lltKILIf 
 
 JBRARY 
 
 CAICFOINIA 
 
 M WESLEY & SON, 
 
 ITr 
 
ESSAY ON DEW 
 
 AND SEVERAL 
 
 APPEARANCES CONNECTED WITH IT, 
 
 BY 
 
 WILLIAM CHARLES WELLS. 
 
 EDITED, WITH ANNOTATIONS, 
 
 BY 
 
 L. P. CASELLA, F.E.A.S.; 
 
 AND AN APPENDIX 
 
 BY R. STRACHAN, F.M.S. 
 
 LONDON: 
 LONGMANS, GREEN, READER, AND DYER. 
 
 1866. 
 
PRINTED BY D. lAHB, 310, STBAKD, LONDON. 
 
PREFACE. 
 
 A reprint of the celebrated Essay on Dew by Dr. William 
 Charles Wells has long been required by meteorologists and 
 scientific students. The last edition dates from 1821, and 
 copies of the work are now very scarce. The subject of 
 which it treats is exceedingly interesting, and has much 
 scientific importance, while the Essay, as a literary produc- 
 tion, is a model worthy of the attentive study of scientific 
 experimentalists and students. It is a lucid exposition of 
 the development of a theory, after the strictly inductive 
 method, from an accumulation of facts which were established 
 by experimental tests. 
 
 Sir J. F. W. Herschel, in his " Discourse on the study of 
 Natural Philosophy," published in 1830, designates the theory 
 of dew, as developed by Wells, " one of the most beautiful 
 specimens we can call to mind of inductive experimental en- 
 quiry lying within a moderate compass. It is not possible in 
 so brief a space to do it justice ; but we earnestly recommend 
 his work (a short and very entertaining one) for perusal to 
 the student of natural philosophy, as a model with which he 
 will do well to become familiar." 
 
 George Harvey, the author of the treatise on Meteorology, 
 in the Encyclopedia Metropolitana, recommended most strongly 
 to the reader's attention, Dr. Wells's Essay on Dew ; and he 
 added : " We know of no work in our day (writing in 1836) 
 which has been more universally admired than the Treatise of 
 Dr. Wells, certainly none that practically exemplifies in a 
 purer and better form the admirable inductive system which 
 it was the object of Bacon to teach." 
 
 Professor John Tyndall, the brilliant experimentalist, 
 whose discoveries relative to the kindred subject, aqueous 
 vapour, have thrown a new light upon meteorology, in the 
 
 612 
 
IV PREFACE. 
 
 second edition of his work entitled "Heat considered as a 
 mode of Motion" says : " A series of experiments, conceived 
 and executed with, admirable clearness and skill, enabled Dr. 
 Wells to propound a Theory of Dew, which has stood the test 
 of all subsequent criticism, and is now universally adopted." 
 
 And he further eloquently remarks : e ' With broken health 
 Wells pursued and completed his beautiful investigation ; and, 
 on the brink of the grave, he composed his Essay. It is a 
 model of wise enquiry and lucid exposition. He made no 
 haste, but he took no rest till he had mastered his subject, 
 looking steadfastly into it until it became transparent to his 
 gaze. Thus he solved his problem, and stated its solution in 
 a fashion which renders his work imperishable." 
 
 In contrast with this unqualified praise, coming from 
 men so eminent in science, neither prompted by friendship, 
 nor uttered for vindication of opinion, but given as the 
 genuine expression of unbiassed judgment, notwithstanding 
 the lapse of half of a century, let us, in fairness, place before 
 the reader a criticism by a writer who has done good service 
 by his enquiry into the history of the subject. Professor 
 Charles Tomlinson in his essay entitled "History of the 
 Modern Theory of Dew" has fairly shown that the phenomena 
 of Dew had been observed and experimented upon by Boyle, 
 Le Eoi, Pictet, Alexander and Patrick Wilson, Six, Prevost, 
 and Young, before Wells ; and memoirs on the subject had 
 been published prior to the appearance of Wells's essay. To 
 the question : What then remains for Wells ? He answers 
 thus : " To Wells belongs the rare merit of seeing clearly 
 where other men saw obscurely; of grasping the whole 
 while other men held only detached parts ; of bringing the 
 scattered and somewhat incoherent labours of other inquirers 
 to bear upon his own experiments, which were undertaken 
 with clearer views, and consequently a more direct purpose 
 than those of his predecessors ; and the final result of his 
 long and patient inquiry was the establishment of a theory of 
 extreme beauty and simplicity, the truth of which subsequent 
 
PREFACE. V 
 
 inquiry has only tended to confirm ; not that Dr. Wells is to 
 be ranked as the author of this theory, but that his essay 
 was, as Dr. Whewell remarks,* ( one of the books which 
 drew most attention to the true doctrine in this country at 
 least.' " The legitimacy of this conclusion may well be 
 questioned ; for, granting that all the main facts and features 
 of the theory were known beforehand, if Wells took and 
 built them up into a theory, he most certainly is fully entitled 
 to be called the author of the theory. That he insufficiently 
 acknowledged the labours of his predecessors may be admitted ; 
 and, in this edition, an endeavour is made in the notes to 
 supply the most important omissions. The notes thus intro- 
 duced are distinguished by numbers running consecutively. 
 
 In the preparation of these notes, the works of the follow- 
 ing authors have been consulted : Daniell, Harvey, Herschel, 
 Glaisher, and Tomlinson, while Tyndall has been carefully 
 studied as the latest discoverer in this branch of physics. 
 
 Not only are the labours of the predecessors of Wells 
 noticed, but the researches of his successors are also related ; 
 so that, as well (it is hoped) as this mode of treatment allows, 
 the present state of our knowledge of the subject is pre- 
 sented in a complete form. 
 
 A few sentences will suffice for such information regard- 
 ing Dr. Wells as modern readers will generally care to have. 
 He dictated a succint memoir of his life, a short time only 
 before his death, which was prefixed to the edition of his 
 two Essays, on Vision and on Dew, published in 1818. From 
 this memoir our account has been drawn. 
 
 William Charles Wells was born in Charlestown, South 
 Carolina, in May, 1757. He was sent to a school in Dumfries 
 before he was eleven, and in 1770 entered Edinburgh Univer- 
 sity. The next year found him at Charlestown, apprenticed 
 to a doctor. In the disputes between the colonies and the 
 mother country, he espoused the Eoyalist cause, and in 1775 
 
 * ' History of the Inductive Sciences," 3rd Edition, vol. II., 1857. 
 
 a 
 
VI PEEPACE. 
 
 found it necessary to leave America. He studied at Edin- 
 burgh and afterwards in London, for the medical profession. 
 In 1781 he returned to America; and, while on a coasting 
 voyage, he suffered shipwreck, but saved himself by swim- 
 ming to shore. In 1784 he returned to London, and com- 
 menced practising as a physician, although he had abandoned 
 his profession while in America. For many years he struggled 
 hard and earned little, but eventually he stood well in his 
 profession and made a good income. 
 
 In 1812 he commenced an enquiry into the nature of dew, 
 which, with failing health, he continued in 1813 and 1814. 
 His physical condition then became so serious that he was 
 advised to desist from his experiments, which necessarily 
 exposed him to the cold night air, as it was thought 
 improbable he could survive more than a few months. 
 "Upon receiving this opinion," he himself says, "I set 
 about immediately composing my Essay on Dew, as my 
 papers containing the facts on which my theory was founded 
 would, after my death, be altogether unintelligible to any 
 person who should look into them. I laboured, in conse- 
 quence, for several months with the greatest eagerness and 
 assiduity, fancying that every page I wrote was something 
 gained from oblivion." The Essay was published in August, 
 1814, and he survived till September 18th, 1817. 
 
 Dr. Wells was admitted into the Eoyal Society of London 
 in 1793, and into that of Edinburgh in 1814. His writings 
 on medical, philosophical and biographical subjects were very 
 numerous. At the present day the Essay on Dew is the only 
 one of them which can lay claim to the attention of the 
 general reader. A list of his writings is given in the edition 
 of his Essays on Vision, and on Dew, published in 1818, and 
 reprinted in 1821. 
 
TO 
 
 JAMES DUNSMURE, ESQUIRE. 
 
 MERCHANT IN LONDON. 
 
 MY DEAR SIR, 
 
 Without your aid, I should, in all probability, never 
 
 have acquired the knowledge, upon which the following Essay is principally 
 grounded ; since I could not, I believe, have found any other place, con- 
 sidering that I was obliged to be daily in London, so well fitted for my ex- 
 periments, as that which you permitted me to use during a very long time, 
 though manifestly to the great inconvenience of yourself and your family. 
 I beg leave to assure you, that I feel this kindness most strongly, and that 
 my gratitude for it will never cease to exist. 
 I am, 
 
 My dear Sir, 
 
 Your most obedient Servant, 
 and faithful Friend, 
 
 WILLIAM CHARLES WELLS. 
 London, August, 25, 1814. 
 
The following notice was prefixed by the Author to the second 
 edition, published in 1815. 
 
 The infirm state of the Author's health having prevented him, since the 
 publication of the former edition of his Essay on Dew, from making experi- 
 ments in the open air during the night, and his reading having in the 
 meanwhile been directed to other objects, the present edition of that 
 Essay will be found to contain almost nothing more than the other. The 
 chief difference between the two arises from a change in the form of 
 several of his expressions. He has, for instance, altered the expression of 
 " saturation with moisture " to that of " repletion with moisture," in 
 order to avoid the appearance of maintaining, that common air is capable 
 of dissolving water ; a tenet unconnected with his theory. Sometimes 
 he has subjoined to the phrase, which he now employs, on that subject, 
 the words " in a pellucid state ;" when this addition has not been made, 
 he wishes it to be understood. 
 
CONTENTS. 
 
 . ESSAY ON DEW 
 
 AND 
 
 SEVEEAL APPEAEANCES CONNECTED WITH IT. 
 
 Page. 
 Introduction ........ 1 
 
 PAET I. 
 
 OF THE PHENOMENA OP DEW. 
 
 SECT. i. Of circumstances which influence the production of dew . 5 
 SECT. ii. Of the cold connected with the formation of dew . .24 
 
 PAET II. 
 
 OF THE THEOEY OF DEW. 
 
 Former theories ........ 46 
 
 A NEW THEOEY PBOPOSEB. 
 
 Dew is the production of a preceding cold in the substances upon which it 
 appears. ......... 49 
 
 That cold precedes the formation of dew ascertained by experiment . 50 
 This fact applied to explain several natural appearances. ^ 
 
 1. The variety in the quantities of dew on different bodies, exposed 
 to the air during the same time of the night, but in different 
 situations . ....... 52 
 
 2. The cold connected with dew, not being always proportional to 
 
 the quantity of that fluid . . . . . .52 
 
 3. The production of heat by the formation of dew . . .53 
 
 4. The fact of more dew being acquired, in very calm nights, by 
 substances placed upon a raised board, than by others of the same 
 kind on the grass ; and that of a slight agitation of the atmos- 
 phere, when very pregnant with moisture, increasing the quantity 
 
 of dew . . . . . . . .55 
 
 5. The fact of dew never being formed in temperate climates upon 
 
 the naked parts of a living and healthy human body . . 55 
 
 G. The fact of hygrometers, formed of animal and vegetable sub- 
 stances, when exposed to a clear sky at night, marking a degree 
 of moisture beyond what is actually resident in the atmosphere . 56 
 
X CONTENTS. 
 
 The cold which produces dew is itself produced by the radiation of heat, 
 from those bodies upon which dew is deposited . . 56 
 
 The cold produced by the radiation of heat from substances upon the 
 surface of the earth, is compensated or over-balanced in the day-time 
 by the heat from the sun, and lessened at night by various causes . 60 
 
 The cold originating in the nightly radiation of heat from bodies upon 
 the surface of the earth, though lessened by various causes, is often 
 very considerable . . . . . . .62 
 
 Some of the useful effects of the radiation of heat from the earth at 
 night . ... .65 
 
 Observations upon, or explanations of the under-mentioned circum- 
 stances : 
 
 1. The prevention, wholly or in part, of cold from radiation, in sub- 
 stances on the ground, by the interposition of any solid body 
 between them and the sky . . . . . .66 
 
 2. The prevention, wholly or in part, of cold from radiation in sub- 
 stances on the ground, by the interposition of clouds . . 67 
 
 3. The prevention, wholly or in part, of cold from radiation, by fogs 69 
 
 4. The prevention, wholly or in part, of cold from radiation, by con- 
 duction from warmer substances in contact with the radiating 
 substances . . . . . . . .71 
 
 5. The effect of wind in compensating the cold from radiation, and 
 sometimes in lessening, and sometimes in increasing the produc- 
 tion of dew . . . . . . . .72 
 
 6. The cold from radiation, of a thermometer placed on a board, 
 being less diminished than that of one suspended in the air . 72 
 
 7. The hurtful effects of cold occurring chiefly in hollow places 
 according to a remark of Theophrastus . . . .78 
 
 8. Frost being less severe upon hills, than in neighbouring plains, 
 
 in calm and serene nights . . . . . .74 
 
 Reasons assigned for believing that air is actually heated by the sun- 
 beams which enter it, and that it not only absorbs, but radiates heat 75 
 
 9. The leaves of trees often remaining dry throughout the night, 
 while those of grass are covered with dew . . . .84 
 
 10. Bright metals exposed to a clear sky in a calm night being less 
 dewed on their upper surface, than other solid bodies j and those 
 metals which radiate heat most, being most attractive of dew . 84 
 
 11. The difference between black and white bodies with respect to 
 radiation, when exposed to the sky at night . . .89 
 
 Whether dew is the product of vapour emitted during the night by the 
 earth and plants upon it ...... 90 
 
CONTENTS. XI 
 
 PART III. 
 
 OF SEVERAL APPEARANCES CONNECTED WITH DEW. 
 
 1. Of the greater moisture, sometimes observed in winter mornings 
 upon the insides of the panes of glass in windows covered with 
 inside shutters, than upon those nofe covered by them . . 97 
 
 2. Of the greater sensation of cold, which is sometimes experienced 
 upon exposure to the sky in a clear night, than is to be explained 
 
 by the temperature of the atmosphere . . . .98 
 
 3. Of the effect of those means employed by gardeners to protect 
 tender plants from cold during the night, which screen them from 
 
 the sky 100 
 
 4. Of the effect of a covering of snow, or of other matters, during 
 still and serene nights, in protecting vegetables from cold . 104 
 
 5. Of the putrefaction which has been supposed to take place in 
 animal substances exposed to moonshine . . . .105 
 
 6. Of the formation of ice, during the night in Bengal, when the 
 temperature of the air is above 32 .... 107 
 
 Conclusion . . . . . . . .119 
 
 APPENDIX. 
 
 The action of aqueous vapour on radiant heat . . . 123 
 
 Solar radiation ........ 136 
 
 Terrestrial radiation ....... 144 
 
 The spectrum of aqueous vapour ..... 149 
 
ESSAY ON DEW, ETC. 
 
 INTRODUCTION. 
 
 I WAS led, in the autumn of 1784, by the event of a rude 
 experiment, to think it probable that the formation of dew 
 is attended with the production of cold. In 1788, a paper 
 on hoarfrost, by Mr. Patrick Wilson 1 of Glasgow, was 
 published in the first volume of the Transactions of the 
 Royal Society of Edinburgh, by which it appeared that this 
 opinion had been entertained by that gentleman before it 
 had occurred to myself. In the course of the same year, 
 Mr. Six of Canterbury mentioned in a paper communicated to 
 the Royal Society, that, on clear and dewy nights, he always 
 found the mercury lower in a thermometer laid upon the ground 
 in a meadow in his neighbourhood, than it was in a similar 
 thermometer suspended in the air, six feet above the former ; 
 and that, upon one night, the difference amounted to 5 of 
 Fahrenheit's scale. Mr. Six, however, did not suppose, 
 agreeably to the opinion of Mr. Wilson and myself, that the 
 cold was occasioned by the formation of dew ; but imagined 
 that it proceeded partly from the low temperature of the air, 
 through which the dew, already formed in the atmosphere, 
 
 1 Wilson had published a paper on the same subject as early as ]780, in 
 the Transactions of the Koyal Society ; and a second paper in the Philo- 
 sophical Transactions for 1781. 
 
 B 
 
2 INTRODUCTION. 
 
 had descended^ and partly from the evaporation of moisture 
 from the ground on which his thermometer had been placed. 
 The conjecture of Mr. Wilson and the observations of Mr. 
 Six, together with many facts which I afterwards learned in 
 the course of reading, strengthened my opinion ; but I made 
 no attempt, before the autumn of 1811, to ascertain by 
 experiment if it were just, though it had, in the meantime, 
 almost daily occurred to my thoughts. Happening, in that 
 season, to be in the country on a clear and calm night, I laid 
 a thermometer upon grass wet with dew, and suspended a 
 second in the air, two feet above the other. An hour after- 
 wards, the thermometer on the grass was found to be 8 
 lower, by Fahrenheit's division, than the one in the air. 
 Similar results having been obtained from several similar 
 experiments made during the same autumn, I determined, in 
 the next spring, to prosecute the subject with some degree of 
 steadiness, and with this view went frequently to the house 
 of one of my friends in Surrey. At the end of two months, I 
 fancied that I had collected information worthy of being 
 published; but fortunately, while preparing an account of 
 it, I met, by accident, with a small posthumous work 3 of 
 Mr. Six, printed at Canterbury in 1794, in which are related 
 differences observed on dewy nights, between thermometers 
 placed upon grass, and others in the air, that are much 
 greater than those mentioned in the paper presented by him 
 
 2 The work is entitled : " The Construction and Use of p, Thermometer, 
 for showing the extremes of Temperature in the Atmosphere during 
 the observer's absence, together with experiments on the variations of local 
 heat, and other meteorological observations. By James Six, Esq., F.E.S., 
 Maidstone, 1794." 
 
INTRODUCTION. 3 
 
 to the Royal Society in 1788. In this work, too, the cold of 
 the grass is attributed, in agreement with the opinion of Mr. 
 Wilson, altogether to the dew deposited upon it. The 
 value of my own observations appearing to me now much 
 diminished, though they embraced many points left un- 
 touched by Mr. Six, I gave up my intention of making them 
 known. Shortly after, however, upon considering the subject 
 more closely, I began to suspect that Mr. Wilson, Mr. Six, 
 and myself had all committed an error in regarding the cold 
 which accompanies dew as an effect of the formation of 
 that fluid. I, therefore, resumed my experiments, and 
 having, by means of them, I think, not only established the 
 justness of my suspicion, but ascertained the real cause both 
 of dew and of several other natural appearances which have 
 hitherto received no sufficient explanation, I venture now to 
 submit to the consideration of the learned an account of 
 some of my labours, without regard to the order of time 
 in which they were performed, and of various conclusions 
 which may be drawn from them, mixed with facts and 
 opinions already published by others. 
 
PART I. 
 
 OF THE PHENOMENA OF DEW. 
 
 SECTION I. 
 
 Of Circumstances ivhich influence the Production of Dew. 
 
 AKISTOTLE* and many other writers have remarked, that 
 dew appears only on calm and serene nights. The justness 
 of this observation, however, has not been universally admitted. 
 For Musschenbroekf says, that dew forms in Holland while 
 the surface of the country is covered with a low mist ; but, 
 as he mentions at the same time that it is deposited upon all 
 bodies indiscriminately, the moisture of which he speaks 
 cannot properly be called dew, as will be more distinctly seen 
 hereafter. Other writers of considerable reputation have 
 also regarded clearness of the atmosphere as not being 
 requisite for the production of dew, misled, I believe, partly 
 by theory, and partly by observing on misty mornings copious 
 dews, which had been produced during preceding clear nights. 
 Respecting this point I can aver, after much experience, that 
 I never knew dew to be abundant except in serene weather. 
 In regard to the necessity of the air being still, I know of no 
 person who rejects it, except M. Prieur,J a late French author 
 of little consideration, and he affirms, in opposition to the 
 most common observation, that a fresh wind is requisite for 
 the production of dew. 
 
 The remark of Aristotle, however, is not to be received in 
 its strictest sense, as I have frequently found a small 
 
 * Meteor. Lib. I. c. x. et De Mundo, c. iii. 
 
 f Nat. Phil. T. ii. De Rore. 
 
 J Journal de 1'Ecole Poly technique, Tom. ii. 409. 
 
ESSAY ON DEW, ETC. 
 
 quantity of dew on grass, both on windy nights, if the sky 
 was clear or nearly so, and on cloudy nights, if there was no 
 wind. If, indeed, the clouds were high and the weather 
 calm, I have sometimes seen on grass, though the sky was 
 entirely hidden, no very inconsiderable quantity of dew. 
 Again, according to my observation, entire stillness of the 
 atmosphere is so far from being necessary for the formation 
 of this fluid, that its quantity has seemed to me to J)e increased 
 fry a very gentle motion in the air. Dew, however, has never 
 been seen by me on nights both cloudy and windy. 
 
 If, in the course of the night, the weather, from being 
 calm and serene, should become windy and cloudy, not only 
 will dew cease to form, but that which has formed will 
 either disappear or diminish considerably. 
 
 In calm weather, if the sky be partially covered with 
 clouds, more dew will appear than if it were entirely covered, 
 but less than if it were entirely clear. 
 
 Dew probably begins in this country to appear upon 
 grass, in places shaded from the sun, during clear and calm 
 weather, soon after the heat of the atmosphere has declined. 3 
 My opportunities, however, for making such observations 
 have not been numerous, since, while pursuing this subject, 
 
 1 seldom went into the country till late in the afternoon ; 
 but I have frequently felt grass moist in dry weather, several 
 hours before sunset. On the other hand, I have scarcely 
 ever known dew to be present in such quantity upon grass, 
 as to exhibit visible drops, before the sun was very near the 
 horizon, or to be very copious, till some time after sunset. 
 It also continues to form, in shaded places, after sunrise ; but 
 the interval between sunrise and its ceasing to form, is, 
 
 3 The invisible vapour contained in the superincumbent air, although 
 being a powerful radiant, is not so quickly chilled as the grass, because it has 
 not only to discharge its own heat, but also that of the large mass of air in 
 which it is suspended. Hence the condensation of the vapour in contact with 
 the grass and the formation of dew. 
 
 x 
 
ESSAY ON DEW, ETC. 7 
 
 according to my observation, which, upon this point, has not 
 been extensive, considerably shorter than that between its 
 first appearance in the afternoon and sunset. Contrary, 
 however, to what happens at sunset, if the weather be favour- 
 able, more dew forms a little before, and, in shaded places, a 
 little after sunrise, than at any other time. Musschenbroek, 
 therefore, errs greatly when he says, that dew does not form 
 after the sun has risen. The preceding observations, on the 
 early appearance of dew in the afternoon, are to be restricted 
 to what happens to grass or other substances highly attrac- 
 tive of dew placed on the ground ; for it occurs much later on 
 similar substances, which are elevated a few feet above the 
 ground, though upon these it continues to form as long after 
 the rising of the sun, as upon the others, if they be equally 
 sheltered from the rays of that body. 
 
 The formation of dew after it has once commenced, con- 
 tinues during the whole night, if the weather remain still 
 and serene. M. Prieur, indeed, of whom I have already 
 spoken, asserts, that dew forms only in the evening and 
 morning, and that any which occurs in the former season 
 always disappears in the course of the night. I can affirm, 
 however, from long experience, that grass, after having been 
 dewed in the evening, is never found dry until after sunrise, 
 unless the weather has, in the mean time, changed. Upon 
 one serene and still night, I placed fresh parcels of wool 
 upon grass every hour, and by weighing each of them, after 
 exposure for an hour, found that they had all attracted 
 dew. 4 
 
 When dew forms upon a smooth dense body, as glass, and 
 it is only by means of such a body that the process can be 
 
 4 " To measure the quantity of dew deposited each night, an instrument is 
 used called a drosometer. The most simple process consists in exposing to 
 the open air bodies whose exact weight is known, and then weighing them 
 afresh after they are covered with dew." Kcemtz's Course of Meteorology. 
 
8 ESSAY ON DEW, ETC. 
 
 accurately observed, the appearances are altogether similar 
 to those which occur on a like body, when exposed to the 
 steam of water a little warmer than itself. The exposed 
 surface has first its lustre diminished by a slight damp 
 uniformly spread over it. As the moisture increases it 
 gathers into irregularly shaped flat drops, which are, at first, 
 very small, but afterwards enlarge and run into one another, 
 forming streamlets, by means of which a great part escapes 
 from the body which had received it. 
 
 During nights that are equally clear and calm, dew often 
 appears in very unequal quantities, even after allowance has 
 been made for any difference in their lengths. One great 
 source of these differences is very obvious. For it being 
 manifest, whatever theory be adopted concerning the imme- 
 diate cause of dew, that the more replete the atmosphere is 
 with moisture previously to the operation of that cause, the 
 more copious will the precipitation of water be ; after this 
 operation has commenced, all the circumstances, which tend 
 to increase the quantity of moisture in the atmosphere, must 
 likewise tend to increase the production of dew. Thus dew, 
 in equally calm and clear nights, is more abundant shortly 
 after rain than during a long tract of dry weather. It is 
 more abundant, also, throughout Europe, with perhaps a few 
 exceptions, and in some parts of Asia and Africa, during 
 southerly and westerly winds, than during those which blow 
 from the north and the east. Aristotle* says, that Pontus is 
 the only country in which dew is more copious during a 
 northerly than during a southerly wind. But a similar fact 
 occurs in Egypt; for dew is scarcely ever observed there, 
 except while the Etesian winds prevail. Both cases, however, 
 though contrary to the letter, are consonant with the spirit of 
 the rule ; since the north wind, in one country, proceeds from 
 the Euxine sea, and, in the other, from the Mediterranean. 
 
 * Meteor. Lib. 1. c. x. 
 
. ESSAY ON DEW, ETC. 9 
 
 Another circumstance of the same kind with the blow- 
 ing of wind from the south and the west, as showing that 
 the air contains much moisture, is the lessening of the weight 
 of the atmosphere. My experience on this point has not, 
 indeed, been great, as the falling of the mercury in the 
 barometer is very commonly attended with wind or clouds, 
 both unfavourable to the production of dew ; but still the 
 greatest dew I have ever witnessed, occurred while the 
 barometer was sinking. A corresponding observation is 
 made by M. de Luc, who says, that rain may be foretold 
 when dew is uncommonly abundant, in relation to the climate 
 and season.* 5 
 
 To the greater or less quantity of moisture in the atmos- 
 phere, at the time of the action of the immediate cause of 
 dew, are likewise to be referred several other facts respecting 
 its copiousness, the explanation of which is, perhaps, not so 
 apparent as in the preceding examples. 
 
 In the first place, dew is commonly more plentiful in 
 spring and autumn than in summer ; the reason is, that a 
 greater difference is generally found between the tempera- 
 tures of the day and the night, in the former seasons of the 
 year, than in the latter. In spring this circumstance is 
 prevented often from having a considerable effect by the 
 opposite influence of northerly and easterly winds ; but, 
 during still and serene nights in autumn, dew is almost 
 always highly abundant. 6 
 
 In the second place, dew is always very copious on those 
 clear and calm nights which are followed by misty or foggy 
 mornings ; the turbidness of the air in the morning showing 
 
 * Kech. sur. les Mod. de 1'Atm. 725. 
 
 8 And Harvey says : " In Devonshire, three successive frosty mornings 
 ar^ regarded by the farmer as a sure harbinger of rain." 
 
 6 " It was found that the differences between the temperature of the air 
 and of bodies on the earth, at uight, in equally calm and clear weather 
 
10 ESSAY ON DEW,, ETC. 
 
 that it must have contained, during the preceding night, a 
 considerable quantity of moisture. 
 
 Thirdly, I have observed dew to be unusually plentiful on 
 a clear morning which had succeeded a cloudy night. For 
 the air, having in the course of the night lost little or no 
 moisture, was in the morning more charged with watery 
 vapour than it would have been if the night had also been 
 clear. 
 
 Fourthly, heat of the atmosphere, if other circumstances 
 are favourable, which, according to my experience, they 
 seldom are in this country, occasions a great formation of 
 dew. For, as the power of the air to retain watery vapour 
 in a pellucid state, increases considerably faster while its 
 temperature is rising than in proportion to the heat ac- 
 quired, 7 a decrease of its heat, in any small given quantity, 
 
 was the same at every period of the year, but it was found that the amount 
 of dew deposited during such times was much greater in summer than in 
 winter. This is easily accounted for, from the now well-known relation exist- 
 ing between temperature and moisture. At all seasons of the year, at night, 
 the depression of the temperature of the dew-point below that of the air is 
 small, or the air is in a state of saturation nearly, and, therefore, in summer, 
 a certain diminution of temperature would cause much more vapour to be 
 changed into water, than an equal diminution in winter would do." Glaisher 
 in Phil. Trans. 1847. 
 
 " It is no uncommon thing in spring for the dew-point to be more than 
 20 below the temperature of the atmosphere in the shade, and I have even 
 seen the difference amount to 30. The effect of such a degree of dryness is 
 parching in the extreme, and if accompanied with wind ia destructive to the 
 blossoms of tender plants." Daniell's Met. Essays. 
 
 Howard, the meteorologist, found dews so abundant in autumn as to be 
 capable of daily measurement in the rain-gauge. 
 
 7 This fact may be readily illustrated by reference to Glaisher's Hygrome- 
 trical Tables. The atmospheric pressure being represented by a barometric 
 column of 30 inches of mercury, air at the temperature of 
 
 Grains 
 
 50 will be saturated by 4-1 of water. Differences. 
 55 4-9 0-8 
 
 ,60 5-8 0-9 
 
 65 6-8 1-0 
 
 Thus, while the temperature of the air rises by equal gradations, its capa- 
 city for moisture rises by increasing increments. 
 
