completely. The particles of 
 
127 
 
 most such soils are extremely minute, 153, and often contain 
 much silica. In Fig. 51 is represented a type of one of the 
 most successful plows for this class of soils. In form it resem- 
 bles the breaking-plow, and the surface of the mold-board is 
 very hard and susceptible of a high polish. The hard surface 
 in these plows appears to be demanded to prevent it from be- 
 coming roughened by the scratching of hard soil particles ; the 
 less abrupt curvature of the mold-board diminishes the surface 
 pressure and thus the liability to scratching, w^hile the fine 
 polish furnishes the fewest and shallowest depressions into 
 which the extremely minute particles can be wedged by the 
 pressure. It is a matter of great moment, in the care of such 
 plows, that they be kept from rusting, because this quickly 
 destroys the necessary polish. 
 
 199. Pulverizing Function of Plows.— The stubble 
 plows are constructed so as to pulverize the soil at the time it 
 is being overturned. This action of the plow can best be ap- 
 preciated by taking a thick bunch of paper, like the leaves of 
 a book, and bending it abruptly upon itself; when this is done 
 it will be observed that the leaves slide upon one another, and 
 through a greater distance the more abruptly the bending 
 takes place. The steep mold-board of the full-stubble plow 
 shown in Fig. 47 has this shearing action upon the soil as one 
 of its chief functions and this necessarily increases its draft. 
 
 In selecting plows for the naturally mellow soils where pul- 
 verizing is unessential, the type represented in Fig. 50 should 
 be taken, as, other conditions being the same, its draft will be 
 lighter. 
 
 200. Driving Three Horses Abreast.— Much time and 
 expense can be saved in plowing by driving three horses 
 abreast, using a larger plow or a gang of plows, and this 
 method is especially to be commended on all clear land where 
 there is any considerable acreage to be plowed. In Fig. 56 is 
 
 Fig. 56, 
 
128 
 
 represented a very compact type of tliree-liorse evener, handled 
 by the S. L. Sheldon (-o., and Fig. 57 illustrates an approved 
 method of driving three horses abreast. 
 
 Fig. 67. 
 
 201. Care of Plows. — Next in importance to having 
 good tools to work with is the keeping of them in proper 
 working trim. It is extremely wasteful to purchase good tools 
 and convert them into poor ones by lack of care, and in no 
 case do these remarks apply w4th greater force than to plows. 
 
 The John Deere Co., in their catalogues, make some remarks 
 regarding the care of plow-shares, and through their kindness 
 I am permitted to use some of their illustrations. Figs. 58 
 and 59 represent a proper and an improper form of point. A 
 
 I'lg. 5S. 
 dull point may increase the draft of a plow six to eight per 
 cent, and more, besides necessitating poorer work. The tend- 
 ency of wear on the point is to change it fi'om the sharp, 
 slightly di])ping form represented in Fig. 58 tt) the blunt uji- 
 turned form shown in Fig. 59, 
 
 The heel of the share, like the jjoiiit, is especially subject to 
 wear, and soon comes into an im})roper shape. In case the 
 ground is hard and dry, as is often the case during fall plow- 
 
i2y 
 
 iug, the share-heel requires a set shown in Fig. 60, dipping de- 
 cidedly downward, preventing it from lifting out of the ground 
 and tipping the plow to land. On the other hand, when the 
 soil is mellow and damp, the heel of the share should be given 
 a more nearly horizontal attitude, as shown in Fig. 01, to pre- 
 vent it from sucking too deeply mto the ground, and neccssi- 
 
 Mcj. 60. 
 tating a steady pressure at the handles toward the land. It 
 should be remembered that, whenever the plow requires a 
 steady pressure at the handles in any direction in guiding it, 
 there is a defect somew^here that should be remedied ; because 
 a pressure of only a few pounds on the long handles, working 
 as levers, is transformed into friction, increasing the draft on 
 the team and the Avear on the plow. 
 
 In taking the share to the shop for setting or sharpening, 
 the land-side should accompany it, so the blacksmith may have 
 a guide in giving the proper set to it. 
 
 iliWiiiilllilllllllllllllllllllilllllllillPllllI'MlillM 
 
 Fig. 61. 
 202. The Subsoil Plow.— One tj^pe of this instrument 
 is represented in Fig. 62, Its function is nominally to loosen 
 the ground to a greater depth than is practicable with the 
 ordinary plow, thus securing deeper tillage without burying 
 the humus-bearing soil too deeply below the surface. Its use 
 requires great discretion, otherwise more harm than good may 
 result from it. Better aeration, better drainage, deeper de- 
 velopment of roots and less suffering from drought are advan- 
 tages claimed for its use. For large yields of root crops a 
 deep loose soil is indispensable, and one necessity for this is 
 found in the Tact that the thick roots require so much space 
 
130 
 
 which can only be secured by forcing the soil aside. There is 
 gre:^t danger of puddling the soil in the use of the subsoil 
 plow, because the surface may appear dry enough to work 
 when the subsoil is too wet. 
 