ESSAY ON DEW, ETC. 11 
 
 during the night, must bring it, if the temperature be high, 
 much nearer to the point of repletion, before it be acted 
 upon by the immediate cause of dew, than if the tempera- 
 ture were low. 8 We read, accordingly, in the writings of 
 those who have travelled into hot climates, of a copiousness 
 of dew frequently observed by them there, which very much 
 exceeds what occurs at any time in this country. But even 
 here dew, though for the most part scanty in our hottest 
 season, is sometimes very abundant during it, an example 
 of which occurred to me on the night common to the 
 29th and 30th of July, 1813; for on that night, notwith- 
 standing its shortness, more dew appeared than has ever 
 been observed by me on any other. 
 
 In the last place, I always found, when the clearness and 
 stillness of the atmosphere were the same, that more dew was 
 formed between midnight and sunrise than between sunset 
 and midnight, though the positive quantity of moisture in 
 the air must have been less in the former than in the latter 
 time, in consequence of a previous precipitation of part of it. 
 The reason, no doubt, is the cold of the atmosphere being 
 greater in the latter than in the prior part of the night. 
 
 But there are many circumstances influencing the quan- 
 tity of dew, 9 which, though much more open to accurate 
 
 From the contents of note 7, it follows that, supposing air at 65, and 
 saturated with vapour, to fall to 60, it would lose by condensation 1 grain 
 of water ; but, supposing the air at 55, also saturated, to fall to 50, it would 
 lose only 0*8 grains. Therefore, viewing the deposition of dew with regard 
 to temperature alone, it is evident that the higher the temperature the 
 more humid the air will become for any given reduction of temperature. 
 
 Hygrometry, which in the time of Wells was in a very imperfect state, 
 has since been greatly developed by the inventions of Daniell, Mason, Eeg- 
 nault, and others ; while the Observatory at Greenwich has furnished obser- 
 vations which have led to the perfecting of the Tables. It is now an 
 exceedingly useful and interesting branch of meteorology. 
 
 9 Various attempts have been made to determine the annual deposition 
 of dew, but without success. To measure the quantity deposited under the 
 most favourable circumstances is no easj task ; and, although Mr. Daniell 
 
12 ESSAY ON DEW, ETC. 
 
 observation than those hitherto mentioned, are yet much 
 less easy to be understood. 
 
 In my first attempts to compare the quantities of dew 
 formed during different times, or in different situations, 
 I attended only to the appearance which it made on bodies 
 having smooth surfaces. But quickly seeing this method to 
 be very imperfect, I next employed wool to collect dew from 
 the atmosphere, and found it well adapted for my purpose, as 
 it readily admits amongst its fibres the moisture which forms 
 on its outer parts, and retains what it receives so firmly, that 
 I never but once had occasion to suspect that it suffered any 
 portion of what it had thus acquired to pass entirely through 
 it. The wool 10 which I used was white, moderately fine, and 
 already imbued with a little moisture, from having been long 
 exposed to the air of a room in which no fire was kept. I 
 divided it into parcels of 10 grains each, and, immediately 
 before exposure, pulled the fibres of every parcel somewhat 
 asunder, so as to give it the form of a flattened sphere, the 
 greatest diameter of which was about 2 inches. As in doing 
 this I went by the judgment of my sight alone, some little 
 inequality in point of size must have existed among different 
 
 observes that the quantity of vapour existing in the atmosphere in the 
 different seasons, at the level of the sea, seems to follow the course of the 
 mean temperature, such are the varied and uncertain conditions connected 
 with the deposition of dew, that no very accurate results for the average 
 annual deposit can be obtained in the present state of our knowledge. It 
 must, indeed, vary with every climate and every locality. In a general way 
 it has been estimated for England at about five inches annually, which is 
 computed to be about a seventh part of the average quantity of moisture 
 received by the atmosphere in the same time. 
 
 10 In any investigation upon dew it is important to employ a substance 
 which will admit of its ready formation, and retain it for examination. 
 Wool has been found very suitable, but care must be taken in selecting and 
 in using it. Uniformity of fibre is necessary, and the parcels selected or 
 made up for comparative observations should expose equal surfaces to the 
 sky. Equality of mass or weight by no means implies equality of radiating 
 surface. 
 
ESSAY ON DEW, ETC. 13 
 
 parcels, but none, I think, sufficient to affect the accuracy of 
 rny conclusions from the experiments in which they were 
 employed, more especially as my conclusions scarcely ever 
 rested upon single trials. 
 
 Previously to mentioning the results of any of my expe- 
 riments with these parcels of wool, I think it right to de- 
 scribe the place where by far the greater part of my 
 observations on dew were made. This was a garden in 
 Surrey, distant, by the public road, about three miles from 
 the bridge over the Thames at Blackfriars, but not more 
 than a mile and a quarter from a densely-built part of the 
 suburbs on the south side of that river. The form of the 
 garden was oblong, its extent nearly half an acre, and its 
 surface level. At one end was a dwelling-house of moderate 
 size, at the other a range of low buildings ; on one side a row 
 of high trees, on the other a low fence, dividing it from another 
 garden. If this fence had been absent, the garden would 
 have been on the latter side entirely open. Within it were 
 some fruit-trees, but, as it had not been long made, their 
 size was small. Towards one end there was a grassplat, in 
 length 62 feet, and nearly 16 broad, the herbage of which 
 was kept short by frequent mowing. The rest of the garden 
 was employed for the production of culinary vegetables. All 
 of these circumstances, however trifling they may appear, 
 had influence on my experiments, and most of them, as will 
 hereafter be seen, must have rendered the results less re- 
 markable than they would have been if they had occurred 
 on a wide open plain, considerably distant from a large city. 11 
 
 11 The example set by Wells, in giving this careful description of the 
 place where he chiefly made his experiments, is well worthy of the attention 
 of meteorologists. Numerous registers of temperature and rainfall observa- 
 tions have been found next to useless from the absence of these desirable 
 details. They are more especially required in connection with observations 
 
14 * ESSAY ON DEW,, ETC. 
 
 I now proceed to relate the influence which several dif- 
 ferences in the situation,, mechanical state, and real nature 
 of bodies, have upon the production of dew : 
 
 I. One general fact relative to situation is, that whatever 
 diminishes the view of the sky, as seen from the exposed 
 "body, occasions the quantity of dew which is formed upon it 
 to be less than would have occurred if the exposure to the 
 sky had been complete. 
 
 I placed, on several clear and still nights, 10 grains of 
 wool upon the middle of a painted board, 4J feet long, 2 feet 
 wide, and 1 inch thick, elevated 4 feet abo^e the grassplat, 
 by means of four slender wooden props of equal height ; and, 
 at the same time, attached loosely, 10 grains of wool to the 
 middle of its underside. The two parcels were consequently 
 only an inch asunder, and were equally exposed to the action 
 of the air. Upon one night, however, I found that the upper 
 parcel had gained 14 grains in weight, but the lower only 4. 
 On the second night the quantities of moisture, acquired by 
 like parcels of wool, in the same situations as in the first ex- 
 periment, were 19 and 6 grains ; on a third, 11 and 2 ; on a 
 fourth, 20 and 4 ; the smaller quantity being always that which 
 was gained by the wool attached to the lower side of the board* 
 
 I bent a sheet of pasteboard into the shape of a house- 
 roof, making the angle of flexure 90 degrees, and leaving 
 both ends open. This was placed one evening, with its ridge 
 uppermost, upon the same grassplat, in the direction of the 
 wind as well as this could be ascertained, I then laid 10 
 grains of wool on the middle of that part of the grass which 
 was sheltered by the roof, and the same quantity on another 
 
 oipon solar and terrestrial radiation, the rainfall and evaporation, the regis- 
 tration of ozone, the maximum and minimum temperature in the shade. 
 The introduction of such particulars regarding local position and circum- 
 stances should be deemed an essential duty by all who keep a register of 
 .any such meteorological observations. 
 
ESSAY ON DEW, ETC. 15 
 
 part of the grassplat fully exposed to the sky. In the morn- 
 ing the sheltered wool was found to have increased in weight 
 only 2 grains, but that which had been exposed to the sky, 
 16 grains. 
 
 In these experiments the view of the sky was almost en- 
 tirely cut off from the situations in which litble dew was 
 formed. In others, where it was less so, the quantity gained 
 was greater. Thus, 10 grains of wool, placed upon the spot 
 of the grassplat which was directly under the middle of the 
 raised board, and which enjoyed, therefore, a considerable 
 oblique view of the sky, acquired during one night 7, during 
 a second 9, and during a third 12 grains of moisture, while 
 the quantities gained during the same times, by equal parcels 
 of wool laid upon another part of the grassplat, which was 
 entirely exposed to the heavens, were 10, 16, and 20 grains. 
 
 As no moisture, falling like rain from the atmosphere, 
 could, on a calm night, have reached the wool in any of the 
 situations where little dew was formed, it may be thought 
 that the substances under which the wool was placed pre- 
 vented, mechanically, the access of that fluid. But on this 
 supposition it cannot be explained why some dew was always 
 found in the most sheltered places, and why a considerable 
 quantity occurred upon the grass under the middle of l.he 
 raised board. A still stronger proof of the want of justness 
 in this supposition is afforded by the following experiment : 
 I placed, upright, on the grassplat a hollow cylinder of 
 baked clay, the height of which was 2^ feet, and diameter 
 1 foot. On the grass, surrounded by the cylinder, were laid 
 10 grains of wool, which, in this situation, as there was not 
 the least wind, would have received as much rain as a like 
 quantity of wool fully exposed to the sky. But the quantity 
 of moisture obtained by the wool surrounded by the cylinder 
 was only a little more than 2 grains, while that acquired by 
 10 grains of fully exposed wool was 16. This occurred on 
 
16 ESSAY ON DEW, ETC. 
 
 the night, during which the wool under the bent pasteboard 
 gained only 2 grains of moisture. 12 
 
 Dew, however, will, in consequence of other varieties of 
 situation, form in very different quantities, upon substances 
 of the same kind, although these should be similarly exposed 
 to the sky. 
 
 In the first place, it is requisite for the most abundant 
 formation of dew, that the substance attracting it should 
 rest on a stable horizontal body of some extent. Thus, upon 
 one night, while 10 grains of wool, laid upon the raised 
 board, increased 20 grains in weight, an equal quantity, sus- 
 pended in the open air, 5J feet above the ground, increased 
 only 11 grains, notwithstanding that it presented a greater 
 surface to the air than the other parcel. On another night, 
 10 grains of wool gained on the raised board 19 grains, but 
 the same quantity suspended in the air, on a level with the 
 board, only 13 ; and on a third, 10 grains of wool acquired, 
 on the same board, 2J grains of weight, during the time in 
 which other 10 grains, hung in the air, at the same height, 
 acquired only 4 a grain. 
 
 In the second place, the quantities of dew attracted by 
 equal masses of wool, similarly exposed to the sky, and rest- 
 ing on equally stable and extended bodies, oftentimes vary 
 considerably, in consequence of some difference in the other 
 circumstances of these bodies. Ten grains of wool, for in- 
 stance, haying been placed upon the grassplat on a dewy 
 evening, 10 grains upon a gravel walk which bounded the 
 
 12 " Harvey traced the effect f of a diminished aspect of the sky by em- 
 ploying cylindrical, and the frustums of conical, vessels of a common 
 diameter at the base and of a common altitude. The greater the upper 
 diameter of a vessel, all other things being the same, the greater was the 
 amount of dew on equal masses of wool placed at the bottom ; and a sensible 
 diminution in the quantity of dew was perceived, whenever the upper end 
 of a vessel had only a few narrow detached slips of wood placed above it." 
 Enc. Met., p. 134, vol. v. 
 
ESSAY ON DEW, ETC. 17 
 
 grassplat, and 10 grains upon a bed of bare garden mould, 
 immediately adjoining the gravel walk ; in the morning, the 
 wool on the grass was found to have increased 16 grains in 
 weight, but that on the gravel walk only 9, and that on the 
 garden mould only 8. On another night, during the time 
 that 10 grains of wool, laid upon grass, acquired 2J grains 
 of moisture, the same quantity gained only i a grain upon 
 the bed of garden mould, and a like quantity, placed upon 
 the gravel walk, received no accession of weight whatever. 
 
 Two objections will probably be made against the accu- 
 racy of these as well as my other experiments with wool. 
 One is, that wool placed on grass may, by a kind of capillary- 
 attraction, receive dew previously formed on the grass, in 
 addition to its own. To this I answer, that wool in a china 
 saucer, placed on the grass, acquired very nearly as much 
 weight as an equal parcel immediately touching the grass. 
 The second objection is, that a part of the increased weight 
 in the wool might arise from its imbibing moisture as a 
 hygroscopic substance. I do not deny that some weight was 
 given to the wool in this way ; but it may be safely affirmed 
 that this quantity must have been very small. For, on very 
 cloudy nights, apparently the best fitted to increase the 
 weight of hygroscopic substances, wool upon the raised board 
 would, in the course of many hours, acquire little or no 
 weight; and in London, I have never found 10 grains of 
 wool, exposed to the air on the outside of one of my chamber 
 windows, to increase during a whole night more than half a 
 grain in weight. When this weight was gained the weather 
 was clear and still; if the weather was cloudy and windy, 
 the wool received either less or no weight. This window is 
 so situated as to be, in great measure, deprived of the aspect 
 of the sky. 
 
 It being shown that wool, though highly attractive of 
 dew, was prevented by the mere vicinity of a gravel walk, or 
 
 c 
 
18 ' ESSAY ON DEW, ETC. 
 
 a bed of garden mould, for only a small part of it actually 
 touched those bodies, from acquiring nearly as much dew as 
 an equal parcel laid upon grass, it may be readily inferred 
 that little was formed upon themselves. In confirmation of 
 this conclusion I shall mention that I never saw dew upon 
 either of them. Another fact of the same kind is, that while 
 returning to London from the scene of my experiments about 
 sunrise, I never observed if the atmosphere was clear, the 
 public road or any stone pavement on the side of it to be 
 moistened with dew, though grass within a few feet of it and 
 painted doors and windows of houses not far from it were 
 frequently very wet. If, indeed, there was a foggy morning, 
 after a clear and calm night, even the streets of London 
 would sometimes be moist though they had been dry the day 
 before, and no rain had in the meanwhile fallen. This entire, 
 or almost entire, freedom of certain situations from dew 
 depends, however, much more upon extraneous circumstances 
 than upon the nature of the substances found there; for 
 river sand, though of the same nature with gravel, when 
 placed upon the raised board, or upon grass, attracted dew 
 copiously. 
 
 A third difference from situation in the quantity of dew 
 collected by similar bodies similarly exposed to the sky 
 depends upon their position with respect to the ground. 
 Thus, a substance placed several feet above the ground, 
 though in this situation later dewed than if it touched the 
 earth, would, notwithstanding, if it lay upon a stable body of 
 some extent, such as the raised board lately mentioned, acquire 
 more dew during a very still night than a similar substance 
 lying on grass. 
 
 A fourth difference of this kind occurred among bodies 
 placed on different parts of the raised board. For one that 
 was placed at the leeward end of it, generally acquired more 
 dew than a similar body at the windward extremity. 
 
ESSAY ON DEW, ETC. 19 
 
 II. Difference in the mechanical state 'of bodies, though 
 all other circumstances be similar, has likewise an effect on 
 the quantity of dew which they attract. Thus more dew is 
 formed upon fine shavings of wood than upon a thick piece of 
 the same substance. It is chiefly for a similar reason, I 
 believe, that fine raw silk, fine unwrought cotton, and 
 flax were found by me to attract somewhat more dew than 
 the wool I employed, the fibres of which were thicker than 
 those of the other substances just mentioned. 
 
 III. Bright metals, in consequence of some circumstance ; 
 in their constitution, attract dew much less powerfully than 
 other bodies ; all of which, after allowance has been made for 
 any difference which may exist in their mechanical states, 
 seem to attract dew in quantities not very unequal if they be i 
 similarly situated. 
 
 Musschenbroek was the first who distinctly remarked this 
 peculiarity of metals, but Dufay,* I believe, published it 
 before him, referring, at the same time, the discovery to its 
 proper author. Both Musschenbroek and Dufay, however, 1 
 made too large an inference from their experiments, for they 
 asserted that dew never appears on the upper surface of 
 bright metals, whereas the contrary has since been observed 
 by many persons, and I have myself known dew to form on 
 gold, silver, copper, tin, platina, iron, steel, zinc, and lead._J 
 Dew, however, when it does form upon metals, commonly 
 sullies only the lustre of their surface ; and even when it is 
 sufficiently abundant to gather into drops, these are almost 
 always small and distinct. Two other facts of the same kind 
 are -first, that the dew which has formed upon a metal will 
 often disappear, while other substances in their neighbour- 
 hood remain wet ; and secondly, that a metal which has been 
 purposely moistened will often become dry, though similarly 
 exposed with bodies which are attracting dew. This inaptitude ' 
 
 * Mem. de 1'Acad. Fran. 1736. 
 
 c2 
 
20 ESSAY ON DEW, ETC. 
 
 to attract dew in metals is communicated to bodies of a 
 ^ very different nature which, touch, or are near to them. For 
 I have found that wool laid upon a metal will acquire much 
 less dew than an equal quantity laid upon grass in the imme- 
 diate vicinit} r . 
 
 A large metallic plate, lying on grass, resists the 
 formation of dew more powerfully than a very small one 
 similarly situated. I conclude from various collateral facts 
 that a considerable difference in the thickness of two pieces 
 of metal exposing equal surfaces to the sky will be attended 
 with a similar consequence wherever they be placed, though 
 I have no observation which proves this directly. If, how- 
 ever, a large and a very small plate be suspended horizontally 
 at the same height in the air the small plate will resist the 
 formation of dew more powerfully than the large. 
 
 If a metal be closely attached to a substance of some 
 thickness which attracts dew powerfully, the attraction of 
 the metal itself for dew, instead of being increased from this 
 circumstance, becomes diminished, provided the metal cover 
 the whole of the upper surface of the other body. If only a 
 part of this body be covered the production of dew on the 
 metal is forwarded by the conjunction, and this somewhat in 
 proportion to the quantity of surface in the lower body left 
 uncovered. The justness of the first of these observations is 
 proved by the following experiment. I joined, in the form of 
 a cross, two pieces of very light wood, each 4 inches long, a 
 third of an inch in breadth, and 1 line in thickness. To one 
 side of this cross I fastened, by means of mucilage, a square 
 piece of gilt paper, and then exposed the instrument to the 
 sky with its metallic side uppermost on a dewy night, by 
 suspending it in a horizontal position, about 6 inches above 
 the ground, A few hours after the unattached parts of the 
 metalled paper were found covered with minute drops of 
 dew, while those which adhered to the cross were dry. 
 
ESSAY ON DEW, ETC. 21 
 
 A large metallic plate laid upon grass was dewed with 
 more difficulty on its upper surface than a similar plate 
 elevated a few inches above the grass by means of slender 
 props, which allowed the air to pass freely under the metal. 
 But the case with respect to small pieces was the reverse ; 
 for I have often seen, covered with dew, the metallic sheath 
 of a small thermometer lying upon grass, while the similar 
 sheath of another thermometer, suspended in the air, 
 remained dry. 
 
 Removing a metal several times in the course of the 
 night from one part of the grassplat to another, facili- 
 tated its being dewed. The same effect was produced on 
 gilt and silvered paper, by first exposing them to the 
 sky for some time, with the bare side uppermost, and then 
 turning them. 
 
 If a piece of glass, covered on one side with a metal, be | 
 placed upon the ground with this side downwards, the upper 
 surface will attract dew, precisely as if no metal were 
 attached to the lower surface. 
 
 The upper surfaces of metals are most readily and most < 
 copiously dewed on those nights and in those parts of the 
 night, during which other substances are the most readily 
 and the most copiously dewed. 
 
 If a metallic plate had been laid upon grass, before dew 
 began to form anywhere, its lower side, notwithstanding, 
 always became moist in the course of the night ; and the 
 same effect was almost always observed, if the plate had been 
 placed horizontally in the air, a few inches above the grass. 
 While the undersides were thus moist the upper surfaces 
 were very often dry. If, however, the plate was elevated 
 several feet in the air, the condition of both sides was always 
 the same, whether this was dry or moist. 
 
 The remarks hitherto made on the relation of metals to 
 dew, apply to the class generally ; but it is now to be 
 
22 ESSAY ON DEW, ETC. 
 
 mentioned that they do not all resist the formation of that 
 fluid with the same force. 
 
 I saw, for example, platina one night distinctly dewed, 
 while gold, silver, copper, and tin, though similarly situated, 
 were entirely dry ; and I have also several times seen these 
 four metals free from dew, while iron, steel, zinc, and lead 
 were covered with it. 
 
 I once supposed, in consequence of the difficulty with 
 which metals are dewed, that they might in all circumstances 
 resist, in a greater degree than other bodies, the condensation 
 of watery vapour upon their surface; and I afterwards found 
 that Le Boi* asserts this to be the case. But having exposed 
 at the same time, to the steam of warm water, pieces of glass 
 and of metal, I did not see that moisture formed in the least 
 more readily upon the former than upon the latter. I have 
 since learned that Saussuref once entertained a similar suspi- 
 cion, which was also proved by an experiment to be groundless. 
 
 All my experiments hitherto spoken of were made in the 
 country. Bat Le Roi having said that dew is never de- 
 posited by the air of cities, I determined to ascertain if his 
 assertion was just. With this view I frequently exposed at 
 night 10 grains of wool upon a slight wooden frame, placed 
 in such a manner between two ridges of the top of my house, 
 which is situated in one of the most crowded districts of 
 London, as to be 3 feet distant from the nearest part of the 
 roof. The event was, that upon clear and calm nights dew 
 was always acquired by the wool, though never in any con- 
 siderable quantity ; probably, however, more from the wooden 
 frame being nearly surrounded by buildings much more 
 elevated than itself, than from any particular condition of the 
 
 * Mem. de 1'Acad. Fran. 1751. f Fygronometrie, page 329. 
 
ESSAY ON DEW, ETC. 23 
 
 air in cities. The formation of dew in this situation pro- 
 ceeded much less regularly than in the country. For, upon 
 one evening- 10 grains of wool gained in it 3 grains of 
 moisture in 1 hour and 18 minutes, though I scarcely ever 
 knew a greater quantity to be collected by a similar parcel of 
 wool in the same place during a whole night. These experi- 
 ments will no doubt seem to many superfluous, since dew may 
 be observed every fine evening upon grass in London. But as 
 dew upon grass is said by Le Roi to proceed from the ground 
 and not from the atmosphere, the argument derived from its 
 appearance there, in cities, against his assertion is thus eluded 
 by him. 
 
 The last subject which I shall here touch upon, is that of 
 hoarfrost. 
 
 This substance has, I believe, from the time of Aristotle,* 
 been uniformly, and, according to my observations, justly 
 considered as frozen dew. I shall, therefore, frequently refer 
 hereafter to the experiments of the late Mr. Patrick Wilson, 
 of Glasgow, respecting it, as if they had been actually made 
 upon that fluid. Indeed several of my experiments upon dew 
 were only imitations of some which had been previously made 
 upon hoarfrost by that ingenious and most worthy man. 
 
 * Meteor. Lib. I. c. x. 
 
24 ESSAY ON DEW, ETC, 
 
 SECTION II. 
 
 Of the Cold connected with the Formation of Dew. 13 
 
 DEW is often spoken of as being 'cold, by popular writers. 
 Thus Cicero and Virgil apply to it the epithet of " gelidus," 
 Milton that of " chill," and Collins that of " cold." Of the 
 same import is a passage in Herodotus, in which it is said, 
 that in Egypt the crocodile passes a great part of the day on 
 dry land, but the whole of the night in the Nile, this being 
 warmer than the atmosphere and the dew. Among philoso- 
 phers, however, Mr. Wilson was the first, I believe, who ever 
 suspected the existence of such a conjunction. u 
 
 In my experiments on the temperature of bodies moist- 
 ened with dew, small thermometers were employed (the 
 largest being only 8 inches long) having globular bulbs, 
 which in most of them were not more than from 2 to 2J lines 
 in diameter. Their scales, which were marked in the manner 
 
 13 All the writers and experimentalists on dew, preceding Wells, considered 
 the cold as the effect of the dew instead of its being the cause. Patrick 
 Wilson, however, appears to have been close upon a true conviction on the 
 subject, for in giving the account of his experiments in 1780 he said, it would 
 be going too far to conclude " that very cold air is never disposed to deposit its 
 contents except upon bodies as cold, or colder than itself; and yet that this is fre- 
 quently the case seems probable from a number of common appearances." 
 
 14 That Wilson was the first philosopher who suspected that dew was 
 colder than the atmosphere may be questioned. The Hon. Robert Boyle's 
 essay entitled "Experimental History of Cold," published in 1665, contains 
 evidence that he had experimentally shown that dew and hoarfrost are formed 
 by the precipitation of aqueous vapour contained in the air upon a colder 
 body; he, however, appears never to have realised the resemblance between 
 the production of dew artificially upon a cold body, and its natural formation 
 upon the cold surface of the earth at night. 
 
ESSAY ON DEW, ETC. 25 
 
 of Fahrenheit, were of ivory or wood, and were furnished, 
 almost all of them, with hinges. They were always employed 
 naked, except I wished to know the effect of covering them 
 with any particular substance. 15 
 
 By means of these instruments I have very many times, 
 during serene and still nights, examined the temperature of 
 dewed grass, and have constantly observed it to be less than 
 that of the air, anywhere between 1 inch and 9 feet above the 
 ground, the latter being the greatest height at which I 
 ever marked the heat of the atmosphere in these experiments. 
 I generally, however, compared the temperature of dewed 
 grass with that of the air 4 feet above the ground ; and on 
 nights that were calm and clear very frequently found the 
 grass at the ordinary place of my observations 7, 8, or 9 
 degrees colder than the air at that height. Several times it 
 was 10 and 11 colder than the air, and once 12. These 
 differences are not so great as those related in Mr. Six's post- 
 humous work. But in his experiments the temperature of 
 grass was compared with that of the air 7 feet above the 
 ground, which, in clear and calm nights may be regarded as 
 half a degree warmer than the air at the height of 4 feet. 
 Besides, the most considerable differences mentioned by Mr. 
 Six, occurred in winter, when he says a greater degree of cold 
 is occasioned by dew than at any other time, whereas very 
 few of my experiments on the temperature of grass were 
 instituted in that season. In the last place my experiments 
 
 15 During the last fifty years improvements have been effected in the con- 
 struction of thermometers, as in other things. Such instruments as Wells 
 employed would now be deemed ill-adapted for investigations upon nocturnal 
 chilling and the phenomena of dew. It would appear that they were gra- 
 duated only on their frames. The improvements of engraving the scale on 
 the stem itself and of enamelling white the tube behind the scale, have 
 rendered the thermometer an instrument of the utmost precision if skilfully 
 made. J. Glaisher, F.E-.S., was the first to employ really good thermometers 
 for the purpose of prosecuting experiments similar to those of Wells. 
 
ESSAY ON DEW, ETC. 
 
 were almost always made on very short grass, while Mr. Six's 
 thermometers were laid upon long grass bent by strong pres- 
 sure towards the earth, in which state they marked a 
 temperature 1, 2, and 3 degrees lower than that shown by 
 similar thermometers placed upon grass less than an inch in 
 height. 16 Had it not been for these circumstances and the 
 unfitness in various respects, besides the shortness of the 
 grass, for the production of a great cold, of the common scene 
 of my operations, I believe that in consequence of my thermo- 
 meters being much better adapted to mark a superficial or 
 transitory cold, than those of Mr. Six, I should at some time 
 have seen a difference several degrees greater than the 
 greatest ever seen by that gentleman, which was one of 13 . 
 In confirmation of this opinion I shall mention, that having, 
 during a short visit to a more distant part of the country, 
 exposed in the evening a thermometer upon the surface of 
 an open grass field, I found it soon after, although the 
 grass was short and the weather warm, 14 lower than a 
 similar thermometer suspended in the air, 4 feet above the 
 grass. If to this quantity be added i a degree, on account 
 of the difference in elevation between our suspended thermo- 
 
 16 James Six, F.R.S., writing in 1783, said : " On green turf where the 
 grass is not an inch long, the diminution [of temperature] will be less on some 
 nights by 1, 2, or 3, than where the grass is longer, and accordingly as the 
 cold is more or less intense." 
 
 The radiating power of grass appears from Mr. Glaisher's experiments to 
 be such that a thermometer when placed on long grass reads less than when 
 placed on short grass by 1 0< 1 ; that the temperature of the soil under long 
 grass exceeds that under short grass by 1 0> 1 ; being exactly the same amount 
 of excess under as it was in defect on the top. " Hence the cause of the 
 difference of the readings on the top of long and short grass arises solely 
 from the greater quantity of heat conducted to the surface of the latter from 
 the soil, over that conducted to the surface of the former, and not from the 
 greater quantity of heat radiated into space from the long, over that 
 radiated from the short ; such being the case, it was to be expected that the 
 readings of a thermometer would vary with every variation of the length of 
 grass upon which it was placed, and such was found to be the case." 
 
ESSAY ON DEW, ETC. 27 
 
 meters, the cold connected with dew observed by me this 
 night on grass will exceed the greatest ever observed by Mr. 
 Six by 1 degree. 
 
 According to a few observations made by me, the greater 
 coldness of grass than that of the air, begins to appear in 
 clear and calm weather, in places sheltered in the afternoon 
 from the sun, but still open to a considerable portion of the sky, 
 soon after the heat of the atmosphere has declined. A similar 
 coldness continues upon grass in still and serene mornings 
 for some time after the rising of the sun, in places shaded from 
 its direct light but otherwise open to the sky. My experi- 
 ments on this point have also not been many, and none of them 
 were made in winter ; which, I presume, are the reasons that 
 I never observed a cold from this cause later in the morning 
 than an hour after sunrise. The surface of snow, however, 
 was once, in the depth of winter, observed by Mr. Wilson, 17 
 
 tf In January, 1768, Professor Alexander Wilson noticed that a thermo- 
 meter placed on snow marked a temperature 8 lower than when suspended 
 in the air, and that the mercury always rose a trifle when a mistiness came 
 on, and vice versa. (Paper in Phil. Trans., 1771.) His son, Prof essor Patrick 
 Wilson, afterwards took up the investigation of this phenomenon ; and to him 
 must be assigned the credit of having been the first to tecognise the im- 
 portant fact that the ground at night, during clear weather, is colder than 
 the superincumbent air. Tomlinson says that the following interesting 
 Table, made by P. Wilson, was the first of the kind ever published : 
 
 Thursday Evening, January 13th, 1780. 
 
 Therm, in Air. 
 
 
 2 
 
 4 
 
 6 
 
 6 
 
 8 
 
 7 
 
 8 
 
 9 
 
 8 
 
 9 
 
 10 
 
 12 
 
 12 
 
 r- 12 
 
 
 Time. 
 
 Cherm. on Snow. 
 
 8h 
 
 . 30m. 
 
 12 
 
 9 
 
 
 14 
 
 10 
 
 
 14 
 
 11 
 
 
 17 
 
 11 
 
 30 
 
 18 
 
 12 
 1 
 
 30 Friday 
 Morning 
 
 20 
 23 
 
 1 
 
 30 
 
 22 
 
 2 
 
 
 22 
 
 2 
 
 30 
 
 21 
 
 3 
 
 
 . . . 
 
 3 
 
 30 
 
 . . . 
 
 4 
 
 
 . . . 
 
 A 
 
 30 
 
 
 5 
 
 
 . 
 
28 ESSAY ON DEW, ETC. 
 
 of Glasgow, to be considerably colder than the air, till a little 
 after midday.* 18 
 
 In cloudy nights, particularly if there was wind, the grass was 
 never much colder than the air. On such nights the tempera- 
 tures of both were sometimes the same ; at other times that 
 of the grass was the higher of the two, even when the grass 
 was wet from preceding rain, and when consequently it must 
 have been in some measure cooled by evaporation. On one 
 such night the grass was found to be 4 colder than the earth 
 an inch beneath the surface of the plat, which afforded a 
 sufficient reason for the grass itself being warmer than the 
 air. In windy weather, however, if the sky was clear some 
 degree of cold in addition to that of the air was always 
 observed upon the grass; and in calm weather very high 
 
 5 30 
 
 
 12 
 
 6 
 
 
 14 
 
 6 30 
 
 7 
 7 30 
 8 
 
 22 
 22 
 22 
 
 19 
 
 13 
 13 
 13 
 10 
 
 " Wilson soon found that the cause of these remarkable differences could 
 not be attributed to the evaporation of the snow, seeing that hoar-frost was 
 being abundantly deposited at the time when the readings were lowest. On 
 the night of the 23rd January ' several things were laid out at the Observatory, 
 such as sheets of brown paper, pieces of boards, plates of metal, glasses of 
 several kinds, &c., which all began to attract hoar-frost, seemingly as soon as 
 they had time to cool down to the temperature of the air. The sheets of brown 
 paper, being thin and easily cooled, acquired it soonest, and when beheld by 
 candlelight were beautifully spangled over by innumerable reflections from 
 the minute crystals of hoar-frost which had parted from the air.' 
 
 " If, in the above passage, the writer, instead of the little word to, which 
 I have printed in italics, could have written the word below, he would 
 have given the true theory of the formation of dew and hoar-frost." 
 Tomlinson. 
 
 * Paper in Phil. Trans. 1781. 
 
 18 The observations were taken on 25th January, 1781, and are as 
 follows : 
 
 Time. Thermometer in Air. Thermometer on Snow. 
 
 9h. 45m. A.M. 10 3 
 
 10 30 14 4 
 
 11 30 17 9 
 
 12 30 P.M , cloudy 22 20 
 1 more cloudy 25 
 
 1 30 cloudy all over 27 27 
 
ESSAY ON DEW, ETC. 29 
 
 clouds, though sufficiently extensive and dense to conceal 
 the sky completely, would yet frequently allow of the grass 
 being several degrees colder than the air. 19 I once observed 
 upon a night of this kind a difference of 5 between the 
 temperatures of those bodies. 
 
 If the night became cloudy after having been very clear, > 
 though there might be no change with respect to calmness, a 
 considerable alteration in the temperature of the grass always j 
 ensued ; and this sometimes very suddenly. Upon one such 
 night the grass, after having been 12 colder than the air, 
 became only 2 colder than it, the temperature of the 
 air being the same at both observations. On the second 
 night, grass became 9 warmer in the space of an hour and 
 a half. On a third night, in less than forty-five minutes 
 for the whole change occurred while I was absent forty-five 
 minutes, the temperature of the grass rose 15, while that 
 of the neighbouring air increased 3. During a fourth 
 night the temperature of the grass at half-past 9 o'clock, 
 was 32. In twenty minutes afterwards it was found to be 
 39, the sky having in the mean time become cloudy. At 
 the end of twenty minutes more, the sky being clear, the 
 temperature of the grass was again 32. These were the 
 most remarkable of my observations on this subject ; but I 
 may add to them, that I have frequently seen, during nights 
 thafc were generally clear, a thermometer lying on the grass- 
 plat rise several degrees, upon the zenith being occupied only 
 a few minutes by a cloud. On the other hand, upon two 
 nights I observed a very great degree of cold to occur on 
 the ground, in addition to that of the atmosphere, during 
 
 19 By virtue of the law of the reciprocal radiation of bodies, " the clouds 
 return to the earth the whole, or a great part, of the heat which may have 
 been radiated from it; although dense clouds may sometimes be formed so 
 high as to be colder than the earth, and thus to radiate to it less heat than 
 the ground imparts; accounting, therefore, for the phenomenon sometimes 
 observed on cloudy nights of good radiating bodies being several degrees 
 colder than the air." Harvey. 
 
30 ESSAY ON DEW, ETC. 
 
 short intervals of clearness of sky, between very cloudy 
 states of it. 20 
 
 I did not speak in the preceding section of another ob- 
 scure state of the atmosphere that occasioned by fog, or 
 mist as the moisture deposited in it attaches to all bodies 
 indiscriminately; on which account I was unable to deter- 
 mine whether or not dew forms during its continuance. But, 
 with respect to the connexion of this condition of the atmos- 
 phere with cold, I have to remark that I have several times, 
 on its appearance betwixt daybreak and sunrise, found the 
 difference between thermometers on grass and in the air, 
 which had been considerable during the night, to diminish 
 greatly. I never, indeed, observed it to vanish ; but this I 
 used to impute to the air being not very much obscured. 
 I have now, however, reason to doubt the justness of this 
 conclusion ; for on the evening of the 1st of January in the 
 present year, 1814, I found, during a dense fog, while the 
 weather was very calm, a thermometer lying on grass, thickly 
 covered with hoarfrost, 9 lower than another suspended in 
 the air, four feet above the former. On the following even- 
 ing, when the air was equally calm, but the fog sufficiently 
 attenuated to allow me to see that the sky was almost en- 
 tirely covered with clouds, the difference between two ther- 
 mometers, similarly placed with the former, was only 1. On 
 
 20 By tabulation of numerous observations, Mr. Glashier says, " it ap- 
 pears that at times when the sky was entirely covered with low cirrostratus 
 clouds, the readings of a thermometer placed on long grass was the same as 
 that in the air; that, with the same clouds at a moderate elevation, the 
 reading of the thermometer in air has exceeded that on long grass by 3; 
 and on those clouds being high, this excess has amounted frequently to 5; 
 and if other clouds than cirrostratus covered the whole sky, this excess has 
 been as large as 10. At times when the sky has been free from clouds, but 
 not bright, haze and vapour being prevalent, the above excess has amounted 
 to 10, 11, or 12 ; and at times when the sky has been both bright and clear, 
 with the air calm, no mist, haze, vapour, or fog being prevalent, this dif- 
 ference has frequently amounted to 14, less frequently to 19, and some- 
 times to 20." 
 
ESSAY ON DEW, ETC. 31 
 
 comparing the observations of these two evenings, I conclude 
 that, on the first, few or no clouds existed above the fog, and 
 consequently that fog, if there be no clouds above it, may, 
 in a very calm air, admit of the appearance of a considerable 
 degree of cold at night upon the surface of the earth, in 
 addition to that of the atmosphere. Mr. Six, indeed, says, 
 while speaking of the cold connected with dew, in his paper 
 in the Philosophical Transactions for 1788, "fogs did not, as 
 far as I could perceive, at all impede, but rather increase, 
 the refrigeration." But this was a mistake, which in all 
 probability arose from his ascribing the effect of a clear night 
 to an ensuing foggy morning, as he examined his thermo- 
 meters only in the daytime. He afterwards discovered his 
 error; for, in his posthumous work, thick fogs are ranked 
 among the circumstances which always impede, and some- 
 times prevent altogether, the appearance of a cold upon the 
 surface of the earth, greater than that of the atmosphere. 
 During a very dense fog, Mr. Wilson found no difference at 
 night between a thermometer laid upon snow and another 
 suspended in the air.* 
 
 When, during a clear and still night, different thermo- 
 meters were examined at the 'same time, which had been 
 placed in different situations, those which were situated where 
 most dew was formed were always found to be the lowest. 21 
 Thus, upon one such night I found a thermometer placed 
 upon a little wool, lying upon the middle of the upper side 
 of the raised board, to be 9 lower than another thermo- 
 meter in contact with an equal quantity of wool attached to 
 the middle of the underside of the board. On two other 
 nights the difference between two thermometers in the same 
 situations was 8. I found also, on two other serene and 
 calm nights, a spot of grass covered by the pasteboard roof, 
 
 * Edin. Phil. Trans. I. 170. 
 
 21 " Starlight nights, yea, and bright moonshine nights, are colder than 
 cloudy nights." Lord Bacon. 
 
32 ESSAY ON DEW, ETC. 
 
 and another spot surrounded by the earthen cylinder, to be 
 both 10 warmer than neighbouring grass fully exposed to the 
 sky. Thinking it possible that the cylinder, which had been 
 exposed to the sun the preceding day, might still possess 
 some of the heat which it had then imbibed, I placed near to 
 it, on another night, a cylinder made of very thin paste- 
 board ; but this was equally efficacious with the earthen one 
 in preventing cold from occurring on grass. When the ex- 
 posure was greater than in the preceding examples, and more 
 dew was in consequence formed, the cold was also greater, 
 but still less than where the exposure was complete. 22 For 
 instance, upon the night during which 10 grains of wool, 
 placed upon the middle of the grass, which was sheltered by 
 the raised board, had gained 7 grains, and the same quantity 
 on grass fully exposed to the sky had gained 10 grains, the 
 difference between the temperatures of the two portions of 
 grass was only 2i. 
 
 The same correspondence was observed when the dif- 
 ferences in the quantity of dew did not depend, as in the 
 
 22 "I placed a large sheet of pasteboard vertically on tlie grassplat, and 
 laid a thermometer close to its lower edge on each side of it ; the readings 
 of both these thermometers thus placed were found to be identical and 
 intermediate to that placed in air at the height of four feet, and protected 
 from the effects of radiation, and that placed on grass fully exposed to the 
 sky ; the same relation was found to exist in what azimuth soever the board 
 was placed. 
 
 " I then placed the pasteboard at an angle of 45 nearly with the horizon, 
 and laid the thermometers as before. In this situation the readings were 
 found to be intermediate as before ; but that which was exposed to three- 
 fourths of the sky read most nearly to that on the grass fully exposed to the 
 sky, and the one which was exposed to one-fourth part of the sky only, read 
 most nearly to that in air, The amount of these differences of readings 
 was as nearly as could be d etermined exactly proportional to the amount of 
 the exposed sky. 
 
 " Hence, as a general fact if may be considered, that whatever diminishes 
 the view of the sky as seen from an exposed body, causes its temperature to 
 decrease less than it would if the exposure to the sky be complete." 
 Glashier, Phil Trans. 1847. 
 
ESSAY ON DEW, ETC. 33 
 
 preceding instances, upon any diversity of exposure to the 
 sky. Thus, the mercury in a thermometer placed upon wool, 
 lying on the raised board, was found to be at the 44th degree, 
 while that in another, pendent in the air, at the same height 
 from the ground, and wrapped in wool, was at the 48th. 
 Wool also, on the raised board,* was commonly a little colder 
 than the same substance on grass, when the night was very 
 still ; and the leeward end of that board was generally colder 
 than the windward extremity. 
 
 But the most remarkable examples of this kind were 
 exhibited by the gravel walk and the bare garden mould. 
 In still and serene nights the surfaces of these bodies were 
 always warmer than the neighbouring grass, and frequently 
 warmer than the air. On one night of this description I 
 observed, two hours and a half after sunset, the surface of 
 the gravel walk to be 16i, and that of the garden mould to 
 be 124 warmer than grass very near to them; and similarly 
 exposed to the heavens. As the night proceeded clouds 
 formed and accumulated; in consequence of which the dif- 
 ference at sunrise, between the temperature of the grass and 
 the gravel walk was only 6, and between those of the grass 
 and the mould only 4, the temperature of the grass having 
 in the mean time increased considerably, while that of the 
 other bodies had decreased a little. At another time, shortly 
 before sunrise, a very clear morning having succeeded a 
 cloudy night, I found the gravel walk to be 10 and the 
 garden bed to be 9 warmer than neighbouring grass, which 
 
 * The greater cold of the raised board, in my experiments, most probably 
 depended on the grass being very short ; since Mr. Wilson found that snow 
 on the ground was colder than the same body on a raised board. If one, 
 two, or three degrees were added to the cold of the grass at my place of 
 observation, agreeably to the difference found by Mr. Six, between the tem- 
 peratures of long and short grass in dewy nights, the cold on my raised 
 board would, upon such night?, have been always less than that of the grass- 
 plat. 
 
 P 
 
34 ESSAY ON DEW, ETC. 
 
 was 8 colder than the air. Both of these examples occurred 
 in summer, and I believe that such considerable differences 
 will occur in that season only. It was on the first of these 
 two nights that 10 grains of wool gained only half a grain of 
 moisture on the mould, and that the same quantity gained 
 no weight on the gravel walk. That the uiifitness of the 
 gravel walk, however, to become cold, like its unfitness to 
 attract dew, arose from its situation, and not from the nature 
 of the substance of which it was made, is proved by this cir- 
 cumstance that river sand, placed on the raised board, was 
 on four different nights, none of them highly favourable for 
 the production of cold, 7, 7, 8 and 8J degrees colder than the 
 air at the same height. 
 
 It may be added here, that I have always found, on dewy 
 nights, the temperature of the earth, half an inch or an inch 
 beneath its surface, much warmer than the grass upon it. 
 On five such nights the differences were from 12 to 16 
 degrees. The earth, at the above-mentioned depth, was also 
 almost constantly warmer on dewy nights than the air ; 
 sometimes it was considerably so, for I once observed it to be 
 10 warmer, at another time 9, and at a third 7J. An 
 exception will no doubt occur, if very mild weather should 
 follow a long frost ; but of this I have had no experience. 
 
 In the experiments upon my housetop in London, I 
 always found, during clear and calm nights, wool lying on 
 the wooden frame to be colder than the air at the same 
 height ; but the difference was seldom more than 3. On 
 the evening, however, during which dew formed there more 
 copiously than usual, the difference was 5. That the small- 
 ness of these differences was not wholly occasioned by any- 
 thing special in the air of cities was afterwards proved by my 
 finding others much greater, in a garden nearly in the 
 middle of London, from which almost the whole of the sky 
 was visible. 
 
ESSAY ON DEW, ETC. 35 
 
 Metals, likewise, furnish proofs of the connexion of dew 
 with a cold in the substance on which it forms, superior to 
 that of the neighbouring atmosphere. My observations, 
 however, on the temperature of metals when exposed to the 
 sky on dewy nights, were less numerous than those on 
 several other subjects treated in this Essay, by reason of the 
 less frequent opportunity I enjoyed of making them ; and 
 many of those which I did make were afterwards found by 
 me to have been improperly conducted. I thought, for 
 instance, for some time that the temperature of a metal on a 
 dewy night might easily be learned in the way in which I 
 had been accustomed to ascertain the temperature of dewed 
 grass. But, observing dew one night on the glass tube of a 
 thermometer, which was lying on a metal placed upon grass, 
 while the metal itself was free from moisture, I conceived it 
 probable that the cold then indicated by the thermometer 
 was not the real temperature of the body to which it was 
 applied. To determine the point, I placed on the same 
 metal a second thermometer, covered with gilt paper, upon 
 which this was found at three observations to be 6J, 7, and 
 7 higher than the other .* In this experiment, the bulb of 
 the naked thermometer, from being very small, did not 
 project as far as the outer surface of the scale, and, conse- 
 quently, did not come in contact with the metal. But, even 
 when the ball of a thermometer was applied directly to a 
 metal on a clear and calm night, a temperature was marked 
 by it, commonly 2 aiid 3, and sometimes more degrees less 
 than that marked by a similar thermometer enclosed in gilt 
 paper, and similarly placed. I found it likewise necessary, n ' 
 in this enquiry, to correct the temperature of the air, as 
 given by a naked thermometer. For, on still and serene 
 nights, a thermometer inclosed in a case of gilt or silvered 
 paper, and suspended in the air 4 feet above the grassplat, 
 
 * Page 42. 
 
 D2 
 
36 ESSAY ON DEW, ETC. 
 
 was usually observed to be 1J or 2 higher than a bare ther- 
 mometer of the same construction suspended near to it. The 
 difference of two such thermometers thus placed was once 
 observed by me to be 2J, and once 3i. 23 It may be thought, 
 perhaps, that these differences were caused by the metalled 
 case obstructing the transmission of the temperature of the 
 air to the enclosed instrument. But that this was not the 
 reason is shown by my observing that on cloudy nights there 
 existed no difference between the two thermometers; that, 
 even on clear nights, a thermometer contained in a case of 
 white paper, somewhat thicker than the metalled, was always 
 nearly of the same temperature with a naked one which was 
 suspended close to it ; and that, when a difference did exist 
 
 23 " The formation of dew was found to depend solely on the temperature 
 of the bodies upon which it was deposited, and it never appeared upon them 
 till their temperatures had descended below that of the dew-point in their 
 locality, as found by observations of a dry and wet-bulb thermometer placed 
 in their vicinity. 
 
 " The amount of water thus deposited was the greatest upon the sub- 
 stances whose temperatures were the lowest ; among these bodies glass was 
 found to radiate heat freely, and it very readily became wet with dew. In 
 consequence of this property the tube of a naked thermometer, which was 
 lying on a substance entirely free from moisture, was frequently found 
 covered by dew, and therefore it seemed probable that the temperature exhi- 
 bited by the instrument was not that of the body in question. On such 
 instances occurring an attempt was made to correct the error by enclosing 
 the thermometer stem in a tube made of gilt paper; the bulb alone, resting 
 on the substance, remained exposed to the sky. The differences between 
 the readings of a thermometer thus enclosed, and when naked, were found 
 to be sensible, but small in amount. It was observed that when the thermo- 
 meter was wholly naked, the stem was at times wet when the bulb itself was 
 dry ; and at all times much less moisture appeared on the bulb than on the 
 stem, unless the disposition of the substance in question to become cold was 
 the same [as] or greater than, that of glass. The error arising from this 
 cause was chiefly confined to the consequent contraction of the mercury in 
 the stem, and not in the bulb, and which was considered to be avoided by 
 the use of gilt paper : the error in all cases must have been small/' 
 Glaisher, Phil. Trans., 1847. 
 
ESSAY ON DEW, ETC. 37 
 
 between the two latter, the thermometer in the white paper 
 case wa,s commonly lower than the other. 24 
 
 The estimation of the heat, both of air and of metals, on 
 a dewy night, is liable to errors from other causes. As 
 these, however, are trifling, I shall not mention them, but 
 proceed to state the results of my observations upon the 
 temperature of metals exposed to the sky at night, though 
 unable to vouch for their entire accuracy. 
 
 Thin bright metallic plates, the least having a surface of 
 25 square inches, and some of them a surface of more than 100 
 such inches, were several times observed, while lying on grass 
 which was attracting dew, to be 1 and 2, and once 3 degrees 
 warmer than the air 4 feet above them. At other times 
 their temperature was the same with that of the air. In both 
 of these cases their upper surfaces were always free from dew. 
 Metals thus situated were, consequently, often much warmer 
 than the grass which surrounded them. I made no experi- 
 ments on this point during the nights on which occurred the 
 greatest instances of cold on grass, relatively to the tempe- 
 rature of the air ; but I found, notwithstanding, during one 
 night, a metal on grass to be 10 warmer than the exposed 
 grass near to it. On two other nights the differences were 
 9 and 8. The superiority of the heat of metals on grass 
 over that of the air, when it did exist, was evidently con- 
 nected with the temperature of the grass which they covered, 
 
 24 " A glass thermometer, suspended in the air, will not give the tempe- 
 rature of the air; its own power as a radiant or an absorbent comes into 
 play. On a clear day, when the sun shines, the thermometer will be warmer 
 than the air; on a clear night, on the contrary, the thermometer will be 
 colder than the air." Tyndall. 
 
 " There is no doubt that if a thermometer be freely suspended in the air 
 with its bulb at the height of thirteen feet above the soil, and far from any 
 object to reflect heat to it, its readings will represent the true temperature 
 of the air at the time, and much more truly than those of any one placed 
 near the ground, or within a few feet of walls or buildings." Glaisher, Phil. 
 Trans., 1847. 
 
38 
 
 ESSAY ON DEW, ETC. 
 
 and this again with that of the earth under the same portion 
 of "grass; for this portion was always a little warmer than 
 the metal, but not so warm as the earth. 
 
 On the other hand, metals on which dew was forming 
 while they lay upon grass were always colder than the air. 
 In like manner, if one metal upon the grassplat were dewed, 
 while another similarly situated remained dry, the former was 
 always colder than the latter. 
 
 When a metal lying on the grassplat became dewed,- the 
 grass under it was always colder than that under another 
 metal which was undewed. 
 
 A metal, while receiving dew, in consequence of being 
 elevated in the air, was always colder than a similar metal 
 which remained undewed on the grass. 
 
 The greatest instances of cold observed by me on metals 
 occurred at times when other bodies near to them had 
 become considerably colder than the atmosphere. 
 
 The cold, however, contracted by metals, from exposure 
 to the sky in a clear and still night, was always less than 
 that of other bodies similarly situated, the greatest excess of 
 cold ever observed by me in the larger metallic plates from this 
 cause over that of the air being not more than 3 or 4 degrees. 
 If much smaller pieces were placed upon grass, the result was 
 different. For I have found a small thermometer placed in 
 this situation, while inclosed in a sheath of gilt paper, to be 
 only 3 less cold than the surrounding grass during a night 
 favourable to the production of cold on the surface of the 
 earth. 
 
 I collected only a few facts respecting the comparative 
 temperatures of different metals when they were exposed 
 together to the sky on dewy nights ; but such as I did 
 collect tend to prove that the most readily dewed metals 
 become colder than the air, sooner than those which receive 
 dew with greater difficulty. 
 
ESSAY ON PEW, ETC. 39 
 
 Many of the experiments which have been mentioned in 
 this section show that, when bodies which had been equally 
 exposed to the night air were examined at the same time, 
 those which were most dewed were also the coldest. No such 
 correspondence, however, was found in the experiments of 
 diiferent nights, or even of different parts of the same night. 
 Thus, during two nights on which grass was 12 and 14 colder 
 than the air, there was little dew ; while on the night which 
 afforded the most copious dew ever observed by me, the cold 
 possessed by the grass beyond that of the air was, for the most 
 part, only 3 and 4 ; and I have always seen less dew about 
 sunset than about sunrise, when the weather has been calm and 
 clear at both times, though there is commonly, in this country 
 at least, a greater difference between the temperature of grass 
 and of air in the evening than in the morning. I had early 
 observed, also, bodies exposed to the sky, on a cloudy but 
 calm night, to be sometimes 2 or 8 colder than the air, 
 without having any appearance of dew ; and when two 
 metals possessing different relations to dew were exposed 
 together, I have seen the one which was the fitter to attract 
 that fluid, colder than the other, though both were dry. 
 
 I shall conclude this part of my Essay with relating the 
 results of some experiments which were made for the purpose 
 of ascertaining the tendencies of various bodies to become 
 cold upon exposure to the sky at night. Unfortunately, the 
 weather was not always favourable to my views ; but what 
 occurred appears to me, notwithstanding, worthy of being 
 related. 
 
 In the observations hitherto given by me on the cold con- 
 nected with dew, the temperature of grass has been chiefly 
 considered, partly because my first experiments had been 
 made upon it, and partly from a wish, which arose alter- 
 
40 ESSAY ON DEW, ETC. 
 
 wards, to compare my own experiments with those of Mr. 
 Six, which had been confined to that substance. I found it, 
 however, very unfit to furnish the means of comparing the 
 degrees of cold produced at night on the surface of the earth, 
 at different times and places ; as its state on different nights, 
 on the same parts of the plat I commonly made use of, and 
 in different parts of the plat on the same nights, was often 
 very unequal in point of height, thickness, and fineness ; all 
 of which circumstances influenced the degree of cold pro- 
 duced by it. 25 I observed, in consequence, a much greater 
 uniformity in the results of experiments made with various 
 other bodies, whose condition, when first exposed to the air, 
 was always the same. Of these the most productive of cold 
 were the filamentous and downy as wool of moderate fine- 
 ness, very fine raw silk, very fine unspun cotton, fine flax, 
 and swansdown, all of which were not only more steadily cold, 
 upon clear and calm nights, than grass, but also gave rise 
 to a greater degree of cold than was almost at any time 
 / observed upon it, even in its best state. Among the bodies 
 of this class wool produced the least cold, and I formerly 
 
 25 Notwithstanding this assertion, and the confirmation given to it by 
 the experiments made by Glaisher, Melloni, and, indeed, every meteorologist 
 subsequent to Wells, the observations on nocturnal radiation from the 
 earth's surface continue to be made by expositing naked thermometers on 
 grass. It is in every respect highly desirable that some settled plan should 
 be followed by meteorologists by which observations on radiation from the 
 sun, or from the earth, may be obtained which will admit of comparison. 
 " The readings of thermometers placed on grass being found so variable, the 
 unfitness of it to furnish the means of comparing the degrees of cold at night 
 on the surface of the earth was evident. A much greater uniformity was 
 observed in the results of experiments made with other substances, which 
 were bad conductors of heat, and whose condition was always the same." 
 Glaisher, Phil. Trans., 1847. 
 
 Probably glass of similar quality to the material of the thermometer would 
 give the desired uniformity in the method of observing, and would furnish 
 comparable results. It might be used as a slab, 20 inches long, 10 wide, and 
 a quarter of an inch thick, and kept clean on its upper surface. 
 
ESSAY ON DEW, ETC. 41 
 
 mentioned that it attracted less dew than silk, cotton, and 
 flax. The last-mentioned substances, and swansdown, were 
 found equal, or nearly so, in their tendency to become cold. 
 Swansdown, however, exhibited the greatest cold rather more 
 frequently than any of the rest ; on which account, and from 
 its being more easily managed, as it was used while adhering 
 to the skin of the bird, I at length scarcely ever employed 
 any other body of the same class. On the night during 
 which grass was observed to be 14 colder than the air, 
 swansdown, lying upon a neighbouring piece of grass, was 
 still one degree lower. This difference of 15 between the 
 temperature, at night, of a body on the surface of the earth 
 and that of the air, a few feet above the earth, is the greatest 
 which I have hitherto seen. 
 
 Fresh unbroken straw and shreds of white paper, 
 though not properly to be ranked among filamentous sub- 
 stances, were also found to be a little more productive of 
 cold than the wool which I used. 
 
 The next class consisted of bodies in the state of a powder 
 more or less fine. These were clean, river sand, glass, chalk, 
 charcoal, lampblack, and a brown calx of iron. Chalk pro- j 
 duced the least, and the three last substances the greatest 
 cold. They were, all, however, inferior in this respect to 
 bodies of the first class. 
 
 Solid bodies having a surface exposed to the sky of at 
 least 25 inches square formed a third class, on which such 
 experiments were made. The particular substances of this 
 description subjected to trial were glass, brick, cork, oak- 
 wood, and wax ; all of which were, likewise, found inferior to 
 the filamentous substances. From these last experiments it 
 follows that when a glass bulb of a thermometer is applied 
 at night to a body exposed to a clear sky, the temperature 
 exhibited by the instrument will not be accurately that of the 
 body in question, except the disposition of the latter to 
 
42 ESSAY ON DEW, ETC. 
 
 become cold in such a situation be the same as that of glass. 
 An example of this fact has been given in this Essay.* 
 
 My principal experiments, however, of this kind were 
 made with snow. 
 