 MOLINE.ILL. 
 
 Fig. 62. 
 
 203. The Harrow. — As implements of tillage, harrows 
 are used to secure several quite distinct ends : 
 
 1. To produce a shallow seed-bed. 
 
 2. To dry the soil preparatory to seeding. 
 
 3. To render the surface of the ground more even. 
 
 4. To pulverize the soil and secure a more even texture. 
 
 5. To cover seed. 
 
 6. To destroy young weeds. 
 
 7. To work manure into the surface soil. 
 
 8. To aerate the soil. 
 
 9. To check evaporation by developing a soil mulch. 
 According as one or another of these ends is to be secured, 
 
 uhe character of the harrow should be different. In Figs. 
 63, G-i and 65 are represented three of the strongly marked 
 types of harrows. 
 
 204. The Disc Harrow.— This harrow, Fig. 63, is dis- 
 tmctly a seed-bed-preparing and soil-drying tool and, in its 
 adjustable ty])es, may be made to work to a remarkable depth 
 in fall plowing and in corn ground in the spring. An immense 
 
131 
 
 amount of work can be done with it where there is the neces- 
 sary power to move it, which, although large when running 
 deep, is really small when compared with the amount of soil 
 
 Fl<j. 6 J. 
 
 moved. Its rolling, concave, thin discs, when set obliquely, 
 enable it to enter the. soil and overturn it with less compression 
 and relatively less friction than almost any other tool. As a 
 first tool to loosen the soil and dry it rapidly it does excellent 
 work. It is also very eflFective in pulverizing sod and may be 
 used to advantage in covering soAved peas. This is also an ex- 
 cellent tool to work in a surface dressing of manure. 
 
 205. The Acme Harrow.— This tool, so far as its effects 
 upon the soil are concerned, is like the disc harrow, but while 
 it sUces the soil and turns it over it does so with moxe com- 
 pression, more friction and less movement. Like the disc har- 
 
 Fig. 
 
132 
 
 row it can be used to cut sod but bas a greater tendency to 
 drag them out of place. 
 
 206. The Tooth Harrows.— These tools, in theu- great 
 variety of forms, are best adapted to secure the ends 3 to 9 
 named in 203. The heavier ty|)es are, however, fair drying 
 tools, especially on the more mellow soils, and in such situa- 
 tions, too, they give a sufficiently deep seed-bed for most of 
 the small grains.. To kill weeds, when just emerging from the 
 ground, in potato and corn fields, and in developing a light 
 
 Fig. 65. 
 
 mulch to retard evaporation from the soil, there is no tool 
 more effective or rapid in its execution than the light, many- 
 toothed harrows. 
 
 207. Cultivators. — We have much to learn yet in regard 
 to the real objects to be secured by summer tillage or cuilt- 
 vation. Three chief objects appear to control present practice ; 
 they are : 
 
 1. To kill weeds. 
 
 2. To lessen surface evaporation. 
 
 3. To cover the roots of plants more deeply. 
 
 I believe we shall find, however, that one of the most im- 
 portant functions is 
 
 4. To secure better soil aeration. 
 
 When we remember that good aeration, plenty of moisture, 
 and a warm temperature are among the essentials both to 
 soil nitrification and root-growth, and that nature's ways of 
 soil aeration are decidedly interfered with by our methods 
 of tillage, it seems but natural that some equivalent should be 
 supplied by our manner of working soil. 
 
 If soil aeration is conducive to its fertility it would appear 
 
133 
 
 to be rational practice with corn, potatoes and similar crops 
 to adopt deep tillage during the early portion of the season 
 before the roots have come to occupy the soil, to facilitate nitri- 
 fication, and then to adopt purely surface tillage, to check 
 evaporation and kill weeds, after the roots are well developed. 
 208. The Roller.— The firming of land with the roller, 
 "if used on the soil in the proper condition, has several benefi- 
 cial effects : 
 
 1. It makes the soil warmer, 187. 
 
 2. It increases the capacity of the surface soil for water and 
 its capillary power, 183. 
 
 3. In cases of broadcast seeding, the germination of seeds is 
 more rapid and more complete on rolled than on unrolled 
 ground. 
 
 4. It is maintained by many that larger yields are secured 
 from rolling land. 
 
 In cases where the soil is too damp and cold the alternate 
 use of the harrow and the roller will hasten its dr3dng very 
 much. Many farmers advocate the use of the roller on lands 
 sowed to small grains after the grain is up, especially if a 
 drought is threatened, the advantage claimed being the forma- 
 tion of a mulch by crushing the surface inequahties. It is one 
 of those practices, however, which demands careful study and 
 experiment to ascertain to what the advantage, if any, is due. 
 
ELEMENTARY LESSONS 
 
 PHYSICS OF AGRICULTURE. 
 
 R H. KING, 
 
 Professor of Agricultural Physics in the 
 University of Wisconsin. 
 
 STATE JOURNAL PRINTING COMPANY, 
 Printers and Stereotypers. 
 
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