 On the 25th of January, 1813, the ground being then 
 covered with snow about an inch deep, I went to my usual 
 place of experiment in the country ; but, during 8 hours that 
 I attended to my thermometers, the whole sky was constantly 
 overcast with clouds. The atmosphere was, for the greater 
 part of that time, very still, and a thermometer on the snow 
 was generally about 2 lower than another in the air. That 
 this difference was not owing to evaporation was proved by 
 the thermometer on the snow always rising from a half to a 
 whole degree whenever the air was a little moved, and falling 
 the same quantity as soon as a great stillness again took 
 place. 
 
 I had no opportunity of renewing my observations upon 
 snow before the beginning of the present year, 1814. The 
 state of my health rendering it improper that I should incur 
 much fatigue, or be long exposed to night air, I restricted 
 myself to the making a few experiments in the large garden 
 in Lincoln's Inn Fields. I went thither, for the first time, 
 on the evening of the 4th of January, immediately after 
 a considerable snowfall had ceased, wishing to begin my 
 observations before any cold should arise on the snow's 
 surface from exposure to the sky. This was desirable on 
 another account; for Mr. Kirwan, in direct opposition to 
 indisputable facts most clearly stated by Mr. Wilson, had 
 said that the great cold observed by that gentleman on snow 
 was occasioned by this substance having retained the tempe- 
 rature of the high region from which it had fallen. f The 
 result of my enquiry was that the surface of the snow and 
 
 * Page 35. t On Temperatures, p. 30. 
 
ESSAY ON DEW, ETC. 48 
 
 the air 4 feet above it had precisely the same heat. The 
 depth of the snow was 4 inches. 
 
 My next experiment took place on the evening of the 6th 
 the intervening day having been snowy. The sky was clear, 
 but the air had a considerable motion. The heat of the 
 atmosphere, at the height of four feet, was at 9J h. 26 ; 
 while that of the surface of the snow and of swansdown lying 
 upon it was 22. The depth of the snow was now about 5 
 inches. 
 
 On the 7th, a little after sunset, the heat of the air in the 
 garden was 23, that of the surface of snow 19, but that of 
 swansdown lying upon the snow only 15 . There was then a 
 gentle breeze ; some parts of the sky were covered with 
 clouds, and the lower atmosphere was a little obscure. 
 While the exposed surface of the snow was 19, a part 
 of its surface, which had been covered about 20 minutes 
 with a piece of pasteboard, was 22 . Grass at the bottom 
 of the snow was 31, and the earth an inch beneath the 
 grass, 32. 
 
 After this, there was no fit time for observation until the 
 13th. The thermometers were exposed at 8 h. on the even- 
 ing of that day, the sky being then without clouds ; but the 
 stars were not bright, and there was a perceptible motion in 
 the air. At 8J h. the temperature of the air was 22i, that 
 of the surface of the snow 13, and that of swansdown lying on 
 the snow 8. At 9 h. the air was 23^, snow 17, and swans- 
 down 15, The sky being now, in great measure, covered 
 with high thin clouds, my experiments ceased. At 10 J h. the 
 sky was very bright, and the atmosphere very calm; but it was 
 not then convenient to me to renew my observations. Had I 
 repeated them at that time, I should probably have found a 
 difference between the temperature of the swansdown and 
 air several degrees more considerable than the one of 14 4, 
 
44 ESSAY ON DEW, ETC. 
 
 wliicli had already occurred on this evening, and conse- 
 quently greater than the greatest observed by Mr. Wilson 
 between the temperatures of snow and of the atmosphere, 
 which was one of 16. 
 
 The next favourable evening was that of the 21st. Much 
 snow having in the meanwhile fallen, its depth was now 
 more than a foot. The thermometers were observed 5 times 
 between 4 h. 15 m. and 4 h. 55 m. At 4 of those times the 
 swansdown was 13, and at one of them 13^ colder than 
 the air, the heat of which at the 4 first observations was 26, 
 and at the last 25J. The temperature of the surface of the 
 snow during the whole period of observation was 17, and 
 consequently 4 times it was 4, and once 5, less cold 
 than that of the swansdown. The atmosphere was alto- 
 gether free from clouds and nearly quite calm, but a good 
 deal hazy. 
 
 Before another proper evening arrived my health became 
 so infirm that I was obliged to relinquish this pursuit. I 
 conclude, therefore, my account of it with two remarks. 
 1. If Mr. Wilson had been accustomed to examine the tempe- 
 rature of swansdown, or any similar substance placed upon 
 snow, he would probably have observed a cold on the 
 surface of the earth exceeding that of the atmosphere by 
 20 or more on the night of his actually observing an 
 excess of 16. 2. Since upon one evening, when the atmos- 
 phere was neither very clear nor very still, a difference of 
 ] 4i was found by me between the temperatures of air and of 
 swansdown, which is only J a degree less than the greatest 
 difference I have ever observed between the same' substances 
 on the stillest and clearest nights in summer, a corroboration 
 is hence derived of a conclusion made by Mr. Six from his 
 experiments, that the greatest differences at night, in point 
 of temperature, between bodies on the surface of the earth 
 
ESSAY ON DEW, ETC. 45 
 
 and the atmosphere near to it are those which take place in 
 very cold weather. 26 
 
 se i< When a radiator is exposed to a clear sky, it tends to keep a certain 
 therm ometric distance if I may use the term between its temperature 
 and that of thp surrounding air. This distance will depend upon the energy 
 of the radiator, but it is to a great extent independent of the temperature 
 of the air. Thus, M. Pouillet has proved that in the month of April, when the 
 temperature of the air was 38 0< 5 F., swansdown fell by radiation to 25'7; the 
 whole chilling effect, therefore, was 12'8. In the month of June, when the 
 temperature of the air was 64, the temperature of the radiating swansdown 
 was 41; the chilling of swansdown by radiation is here 13, almost precisely 
 the same as that which occurred in April. Thus, while the general tempe- 
 rature varies within wide limits, the difference of temperature between the 
 radiating body and the surrounding air remains sensibly constant. 
 
 " These facts enabled Melloni to make an important addition to the 
 theory of dew. He found that a glass thermometer, placed on the ground, 
 is never chilled more than 3'6 F. below an adjacent thermometer, with sil- 
 vered bulb, which hardly radiates at all. These 3'6, or thereabouts, mark the 
 thermometric distance above referred to, which the grass tends to preserve 
 between it and the surrounding air. But Six, Wilson, Wells, Parry, Scoresby, 
 Glaisher, and others, have found differences of more than 18 F. between a 
 thermometer on grass and a second thermometer hung a few feet above the 
 grass. How is this to be accounted for ? Very simply ; according to Mel- 
 loni, thus : The grass blades first chill themselves by radiation, 3 0< 6 F. 
 below the surrounding air : the air is then chilled by contact with the grass, 
 and forms around it a cold aerial bath. But the tendency of the grass is to 
 keep the above constant difference between its own temperature and that 
 of the surrounding medium. It therefore sinks lower. The air sinks in its 
 turn, being still further chilled by contact with the grass ; the grass, how- 
 ever, again seeks to re-establish the former difference ; it is again followed > 
 by the air, and thus, by a series of actions and reactions, the entire stratum I 
 of air in contact with the grass becomes lowered to a temperature far below 
 that which corresponds to the actual radiative energy of the grass." 
 Tyndall, "Heat as a Mode of Motion." 
 
 It does not appear that the observations recorded by Wells support 
 Melloni's addition to the theory ; and, indeed, before the notion of a " ther- 
 mometric distance " can be accepted as a fact, the matter must be rouoh / 
 more carefully observed and worked out. 
 
PART II. 
 
 ON THE THEORY OF DEW. 
 
 , according to Aristotle*, is a species of rain, formed 
 in the lower atmosphere, in consequence of its moisture 
 being condensed by the cold of the night into minute drops. 
 Opinions of this kind respecting the cause of dew are still 
 entertained by many persons, among whom is the very inge- 
 nious Mr. Leslie of Edinburghf. A fact, however, first 
 taken notice of by Gersten, who published his treatise on 
 dew in 1733, proves them to be erroneous ; for he found that 
 bodies a little elevated in the air often become moist with 
 dew, while similar bodies lying on the ground remain dry, 
 though necessarily, from their position, as liable to be wetted 
 by whatever falls from the heavens as the former. 
 
 Shortly after the appearance of Gersten's treatise, Mus- 
 schenbroek made the remark, already mentioned in this 
 Essay, that metals will be free from dew while other bodies 
 attract it copiously. This philosopher contented himself 
 with publishing his discovery; but his friend Dufay con- 
 cluded from it that dew is an electric phenomenon, since it 
 leaves untouched the bodies which conduct electricity, while 
 it appears upon those which cannot transmit that influence. 
 If dew, however, were to form on the latter only, its quantity 
 would never be sufficiently great to admit its being distinctly 
 seen ; for the non-conductors, as soon as they became in the 
 least moist, would be changed into conductors. Charcoal, 
 
 * Meteor. Lib. 1. c. x. et De Mundo, c. iii. 
 f Relations of Heat and Moisture, p. 37 and 132. 
 
ESSAY ON DEW, ETC. 47 
 
 too, it is now known, though the best solid conductor of 
 electricity after the metals, attracts dew very powerfully ; 
 and, in the last place, contrary to the assertion of Dufay, 
 dew frequently forms upon metals themselves. 
 
 Other authors have ascribed the production of dew to 
 electricity, for reasons different to that from Dufay. But 
 there are several considerations which seem to me to prove 
 that no such opinion can be just. 1. When dew is produced 
 in a clear atmosphere, the portion of air by which it is depo- 
 sited must necessarily be unable, at that moment, to retain 
 in a state of pellucid vapour all the moisture which it had 
 immediately before held in that form. But I know of no 
 experiment which shows that air, by becoming positively 
 electrical, which is said to be its condition on the evenings 
 during which dew is most abundant, is rendered less able 
 than it had previously been to contain watery vapour in a 
 state of transparency. 2. Bodies in similar circumstances, as 
 far as electricity is concerned, acquire very different quan- 
 tities of dew. Wool placed on the raised board, for example, 
 attracted very much more dew than wool attached to the 
 lower side of the same board, and even considerably more 
 than the same substance freely suspended in the air and 
 entirely exposed to the sky. 3. Dew forms in different parts 
 of the night in quantities no way proportioned to the degrees 
 of electricity found in the atmosphere at the same times. 
 Thus, it is commonly more copious in the morning than in 
 the evening, notwithstanding that the air is observed to be, 
 in the latter season, more highly electrical than in the 
 former. 4. I have several nights held a glass bottle, upon 
 which dew was forming, close to the top of a Bennett's electro- 
 meter, which had been previously kept in a dry place ; but 1 
 never saw the slips of gold leaf to move in consequence. It 
 is very probable, however, that more refined experiments will 
 show that electrical appearances attend the production of 
 
48 ESSAY ON DEW, ETC. 
 
 dew. These, perhaps, accompany every change in the 
 chemical form of bodies. But the facts which have been 
 stated seem sufficient to establish that any such appearances 
 which may be hereafter remarked during the formation of 
 dew must be considered as effects, and not as the cause, of 
 the conversion of the watery vapour of a clear atmosphere 
 into a fluid. 
 
 A remaining argument applies equally to all the theories 
 which have hitherto been made public on the cause of dew. 
 This is, that none of them include the important fact that its 
 production is attended with cold ; since no explanation of a 
 natural appearance can be well founded which has been built 
 without the knowledge of one of its principal circumstances. 
 It may seem strange to many that neither Mr. Wilson nor 
 Mr. Six applied this fact to the improvement of the theory of 
 dew. 27 But, according to their view of the subject, no such 
 
 27 Wells might have added also, Boyle, Le Roi, Pictet and Prevost. 
 
 Le Eoi, in 1752, by an experiment of singular simplicity, satisfied him- 
 self that dew was deposited by the air. He sealed in a glass bottle atmos- 
 pheric air at a temperature of 77, and he found that as the temperature of 
 the day declined the interior of the bottle became bedewed, and he concluded 
 that the quantity of invisible vapour suspended in the air must be dependent 
 on the temperature, and that, by cooling the air sufficiently, some of the 
 vapour would be deposited as moisture. He termed the temperature at 
 which the deposition took place the degr de saturation de I' air. "Le Eoi 
 noticed that the point of saturation, or the dew-point as we now call it, was 
 often, during the day, very near the temperature of the air ; but at night, 
 when the air was many degrees colder than during the day, it was natural to 
 suppose that on some nights it would fall below its point of saturation; and 
 that, when such was the case, all the moisture in excess of that proper to the 
 air temperature would be precipitated and form dew, and thus prove that 
 dew is really formed by condensation of the moisture of the air." Tomlinson. 
 Experimentally he obtained this result, and, moreover, made the important 
 observation that, although the nocturnal temperature might fall below the 
 point of saturation, a change of wind would sometimes prevent the forma- 
 tion of dew. His method of determining the dew-point was as follows : 
 
 " I take water sufficiently cooled to precipitate the moisture of the air on 
 the exterior surface of the vessel that contains it. I pour this water into a 
 a large glass, very dry on the outside, and plunge into it the bulb of a 
 
ESSAY ON DEW, ETC. 49 
 
 use could have been made of it by them, as they held the 
 formation of that fluid to be the cause of the cold observed 
 with it. I had many years, as was formerly mentioned, 
 held the same opinion ; but I began to see reason not long 
 after my regular course of experiments commenced to doubt 
 its truth, as I found that bodies would sometimes become 
 colder than the air without being dewed; and that, when 
 dew was formed, if different times were compared, its 
 quantity and the degree of cold -which appeared with it 
 were very far from being always in the same proportion to 
 each other. The frequent recurrence of such observations 
 at length converted the doubt of the justness of my ancient 
 opinion into a conviction of its error, and at the same time 
 occasioned me to conclude that dew is the production of a 
 preceding cold in the substances upon which it appears. 
 Wishing, however, to obtain proofs more striking in degree 
 of the validity of these inferences than such as had been 
 afforded to me by casual observation, while attending to 
 other parts of my subject, I instituted the experiments which 
 will be next related. 
 
 I had frequently remarked, early in the evening, a consider- 
 able degree of cold on substances exposed in calm weather to 
 a clear sky, and I had also sometimes seen, early in the evening, 
 
 thermometer, in order to note its temperature. I allow it to become warmer 
 by kalf a degree, and then pour it into another goblet. If the outer surface 
 of the globe be still bedewed, I allow the water to become warmer half a 
 degree at a time, until I have seized the exact point above which there will be 
 no further precipitation. This point is the point of saturation of the air. 
 For example, on the 5th October, 1752, the temperature of the air was 13E.., 
 at 5 the surface of a cooled glass became bedewed; above this point it 
 remained dry ; below it moisture from this air was precipitated upon it, and 
 that in greater quantity as the temperature was reduced." This very 
 ingenious method would now be deemed far too fude, but, in principle, it is 
 the same as that employed for finding the dew-point by means of Daniell's 
 or Eegnault's hygrometer. Le Eoi's account of his experiments is given 
 in the Memoires dc I' Academic Royale des Sciences, 1752. 
 
50 ESSAY ON DEW, ETC. 
 
 the raised board altogether dry while the grass was much 
 moistened. I therefore determined to make the experiments 
 in view on the raised board, and to commence them as soon 
 as the sun should cease to shine upon it. The first day I 
 went to the country for this purpose., the 19th of August,, 
 1813; almost every circumstance was favourable to its com- 
 pletion. There had been no rain for three weeks, the wind 
 was northerly, and the barometer was rising; all which 
 indicated that the atmosphere contained little moisture. The 
 air, too, was extremely still. The only appearance in the 
 least unfavourable was, that the sky was not entirely free 
 from clouds ; but these were few, of smal] extent, thin, and 
 high. 
 
 At 6 h. 25 m. immediately after the sun had ceased to shine 
 upon the spot where my experiments were to be carried on, 
 though the time of its setting was still 47 minutes distant, I 
 placed upon the raised board 10 grains of wool and a small 
 bag, made of the skin of a swan's breast with the down 
 adhering, and stuffed with wool, the whole weighing nearly 
 5 drachms. On each of these substances the naked bulb of 
 a small and delicate thermometer was laid. A similar 
 thermometer with its bulb also naked was suspended in the 
 air over the grassplat, at the same height with the board. 
 Two thermometers were placed in other situations, as will be 
 seen in the annexed table. After an exposure of 20 minutes 
 the wool was 7 colder than the air, but the swansdown bag 
 only 6, no doubt in consequence of its comparatively great 
 quantity of matter. Neither, however, had gained the least 
 weight according to the scales employed by me, which were 
 sensibly moved by the 16th of a grain. These observations 
 were repeated several times during the following hour, as will 
 be seen by the Table, at none of which, except the last, was 
 either the wool or swansdown found in the least heavier 
 than when first placed on the board. At this last observation 
 
ESSAY ON DEW, ETC, 
 
 51 
 
 the wool, though 9i colder than the air, was still without any 
 increase in weight ; but the swansdown, which was 1 colder 
 than the wool, had gained half a grain. My experiments 
 now properly ceased, but having suffered the thermometers, 
 which had been placed on the wool and swansdown and in the 
 air, to remain in those situations, I examined them again at 
 8 h. 45 m., that is, 2 h. 20 m. after they had been first exposed. 
 The wool, which was still 9i colder than the air, had gained 
 somewhat less than half a grain ; and the swansdown, which 
 was now lli colder than the air, had gained 2 grains, 
 including the half grain already mentioned. When these last 
 observations were made, the sky was entirely cloudless and 
 the atmosphere very calm. 
 
 TABULAR VIEW OF OBSERVATIONS 
 
 on the Evening of August 19, 1813. 
 
 Heat of air 4 feet above the grass 
 
 wool on the raised board 
 
 swansdown on the same 
 
 surface of the raised board 
 
 grassplat* 
 
 Similar experiments made at the same place on the 
 evenings of the 25th of August and 17th of September in the 
 same year, had results which were also similar, but less in 
 degree ; the greatest difference between the temperature of 
 wool or swansdown while they were without any increase of 
 weight, and the temperature of the air, having been on the 
 first of those evenings only 4, and on the second only 5. 
 The reasons were in great measure, if not wholly, that a 
 considerable part of the sky was covered with clouds, and 
 that the air was commonly in that state of motion which is 
 denominated a gentle breeze. 
 
 * In these experiments, contrary to what usually happens, the grass waa 
 almost constantly colder than the filamentous substances, although they were 
 placed upon the raised board. 
 
 E2 
 
 6h. 45m. 
 
 7h. 
 
 7h. 20m. 
 
 7h. 40m. 
 
 8h. 45m. 
 
 60* 
 
 GO r 
 
 59 
 
 58 
 
 54 
 
 68* 
 
 541 
 
 51* 
 
 48* 
 
 44* 
 
 54* 
 
 53 
 
 51 
 
 47* 
 
 42* 
 
 58 
 
 57 
 
 55* 
 
 
 
 53 
 
 51 
 
 49* 
 
 49 
 
 42 
 
52 ESSAY ON DEW, ETC. 
 
 On the evening of my first experiments I had omitted to 
 measure the heat of the raised board before the ther- 
 mometers were placed upon it. This was attended to on the 
 two latter evenings, on the first of which its upper surface was 
 found at the commencement of the experiments, 4^ warmer than 
 the air ; on the second, both it and the air were of the same 
 temperature. Again, on the first of the latter evenings 10 
 grains of wool, to which 3 grains of water had been added, 
 having been laid on the raised board near the thermometers, 
 at the end of 45 minutes the parcel was found to have lost 
 2J grains of moisture by evaporation, during the time that 
 dry wool had become several degrees colder than the air. 
 
 A fourth experiment of this kind was made by me on the 
 7th of January, 1814, in the garden of Lincoln's Inn Fields, 
 by placing 10 grains of wool on a sheet of pasteboard, which 
 lay upon the snow. At the end of 35 minutes the wool was 
 5 colder than the air without possessing any additional 
 weight. 
 
 Having thus shown the justness of my former conclusion 
 that the cold observed with dew, is the previous occurrence, 
 and, consequently, that the formation of this fluid has 
 precisely the same immediate cause as the presence of 
 moisture upon the outside of a glass or metallic vessel, when 
 a liquid considerably colder than the air has been poured into 
 it shortly before ; I shall next apply this fact to the 
 explanation of several atmospherical appearances. 
 
 I. The variety in the quantities of dew, which were found 
 by me upon bodies of the same kind, exposed to the air during 
 the same time of the night, but in different situations, is now 
 seen to have been occasioned by the diversity of temperature 
 which existed among them. 
 
 II. Agreeably to the opinion of Mr. Wilson and Mr. Six, 
 the cold connected with dew ought always to be proportional 
 to the quantity of that fluid ; but this is contradicted by 
 
ESSAY ON DEW, ETC. 53 
 
 experience. On the other hand, if it be granted, that dew ~] 
 is water precipitated from the atmosphere, by the cold of the 
 body on which it appears, the same degree of cold, in the 
 precipitating body, may be attended with much, with little, 
 or with no dew, according to the existing state of the air in 
 regard to moisture; all of which circumstances are found 
 actually to take place. 
 
 III. The formation of dew, indeed, not only does not 
 ^produce cold, but like every other precipitation of water from 
 the atmosphere, produces heat. I infer this, partly because < 
 very little dew appeared upon the two nights of the greatest 
 cold I have ever observed on the surface of the earth, relatively 
 to the temperature of the air, both of them having occurred 
 after a long tract of dry weather ; and partly from the most 
 dewy night which I have ever seen, having been attended, 
 during the greater part of it, with no considerable degree of 
 cold. On this night, the difference between the temperatures 
 of grass and of air was at first 74, the dew being then not 
 very abundant. But, after the dew had become very abun- 
 dant, the difference of those temperatures never exceeded 4 , 
 and was frequently only &. 
 
 With the view of obtaining, though indirectly, some know- 
 ledge of the quantity of cold, which had been prevented, by 
 the formation of dew, from appearing on the surface of the 
 earth, in the night just spoken of, I made the following 
 experiment. To 10 grains of wool having the same form 
 and extension, as the parcels employed for the collection of 
 that fluid, were added 21 grains of water, this being the 
 quantity of moisture which had been attracted by 10 grains 
 of wool, lying on the grassplat, in the space of 8 hours on 
 that night. The wet wool having been then placed in a china 
 saucer, laid on a feather-bed in a room, the door and windows 
 of which were shut, its heat during the following 8 hours 
 
54 ESSAY ON DEW, ETC. 
 
 was, at frequent examinations, uniformly found to be about 4 
 less than that of a dry china saucer on the same bed ; the tem- 
 perature of the air in the room not having altered more than 
 half a degree in the course of the experiment. At the end of the 
 8 hours, the wool still retained 2J grains of moisture. If this 
 quantity kad also evaporated, the cold uniformly produced 
 during the 8 hours would, in all probability, have been about 
 4J. From this experiment, therefore, I think it may be in- 
 ferred, that the mean quantity of cold, which was prevented, by 
 the formation of dew, from appearing on the ground, during 
 the night which has been mentioned, was also about 4J. But, 
 as the production of dew, during some parts of the night, 
 was at a greater rate than that of 21 grains for 8 hours, 1 or 2 
 degrees may be added for those times, which will raise the 
 effect of the dew in diminishing the appearance of cold during 
 them to about 6, on the supposition, which cannot be far 
 from the truth, that dew had been attracted as copiously by 
 the grass as by wool which lay upon it. 
 
 The less difference commonly observed between the tem- 
 peratures of grass and of air, in the morning, than what 
 occurs in the evening, is likewise to be, in part, attributed to 
 a greater quantity of dew appearing in the former than in 
 the latter season. 
 
 A more remarkable fact, deriving an explanation from the 
 same source, is the greater difference which takes place 
 in very cold weather, if it be calm and clear, between the 
 temperatures of the air and of bodies on the earth, at night, 
 than in equally clear and calm weather in summer ; since, in 
 very cold weather, any diminution of the temperature of a 
 portion of air, in contact with a cold body, will be attended, 
 in consequence of the well known relations of the atmosphere 
 to moisture, with a much less formation of water, than an 
 equal diminution would be in summer, supposing the air, 
 
ESSAY ON DEW, ETC. 55 
 
 before it touches the cold body, to be at both times equally 
 near to its point of repletion with moisture. 28 
 
 IV. In very calm nights, a portion of air, which comes in 
 contact with cold grass, will not, when the surface is level, 
 immediately quit it, more especially as this air has become 
 specifically heavier than the higher, from a diminution of its J 
 heat, but will proceed horizontally, and be applied succes- 
 sively to different parts of the same surface. The air, there- 
 fore, which makes this progress, must at length have no 
 moisture to be precipitated, unless the cold of the grass which 
 
 it touches should increase. Hence in great measure is to be 
 explained, why on such nights, as have been just mentioned, 
 more dew was acquired by substances placed on the raised 
 board, than by others of the same kind on the grass, though 
 it began to form much sooner in the latter than in the for- 
 mer situation, those on the raised board having received air, 
 which had previously deposited less of its moisture. 
 
 A reason is now also afforded, why a slight agitation of 
 the atmosphere, when very pregnant with moisture, should 
 increase the quantity of dew ; since fresh parcels of air will 
 hence be more frequently brought into contact with the cold 
 surface of the earth, than if the atmosphere were entirely 
 calm. 
 
 V. Dew, in agreement with the immediate cause which 
 has been assigned by me for its production, can never be 
 formed, in temperate climates, upon the naked parts of a 
 living and healthy human body, during the night ; since their 
 
 28 " The formation of dew is one of the circumstances which modify and 
 check the refrigerating effect of radiation ; for, as the vapour is condensed, 
 it gives out the latent heat with which it was combined in its elastic form, 
 and thus, no doubt, prevents an excess of depression which might in many 
 cases prove injurious to vegetation. A compensating arrangement is thus 
 established, which, while it produces all the advantages of this gentle effusion 
 of moisture, guards against the injurious concentration of the cause by which 
 it is produced." Daniell's Met. Essays. 
 
56 ESSAY ON DEW, ETC. 
 
 heat is never less in this season, in such climates, than that 
 of the atmosphere. I have, in fact, never perceived dew on 
 any naked part of my own body at night, though my atten- 
 tion was much occupied, for three years, with every thing 
 relative to this fluid, and though I had been, during that 
 period, much exposed to the night air. On the other hand, 
 in very hot countries, the uncovered parts of a human body 
 may sometimes, from being considerably colder than the air, 
 condense the watery vapour of the atmosphere, and hence be 
 covered with a real dew, even in the day-time. 
 
 VI. Hygrometers formed of animal or vegetable sub- 
 stances, when exposed to a clear sky at night, will become 
 colder than the atmosphere ; and hence, by attracting dew, 
 or, according to an observation of Saussure,* by merely 
 cooling the air contiguous to them, mark a degree of mois- 
 ture, beyond what the atmosphere actually contains. This 
 serves to explain an observation made by M. De Luc f, that 
 in serene and calm weather, the humidity of the air, ss de- 
 termined by an hygrometer, increases about, and after sunset, 
 with a greater rapidity than can be attributed to a diminu- 
 tion of the general heat of the atmosphere. 
 
 These examples are sufficient to show the value of the 
 fact, that bodies become colder than the neighbouring air, 
 before they are dewed, in explaining many atmospherical 
 appearances. To this point, the investigation of the cause of 
 dew might have been carried at any time since the invention 
 of thermometers ; but its complete theory could not possibly, 
 in my opinion, have been attained, before the discoveries on 
 heat were made, which are contained in the works of Mr. 
 Leslie and Count Eumford. 
 
 * Hygronornetrie, p. 25. 
 
 f Introduction a la Physique Terrestre, II. 491. 
 
ESSAY ON DEWj ETC. 57 
 
 The experience of most persons, respecting tHe communi- 
 cation of heat among bodies in the open air, is confined to 
 what happens during the day ; at which time, those that are 
 situated near to one another are always found to possess the 
 'same temperatiire, unless some very evident reason for the 
 contrary should exist. To many, therefore, it may appear 
 incredible, that a perfectly dry body, placed in contact, on all 
 sides, with other bodies of the same temperature with itself, 
 shall afterwards, without undergoing any chemical change, 
 become much colder than they are, and shall remain so for 
 many hours ; yet these circumstances are found to occur in ^ 
 substances attractive of dew when laid on the surface of the 
 earth in a still and serene night, and are in perfect agreement 
 with the doctrine of heat, now universally admitted to bej 
 just. 29 
 
 To render this more easy of apprehension, let a small 
 body which radiates heat freely, and possesses a temperature 
 in common with the atmosphere, higher than 32, be placed 
 while the air is clear and still, on a slow conductor of heat 
 lying on the surface of a large open plain, and let a firmament 
 of ice be supposed to exist at any height in the atmosphere ; ^ 
 the consequence must be that the small body will, from its 
 situation, quickly become colder than the neighbouring air. 
 For, while it radiates its own heat upwards, it cannot receive 
 a sufficient quantity from the ice to compensate this loss ; 
 
 investigation of the phenomenon of dew developes the singular 
 relations of radiant heat, and the laws by which caloric is communicated from 
 one body to another. The vegetable world in particular opens a fertile and 
 most interesting train of observation. Of the different grasses each draws 
 from the atmosphere during the night a supply of dew to recruit its energies, 
 dependent on its form and its peculiar radiating power. Every flower has a 
 force of radiation of its own, subject to changes during the day and the night ; 
 and the deposition of moisture on it is regulated by the peculiar law which this 
 radiating power obeys ; and this power will itself be influenced by the aspect 
 which the flower presents to the sky, unfolding to the contemplative mind 
 the most beautiful examples of creative wisdom." Harvey, Enc. Met. vol. V. 
 
58 ESSAY ON DEW, ETC. 
 
 little also can be conveyed to it from the earth, as a bad con- 
 ductor is interposed between them ; and there is no solid or 
 fluid except the air, to communicate it laterally either by 
 
 r radiation or conduction. This small body, therefore, unless 
 it shall receive from the air nearly as much heat as it has 
 emitted, which, considering the little that can be communi- 
 cated from one part of the atmosphere to another, in its 
 
 'present calm state, must be regarded as impossible, will 
 become colder than the air, and condense the watery vapour 
 of the contiguous parts of it, if they should contain a sufficient 
 quantity to admit of this effect. But events similar to these 
 occur when dew appears in an open and level grass field 
 during a still and serene night. The upper parts of the 
 grass radiate their heat into regions of empty space, which 
 consequently send back no heat in return ; its lower parts, 
 from the smallness of their conducting power, transmit little 
 of the earth's heat to the upper parts, which, at the same 
 time, receiving only a small quantity from the atmosphere, 
 and none from any other lateral body, must remain colder 
 than the air, and condense into dew its watery vapour, if this 
 be sufficiently abundant, in respect to the decreased tempera- 
 ture of the grass. * 80 
 
 * I have adopted in this explanation the hypothesis of M. Prevost, 
 of Geneva, on the constant radiation of heat by bodies in contact with the 
 atmosphere, even at the time that they are exposed to the influence of bodies 
 warmer than themselves ; as it appears to agree perfectly with all the pheno- 
 mena of the communication of heat, which do not depend upon conduction. 
 ' I shall hereafter make frequent use of this hypothesis. 
 
 30 M. Prevost, in 1792, published a work entitled Recherches Physico- 
 Mecaniques sur la Chaleur, in which he "distinctly recognises the fact that when 
 at night the sky is clear the air is generally colder near the earth than at a 
 higher elevation ; that in spring and autumn it seldom freezes under a cloudy 
 sky, and that often on a calm night, when a cloud passes over the zenith of 
 the observer, the thermometer instantly begins to flse. He explains this on 
 the doctrine of radiant heat, which he says, passes easily through the air, but 
 that clouds are opaque for heat as well as for light, they absorb both, or only 
 allow it to pass slowly. The heat radiating from the earth traverses the pure 
 
ESSAY ON DEW, ETC. 59 
 
 This subject may be further illustrated by a reference to 
 what happens in the experiment which has been used to 
 prove the reflection of cold. 
 
 In the simplest form of this experiment, a small body, the 
 bulb of a thermometer, possessing the temperature of the 
 atmosphere, is placed before a larger cold body, rendered 
 equal in effect to one still larger by means of a concave metallic 
 mirror. In this situation the small body radiates heat to 
 the larger, without receiving an equivalent from it, and, in 
 consequence, becomes colder than the air through which its 
 heat is sent, notwithstanding that it is continually gaining 
 some heat, both from the air which surrounds it and from 
 the walls and contents of the apartment in which the experi- 
 ment is made. Dew, therefore, would as readily form upon 
 the thermometer in this experiment as it would upon one 
 suspended in the open air at night, under a clear sky, provided 
 that the two instruments were equally colder than the atmos- 
 phere, and that this was in both cases equally near to being 
 replete with moisture. 1 * 
 
 atmosphere with facility, but is intercepted by clouds, which form a sort of 
 vestment for the earth ; receiving the radiant heat on their lower surfaces 
 they become warm there, as a garment does close to the body, and restore to 
 the earth more heat than transparent air can do. Meanwhile the surface of 
 the cloud cools from the ease with which it emits heat into the rarefied air 
 above . " Tomlinson. 
 
 This explanation of the effect of clouds is almost identical with that which 
 a modern philosopher would give. Dr. Tyndall's experimental discovery 
 that aqueous vapour is a good radiant and a good absorbent of heat, whereas 
 dry air is devoid of these properties, would only serve to modify the language 
 employed. 
 
 Harvey has a note in which these words occur (Enc. Met. vol. V. p. 128) : 
 1 ' Above the clouds, however, the heavens are serene, and consequently radi- 
 ation goes on freely from their upper surface. Hence the principal cause of 
 the refrigeration upon which depends the formation of the nucleus of the 
 hailstone." This is in reference to Volta's theory of hail ; but, he adds "] 
 " It is proper to observe that this theory of the radiation of heat from the.J 
 superior surfaces of clouds had already been proposed by Gay Lussac." 
 
 to 
 
 * The invention of this experiment having been ascribed a few years ago | 
 M. Pictet, of Geneva, various English writers have shown that it occurs ' 
 
60 ESSAY ON DEW, ETC. 
 
 Regarding now as established that bodies situated on or 
 near to the surface of the earth become, under certain cir- 
 cumstances, colder than the neighbouring air, by radiating 
 more heat to the heavens than they receive in every way,* 
 I shall in the first place offer a few remarks on the extent 
 and use of this occurrence, and shall afterwards apply the 
 knowledge of it to the explanation of several more of the 
 appearances described in the former part of this Essay, and 
 of some others, which have not hitherto been mentioned 
 by me. 
 
 Radiation of heat by the earth to the heavens must exist 
 at all times ; but, if the sun be at some height above the 
 horizon, the degree of which is hitherto undetermined, and 
 probably varies according to season, and several other cir- 
 cumstances, the heat emitted by it to the earth will over- 
 balance, even in places shaded from its direct beams, that 
 which the earth radiates upwards. I suspended at midday 
 on the 24th of July, 1813, in the open air over a grassplat, 
 
 in several much older foreign authors. But I have not seen any mention 
 made of its having been also long since known in this country. That it was 
 so appears from the following extract of a letter, written by Mr. Oldenburgh 
 to Mr. Boyle, in 1665. " I met the other day in the Astrological Discourse 
 of Sir Christopher Heydon, with an experiment, which he affirms to have tried 
 himself, importing that cold accompanies reflected light, by employing burning 
 spherical concaves, or parabolical sections, which, he saith, will as sensibly 
 reflect the actual cold of snow or ice, as they will the heat of the sun." 
 Boyle's Works, folio, vol. V. p. 345. 
 
 * Count Eumford offered the following conjecture, in a paper printed in 
 the Philosophical Transactions for 1804. " The excessive cold which is known 
 to reign, in all seasons, on the tops of very high mountains, and in the higher 
 regions of the atmosphere, and the frosts at night, which so frequently take 
 place on the surface of the plains below, in very clear and still weather, in 
 spring and autumn, seem to indicate, that frigorific rays arrive continually 
 L at the surface of the earth, from every part of the heavens." But he gave 
 no experiments to prove that such a communication actually exists between 
 the heavens and the earth at night. Neither does it appear from any of his 
 writings which I have seen that he ever supposed that the surface of the 
 earth is more cooled by these frigorific rays, than the air through which they 
 pass, or that some solid bodies are more cooled by them than others. 
 
ESSAY ON DEW, ETC. 61 
 
 while the sky was wholly covered with very dense clouds and 
 the weather calm, two delicate thermometers, one of which 
 was naked, but the other cased in gold paper. At two ob- 
 servations, having an interval of 10 minutes between them, 
 the thermometer in the gilt case was 2 lower than that 
 which was naked. A white paper case was then drawn over 
 the gilt one, upon which, after 5 minutes, the covered in- 
 strument was observed to be at the same height with the 
 naked. The outer white case having, in the next place, been 
 taken from the covered thermometer, but that which was 
 gilt suffered to remain, the two instruments were in a few 
 minutes found again to differ 2. A thermometer on the 
 grassplat was, during these experiments, higher than the 
 naked instrument in the air by 2, and than that in the gilt 
 case by 4. It is evident, therefore, that heat radiated by 
 the sun must, on this day, have been transmitted in consi- 
 derable quantity through the thickest clouds, since not only 
 was the earth's surface warmer than the air, but a small 
 body, covered with a substance not readily admitting the 
 entrance of radiant heat, was colder than a similar body 
 which was uncovered. In like manner I observed at noon 
 on the 2nd of January, 1814, during the prevalence of a 
 dense fog, a thermometer placed upon swansdown, which was 
 lying upon grass thickly incrusted with hoarfrost, to be 2 
 warmer than the air, and 1 warmer than the grass. * 
 
 In a calm and serene night, however, when consequently 
 little impediment exists to the escape by radiation of the 
 earth's heat to the heavens, and when no heat can be 
 radiated by the sun to the place of observation, an immense 
 
 * Another fact of the same kind, which occurred at the same time, is that 
 although the temperature of the air was 30, the hoarfrost on trees rapidly 
 decreased, the solid matter of the trees intercepting radiant heat, which had 
 penetrated through the fog from the sun, and converting it into heat of 
 temperature. 
 
62 ESSAY ON DEW,, ETC. 
 
 degree of cold would occur on the ground, if the following 
 circumstances did not combine to lessen it. 1. The 
 incapacity of all bodies to prevent entirely the passing of 
 heat by conduction from the earth to substances placed 
 upon them. 2. The heat radiated to these substances by 
 lateral objects. 3. The heat communicated to the same 
 substances by the air. 4. The heat which is evolved during 
 the condensation of the watery vapour of the atmosphere 
 into dew. 
 
 The extent of the effect of all these checks upon the 
 production of cold by the nightly radiation of heat from 
 bodies on the surface of the earth, cannot in the present 
 state of our knowledge be properly estimated; but facts 
 show that notwithstanding their operation, the cold origi- 
 nating in this source must be often very considerable. 
 
 1. Mr. Wilson once observed a difference of 16 from 
 this cause between the temperatures of snow and of air. In 
 taking the latter temperature, however, he employed a naked 
 thermometer, on which account, in consequence of what has 
 already been mentioned to me, about 2 are to be added to 
 the 16 noted by him, in order to obtain the real difference 
 between the heat of the snow and the air at that time.* 
 
 2. If Mr. Wilson, as was formerly said, had laid a 
 thermometer on any downy substance in contact with the 
 snow, he would, in all probability, have found a cold indicated 
 by it at least 20 greater than that of the air, as marked by a 
 naked instrument, and consequently at least 22 greater than 
 the real cold of the surrounding atmosphere. 
 
 * As bright metals, when suspended in the air, and exposed to a clear sky 
 on a calm night, become colder than the surrounding atmosphere, a ther- 
 mometer covered with metalled paper, and placed in the circumstances which 
 have been just mentioned, will mark a temperature less than that of the air 
 near to it. But, as the difference must be small, and as I know of no way 
 to estimate it accurately, I have hitherto always neglected to consider it. 
 
ESSAY ON DEW, ETC. 63 
 
 3. Mr. Wilson's place of observation was not very favour- 
 able to the occurrence of a great cold from radiation of 
 heat at night, it being near to a large smoky city in the 
 immediate vicinity, also, as appears to me from what he says 
 of it, of one or more considerable buildings, and in a climate 
 abounding in moisture. 
 
 4. None of Mr. Wilson's experiments, in which a very 
 great degree of cold occurred, were made within an hour or 
 two after sunset, during which time according to my obser- 
 vation, the most considerable differences between the tem- 
 peratures of the air and of bodies on the surface of the 
 earth commonly happen. 
 
 If then such experiments should be made in an atmos- 
 phere still colder than that in which Mr. Wilson made 
 his, on a large plain remote from any city, and free from 
 objects of every kind that are elevated above the ground, 
 and in a country remarkable for the dryness of its air, all 
 which circumstances may be found in Russia during the 
 winter ; a difference of at least 30 would probably appear, 
 on some still and serene night, between a small thermometer 
 placed with its bulb naked* on the middle or leeward side of 
 a stratum of a downy substance, occupying a space upon a 
 grass field or bed of snow, one or two square yards in extent, 
 and a similar thermometer inclosed in a case of gilt paper, 
 and suspended in the air a few feet above the other. Two 
 thermometers thus placed would, I think, be sometimes found 
 even in this country to differ not much less than 30. I have 
 myself never made any such experiments with a downy 
 substance which had a surface of more than a few square 
 inches, or in a very cold night when the atmosphere was 
 clear and calm, and the scene of observation remote from 
 large masses of building. 
 
 * The effect would, perhaps, be a little increased, by covering the bulb 
 with a very thin layer of lamp-black. 
 
64 ESSAY ON DEW, ETC. 
 
 But even a cold of 30 appears not to be the greatest that 
 can be thought to occur from the radiation of heat to the 
 heavens, at night, by substances on the surface of the earth. 
 For experiments by M. Pictet,* Mr. Six,f and I may add by 
 myself, establish that in exception to the common rule, the heat 
 of the atmosphere in clear and calm nights increases with the 
 distance from the earth. Agreeably to Mr. Six's experiments 
 the atmosphere at the height of 220 feet is often upon such 
 nights, 10 warmer than what it is 7 feet above the ground. 31 
 If, therefore, I am able to show, as I expect I shall be in the 
 course of a few pages, that the air at the smaller height 
 becomes colder than that of the greater, from its vicinity 
 to the surface of the earth, previously rendered cold by 
 radiating its heat to the heavens, it will follow that these 
 10 must be added to the quantity of cold already men- 
 tioned; and, consequently, that a body on the ground may 
 become at night at least 40 colder than the air two or 
 three hundred feet above it, by the radiation of its heat to a 
 clear sky. 32 
 
 I shall add, with the greatest diffidence, a few words 
 upon a final cause of the radiation of heat from the earth at 
 
 * Essai sur le Feu, c. x. 
 
 t Phil. Trans. 1784, and 1788. 
 
 31 The following observations made by Mr. Glaisher, in his balloon 
 ascents, and published in the newspapers, will be read with interest in con- 
 nection with Wells' s statement. 
 
 July 23rd, 1863. About 5 h. 40 m. p.m., at 600 feet the temperature of 
 the air was 61 0> 5, there was no change in descending to the earth, where it was 
 still 61-5. . 
 
 Dec. 1st. About 2h. 40m. p.m., temperature 48, remained unchanged till 
 a height of 600 feet was passed. 
 
 June 13th, 1864. About 8 p.m. at 2000 feet, 53, no alteration in tem- 
 perature in reaching the earth, which occupied 20 minutes. 
 
 Dec. 30th. About 3 p.m. at 900 feet, 38i, no change at 300, and the 
 same at the surface. 
 
 32 f( The reading of a thermometer on raw wool was 25 less, whilst another 
 placed at 8 feet from the ground and fully exposed to the sky, was 3'5. 
 
ESSAY ON DEW, ETC. 65 
 
 night, and upon some of the circumstances which modify its 
 action, though fully conscious of the danger of error which 
 is always incurred in the attempt to appreciate the works of 
 our Creator. 
 
 The heat which is radiated by the sun to the earth, if 
 suffered to accumulate, would quickly destroy the present 
 constitution of our globe.* This evil is prevented by the 
 radiation of heat by the earth to the heavens during the 
 night, when it receives from them little or no heat in return. 
 But through the wise economy of means, which is witnessed 
 in all the operations of Nature, the prevention of this evil is 
 made the source of great positive good. For the surface of the " 
 earth having thus become colder than the neighbouring air, 
 condenses a part of the watery vapour of the atmosphere into 
 dew, the utility of which is too manifest to require my . 
 speaking of it. I may - remark, however, that this fluid - 
 appears chiefly where it is most wanted, on herbage and low 
 plants, avoiding in great measure, rocks, bare earth, and 
 considerable masses of water, f Its production too, by another 
 wise arrangement, tends to prevent the injury that might 
 arise from its own cause; since the precipitation of water 
 
 greater than that in air at the height of 4 feet and protected from radiation, 
 and thus a difference of 28'5 existed between the readings of two thermome- 
 ters, the one placed on raw wool, and the other in air at the height of 8 feet. 
 This difference was the greatest that I have ever seen." Glaisher, Phil. 
 Trans., 1847. 
 
 * Count Rumf ord says : " May it not be by the action of these (frigorific) / 
 rays that our planet is cooled continually and enabled to preserve the same 
 mean temperature for ages, notwithstanding the immense quantities of heat ' 
 that are generated at its surface by the continual action of the solar rays ?" 
 Phil. Trans. 1804, p. 181. 
 
 f I have no direct observations for the foundation of this assertion con- 
 cerning considerable masses of water. But I hold it, notwithstanding, to be 
 just, because as soon as the surface of the water is in the least cooled by 
 radiation, the particles composing it must fall downwards from their increased 
 gravity and be replaced by others that are warmer. The whole mass, there- 
 fore, can never in the course of a single night be sufficiently cooled to con- 
 
 P 
 
66 ESSAY ON DEW, ETC. 
 
 upon the tender parts of plants must lessen the cold in them 
 which occasions it. I shall observe in the last place,, that the 
 appearance of dew is not confined to any one part of the 
 night, but occurs during its whole course,, from means the 
 most simple and efficacious. For after one part of the air 
 has deposited its moisture on the colder surface of the earth, 
 it is removed in consequence of that agitation in the 
 atmosphere which exists during its stillest states, and gives 
 place to another having its quantity of water un diminished ; 
 and again, as the night proceeds, a portion of air which had 
 before deposited all the moisture which circumstances at that 
 time permitted, is rendered fit by the general increase of the 
 cold of the atmosphere, to give out a fresh parcel when it 
 comes anew into contact with the ground. 
 
 I. The first fact which I shall here attempt to explain is 
 the prevention, either- wholly or in part, of cold from radia- 
 tion, in substances on the ground, by the interposition of any 
 solid body between them and the sky. This evidently ap- 
 \ pears to arise in the following manner. The lower body 
 radiates its heat upwards, as if no other intervened between 
 it and the sky ; but the loss, which it hence suffers, is more 
 or less compensated by what is radiated to it, from the body 
 / above, the under- surface of which possesses always the same, 
 or very nearly the same temperature as the air. In this way, 
 therefore, is to be accounted for the warmth of the substances, 
 which were sheltered from the sky by the raised board, the 
 pasteboard roof, and the hollow cylinders of earth and paste- 
 board. In these examples the interposed substances cannot 
 be supposed to have remitted more heat than they received. 
 But in situations where large masses of bare solid matter 
 
 dense into dew any great quantity of the watery vapour of the atmosphere. 
 Besides, I have found that even a small mass of water, as will be more 
 particularly mentioned in the last part of this Essay, sometimes acquires no 
 weight from the reception of dew in the space of a whole night favourable to 
 the formation of that fluid. 
 
ESSAY ON DEW, ETC. 67 
 
 exist, which are warmer than the atmosphere, from the heat 
 of the preceding day or other causes, a greater heat will be 
 received by the exposed body than what is radiated by itself. 
 For example, it seems certain to me that the houses sur- 
 rounding Lincoln's Inn Fields had an influence upon my ther- 
 mometers during my experiments there at night, beyond what 
 arose from their merely returning a quantity of heat, equi- 
 valent to that which they received from the surface of the 
 garden. It is not, however, absolutely requisite that a body 
 should be itself exposed to the sky on a clear and calm night 
 in order to become colder than the atmosphere ; exposure to 
 the influence of another body, so situated, is sufficient for the 
 production of a slight degree of this effect. Thus, I have 
 always found wool attached to the underside of my raised 
 board on such a night to be a little colder than the air ; and 
 it has appeared to me a sufficient reason for the fact that the 
 wool in this situation was, in some degree, exposed to the 
 influence of grass, which had become considerably colder than 
 the atmosphere by radiating its heat to the sky. 
 
 II. No direct experiments can be made to ascertain the 
 manner in which clouds prevent or occasion to be small 
 the appearance of a cold at night upon the surface of the 
 earth greater than that of the atmosphere ; but it may, I 
 think, be firmly concluded, from what has been said in the 
 preceding article, that they produce this effect, almost 
 entirely by radiating heat to the earth in return for that 
 which they intercept in its progress from the earth towards 
 the heavens. For although, upon the sky becoming suddenly 
 cloudy during a calm night, a naked thermometer, suspended 
 in the air, commonly rises 2 or 3 degrees, little of this rise is 
 to be attributed to the heat evolved by the condensation of 
 watery vapour in the atmosphere, as was supposed by Mr* 
 Wilson ;* since, in consequence of the ceasing of that part of 
 
 * Edin. Phil. Trans. I. 157. 
 
 F2 
 
68 ESSAY ON DEW, ETC. 
 
 the cold indicated by the thermometer, which was owing to 
 its own radiation to a clear sky, the temperature of the 
 atmosphere may seem to increase 2, or more, notwithstand- 
 ing that it has received no real addition. Besides, the heat 
 which is extricated by the condensation of vapour during the 
 formation of a cloud must soon be dissipated ; whereas 
 the effect of greatly lessening, or preventing altogether, 
 the appearance of a superior cold on the earth to that of the 
 air, will be produced by a cloudy sky, during the whole of a 
 long night. 
 
 Dense clouds near the earth must possess the same heat 
 as the lower atmosphere, and will therefore send to the earth, 
 as much, or nearly as much heat as they receive from it by 
 radiation. But similarly dense clouds, if very high, though 
 they equally intercept the communication of the earth with 
 the sky, yet being, from their elevated situation, colder than 
 the earth, will radiate to it less heat than they receive from 
 it, and may, consequently, admit of bodies on its surface 
 becoming several degrees colder than the air. In the first- 
 part of this Essay an example was given of a body on the 
 ground becoming at night 5 colder than the air, though the 
 whole sky was thickly covered with high clouds.* 
 
 Islands, and parts of continents close to the sea, being, by 
 their situation, subject to a cloudy sky, will, from the smaller 
 quantity of heat lost by them through radiation to the 
 
 * M. Prevost of Geneva, in his work on " Eadiant Heat/' p. 382,, has 
 already in this way, conjecturally, accounted for the effect of clouds in dimi- 
 nishing at night the cold of the atmosphere and of the surface of the earth ; 
 but he seems not to have known that their effect on the temperature of the 
 latter is much greater than that which they produce upon the air. My 
 explanation of this influence of clouds on the temperature of the surface of 
 the earth during the night is a direct consequence from the facts which I had 
 observed respecting the prevention of cold on the ground from radiation by 
 the interposition of solid bodies between it and the heavens, and occurred to 
 me in 1812. M. Prevost's work, indeed, was published in 1809, but I did 
 not see it before the summer of 1813, when it was lent to me by his relation, 
 
ESSAY ON DEW, ETC. 09 
 
 heavens at night, in addition to the reasons commonly 
 assigned, be less cold in winter than countries considerably 
 distant from any ocean. 
 
 III. -Fogs, like clouds, will arrest heat, which is radiated 
 upwards by the earth, and if they be very dense and of con- 
 siderable perpendicular extent, may remit to it as much as 
 they receive. Accordingly Mr. Wilson foimd no difference 
 at night, in very foggy weather, between the temperature of 
 the surface of snow and that of the air. Several observations 
 by myself tend to confirm that of Mr. Wilson. An instance, 
 however, as was formerly said, occurred to me of a difference 
 at night of 9 between the temperatures of grass crusted over 
 with hoarfrost, and of air, during a very dense fog. A fact, 
 remarked by Mr. Leslie, respecting fogs, serves to explain 
 this apparent anomaly. For it was found by that philoso- 
 pher* from experiments made with his photometer, that in 
 mists and low fogs the diminution of the sun's heat is small, 
 when compared with what occurs when the sky is obscured 
 by a dense body of clouds ; and it will, I presume, be readily 
 granted, that the same state of the atmosphere which allows 
 the heat of the sun to pass copiously will also give a ready 
 transit to heat radiated by the earth. Now there are several 
 reasons for believing that the fog, during which grass was 9 
 colder than the air, did not ascend far above the ground. 
 1. The barometer had been falling for some days before, and 
 it is a matter of common observation that great fogs seldom 
 
 Dr. Marcet of London, who at the same time said that he believed there was no 
 other copy of it in Great Britain, except one, which had been sent by him- 
 self to Edinburgh. 
 
 Note to second edition.'] I did not know until after the first edition of this 
 Essay was printed, that M. Prevost had published his opinion on the effect 
 of clouds in preventing the occurrence of cold at night in the atmosphere 
 and upon the surface of the earth, as early as 1792, in a work entitled 
 " Eecherches sur la Chaleur." 
 
 * On Heat and Moisture, p. 57. 
 
70 ESSAY ON DEW, ETC. 
 
 occur, except it be high. 2. On the day preceding the 
 observation, the air, after having been extremely foggy for 
 nearly a week, had become clear enough to allow the sun's 
 being distinctly seen during the whole of the afternoon, 
 though there was still a sufficient obscurity in the lowermost 
 parts of the atmosphere, to obstruct considerably the view of 
 objects on the ground and very near to it. 3. On the day 
 following the observation the fog was again much less, on the 
 next it disappeared, and was succeeded by snow. It is to be 
 mentioned, likewise, that on the evening in question the state 
 of the grass, which was the subject of experiment, was 
 unusually favourable to the production of cold; since, 
 contrary to general experience, it was as cold as swansdown. 
 If, then, the latter substance, from the much greater regu- 
 larity of the appearances exhibited by it, be taken as the 
 standard by which the occurrences of different nights are to 
 be compared together, it will follow that the fog of which I 
 am speaking, though it did not prevent, must have lessened, 
 the production of cold from radiation. For, on the preceding 
 evening, when there was little fog, the atmosphere being 
 equally still on both, the difference between swansdown and 
 the air was 12; and on another, a fortnight after, the 
 difference at the same place of observation between ther- 
 mometers in the same situations, was 14^, the air being now 
 free from fog. If the atmosphere had been as still on this 
 as on the former evenings, a greater difference would doubt- 
 less have been seen. I conclude, therefore, that fogs do not 
 in any instance furnish a real exception to the general rule, 
 that whatever exists in the atmosphere, capable of stopping 
 or impeding the passage of radiant heat, will prevent or 
 lessen the appearance at night of a cold on the surface of the 
 earth, greater than that of the neighbouring air. 
 
 It follows also from what has been said in this article, that 
 the water deposited upon the earth during a fog at night may 
 
ESSAY ON DEW, ETC. 71 
 
 sometimes be derived from two different sources, one of which 
 is a precipitation of moisture from a considerable part of the 
 atmosphere, in consequence of its general cold ; the other, a 
 real formation of dew, from the condensation, by means of the 
 superficial cold of the ground, of the moisture of that portion 
 of the air which comes in contact with it. In such a state 
 of things, all bodies will become moist, but those especially 
 which most readily attract dew in clear weather.* I have had 
 no opportunity, however, of trying this conclusion by the test 
 of observation since it occurred to me. 
 
 IV. When bodies become cold from radiation, the degree 
 of effect observed must depend, not only on their radiating 
 power, but in part also on the greater or less ease with which 
 they C^L derive heat by conduction from warmer substances 
 in contact with them. Thus grass on a clear and still night, 
 was constantly colder, sometimes very much colder, than the 
 gravel walk, though a small quantity of sand placed upon 
 grass was always nearly as cold as this substance. In this 
 case the difference in temperature between the gravel walk and 
 sand evidently depended on the different quantities of heat, 
 which they received from the parts beneath. A like reason is 
 to be given for dew appearing in greater quantity on shavings 
 of wood than on the same substance in a more dense and 
 compact form ; and for filamentous and downy substances^ 
 becoming colder than all others, even than lampblack, which } 
 is placed by M.r. Leslie at the head of the best solid radiators j 
 of heat. For the lampblack exposed by me being about 2 
 lines in depth, possessed, in consequence, a fund of internal 
 heat, which would more readily pass to its cold surface than 
 the heat of the lower parts of the downy substances would to 
 their upper surface. 
 
 * The moisture observed at night by Musscheubroek, in Holland, and 
 called by him dew, appears to ine to have been of this kind. See this 
 Essay, p. 5. 
 
72 ESSAY ON DEW, ETC. 
 
 This subject is illustrated by the following experiment : 
 On a dewy evening I depressed into soft garden mould a 
 drinking glass, having a thick flat bottom, until its brim was 
 upon a level with the surrounding earth, and at the same 
 time placed a similar vessel, with its cavity also towards the 
 sky, on the surface of the mould. In the morning the inside 
 of the depressed glass was entirely dry, while that of the 
 other was dewed. I then applied the bulb of a small ther- 
 mometer to the inside of the bottom of each vessel, on which 
 I found the heat of that part of the depressed one to be 56, 
 but of the same part of that which stood on the mould only 
 49^. At this time the temperature of the air was 53. 
 The cause, therefore, was evident, both of the wetness of the 
 first vessel and of the dryness of the second. 9 
 
 From this source also is to be derived the reason why 
 the prominent parts of various bodies were observed by 
 Mr. Wilson to be crusted with hoarfrost, while their more 
 retired and massy parts were free from it.* 
 
 V. Bodies, exposed in a clear night to the sky, must 
 radiate as much heat to it during the prevalence of wind, as 
 they would do if the air were altogether still. But in the 
 former case little or no cold will be observed upon them 
 above that of the atmosphere, as the frequent application of 
 warm air must quickly return a heat equal, or nearly so, to 
 that which they had lost by radiation. A slight agitation of 
 the air is sufficient to produce some effect of this kind; 
 though, as has already been said, such an agitation, when 
 the air is very pregnant with moisture, will render greater 
 the quantity of dew, one requisite for a considerable produc- 
 tion of this fluid being more increased by it than another is 
 diminished. 
 
 VI. A small body, as a thermometer, suspended in the 
 
 * Paper in Phil. Trans. 1780. 
 
ESSAY ON DEW, ETC. 73 
 
 air, will even in the calmest night exhibit but little cold from 
 radiation, since it is continually exposed to the application of 
 fresh parcels of warmer air, both from the progressive motion 
 of this fluid, and from the downward motion produced in it 
 by the superior gravity of such portions as have been cooled 
 by contact with the suspended body. On the other hand, a 
 thermometer upon a board, raised above the earth and posses- 
 sing a surface of several square yards, will have its cold from 
 radiation much less diminished than the former, as it is 
 exposed to no loss from a downward motion of the air, and 
 as the air, which approaches it horizontally, must, almost 
 always, have had its temperature previously lowered by 
 passing over another part of the board. The reason then 
 of the lee side of the raised board being often colder than the 
 windward is obvious. 
 
 VII. There is a remark by Theophrastus,* which has been 
 confirmed by other writers, that the hurtful effects of cold 
 occur chiefly in hollow places. Jf this be restricted to what 
 happens on serene and calm nights, and it does not, I believe, 
 hold true in any other circumstances, two reasons from 
 different sources are to be assigned for it. The first is, that 
 the air being stiller in such a situation than in any other, the 
 cold, from radiation in the bodies which it contains, will be 
 less diminished by renewed applications of warmer air ; the 
 second, that from the longer continuance of the same air in 
 contact with the ground in depressed places than in others, 
 less dew will be deposited, and therefore less heat extricated 
 during its formation. It will be seen in the last part of this 
 Essay, that in the East Indies depressions in the earth are 
 artificially made for the purpose of increasing the cold which 
 appears in serene nights. On this subject, however, it is to 
 be observed, that if the depressed or hollow places be deep in 
 
 * Lib. v. c. xvi. 
 
74 ESSAY ON DEW, ETC. 
 
 proportion to their horizontal extent, a contrary effect must 
 follow ; as a case will occur more or less similar to that which 
 existed in some experiments formerly related by me, in which 
 a small portion of grass was surrounded by a hollow cylinder. 
 VIII. An observation closely connected with the pre- 
 ceding, namely, that in clear and still nights frosts are less 
 severe upon hills than in neighbouring plains,* has excited 
 more attention, chiefly from its contradicting what is com- 
 monly regarded an established fact, that the cold of the 
 atmosphere always increases with the distance from the 
 earth. This inferior cold of hills is evidently a circumstance 
 of the same kind with that ascertained by M. Pictet and 
 Mr. Six respecting the increasing warmth, in clear and calm 
 nights at all seasons of the year, of the different strata of the 
 atmosphere, in proportion as these are more elevated above 
 the earth. As the greater cold of the lower air is the less 
 complicated fact, I shall attempt to explain it in the first 
 place. M. Pictet, indeed, furnishes an explanation himself 
 by ascribing it to the evaporation of moisture from the 
 ground. But to show that this is not just, it need only be 
 mentioned that the appearance never occurs in any consider- 
 able degree, except upon such nights as are attended with 
 some dew, and that its great degrees are commonly attended 
 with a copious formation of that fluid; since it cannot be 
 thought that the same stratum of air will deposit moisture 
 on the ground from an insufficiency of heat, at the very time 
 it is receiving moisture from the ground in the state of pel- 
 lucid vapour, as this presupposes that it is not yet replete 
 with water. 
 
 Our atmosphere has been very generally regarded as in- 
 capable of being heated directly by the rays of the sun, prin- 
 
 * Theophrastus also remarks that it freezes less on hills than on plains, 
 but without mentioning that this happens only on calm and serene nights. 
 (Lib. v. c. xx.) 
 
ESSAY ON DEW, ETC. 75 
 
 cipally because these give no heat to any particular portion 
 of it, in which they are brought to a focus. I do not know 
 whether this experiment was ever made with all the accuracy 
 of which it is susceptible; but, granting that it has been 
 thus made, my opinion is, notwithstanding, that no reliance 
 can be placed in it. For as air, if heated at all by concen- 
 trated sunbeams, must be heated by them in a very slight 
 degree during the time that their focus may be looked upon 
 as stationary, otherwise the present question would not have 
 arisen, it is necessary for conducting the experiment properly 
 that, during the whole of it, the same individual small portion 
 of air shall constantly receive that focus ; but this, for various 
 manifest reasons, cannot possibly happen. Viewing, there- 
 fore, the argument founded upon this experiment as without 
 force, I shall now offer several considerations which seem to 
 prove that air is actually heated by the sunbeams which 
 enter it. 
 
 1. Air both reflects and refracts light, and all other 
 bodies, as far as I know, acquire heat while they act thus 
 on the light of the sun. 
 
 2. Air suffocates or absorbs the sun's light, which it can- 
 not be supposed to do without increasing in temperature. 
 
 3. If air, considered as a uniform fluid, were even in- 
 capable of gaining heat directly from the sun's rays, heat 
 would be communicated by them to it through the inter- 
 vention of the innumerable particles of solid matter, which 
 the trivial experiment of receiving a sunbeam into a darkened 
 room shows to be present in the atmosphere. Should it be 
 said that this appearance may occur only in the neighbour- 
 hood of the earth from the accidental admixture of solid 
 matter raised from its surface by winds, or in any other way, 
 the answer is, that as my inquiry is concerning the existence 
 of a certain condition of the atmosphere, it matters not how 
 this originates. Nothing more can be demanded than that 
 
J 
 
 76 ESSAY ON DEW, ETC. 
 
 it should always be found, which I believe to be the case ; 
 since, if I can trust my memory with respect to what took 
 place many years ago, I should say that such particles are 
 to be seen, by means of the sun's light, in the air over the 
 middle of the Atlantic ocean. These particles then must 
 receive heat from the sunbeams, which impinge upon them, 
 and this they will communicate to the contiguous pellucid air. 
 4. Unless it be admitted that the atmosphere is capable of 
 intercepting part of the heat, which is radiated into it by the 
 sun, and of converting this into heat of temperature, I deem 
 it impossible to find a sufficient reason for the great warmth 
 which exists, after a long calm, in air incumbent upon the 
 Atlantic and Pacific oceans, at the distance of a thousand 
 miles or more from any considerable body of land. It can- 
 not be derived from the neighbouring water, since this is 
 colder than the lower atmosphere ; and no one will suppose 
 it to be the same heat which the air had acquired from the 
 last continent it had passed over many days before. But, 
 if even this were supposed, another difficulty would remain 
 to be removed, which is, that during the whole of the calm 
 the air is cooled every night and again becomes warm in 
 the day.* 33 
 
 Should what has been said be thought sufficient to estab- 
 lish that the air arrests part of the sun's heat, which is 
 radiated into it bound up with light, two consequences must 
 
 * One reason is hence apparent for the great coldness of the high regions 
 of the atmosphere ; since the air in them must be less fit than that of the 
 lower strata to arrest heat which is radiated into it. 
 
 33 Professor Tyndall has established, by experimental proof, the fact that 
 dry air is as transparent to radiant heat as a vacuum ; while aqueous vapour 
 has a high absorbing power for radiant heat. 
 
 Grlaisher has found, in our latitude, that at certain elevations the air is 
 generally in a state almost absolutely free from aqueous vapour. 
 
 These results of modern researches will serve to considerably modify the 
 arguments and reasoning of Wells on the subject of the heating of the 
 atmosphere. 
 
, ESSAY ON DEW, ETC. 77 
 
 also be allowed. The first is that air will exert a greater \ 
 power of the same kind upon heat radiated into it without 
 light, since the sun's heat passes instantaneously through 
 many bodies, which refuse a similar way to heat radiated by 
 terrestrial substances ; he other, that air must be as capable 
 of becoming cold by radiating its own heat*, as of becoming 
 warm from heat radiated into it, as these two properties are 
 uniformly observed to exist together, and to be proportional 
 to each other. 84 The truth of the latter conclusion may also 
 be inferred from this fact, that in still and calm weather the 
 heat of the air a few feet above the earth will sometimes de- 
 crease, even in this country, 18 or 20 degrees between sunset 
 and sunrise, though no change of wind has in the meantime 
 occurred ; for the inconsiderable conducting power, which air 
 is now known to possess, will permit only a small part of this 
 diminution to arise from heat passing, by means of that 
 power, from the atmosphere to the colder earth. Mr. 
 Leslie f, indeed, ascribes this effect to the descent of cold air 
 from the higher regions of the atmosphere ; but if this were 
 just, a less cold ought to be found on a clear and still night in 
 the lower than in the higher strata, which is contrary to the 
 uniform results of numerous experiments by M. Pictet and 
 Mr. Six. Winds too, which produce such a mixture, always 
 lessen the nocturnal decrease of temperature in the lowermost 
 part of the atmosphere. 
 
 Having thus shown that air is capable, both of absorbing 
 heat, which is radiated into it, and of radiating heat which 
 had before formed a part of its temperature, I proceed to 
 
 * M. Prevost says : " On peut Bupposer que lea molecules de Fair rayon- 
 nent." Du Calorique Rayonnant, p. 24. 
 
 f On Heat and Moisture, p. 11, and 132. 
 
 34 Apply these conclusions to the aqueous vapour of the atmosphere 
 instead of the "air," and they will receive the sanction of the best teaching 
 of the present day. 
 
78 ESSAl! ON DEW,, ETC. 
 
 apply the knowledge of these facts to the explanation of the 
 phenomenon observed by M. Pictet and Mr. Six. 
 
 This phenomenon occurs on those nights only which 
 permit bodies on the surface of the earth to become cold by 
 radiating their heat to the heavens. On other nights 
 when bodies thus situated were not colder than the air, I 
 have observed the atmosphere within the limits of 9 feet from 
 the ground, the boundary of my own experiments,, to decrease a 
 little in temperature, as the distance from the earth increased. 
 Mr. Six likewise found that on cloudy nights the air was 
 sometimes colder 220 feet above the ground than at the dis- 
 tance of 9 feet from it. When, therefore, the earth has become 
 colder from radiation than the neighbouring air, in conse- 
 quence of the latter having, by reason of its small radiating 
 power, emitted a less proportion of its heat to the heavens, 
 the warmer air must radiate a part of its heat to the earth, 
 without receiving a full compensation, and will, therefore, 
 become colder than it otherwise would have been. In pro- 
 portion too as the air is nearer to the earth, must the cold of 
 the former from this cause be the greater. My own conception 
 of this matter is facilitated* by contemplating the occurrence 
 of an opposite effect when the earth is warmer than the air. 
 Let it be supposed then, that while the earth in this state, 
 radiates upwards a quantity of heat, a foot in depth of the 
 incumbent air is capable of stopping a 1000th of what it 
 hence receives, and of converting it into heat of temperature. 
 The consequence must be that the next foot, from receiving 
 only 999 parts of what had been emitted by the earth, will 
 not be so much heated as the first foot, though it should 
 absorb the same proportional quantity of what enters it. In 
 this way every successive foot will acquire a less quantity of 
 heat than the preceding, and a state of the atmosphere be 
 
 * The same facility is afforded by considering cold as a body. 
 
ESSAY ON DEW, ETC. 79 
 
 produced, like to that which is actually observed in a calm and 
 sunny day. 35 In the da} r , however, the phenomena, from the 
 heating of air by rays from the earth, are somewhat confused 
 by the warmed portions rising upwards, and mixing with what 
 is colder ; whereas, at night, the air which has been cooled 
 by radiating heat to the earth, is rendered, by an increase of 
 gravity, the more fit to retain its low position. I have here 
 for the sake of simplifying the argument, taken no notice of 
 the cooling of any considerable mass of the air in consequence 
 of the actual contact of its lowermost stratum with the earth, 
 or by the conduction of the temperature of one portion of it to 
 another. But, in a calm state of the atmosphere these effects 
 must be inconsiderable, though it appears to me impossible, 
 in the present state of our knowledge, to determine them with 
 any precision. 
 
 According to the view which has been given by me of 
 this subject, the heat of the air in a clear and calm night 
 ought to increase within the limits of the phenomenon 
 in some decreasing geometrical ratio as the atmosphere 
 ascends; and this conclusion is so far confirmed by the 
 observations of M. Pictet and Mr. Six taken together, 
 that the increase of temperature is found to be greater in 
 a given space very near to the earth, than in an equal space 
 more remote from it. 
 
 To return to the immediate object of this article, the 
 fact is certain, whatever may be thought of my explanation 
 of it, that in every clear and still night the air near to the 
 earth is colder than that which is more distant from it, to the 
 height at least of 220 feet, this being the greatest to which 
 Mr. Six's experiments relate. If then a hill be supposed to rise 
 
 35 The reader should compare this reasoning from theoretical considera- 
 tions with the conclusions deduced by Tyndall from his most carefully con- 
 ducted and remarkable series of experiments, given in his work, "Heat 
 considered as a Mode of Motion." 
 
80 ESSAY ON DEW, ETC. 
 
 from a plain to the height of 220 feet, having upon its summit 
 a small flat surface covered with grass, and if the atmosphere 
 during a calm and serene night be admitted to be 10 warmer 
 there than it is near the surface of the low ground, which 
 is a less difference, according to the observations of Mr. Six, 
 than what sometimes occurs in such circumstances, it is 
 manifest that should both the grass upon the hill and that 
 upon the plain acquire a cold of 10 by radiation, the former 
 will, notwithstanding, be 10 warmer than the latter. 36 
 
 But the equality here supposed to be in the cold acquired 
 by grass in two such situations can seldom exist. For 
 according to an observation made by Aristotle,* and since 
 frequently repeated, the air of high places is much more 
 agitated than that upon low ground. The frequent renewal, 
 therefore, from this cause, of the air in contact with the 
 grass on the hill, will prevent it from ever becoming much 
 colder than the general mass of the atmosphere at the same 
 height. Consequently, any diminution in this way of the 
 10 of cold, formerly supposed to occur there from radiation, 
 must be added to the difference of temperature in the grass 
 in the two situations. 
 
 What has hitherto been said refers only to the occur- 
 rences on the very summit of the hill. With respect to its 
 sides, these can be only a little colder than the atmosphere 
 upon a level with them, even in its calmest state. For in 
 the first place, they do not enjoy the full aspect of the sky ; 
 and, in the second, the air which is cooled by contact with 
 them, will, from its increased gravity, slide down their 
 declivity, and thus make room for the application of new 
 and warm parcels to the same surface. The motion, too, 
 thus excibed in the air near to the sides of the hill, must 
 
 36 It may be doubted whether much reliance can be placed upon the results 
 of Six's experiments at the height of 223 feet. The supposed case of the hill 
 is certainly opposed to received notions. 
 
 * Meteor, lib. 1. c. x. 
 
ESSAY ON DEW, ETC. 81 
 
 occasion a motion in that upon the summit, which may in 
 some measure account for the last mentioned observation of 
 Aristotle, as far as relates to what happens in a clear night. 
 
 The height of the hill in this example has been supposed 
 to be small, to make it accord with that of the stations, 
 whose temperatures were compared by Mr. Six with the 
 heat of the air near the ground. But observations of the 
 same kind will apply to hills of much greater elevation. 
 For granting, first, that the air at the height of 220 feet 
 is never more than 10 colder than that near to the earth, 
 which is not probable, and is indeed contradicted by some of 
 Mr. Six's observations ; and again, that the increase of the 
 air's heat in a calm and serene night, ceases precisely at the 
 greatest height to which Mr. Six carried his observations, 
 which is also improbable; still a reduction to the extent of 
 10* in the temperature of the air near to the earth, will 
 render the cold of this low portion of the atmosphere greater 
 than that of any other portion, which is not more than 2500 
 or 3000 feet above the former, if the estimate be just, which 
 makes a declension in the heat of the atmosphere of 1 for 
 every 250 or 300 feet of its height, when no counteracting 
 cause exists. 37 
 
 The remarks, however, which have been offered on the 
 
 37 The law of decrease of temperature for height above the earth's surface 
 originated from the experiments made in balloons by Gay Lussac. It was 
 generally received as a fact that for each 300 feet of elevation the tempera- 
 ture of the air was 1 lower than that at the surface. The balloon experiments 
 made by Welsh in 1852 were even considered to confirm the law. The more 
 recent, extensive, and careful meteorological observations made in balloon 
 ascents by Glaisher, throw entire discredit upon this supposed law. 
 
 J. Glaisher, 1?.R.S., from a series of balloon ascents undertaken for 
 meteorological research, deduced the following approximate law of decrease 
 of temperature with elevation : 
 
 When the sky was cloudy 
 
 For the first 300 feet the decrease was 0'5 for every 100 feet 
 From 300 to 3400,, 0-4 
 
 3400 to 5000,, , 0-3 
 
82 
 
 ESSAY ON DEW, ETC. 
 
 greater warmth of Mils at night in a certain state of weather, 
 are strictly applicable to those only which are insulated and 
 
 Therefore in cloudy states of the sky the temperature of the air decreases 
 nearly uniformly with the height above the surface of the earth nearly up to 
 the cloud region. 
 
 When the sky was partially cloudy 
 In the first 100 feet there was a decline of 0-9 
 From 100 to 300 , 0'8 for each 100ft. 
 
 300 to 500 , 0-7 
 
 500 to 900 , 0-6 
 
 900 to 1800 , 0.5 
 
 1800 to 2900 , 0.4 
 
 2900 to 5000 , 0-3 
 
 The decline of temperature near the earth with a partially clear sky is 
 nearly double of that with a cloudy sky; at elevations above 4000ft. the 
 changes for 100 feet seem to be the same in both states of the sky. 
 At heights exceeding 5000 feet 
 
 to 6000 feet the decline was 
 thence to 7000 
 
 8000 
 9000 
 10000 
 
 nooo 
 
 12000 
 
 13000 
 
 14000 
 15000 
 
 16000 
 17000 
 18000 
 19000 
 20000 
 21000 
 22000 
 23000 
 24000 
 25000 
 26000 
 27000 
 28000 
 29000 
 
 2-8 
 2-8 
 2-7 
 2-6 
 2-6 
 2-6 
 2-6 
 2-5 
 2-2 
 2-1 
 2-1 
 1-9 
 1-8 
 1-8 
 1-5 
 1-3 
 1-3 
 1-0 
 1-3 
 1-1 
 1-0 
 1-0 
 0-9 
 0-8 
 
 Approximate Law of Moisture at Elevations. 
 With an overcast sky the degree of humidity being 77 
 
 At 1000 feet it was 77 At 4000 feet it was 80 
 
 2000 77 5000 83 
 
 3000 80 6000 82 
 
 When the sky was partially clear, the degree of humidity 
 
 On the ground was 63 At 4000 feet it was 76 
 
 At 1000 feet it was 68 5000 69 
 
 2000 77 6000 68 
 
 3000 76 
 
ESSAY ON DEW, ETC. 
 
 83 
 
 of inconsiderable lateral extent ; and it is upon such chiefly, 
 if not solely, that this phenomenon has been observed. The 
 superiority of the cold of a low plain, from radiation, over 
 that of a wide expanse of hilly ground will, for obvious 
 reasons, be less; and no superiority of this kind will pro- 
 bably exist in the former situation, when the high ground is 
 not only extensive, but flat on the top, forming what is called 
 a table-land ; unless, indeed, which seems to be actually the 
 case, the air of such an elevated country should be commonly 
 more agitated than that of lower places equally level. 
 
 An explanation may be now easily given of an observation 
 by Mr. Jefferson, of Virginia,* which, however, had also been 
 made by Aristotlef and Plutarch, J that dew is much less 
 copious on hills than it is upon plains. For allowing, at 
 first, the surface of the ground to be in both situations 
 equally colder than the air which is near to it ; still, as the 
 production of dew must be in proportion to the whole de- 
 pression of the temperature of the air which furnishes it, 
 below what its heat had been in the preceding day, and as 
 one part of this depression, the general cooling of the atmos- 
 phere is much more considerable on the plain than on the 
 hill, moisture must necessarily be deposited more copiously 
 
 . Above the clouds the degree of humidity 
 
 at 7000 feet was 64 at 16000 feet was 45 
 
 
 , 8000 
 
 58 
 
 17000 
 
 
 3$ 
 
 
 , 9000 
 
 52 
 
 18000 
 
 
 21 
 
 
 , 10000 
 
 62 
 
 19000 
 
 
 36 
 
 
 , 13000 
 
 48 
 
 20000 
 
 
 33 
 
 
 , 12000 
 
 48 
 
 21000 
 
 
 32 
 
 
 , 13000 
 
 43 
 
 22000 
 
 
 21 
 
 
 , 14000 
 
 58 
 
 23000 
 
 
 16 
 
 
 , 15000 
 
 53 
 
 24000 
 
 
 , unknown. 
 
 It would seem that at the higher elevations there is an almost entire 
 absence of water. Vide Report of British Association, 1862. 
 
 * Notes on Virginia, p. 132. f Meteor. Lib. 1. c. x. 
 
 J De Primo Fiigido. 
 
 G2 
 
81 ESSAY ON DEW, ETC. 
 
 in the former than in the latter place. If the greater agita- 
 tion of the atmosphere, and the less quantity of moisture 
 during clear weather, in its higher region than in the lower 
 be added, it may readily be inferred that dew shall sometimes 
 be altogether wanting on a hill, though abundant on a plain 
 at its foot, agreeably to what has been actually observed by 
 Mr. Jefferson. 
 
 IX. The leaves of trees often remain dry throughout the 
 night, while those of grass are covered with dew. As this is 
 a similar fact to the smallness of dew on hills, I shall in 
 accounting for it do little more than enumerate the circum- 
 stances on which it depends. 
 
 1. The atmosphere is several degrees warmer near the 
 upper parts of trees on dewy nights than close to the ground. 
 2. The air in the higher situation is more agitated than that 
 in the lower. 3. The air at a little distance from the ground, 
 from being nearer to one of its sources of moisture, will on a 
 calm evening contain more of it than that which surrounds 
 the leaves of elevated trees. 4. Only the leaves of the very 
 tops of trees are fully exposed to the sky. 5. The declen- 
 sion of the leaves from an horizontal position will occasion 
 the air, which has been cooled by them, to slide quickly away 
 and be succeeded by warmer parcels. 6. The length of the 
 branches of the trees, the tenderness of their twigs, and the 
 pliancy of the foot-stalks of their leaves, will cause in the 
 leaves an almost perpetual motion, even in states of air that 
 may be denominated calm. I have hence frequently heard, 
 during the stillness of night, a rustling noise in the trees, 
 which formed one of the boundaries of the ordinary place of 
 my observations, while the air below seemed without motion. 
 
 Nearly in the same manner is to be explained why shrubs 
 and bushes also receive dew more readily than lofty trees. 
 
 X. Bright metals, exposed to a clear sky in a calm night, 
 will be less dewed on their upper surface than other solid 
 
ESSAY ON DEW, ETC. 85 
 
 bodies ; since of all bodies they will, in such a situation, lose \ 
 the smallest quantity of heat by radiation to the heavens, at 
 the same time that they are capable of receiving, by conduction, 
 at least as much heat as any others from the atmosphere, and 
 more than any others from the warmer solid substances which 
 they happen to touch. 
 
 If the exposed pieces of metal be not very small, another 
 reason will contribute somewhat to their being later and less 
 dewed than other solid substances. For, in consequence of 
 their great conducting power, dew cannot form upon them, 
 unless their whole mass be sufficiently cold to condense the 
 watery vapour of the atmosphere ; while the same fluid will 
 appear on a bad conductor of heat, though the parts a very 
 little beneath the surface are warmer than the air.* 
 
 From the same ready passage of heat from one part of a 
 metal to another, a metallic plate suspended, horizontally, in 
 the air several feet above the ground, will be found dewed on 
 its lower side if the upper has become so; while the lower 
 surface of other bodies more attractive of dew, but worse 
 conductors of heat, are without dew in a similar situation. 
 
 A metal placed at night in the air near to the ground is, 
 for the most part, sufficiently cold to condense, on its under- 
 side, the vapour which arises from the warmer earth, though 
 its upper surface may be dry, from possessing the same, or 
 almost the same temperature, as the atmosphere near to it. 
 
 As the temperature of metals is never much below that of 
 the neighbouring air, a slight diminution of their cold from 
 radiation will often occasion them to evaporate the dew which 
 
 * I hence think it probable that dew will sometimes form on the bulb of 
 a thermometer, before the mercury in it ia cooled below the temperature of 
 the air. It seems certain to me, also, that dew may appear upon substances 
 which, from the thinness of the layer of matter their cold is confined to, 
 will produce little or no sensible effect upon a thermometer that is applied 
 to them. 
 
86 ESSAY ON DEW, ETC, 
 
 they had previously acquired, though other substances which 
 had been more cooled by radiation are still attracting dew. 
 For a like reason a metal, which has been purposely wetted, 
 will often become dry at night, while other substances are 
 becoming moist. 
 
 A substance highly attractive of dew, such as wool, if laid 
 i;pon a metal, will derive heat from it, and will therefore 
 acquire less dew than an equal portion of the same substance 
 laid upon grass. 
 
 A large metallic plate will be less readily dewed while 
 lying on grass than if it were placed in the air, though only 
 a few inches above the grass, because, in the former situation 
 it receives freely, by means of its great conducting power, 
 heat from the earth; whereas, when placed in the air, it 
 powerfully resists by another property, possessed in a great 
 degree by bright metals, the entrance of heat radiated towards 
 it by the grass beneath. Besides, the grass under the metal 
 possesses now less heat than when this substance was in con- 
 tact with it, partly from having a small oblique aspect of the 
 sky, and partly from receiving air which has been cooled by 
 passing over other grass fully exposed to the heavens. 
 
 When a piece of metal, having closely applied to its under 
 surface a substance of some thickness, which attracts dew 
 powerfully, and, therefore, imbibes readily heat that is radia- 
 ted to it, is exposed to the sky at night, the heat supplied 
 by the attached substance, both from its own original store and 
 from what it has acquired through the radiation of the ground, 
 to it during the exposure, will enable this piece to resist 
 longer than a bare piece the formation of dew, or even than 
 another piece which has only a thin coat of matter consider- 
 ably attractive of dew attached to its underside. The expert 
 ment with the wooden cross, covered with gilt paper, affords 
 an example of the latter fact. 
 
 A very small metallic plate, suspended in the air, is less 
 
ESSAY ON DEW, ETC. 87 
 
 readily dewed than a large one similarly situated, as it 
 receives, in proportion to its size, more heat from the atmos- 
 phere. On the other hand, a very small plate laid upon 
 grass, rendered cold by radiation, will be sooner dewed than 
 a larger one in the same situation, from presenting a greater 
 proportional circumference to the surrounding grass, and 
 therefore losing more quickly its heat by conduction. It 
 will be also sooner dewed than another very small plate sus- 
 pended in the air ; since the latter, like other small bodies 
 similarly placed, must be continually acquiring more heat than 
 the former, in the manner described above in this Essay .* 
 
 A piece of metal, applied to different portions of cold 
 grass in succession, will sooner become cold itself than 
 another piece which is suffered to remain constantl} r upon 
 one portion of the same grass, and will in consequence be 
 sooner dewed. 
 
 If the bare side'of a piece of metalled paper be exposed to 
 a clear and calm sky at night, it will become cold by radia- 
 tion, and receive by conduction, the heat of the inferior 
 metallic surface ; whence, if this surface be afterwards made 
 the upper one, it will sooner acquire dew than a similar 
 metallic surface which has been exposed to the sky during 
 the whole of the experiment. 
 
 When a metal covers, in part only, the upper surface of a 
 piece of glass, the uncovered portion of the glass quickly be- 
 comes cold by radiation, on exposure to a serene sky in a still 
 night, and then, by deriving to itself a part of the heat of the 
 metal,, occasions this body to be more readily dewed than if the 
 whole of the exposed surface had been metallic. In this experi- 
 ment the outer edge of the metallic surface, from being nearest 
 to the colder glass, will be the first and most dewed, while the 
 parts of the uncovered glass, which are contiguous to the 
 
 * Page 73. 
 
88 ESSAY ON DEW, ETC. 
 
 warmer metal, will be the last and the least dewed of their 
 respective substances. 
 
 A piece of glass, covered on one side with a metal, being 
 placed on grass with this side down, its upper surface attracts 
 dew as readily as if no metal were attached to it ; since the 
 metal in this situation has no power to lessen the radiation 
 of heat from the upper surface of the glass. I conclude, 
 however, from general principles, for I have not made the 
 trial, that if the same piece of glass, having its metallic side 
 still undermost, were raised in the air a little above the grass, 
 it would be more readily dewed on its upper surface than if 
 it had been without a metallic coating on the lower, as this 
 coating must resist the introduction of heat radiated by the 
 warmer grass, and thus preserve nearly undiminished the 
 cold acquired from radiation of heat to the sky by the bare 
 upper surface. 
 
 The preceding remarks apply to the whole class of metals 5 
 but the discoveries of Mr. Leslie, respecting the difference in 
 the capacities of these bodies to radiate heat, furnish an 
 explanation of a diversity among themselves in regard to 
 attraction for dew, which was noted in the foregoing part of 
 this Essay. Gold, silver, copper and tin, are there said to resist 
 the formation of dew more strongly than other substances of 
 the same class; but these metals, according to Mr. Leslie, 
 radiate heat the most sparingly. On the other hand, lead, 
 iron, and steel, which, according to the same author, radiate 
 heat more copiously than the former metals, were found by 
 me to acquire dew more readily. I do not know if the 
 r radiating power of platina has been ascertained by direct 
 experiments ; but, as its conducting power is small, its radia- 
 tion must be great, since these qualities exist always in 
 opposite degrees in the same substance; and I have accor- 
 dingly observed it to be dewed while the four first mentioned 
 metals were dry. I am ignorant both of the radiating and 
 
EB8AY ON DEW, ETC. 89 
 
 the conducting power of zinc, as determined by ordinary ; 
 experiments ; but I infer, from its being more easily dewed 
 than gold or silver, that it radiates heat more copiously than ; 
 they do ; unless indeed, the pieces which I used, from having 
 had their surfaces roughened by friction with sand, which 
 was employed to brighten them, had acquired a radiating 
 power greater than that possessed by polished pieces, agree- 
 ably to the results of some of Mr. Leslie's experiments.* 
 
 XI. Thinking it probable that black bodies might radiate 
 .more heat to the sky at night than white, I placed upon 
 grass, on five different evenings, equal parcels of black and 
 white wool. On four of the succeeding mornings the black 
 wool was found to have acquired a little more dew than the . 
 white, whence I inferred that it had, in consequence of its 
 colour, radiated a little more heat. But I afterwards re- 
 marked that the white wool was somewhat coarser than the 
 black ; which circumstance alone was sufficient to occasion a 
 difference in their quantities of moisture. Another night I 
 laid on the raised board a piece of pasteboard covered with 
 white paper, and close to this a second piece, similar to the 
 
 ^ 
 
 * I once intended to subjoin here an explanation of some very curious 
 observations by M. Benedict Prevost on dew, which were published, first 
 in the 44th volume of the French Annals of Chemistry, and afterwards by 
 M. Peter Prevost, of Geneva, in his Essay on Radiant Heat ; but fearing to 
 be very tedious, I have since given up the design. I will say, however, that 
 if to what is now generally known on the diiferent modes in which heat is 
 communicated from one body to another, be added the two following circum- 
 stances : that substances become colder, by radiation, than the air, before 
 they attract dew ; and that bright metals, when exposed to a clear sky at 
 night, become colder than the air much less readily than other bodies; 
 the whole of the appearances observed by M. Prevost may be easily ac- 
 counted for. 
 
 Note to Second Edition. I found, shortly after the publication of the | 
 former edition of this Essay, that the learned Dr. Young had, several years ; 
 before, in his great work on Natural Philosophy, employed the principle of j 
 the radiation of heat to account for several of the facts observed by M. B. I 
 Prevost. On the subject of Dr. Young's explanation, I have spoken some- ; 
 what fully in the 28th number of Dr. Thomson's Annals of Philosophy. 
 
90 ESSAY ON DEW, ETC. 
 
 former in every respect, except that it was covered with 
 paper blackened with ink. At daylight I saw hoarfrost upon 
 both pieces, but the black seemed to have a greater quantity 
 than the white. A doubt, however, afterwards arose upon 
 the accuracy of this experiment likewise, for, as the light was 
 faint when I viewed the two surfaces, the quantity of hoar- 
 frost, though equal on both, might have appeared greater on 
 the black than on the white, from the contrast of its colour 
 with that of the former surface. But trials of this kind, as 
 Mr, Leslie* has ojbserved, never afford firm conclusions, since 
 a black body must always differ from a white in one or 
 more chemical properties, and this difference may of itself be 
 competent to produce a diversity in their powers to radiate 
 heat. 
 
 With the view to render the subject less complicated, I 
 have hitherto treated of dew, as if it were altogether derived 
 from watery vapour previously diffused through the atmos- 
 phere ; this appearing to me to be by far its most considerable 
 source, and none of my conclusions of any importance being 
 liable to be effected, even by the establishment of a contrary 
 opinion. Other writers, however, have regarded dew as being 
 entirely the product of vapour emitted during the night by 
 the earth and plants upon it. According to this theory dew 
 is said to rise. 
 
 The first trace which I have found of the opinion that 
 dew rises from the earth at night, occurs in the History of 
 the Academy of Sciences for 1687. It is mentioned there 
 briefly and obscurely, and was, probably, shortly for- 
 gotten, for Gersten, who advanced it anew in 1733, held 
 himself to be its author. Musschenbroek and Dufay embraced 
 it immediately after Gersten, but the former soon admitted that 
 
 * On Heat, p. 95. 
 
ESSAY ON DEW, ETC. 91 
 
 dew sometimes falls. As far as I have learned, no writer 
 upon dew has since ascribed its total production to vapour, 
 emitted by the earth at night, except Mr. Webster of New 
 England.* But this opinion is frequently advanced in con- 
 versation by persons not much accustomed to philosophical 
 pursuits, chiefly, I think, because it contradicts a popular 
 belief. 
 
 The only argument used by the French academicians in 
 support of their opinion is, if I understand it rightly, that as 
 much dew is observed under an inverted glass-bell as in any 
 other situation. But admitting, for a moment, this to be 
 true, they would not thus prove that the ground is the only 
 source of that fluid. 
 
 Gersten was led to think that dew rises from the earth, 
 by often finding grass and low shrubs moistened with it, 
 while trees were dry. Respecting this fact, I shall add 
 nothing to what I have lately said upon it. But his chief 
 argument is derived from another fact related in the first 
 part of this Essay, which is, that a plate of metal, laid upon 
 bare earth on a dewy night will remain dry on its upper 
 surface, while it becomes moist on the lower. This also is 
 easily explicable by what has already been mentioned by me. 
 For the lower side of the metal, in consequence of the upper 
 being in contact with the air and being exposed to a clear 
 sky, is colder than the earth a little below the surface, and 
 therefore condenses the vapour, which strikes against its 
 bottom ; while the upper side, from being frequently warmer, 
 and never more than a little colder than the air, is for the 
 most part unable to condense the watery vapour of the 
 atmosphere. f Gersten, moreover, describes several appear- 
 ances himself which refute his opinion. He mentions, for 
 
 * Mem. of American Acad. vol. III. 
 
 t I have, in like manner, observed, on a cloudy night, a piece of glass, 
 laid over an earthen pan containing water and placed upon the ground, to 
 
92 ESSAY ON DEW, ETC. 
 
 example, that the higher parts of shrubs are more dewed 
 than the lower ; that metallic plates, placed horizon- 
 tally in the air, are as much dewed on their superior as on 
 their inferior surfaces; and that convex and cylindrical 
 bodies, suspended in the air, the latter having a position 
 parallel to the horizon, are dewed only on their upper parts. 
 
 The principal reason given by Dufay for the rising of dew 
 is, that it appears more early on bodies near to the earth 
 than on those which are at a greater height. But this fact 
 readily admits of an explanation, on other grounds, that have 
 already been mentioned. 1. The lower air, on a clear and 
 calm evening, is colder than the upper, and will, therefore, 
 be sooner in a condition to deposit a part of its moisture. 
 2. It is less liable to agitation than the upper. 3. It con- 
 tains more moisture than the upper, from receiving the last 
 which has risen from the earth, in addition to what it had 
 previously possessed in common with other parts of the 
 atmosphere. Dufay attempted to strengthen his argument 
 by exposing, on three dewy nights, similar substances at 
 different heights from the ground, expecting that Ihe lower 
 would always acquire more moisture than the upper; but, 
 upon all the nights some one of the lower substances acquired 
 less moisture than some one of the higher. 
 
 Mr. Webster has advanced no new fact in favour of the 
 opinion of which I am speaking. 
 
 Enough having been said to pro^e that dew is not entirely 
 the product of vapour rising from the earth at night, I shall 
 next show that it often occurs when this cause can have little 
 or no operation. 
 
 1. It appears from Hasselquist and Bruce that in Egypt, 
 shortly before the rising of the Nile, and consequently when 
 
 be wet on its lower side, while tlie upper was dry, the glass being, in this 
 situation, sufficiently cold to condense the vapour of water heated by the 
 earth, but not enough so to condense the watery vapour of the atmosphere. 
 
ESSAY ON DEW, ETC. 93 
 
 the ground there is in its driest state, dew becomes exceed- 
 ingly plentiful, though little or none had formed before, 
 while the earth was somewhat less dry. The cause evidently 
 is, as was formerly mentioned, the moist air brought from 
 the Mediterranean by the north wind which then prevails. 
 
 2. Mr. Webster, speaking of hoarfrost, which he properly 
 regards as frozen dew, candidly says, though it overthrows 
 his opinion : " This frost appears when the surface of the 
 earth is sealed with frost, and of course the vapour of which 
 it is formed cannot at the time perspire from the earth." 
 
 3. I have myself at all seasons of the year frequently ob- 
 served wool upon the middle of the raised board, and, there- 
 fore, out of the way of vapour rising from the ground, to 
 acquire more dew than wool laid upon the grassplat. 
 
 4. The bodies that condense the rising vapour must neces- 
 sarily be colder than it ; but, as they are likewise, according 
 to the opinion under view, of the same temperature with the 
 air surrounding them, this also should condense the rising 
 vapour. Dew, therefore, should never appear in any con- 
 siderable quantity, without being accompanied with fog or 
 mist. Now I can assert after much attention to this point, 
 that the formation of the most abundant dew is consistent 
 with a pellucid state of the atmosphere. Hasselquist makes 
 a similar observation with regard to Egypt ; where, during 
 the season remarkable for the most profuse dews, "the 
 nights," he says, e( are as resplendent with stars, in the 
 midst of summer, as the lightest and clearest winter nights 
 in the north." 
 
 But, although these facts prove that copious dews may 
 occur with little or no contribution by vapour immediately 
 rising from the earth, it must yet be admitted that some of 
 the moisture which forms during clear and still weather 
 on bodies situated upon or near its surface is in most cases to 
 be attributed to this source; since, in my experiments, 
 
94 ESSAY ON DEW, ETC. 
 
 substances on the raised board became much, later moist than 
 others on the ground, though equally cold with them. The 
 quantity from this cause, however, can never be great. For, 
 in the first place, until the air be cooled, by the substances 
 attractive of dew with which it comes in contact, below its 
 point of repletion with moisture, it will be always in a condi- 
 tion to take up that which has been deposited upon grass, or 
 other low bodies, by warm vapour emitted by the earth ; just 
 as the moisture formed upon a mirror by our breath is, in 
 temperate weather, almost immediately carried away by the 
 surrounding air. Accordingly, I have sometimes in serene 
 and still weather observed dew to appear sparingly upon grass 
 in the shade several hours before sunset, and to continue in 
 nearly the same quantity till about sunset, when it would in- 
 crease considerably at the time that the same fluid began to 
 show itself on the raised board. In the second place, though 
 bodies situated on the ground after they have been made suf- 
 ficiently cold by radiation to condense the vapour of the 
 atmosphere will be able to retain the moisture, which they 
 acquire by condensing the vapour of the earth ; yet, before 
 this happens, the rising vapour must have been greatly 
 diminished by the surface of the ground having become much 
 colder. These considerations, added to the fact that sub- 
 stances on the raised board attracted rather more dew 
 throughout the night than similar substances lying on the 
 grass, warrant me to conclude that on nights favourable to 
 the production of dew, only a very small part of what occurs 
 is owing to vapour rising from the earth ; though I am ac- 
 quainted with no means of determining the proportion of this 
 part to the whole. On the other hand, however, in a cloudy 
 night all the dew that appears upon grass may sometimes be 
 attributed to a condensation of the earth's vapour ; since I 
 have several times in such nights remarked the raised board 
 to be dry, while the grass was moist. These nights were 
 
ESSAY ON DEW, ETC. 95 
 
 calm, and evaporation from the grass consequently not copious. 
 When evaporation on cloudy nights was assisted by wind, dew 
 has never, as was mentioned in the first part of this Essay, 
 been anywhere observed by me.* 
 
 Agreeably to another opinion, the dew found upon grow- 
 ing vegetables is the condensed vapour of the very plants on 
 which it appears. But this also seems to me erroneous, for 
 several reasons. 1. Dew forms as copiously upon dead as 
 upon living vegetable substances. 2. The transpired humour 
 of plants will be carried away by the air which passes over 
 them when they are not sufficiently cold to condense the 
 watery vapour contained in it; unless, which is almost never 
 the case if mist does not already exist, the general mass of 
 the atmosphere be incapable of receiving moistute in a pellucid 
 form. Accordingly, on cloudy nights when the air, conse- 
 quently, can never be cooled more than a little below the 
 point of repletion with moisture, by bodies in contact with it, 
 dew is never observed upon any plants that are elevated a 
 few feet above the ground. 3. If a plant has become, by 
 radiating its heat to the heavens, so cold as to be enabled to 
 bring the air in contact with it below the point of repletion 
 with moisture, that which forms upon it, from its own tran- 
 spiration will not then, indeed, evaporate. But other moisture 
 
 * The interval between the first appearance of dew in the afternoon on 
 grass, in shaded places, and sunset, was formerly said by me on the authority, 
 however, of only a few observations, to be considerably greater than that 
 between sunrise and the ceasing of the formation of dew upon grass in the 
 morning. These observations were made on spots exposed during the greater 
 part of the day to the sun. In such places the heat acquired from the sun, 
 by the uppermost layer of earth, will be longer retained than that acquired 
 by the grass, which will, therefore, be sufficiently coel soon after the heat 
 of the day has declined to condense a part of the vapour then copiously 
 rising from the earth, whereas in the morning both less vapour will rise, the 
 surface of the earth having now lost a great part of its heat, and a less pro- 
 portion of that which does rise will be condensed by the grass, as the tem- 
 perature of this body now more nearly approaches that of the ground, from 
 first receiving the heat of the sun reflected from the atmosphere and other 
 substances. 
 
96 ESSAY ON DEW, ETC. 
 
 will, at the same time, be communicated to it by the 
 atmosphere ; and when the difference in the copiousness of 
 these two sources is considered, it may, I think, be safely 
 concluded that almost the whole of the dew, which will after- 
 wards form on the plant, must be derived from the air ; more 
 especially when the coldness of a clear night, and the general 
 inactivity of plants in the absence of light, both lessening 
 their transpiration, are taken into account. 
 
 An experiment, however, has been appealed to in proof, 
 that the dew of plants actually does originate from fluid 
 transpired by them ; that, namely, in which a plant, shut up 
 in an air-tight case, becomes covered with moisture. But 
 this experiment, if attentively examined, will be found to 
 have little weight. First ; the inclosed plant, being exempt 
 from the cold, which its own radiation would have produced 
 in its natural situation, on a dewy night, will transpire a 
 greater quantity of fluid, than a similar plant exposed at the 
 same time to the open air. Again ; the small quantity of air 
 contained in the case, must soon be replete with moisture, after 
 which the whole of what is further emitted by the plant will 
 necessarily assume the form of a fluid, whatever may be the 
 condition of the external atmosphere ; whereas, during even 
 the clearest night, only a part of the smaller quantity of 
 moisture emitted by the exposed plant, will be condensed on 
 its surface. In the last place, notwithstanding the circum- 
 stances which favour the appearance of moisture upon in- 
 closed plants from their own transpiration, still the quantity 
 observed on them is said to be, for I have made no experi- 
 ment myself respecting this matter, much less considerable 
 than what is seen upon plants of the same kind, exposed to 
 the air for the same time, during a calm and serene night. 
 
PART III. 
 
 OF SEVERAL APPEARANCES CONNECTED WITH 
 
 DEW. 
 
 THERE are various occurrences in nature which seem to me 
 strictly allied to dew, though their relation to it be not 
 always at first sight perceivable. The statement and expla- 
 nation of several of these will form the concluding part of the 
 present Essay. 
 
 I. I observed one morning in winter that the insides of 
 the panes of glass in the windows of my bedchamber were all 
 of them moist, but that those which had been covered by an 
 inside shutter during the night were much more so than 
 others which had been uncovered. Supposing that this 
 diversity of appearance depended upon a difference of tem- 
 perature, I applied the naked bulbs of two delicate thermo- 
 meters to a covered and uncovered pane, on which I found 
 that the former was 8 colder than the latter. The air of 
 the chamber, though no fire was kept in it, was at this time 
 114 warmer than that without. Similar experiments were 
 made on many other mornings, the results of which were : 
 that when the warmth of the internal air exceeded that of 
 the external, from 8 to 18, the temperature of the covered 
 panes would be from 1 to 5 less than that of the uncovered ; 
 that the covered were sometimes dewed while the uncovered 
 were dry ; that at other times both were free from moisture ; 
 that the outsides of the covered and uncovered panes had 
 similar differences with respect to heat, though not so great 
 as those of the inner surfaces ; and that no variation in the 
 quantity of these differences was occasioned by the weather's 
 
 H 
 
98 ESSAY ON DEW, ETC. 
 
 being cloudy or fair, provided the heat of the internal air ex- 
 ceeded that of the external equally in both of those states of 
 the atmosphere. 
 
 The remote reason of these differences did not immediately 
 present itself. I soon, however, saw that the closed shutter 
 shielded the glass, which it covered, from the heat, that was 
 radiated to the windows by the walls and furniture of the 
 room, and thus kept it nearer to the temperature of the ex- 
 ternal air, than those parts could be, which, from being 
 uncovered, received the heat emitted to them by the bodies 
 just mentioned. 
 
 In making these experiments I seldom observed the 
 inside of any pane to be more than a little damped, though 
 it might be from 8 to 12 colder than the general mass of 
 the air in the room ; while, in the open air, I had often found 
 a great dew to form on substances only 3 or 4 colder than 
 the atmosphere. This at first surprised me ; but the cause 
 now seems plain. The air of the chamber had once been a 
 portion of the external atmosphere, and had afterwards been 
 heated, when it could receive little accession to its original 
 moisture. It consequently required being cooled consider- 
 ably, before it was even brought back to its former nearness 
 to repletion with water ; whereas the whole external air is 
 commonly, at night, nearly replete with moisture, and there- 
 fore readily precipitates dew on bodies only a little colder 
 than itself. 
 
 When the air of a room is warmer than the external 
 atmosphere, the effect of an outside shutter on the tempera- 
 ture of the glass of the window will be directly opposite to 
 what has been just stated, since it must prevent the radiation 
 into the atmosphere of the heat of the chamber transmitted 
 through the glass. 
 
 II. Count Rumford* appears to have rightly conjectured 
 
 * Phil. Trans. 1804. p. 182. 
 
ESSAY ON DEW, ETC. 99 
 
 that the inhabitants of certain hot countries, who sleep at 
 nights on the tops of their houses, are cooled during this expo- 
 sure by the radiation of their heat to the sky ; or, according 
 to his manner of expression, by receiving frigorific rays from 
 the heavens. Another fact of this kind seems to be the 
 greater chill which we often experience upon passing, at 
 night, from the cover of a house into the open air, than might 
 have been expected from the cold of the external atmosphere. 
 The cause, indeed, is said to be the quickness of transition 
 from one situation to another. But, if this were the whole 
 reason, an equal chill would be felt in the day, when the 
 difference in point of heat between the internal and external 
 air was the same as at night, which is not the case. Besides, 
 if I can trust my own observation, the feeling of cold from 
 this cause is more remarkable in a clear than in a cloudy 
 night, and in the country than in towns* The following 
 appears to be the manner in which these things are chiefly 
 to be explained. 
 
 During the day our bodies while in the open air> although 
 not immediately exposed to the sun's rays, are yet constantly 
 deriving heat from them, by means of the reflection of the 
 atmosphere. This heat, though it produces little change on ~j 
 the temperature of the air which it traverses, affords us some I 
 compensation for what we radiate to the heavens. At night,J 
 also, if the sky be overcast, some compensation will be made 
 to us, both in towns and in the country, though in a less 
 degree than during the day, as the clouds will remit towards 
 the earth no inconsiderable quantity of heat. But on a clear 
 night, in an open part of the country, nothing almost can be 
 returned to us from above, in place of the heat which we 
 radiate upwards. In towns, however, some compensation 
 will be afforded, even on the clearest nights, for the heat 
 which we lose in the open air, by that which is radiated to 
 us by the surrounding buildings. 
 
100 ESSAY ON DEW, ETC. 
 
 To our loss of heat by radiation, at times that we derive 
 little compensation from the radiation of other bodies, is 
 probably to be attributed a great part of the hurtful effects 
 of the night air. Descartes* says that these are not owing 
 to dew, as was the common opinion of his cotemporaries, but 
 to the descent of certain noxious vapours, which having been 
 exhaled from the earth during the heat of the day, are after- 
 wards condensed by the. cold of a serene night. The effects 
 in question certainly cannot be occasioned by dew, since that 
 fluid does not form upon a healthy human body in temperate 
 climates ; but they may, notwithstanding, arise from the 
 same cause that produces dew on those substances, which do 
 not, like the human body, possess the power of generating 
 heat, for the supply of what they lose by radiation or any 
 other means. 
 
 III. I had often, in the pride of half knowledge, smiled 
 at the means frequently employed by gardeners to protect 
 tender plants from cold, as it appeared to me impossible that 
 a thin mat, or any such flimsy substance, could prevent them 
 from attaining the temperature of the atmosphere, by which 
 alone I thought them liable to be injured. But, when I had 
 learned that bodies on the surface of the earth become, during 
 a still and serene night, colder than the atmosphere, by 
 radiating their heat to the heavens, I perceived immediately 
 a just reason for the practice, which I had before deemed 
 useless. Being desirous, however, of acquiring some precise 
 information on this subject, I fixed, perpendicularly, in the 
 earth of a grassplat, 4 small sticks, and over their upper 
 extremities, which were 6 inches above the grass, and formed 
 the corners of a square, the sides of which were 2 feet long, 
 drew tightly a very thin cambric handkerchief. In this dis- 
 position of things, therefore, nothing existed to prevent the 
 free passage of air from the exposed grass to that which was 
 
 * Meteorolog. c. vi. 
 
ESSAY ON DEW, ETC. 101 
 
 sheltered, except the 4 small sticks, and there was no substance 
 to radiate heat downwards to the latter grass, except the cam- 
 bric handkerchief. The temperature of the grass, which was 
 thus shielded from the sky, was upon many nights afterwards 
 examined by me, and was always found higher than that of 
 neighbouring grass which was uncovered, if this was colder 
 than the air. 88 When the difference in temperature between 
 the air several feet above the ground and the unsheltered 
 grass did not exceed 5, the sheltered grass was about as 
 warm as the air. If that difference, however, exceeded 5, 
 the air was found to be somewhat warmer than the sheltered 
 grass. Thus, upon one night, when fully exposed grass was 
 1 1 colder than the air, the latter was 3 warmer than the 
 sheltered grass ; and the same difference existed on another 
 night, when the air was 14 warmer than the exposed grass. 
 One reason for this difference, no doubt, was that the air, 
 which passed from the exposed grass, by which it had been 
 very much cooled, to that under the handkerchief, had de- 
 prived the latter of part of its heat ; another, that the hand- 
 kerchief, from being made colder than the atmosphere by 
 the radiation of its upper surface to the heavens, would remit 
 somewhat less heat to the grass beneath than what it received 
 from that substance. But still, as the sheltered grass, not- 
 withstanding these drawbacks, was upon one night, as may 
 be collected from the preceding relation, 8, and upon another 
 11, warmer than grass fully exposed to the sky, a sufficient 
 reason was now obtained for the utility of a very slight 
 shelter to plants, in averting or lessening injury from cold 
 on a still and serene night. 
 
 In the next place, in order to learn whether any difference 
 would arise from placing the sheltering substance at a much 
 
 38 Patrick Wilson had previously written : " I fastened with packthread 
 a piece of open gauze to a hoop of 8 inches in diameter, and an inch deep ; 
 and, when the thermometers were sheltered in this manner, the quicksilver 
 commonly rose nearly 2V 
 
102 ESSAY ON DEW, ETC. 
 
 greater distance from the ground, I had 4 slender posts driven 
 perpendicularly into the soil of a grass field, and had them so 
 disposed in other respects that their upper ends were 6 feet 
 above the surface, and formed the angular points of a square 
 having sides 8 feet in length. Lastly, over the tops of the 
 posts was thrown an old ship flag of a very loose texture. 
 Concerning the experiments made by means of this arrange- 
 ment of things, I shall only say that they led to the conclusion, 
 as far as the events of different nights could rightly be com- 
 pared, that the higher shelter had the same efficacy with the 
 lower in preventing the occurrence of cold upon the ground 
 in a clear night, greater than that of the atmosphere, provided 
 the oblique aspect of the sky was equally excluded from the 
 spots on which my thermometers were laid. 
 
 On the other hand, a difference in temperature, of some 
 magnitude, was always observed on still and serene nights 
 between bodies sheltered from the sky by substances touching 
 them, and similar bodies which were sheltered by a substance 
 3, little above them, I found, for example, upon one night, 
 that the warmth of grass, sheltered by a cambric handkerchief 
 raised a few inches in the air, was 3* greater than that of a 
 neighbouring piece of grass, which was sheltered by a similar 
 handkerchief actually in contact with it. On another night 
 the difference between the temperatures of two portions of 
 grass, shielded in the same manner as the two abo^e men^ 
 tioned from the influence of the sky, was 4. Possibly, 
 experience has long ago taught gardeners the superior 
 advantage of defending tender vegetables from the cold of 
 clear and calm nights by means of substances not directly 
 touching them, though I do not recollect ever having seen 
 any contrivance for keeping mats, or such like bodies, at a 
 distance from the plants which they were meant to protect. 39 
 
 39 "Anything which obstructs the free aspect of the sky arrests, in propor- 
 tion, the progress of refrigeration, and the slightest covering of cloth or 
 matting annihilates it altogether. Trees trained upon a wall or paling, or 
 
ESSAY ON DEW, ETC. 103 
 
 Walls, I believe, as far as warmth is concerned, are re- 
 garded as useful during a cold night to the plants which 
 touch them or are near to them, only in two ways ; first, by 
 the mechanical shelter which they afford against cold winds, 
 and secondly, by giving out the heat which they had acquired 
 during the day. It appearing to me, however, that on clear 
 and calm nights, those on which plants frequently receive 
 much injury from cold, walls must be beneficial in a third 
 way, namely, by preventing, in part, the loss of heat which 
 they would sustain from radiation if they were fully exposed 
 to the sky, the following experiment was made for the 
 purpose of determining the justness of this opinion. 
 
 A cambric handkerchief having been placed, by means of 
 two upright sticks, perpendicularly to a grassplat, and at 
 right angles to the course of the air, a thermometer was laid 
 
 plants sown under their protection, are at once cut off from a large portion of 
 this evil; and are still further protected if within a moderate distance 
 of another opposing screen. * * * * 
 
 "Experience has taught gardeners the advantages of warding off the 
 effects of frost from tender vegetables, by loose straw or other litter ; but 
 the system of matting does not appear to be carried to that extent which its 
 simplicity and efficacy would suggest. Neither does the manner of fixing the 
 screen exhibit a proper acquaintance with the principle upon which it is 
 resorted to : it is generally bound tight round the tree which it is required 
 to protect, or nailed in close contact with its foliage. 
 
 " Now it should be borne in mind that the radiation is only transferred 
 from the tree to the mat, and the cold of the latter will be conducted to the 
 former in every point where it touches. Contact should, therefore, be pre- 
 vented by hoops or other means properly applied, and the stratum of air 
 which is enclosed will, by its low conducting power, effectually secure the 
 plant. With their foliage thus protected, and their roots well covered with 
 litter, many evergreens might doubtless be brought to survive the rigour of 
 our winters, which are now confined to the stunted growth of the greenhouse 
 and conservatory." Daniell: Climate considered with regard to Horticulture. 
 " Various experiments were made to ascertain the effect of covering plants 
 at night by matting, or other thin substances ; and it was always found that 
 when the protecting substances touched the plant much heat was conducted 
 away from it, and such plant was at a lower temperature than when the sub- 
 stance was merely interposed between it and the sky ; the thinnest substance 
 
104 ESSAY ON DEW, ETC. 
 
 upon the grass close to the lower edge of the handkerchief 
 on its windward side. The thermometer thus situated was 
 several nights compared with another lying on the same 
 grassplat, but on a part of it fully exposed to the sky. On 
 two of these nights, the air being clear and calm, the grass 
 close to the handkerchief was found to be 4 warmer than 
 the fully exposed grass. On a third the difference was 6. 
 An analogous fact is mentioned by Gersten, who says that 
 a horizontal surface is more abundantly dewed than one 
 which is perpendicular to the ground. 
 
 IY. The covering of snow, which countries in high lati- 
 tudes enjoy during the winter, has been very commonly 
 thought to be beneficial to vegetable substances on the surface 
 of the earth, as far as their temperature is concerned, solely 
 by protecting them from the cold of the atmosphere. But 
 were this supposition just, the advantage of the covering 
 would be greatly circumscribed, since the upper parts of trees 
 and of tall shrubs are still exposed to the influence of the air. 
 Another reason, however, is furnished for its usefulness, by 
 what has been said in this Essay ; which is, that it prevents 
 the occurrence of the cold, which bodies on the earth acquire, 
 in addition to that of the atmosphere, by the radiation of their 
 heat to the heavens during still and clear nights. The cause, 
 indeed, of this additional cold does not constantly operate, 
 
 thus interposed, at any distance from the plant, was found to be effectual iu 
 preventing the loss of its heat by radiation. It was found, however, that 
 when a plant was thus itself protected, but yet was exposed to any body 
 which was exposed to the sky, more heat radiated from the former to the 
 latter, than from the exposed body to the plant, and thus it lost some heat." 
 Glaisher : Phil. Trans. 1847. 
 
 " In the clear blue sky of the Valley of Chamouni, if the crops should 
 not have ripened towards the end of the season, the peasants make fires 
 of green wood on the two sides of the enclosing mountain, the smoke of 
 which, uniting in the middle, forms a kind of cloudy canopy, which not 
 only prevents the escape of radiant heat but increases its intensity, and 
 prevents the formation of frost." Quarterly Review, Vol. xxii. 
 
ESSAY ON DEW, ETC. 105 
 
 but its presence during only a few hours might effectually 
 destroy plants which now pass unhurt through the winter. 
 Again, as things are, while low vegetable productions are 
 prevented, by their covering of snow, from becoming colder 
 than the atmosphere in consequence of their own radiation, 
 the parts of trees and tall shrubs which rise above the snow 
 are little affected by cold from this cause. For their outer- 
 most twigs, now that they are destitute of leaves, are much 
 smaller than the thermometers suspended by me in the air, 
 which in this situation very seldom became more than 2 
 colder than the atmosphere. The larger branches too, which 
 if fully exposed to the sky, would become colder than the 
 extreme parts, are, in a great degree, sheltered by them ; and, 
 in the last place, the trunks are sheltered both by the smaller 
 and the larger parts, not to mention that the trunks must 
 derive heat by conduction through the roots, from the earth 
 kept warm by the snow.* 
 
 In a similar way is partly to be explained the manner in 
 which a layer of earth or straw preserves vegetable matters 
 in our own fields from the injurious effects of cold in winter. 
 
 V. The bare mention of the subject of this article will be 
 apt to excite ridicule, it being an attempt to show in what 
 way the exposure of animal substances to the moon's light 
 promotes their putrefaction. I have no certain knowledge 
 that such an opinion prevails anywhere at present, except in 
 the West Indies ; but I conclude, from various circumstances, 
 that it exists also in Africa, and that it was carried thence by 
 negro slaves to America. It was entertained, however, by 
 persons of considerable rank and intelligence among the 
 
 * It may be remarked here, however, that a thick covering of snow, while 
 it renders the surface of the earth warmer than it would otherwise be, must 
 occasion the lower atmosphere to be colder, by preventing the passage of the 
 heat of the ground to the air, either by radiation or conduction. 
 
106 ESSAY ON DEW, ETC. 
 
 ancients ; for Pliny* affirms it to be true, and Plutarch, after 
 making it a subject of discussion in one of his Symposia, f 
 admits it to be well founded. f 
 
 As moonbeams communicate no sensible heat to the 
 bodies on which they fall it seems impossible that they can 
 directly promote putrefaction. But still a reason for ascrib- 
 ing such a power to them may be derived from their being 
 received by animal substances, at the very time that a real, 
 but generally unnoticed, cause of putrefaction in warm 
 climates (and it is in these alone the opinion I am treating of 
 has ever prevailed) is taking place, which ceases to act as soon 
 as the moon's light is excluded. 
 
 The nights, on which a steady moonshine occurs, must 
 necessarily be clear ; and nights which are clear are almost 
 always calm.f A moonshiny night, therefore, is one on 
 which dew forms plentifully ; hence the expressions ' roscida : 
 and e rorifera luna* employed by Yirgil and Statius ; and 
 hence also an opinion, held, as appears from Plutarch, even 
 by philosophers among the ancients, that the moon communi- 
 cates moisture to the bodies which are exposed to its light. 
 
 Animal substances are among those which acquire dew in 
 the greatest quantity. To do this, indeed, they must pre- 
 viously become colder than the atmosphere ; but, having 
 acquired the moisture of dew, in addition to their own, they 
 will, on the following day, be in that condition, which is 
 
 * Lib. ii. . civ. f Lib. iii. Prob. x. 
 
 $ M. De Luc lias remarked that clouds frequently disappear soon after 
 sunset. Idees sur la Meteorologie, II. 98. I have often observed this myself, 
 and at the same time another fact of which he takes no notice ; namely, that 
 the atmosphere is then calmer than it had been before sunset. This calmness 
 of the air very commonly, if not always, precedes the dissipation of the clouds. 
 
 Akin to this opinion of the ancients respecting the humefying quality 
 of the moon, is one which has been held by modern writers as well as ancient, 
 upon that planet's being a cause of cold to the bodies which receive its rays ; 
 though I know of no author who has taken notice of this affinity. 
 
ESSAY ON DEW, ETC. 107 
 
 known, by experience, to favour putrefaction most powerfully 
 in hot climates. 
 
 The immediate cause assigned here for the quick putre- 
 faction of animal substances which have been exposed to the 
 moon's rays in a hot country, is the same as that given by 
 Pliny and Plutarch ; but they attributed the origin of this 
 immediate cause, the additional moisture, to the peculiar 
 Jiumefying quality which they supposed that luminary to 
 possess. This false theory has, probably, contributed to 
 discredit, with the moderns, the circumstance which it was 
 employed to explain. 
 
 VI. The last fact, of which I shall treat in this Essay, is 
 the formation of ice during the night in Bengal, while the 
 temperature of the air is above 32 . 
 
 I have seen only two original descriptions of this process, 
 both of which are contained in the Philosophical Transac- 
 tions ; the first, by Sir Robert Barker, in the 65th volume ; 
 the other in the 83rd, by Mr, Williams. 
 
 According to the method followed by Sir R. Barker's ice- 
 maker, square excavations, 2 feet deep and 30 wide, having 
 been formed in a large open plain, their bottoms are covered 
 with sugar-cane, or stems of Indian corn, dried, to the 
 thickness of 8 inches or 1 foot. On this layer are afterwards 
 placed, in rows, near to each other, small, unglazed earthen 
 pans, a quarter of an inch thick, and 1 inch and quarter deep, 
 filled with boiled soft water. The pans are sufiiciently porous 
 to allow their outer surface to appear moist after water has 
 been poured into them. Sir R. Barker adds ; that the nights, 
 the most favourable for the production of ice, are those which 
 are the calmest and most serene, and on which very little 
 dew appears after midnight ; that clouds and frequent 
 changes of wind are certain preventives of its formation; 
 and that, although ice is thus very readily procured by art in 
 
108 ESSAY ON DEW, ETC. 
 
 Bengal during the winter, it scarcely ever occurs there 
 naturally. 
 
 The process described by Mr. Williams must, from its 
 extent, 300 persons being employed in it, have been carried 
 on for profit, and would, consequently, be conducted in the 
 most economical manner. A piece of ground, nearly level, 
 containing about 4 acres, was divided into square plats, from 
 4 to 5 feet wide, which were surrounded by little mounds of 
 earth, 4 inches high. In these inclosures, previously filled 
 with dry straw, or sugar-cane haum, were placed as many 
 broad, shallow, unglazed earthen pans, containing unboiled 
 pump water as they could hold. The air was generally very 
 still, when much ice was formed ; wind prevented its forma- 
 tion altogether. In the morning, between 5 and 6h., at 
 which time alone Mr. Williams made his observations, a 
 thermometer, with its bulb naked, placed on the straw 
 amidst the freezing vessels, was never found by him lower 
 than 35 ; and he has observed ice, when a thermometer so 
 placed was 42. Another thermometer, suspended 5J feet 
 above the ground, was commonly 4 higher than that among 
 the pans. It is possible, therefore, that Mr. Williams may 
 have seen ice a little before sunrise, when the temperature of 
 the air was 46. But granting this were the fact, it would 
 not hence follow that the ice was formed while the air pos- 
 sessed that heat. For, although the air is generally held to 
 be in all countries colder about sunrise than at any other time, 
 I know from my own observations, that this is not always the 
 case in England ; and similar exceptions may occur in Bengal. 
 f Sir H. Davy has said, in his Elements of Chemistry, that ice 
 will form in Bengal when the temperature of the air is not 
 below 50 ; but he has given no authority for this assertion. 
 
 The formation of ice in the circumstances which have 
 been just mentioned, was attributed by Sir E. Barker alto- 
 gether, and by Mr. Williams in great measure, to cold 
 
ESSAY ON DEW, ETC. 109 
 
 produced by evaporation. Sir E. Barker's opinion has since ' 
 been adopted by some of our most distinguished writers on 
 Natural Philosophy, as Watson, Thompson, Young, Davy, 
 and Leslie, apparently, however, without their having fully ; 
 considered it, as I shall now attempt to show. 
 
 1. It is necessary for the complete success of the process 
 that the air should be very still; wind, which so greatly 
 promotes evaporation, prevents the freezing altogether. Sir 
 R. Barker admits that the excavations in the earth are made 
 to increase the stillness of the air in contact with the water 
 in the pans ; but, with the view to explain the utility of this 
 stillness he supposes, in opposition to all experience, that 
 water kept very quiet freezes more readily, when other 
 circumstances are the same, than if it were a little agitated. 
 
 2. No proof is given that evaporation from the pans 
 actually does occur at the times which are the most favour- 
 able for the appearance of ice. At any rate it cannot be 
 considerable, since, agreeably to what is mentioned by Sir E. 
 Barker, dew forms in a greater or less degree during the whole 
 of the nights the most productive of ice ; and it is not to be 
 thought, as was said upon a former occasion, that one portion 
 of air will be depositing moisture, from possessing a super- 
 abundance of it, while another in the immediate vicinity is 
 receiving moisture in great quantity in the state of pellucid 
 vapour, as the latter fact can exist only when the air is far 
 removed from a state of repletion with water. 
 
 3. If evaporation produced the cold under consideration, 
 the wetting of the straw or other matter upon which the 
 pans are placed would tend to increase it ; and, accordingly, 
 Sir H. Davy affirms this to be the case. ^But Mr. Williams, 
 who must here be regarded as the better authority, says that 
 it is necessary to the success of the process that the straw be 
 dry; in proof of which he mentions that when the straw 
 becomes wet by accident it is replaced ; and that when he 
 
110 ESSAY ON DEW, ETC. 
 
 purposely wetted it in some of the inclosures the formation of 
 ice there was always prevented. The reasons are clear. The 
 water, by softening the straw, renders it easily compressible 
 by the weight of the pans, and at the same time fills up 
 what would otherwise be vacant spaces among its parts. 
 The straw, therefore, in this condensed state, must afford a 
 ready passage to heat from the earth to the pans, the hin- 
 drance of which is allowed by every person to be the use of it 
 in this process when dry. Again ; the moisture which passes 
 through the straw to the earth it covers will rise afterwards 
 in the form of vapour, having the same temperature with the 
 warm ground, and will communicate heat to the pans. In 
 the last place ; a part of this vapour will be condensed into 
 water by the pans, in consequence of which heat must be 
 extricated. 
 
 4. It is mentioned both by Sir E. Barker and Mr. Williams, 
 in support of their opinions, that the pans, when new, are so 
 porous that they readily permit water to transude them ; 
 and that old pans, which permit this in a less degree, are 
 less fit for the making of ice. But the argument, which is 
 hence derived by them, is completely refuted by a fact related 
 by Mr. Williams himself, for he says that the pans are greased 
 before they are used, to prevent the adhesion of the ice to 
 their sides, since, if this purpose be answered, the water can 
 never be in contact with the pans, and therefore can never 
 pass through them. 
 
 The real reason of the less fitness of old pans for the 
 making of ice is perhaps the following : The production of 
 the cold which occurs in this process must take place in the 
 water; since neither the straw upon which the pans are 
 placed, nor the air above them, was ever found by Mr. 
 Williams of so low a temperature as 32. Whatever, there- 
 fore, obstructs the passage of heat from the straw to the 
 water mast favour the freezing of the latter. But this will 
 
ESSAY ON DEW, ETC. Ill 
 
 be less effectually done by an old than by a new pan, RS the 
 density of the former is greater, from the grease forced into it 
 by rubbing, and from the slime and sand that will enter with 
 the water into its pores, when these are not entirely closed by 
 the grease; which must often happen, as the smearing is 
 performed only once in three or four days. The difference, 
 however, in effect betwixt old and new pans must be very 
 small, as it does not appear that the old are ever laid aside on 
 account of their unfitness. 
 
 In a like way may be explained, without the aid of cold 
 produced by the evaporation of moisture from the outsides of 
 the pans, another fact mentioned by Mr. Williams, that ice 
 was often found by him in. those vessels, while water con- 
 tained in a china plate surrounded by them, had none ; since 
 the thin and dense substance of the plate must have trans- 
 mitted more readily than the thick and rare substance of the 
 pans, the heat of the straw to the water. 
 
 5. In accounting for the making of ice in Bengal, it is 
 requisite to show, not only how the first film is produced, 
 but also in what way the thickness of this film is after- 
 wards increased. If evaporation be the cause of this increase, 
 it follows, that a plate of ice in the night-time, and in the 
 stillest air, both unfavourable to that process, must yet 
 emit as much moisture as is necessary for the production of 
 a cold, according to Mr. Williams, of at least 14, and 
 according to Sir H. Davy, of at least 18, a conclusion, as it 
 appears to me, of itself sufficient to destroy the credit of the 
 theory from which it is drawn. 
 
 While attending to this subject, I became desirous of 
 acquiring some knowledge of the degree of cold which might 
 be produced by evaporation from water contained in a shallow 
 vessel. With this view I placed on a featherbed, situated 
 between the door and window of a room in my house in 
 London, two china plates, into one of which as much water 
 
112 ESSAY ON DEWj ETC. 
 
 was poured, as covered its bottom to the depth of a quarter 
 of an inch. The other plate was kept dry. The bulb of a 
 small thermometer being then applied to the inside of the 
 bottom of each plate, I observed upon many days, in various 
 seasons of the year, the difference between these instruments 
 while the door and window were open. I found, in con- 
 sequence, that when the temperature of the air in the room 
 was 75, the highest at which any experiment was made, the 
 thermometer in the plate containing water, was between 6 
 and 7 degrees lower than the one in the dry plate ; that the 
 difference between these thermometers diminished gradually 
 as the air became colder ; and that when the temperature of 
 the air was 40, the lowest for which I have any observation, 
 the difference was only 1J. At 82, therefore, it would have 
 been very small, and at a few degrees below 32 it would 
 probably have vanished. This supposition agrees with an 
 observation made by Mr. Wilson of Glasgow, who found 
 that no cold was produced by evaporation from snow 
 possessing a temperature of 27, though the air in the 
 immediate neighbourhood was purposely much agitated 
 by him. 40 
 
 The conclusions here given by me respecting the cold 
 produced by the evaporation of water, were drawn from 
 experiments made in the day, while the sky was clear, the 
 air very calm, and the temperature of the atmosphere 
 stationary. At night, and during a cloudy day, the differences 
 were less. On the other hand, if there was any perceptible 
 motion in the air, they were greater. They were also 
 greater if the heat of the atmosphere was increasing, but 
 less, if this was decreasing. 
 
 Having thus, I think, placed beyond doubt that the for- 
 mation of ice in Bengal is not occasioned by evaporation, 
 
 40 The reader may wonder at the rudeness of these experiments ; but the 
 wet-bulb thermometer was not yet thought of. 
 
I 
 
 ESSAY ON DEW, ETC. 113 
 
 I shall now state several reasons which have induced me to 
 believe that it depends upon the radiation of heat to the 
 heavens. 
 
 1. This cause not only exists, but exists in a degree 
 sufficient for the production of the effect which I attribute to 
 it. For Mr. Wilson found the surface of snow during a clear 
 and calm night to be 16 colder than air 2 feet above it, the 
 temperature of the latter being taken by a naked thermometer; 
 whereas the greatest heat of the atmosphere ever observed by 
 Mr. Williams, at the distance of 5 feet from the ground, 
 during the time that he supposed ice to be forming, was only 
 14 higher than the freezing point of water. I need say 
 nothing of the difference of 18 related by Sir H. Davy, as he 
 does not speak from his own observation, and as he gives no 
 authority for what he advances ; though even this difference ' 
 is considerably less than what I have attempted to show must " 
 sometimes occur, from the radiation of heat at night, between 
 the temperature of air a few feet above the earth, and that 
 of bodies placed on its surface. 
 
 It is to be mentioned here also, that, according to Mr. 
 Leslie* the power of water to radiate heat exceeds, perhaps, 
 that of all other substances. 
 
 2. Ice is chiefly formed in Bengal during the clearest and 
 calmest nights ; and it is on such nights that the greatest 
 cold from radiation is observed on the surface of the earth. 
 In Sir R. Barker's more refined mode of conducting the pro- 
 cess, an unusual stillness of the air, in contact with the water 
 to be frozen, is procured by placing the pans containing it a 
 little below the level of the ground ; in which situation it was 
 formerly shown bodies must grow colder from radiation to the 
 heavens at night than in any other. 
 
 3. The cold, by means of which ice is produced in Bengal, 
 appears, as I think may be inferred from what is said by Sir 
 R. Barker, in its greatest degree, like cold from radiation in 
 
 * On Ueat, p. 80. 
 
114 ESSA" ON DEW, ETC. 
 
 other substances, on those still and serene nights, during 
 which little dew is deposited by the atmosphere. 
 
 4. Clouds and wind prevent the formation of ice in Bengal; 
 and the same states of the atmosphere either prevent, or con- 
 siderably diminish, the occurrence of cold from the radiation 
 of heat at night by bodies on the ground. 41 
 
 I shall close this subject by giving some account of a few 
 attempts to procure the freezing of water at night in this 
 country, by exposing it to air of a temperature higher than 
 that of 32. These were made by me in 1812, at my usual 
 place of experiment, which was formerly stated to be not 
 well adapted for the appearance of a great cold from radia- 
 tion, and on nights not among the most favourable to such 
 an undertaking, even of those which occur in this country. 
 It is proper also to mention that I was then less able to 
 conduct such experiments and to make use of them than I 
 afterwards became from a longer attention to similar objects. 
 
 EXPERIMENT FIRST. 
 
 With a view to imitate the method of making ice described 
 by Sir K. Barker, I had a pit dug, on the evening of the 3rd 
 of May, in the middle of the. garden so often spoken of, 
 4J feet long, 3 wide, and 2 deep. It consequently had the 
 same depth as the excavations mentioned by that gentleman, 
 
 41 ( ' This explanation of Wells, is, no doubt, the true one. I think, however, 
 it needs supplementing. It appears, from the description, that the condition 
 most suitable for the formation of ice, is not only a clear air, but a dry air. 
 The nights, says Sir Robert Barker, most favourable for the production of 
 ice, are those which are clearest and most serene, and in which very little dew 
 appears after midnight. I have italicised a very significant phrase. To pro- 
 duce the ice in abundance, the atmosphere must not only be clear, but it 
 must be comparatively free from aqueous vapour. When the straw in which 
 the pans were laid became wet, it was always changed for dry straw ; and 
 the reason Wells assigned for this was, that the straw by being wetted was 
 rendered more compact and efficient as a conductor. This may have been- 
 the case, but it is also certain that the vapour rising from the wet straw and 
 overspreading the pans like a screen, would check the chill and retard the 
 congelation." Tyndall On Heat. 
 
ESSAY ON DEW, ETC. 115 
 
 but was considerably less in its other dimensions. Clean dry 
 straw was then strewed, to the height of a foot, over the 
 bottom of the pit. On the straw were next laid a number 
 of small shallow earthen pans, a part of which were glazed 
 and a part unglazed. In the last place, all the pans were 
 filled with soft water, which had been boiled on the same 
 evening. Contrary to my expectation, the unglazed pans 
 remained as dry on the outside, after water had been poured 
 into them, as those which were glazed. I conclude, there- 
 fore, that the former were more dense in their substance than 
 the unglazed pans used in India, and that their density was 
 probably the reason why ice did not afterwards form in them 
 sooner than in the glazed pans which were employed by me. 
 
 Two pans, containing boiled water, were set upon the 
 grassplat at a little distance from the pit. A watch-glass 
 filled with boiled water was also placed upon the grassplat, 
 and another was laid upon the raised board, which had been 
 thinly covered with sand. All these arrangements were not 
 completed before 10 h. at night. 
 
 At 1 h. in the morning, ice appeared in the watch-glasses 
 on the grassplat and raised board, the heat of the air, as 
 measured by a naked thermometer, being then, at 4 feet 
 above the ground, 39 J, and at 7 feet, 40% At 2 h. ice was 
 observed in the pans in the pit, while a thermometer in the 
 air, 2J feet above the ground, was 36i. Shortly afterwards 
 ice began also to form in the pans upon the grassplat. The 
 temperature of grass fully exposed to the sky was at the 
 same time 30, while that of the earth an inch below the 
 bottom of the grass was 45. During the time of these 
 observations dew formed copiously. 
 
 EXPERIMENT SECOND. 
 
 My next attempt was in the manner mentioned by Mr. 
 Williams. 
 
 On the evening of the 22nd of May I encompassed a 
 
 i2 
 
116 ESSAY ON DEW, ETC. 
 
 square piece of level ground, the sides of which were 3 feet 
 long, with a border of earth 4 inches high, and filled the 
 area with dry straw. On this were placed several of the 
 earthen pans which had been formerly used, and a few smaller 
 vessels, all containing unboiled water. After an exposure of 
 little more than an hour, water in a watch-glass upon the 
 straw was found frozen, the temperature of the air 2 feet 
 above the straw being then 37. In half an hour more ice 
 began to appear in the earthen pans, while a thermometer 
 54 feet above them, this being the height at which Mr. 
 Williams used to suspend his instrument, was 36. The air 
 soon after became colder, but its temperature was never less 
 than 33, though taken by a naked thermometer, which, as 
 was before said, upon a clear and calm night, occasions the 
 air to seem about 2 colder than it really is. 
 
 It might be inferred, from what is mentioned by Mr. 
 Williams, that the temperature of the straw beds, on which 
 the ice pans were set at Benares, was always found by him 
 above the freezing point, for this reason, that the straw, from 
 containing no moisture, could not, like the water, grow cold 
 by evaporation. I had, therefore, been surprised during the 
 first experiment, for I had then but little acquaintance with 
 the phenomena of cold observed with dew, that a thermometer, 
 laid upon an exposed part of the straw, was always below the 
 freezing point after ice had begun to form in the pans. On 
 reading, however, his account of the process a second time 
 with increased attention, my wonder ceased. For, as the 
 pans he speaks of were large, and touched one another, and 
 as all the pans employed in India for the making of ice widen 
 as they rise from the bottom, like our milk pans, the ther- 
 mometer, placed by him. on the straw, must have been 
 secluded from all view of the sky, and would therefore mark 
 a temperature much higher than if it had been laid, as in my 
 experiment, upon straw fully exposed to the heavens. On 
 
ES8AY ON DEW, ETC. 117 
 
 this, the second night, therefore, I placed a thermometer 
 under the edge of one of the pans lying on the straw bed, 
 and found it some time afterwards 6 higher than a similar 
 instrument upon a part of the straw bed which was un- 
 covered. Generally, however, the difference was not so 
 great. If my pans had been large, like those of Mr. Williams, 
 I should, no doubt, have observed more considerable dif- 
 ferences ; for, in consequence of their smallness, I could not 
 lay a thermometer on the straw bed so as to be fully screened 
 from the sky by the edge of any of them, without its being 
 almost in contact with the vessel, every part of which was 
 always colder than the sheltered straw. 
 
 Much dew formed in the course of this night. The 
 greatest difference remarked by me during it, between the 
 temperatures of grass and of air, was 0, and between those 
 of air and a fully exposed part of the straw bed 9. 
 
 EXPERIMENT THIED. 
 
 This was begun on the evening of the 16th of October, 
 and was likewise made agreeably to the method related by 
 Mr. Williams. 
 
 Ice appeared in the pans when the temperature of the 
 air, at the height of 5J feet, was, according to a naked 
 thermometer, 37. 
 
 On this night I placed upon the straw bed a dry earthen 
 pan among those which contained water, and found the 
 inside of its bottom to be as much colder than the air as the 
 water was in the other pans before ice appeared in them. 
 After the water had begun to freeze, no proper comparison 
 could be made between its temperature and that of the 
 empty pan. This pan in the course of the night attracted 
 moisture, which was afterwards converted into a film of ice. 
 
 But the chief fact established by the present experiment 
 was, that water may freeze at night in air of a temperature 
 higher than 32, not only without any loss of weight from 
 
118 ESSAY ON DEW, ETC. 
 
 evaporation, but with a gain of weight from an opposite 
 process. 
 
 I had observed that water, exposed early in the evening 
 in the open air to the sky, lost a little weight in the course 
 of a clear night. This I imputed to evaporation taking place 
 before the water had been cooled enough to condense the 
 vapour of the atmosphere, and to the weight gained after- 
 wards being insufficient to compensate the previous loss. I 
 exposed, therefore, on this night, water to the influence of 
 the sky until it was cooled to 34. Of this I put 2 ounces 
 into each of two china saucers, which had also been exposed 
 to the air, and then placed the saucers upon the straw bed. 
 In the morning a thin cake of ice was found in both saucers, 
 one of which had gained 2 J and the other 3 grains in weight. 
 
 Dew was also copious on this night. At one time grass 
 was 9i, and the exposed part of the straw bed 12 colder 
 than the air.* 
 
 It must be evident to every person that the formation of 
 ice, in the three preceding experiments, was the effect of a 
 natural operation, similar to that by which the same sub- 
 stance is produced in Bengal. These two facts must, there- 
 fore, have a common cause, and this has been shown by the 
 last experiment, independently of what was said before in 
 this Essay, not to be evaporation. It is also clear that the 
 cold, induced on the water in those experiments, had a 
 common cause with that observed at the same time upon the 
 grass and the straw, which latter cold must, in consequence 
 of proofs formerly given, be admitted to have arisen from the 
 radiation of the heat of those substances to the heavens. A 
 necessary inference, therefore, appears to be, that the forma- 
 tion of ice in Bengal, in the circumstances described by Sir 
 
 * The greater cold observed in this and the preceding experiment upon 
 straw than upon grass, is to be referred to the shortness of the latter, by 
 reason of which heat was readily communicated to its upper parts by the 
 earth. 
 
ESSAY ON DEW, ETC. 119 
 
 R. Barker and Mr. Williams, must likewise be attributed in 
 by far the greater measure, if not altogether, to a loss of heat, 
 which the water suffers by its own radiation while situated in 
 such a manner that it can receive little heat from other bodies, 
 either by radiation or conduction.* 
 
 CONCLUSION. 
 
 THE experiments which were made by me on dew, and 
 other subjects treated of in the preceding Essay, were un- 
 avoidably attended with many inconveniences, which were 
 the more felt as my health had long been feeble, and as my 
 professional duties obliged me often to return to London in 
 the morning, without having previously taken rest after the 
 whole of a night had been spent in attending to the objects of 
 my pursuit. The inconveniences here alluded to were, indeed, 
 so great, that I was twice or thrice obliged to intermit my 
 labours for several months together, and at length found it 
 necessary to cease from them entirely, before I had nearly 
 completed the plan which I had formed. I take the liberty 
 of mentioning these things to excuse, in part, the imperfec- 
 
 * On the evenings preceding the nights, during which ice is produced in 
 Bengal, the temperature of the water exposed in the pans is, probably, often 
 60 or more. But water of the heat of 60, if exposed in a shallow earthen 
 vessel to air of the same temperature during the day, while the weather is 
 calm and clear, will lose about 3 of heat by evaporation. A cold from thia 
 cause may, therefore, concur with that from radiation, and, consequently, 
 may, in Bengal, accelerate somewhat the formation of ice. The influence, 
 however, of evaporation there, in this respect, should the state of the air 
 with regard to moisture still permit it, which must often not be the case 
 while dew is forming, will, as the night proceeds, gradually diminish, and at 
 length almost disappear before the freezing of the water commences ; since I 
 have lately shown that evaporation from water of 32 produces very little 
 cold, even in the day time. Indeed, it seems to me much more probable, 
 that on a clear and calm night, though in a dry winter of Bengal, water at the 
 temperature of 32 will acquire warmth from the formation of dew upon it 
 than that it will become cold from evaporation. 
 
120 ESSAY ON DEW, ETC. 
 
 tions which will be observed in what I have written, as some 
 of them would, no doubt, have been removed by a further 
 interrogation of Nature.* 
 
 London, September 25, 1815. 
 
 * Of the experiments related in the beginning of the second Part of this 
 Essay, with the view of proving, that the formation of dew is an effect of 
 previous cold in the substances on which it appears, those of only one even- 
 ing were remarkable for the greatness of their results, the weather upon the 
 other evenings not having favoured much my purpose. I took advantage, 
 therefore, of being in the country, at the distance of a few miles from London, 
 on the 21st of the present month, the last day but one of an unusually long 
 tract of dry weather, to expose to the sky, 28 minutes before sunset, weighed 
 parcels of wool and swansdown, upon a smooth, unpainted, and perfectly dry 
 fir table, 5 feet long, 3 broad, and nearly 3 in height, which had been placed 
 an hour before in the sunshine in a large level grass field. At this time, and 
 throughout my experiments, the air was very still, and the sky very serene. 
 The atmosphere, too, in all probability, contained but little moisture, in 
 consequence of the long absence of rain; and the surface of the ground 
 apparently contained none. The wool, 12 minutes after sunset, was found 
 to be 14 colder than the air, the temperature of the latter being measured 
 by a naked thermometer suspended 4 feet above the ground, and to have 
 acquired no weight. The swansdown, the quantity of which was much 
 greater than that of the wool, was at the same time 13 colder than the air, 
 and was also without any additional weight. In 20 minutes more the 
 swansdown was 14^ colder than the neighbouring air, and was still without 
 any increase of its weight. My experiments now ceased from a failure of 
 daylight. 
 
 In my former experiments of this kind, the greatest cold observed by me 
 from radiation, without the appearance of dew, was only 9-|. 
 
 While making the experiments on wool and swansdown, I attended fre- 
 quently to the temperature of the grass, and found it at one time 15 colder 
 than that of the air 4 feet above the ground. This difference is 1 greater 
 than any I had ever before seen between the temperatures of the same sub- 
 stances, and is equal to the greatest which I had ever known to occur 
 between those of the atmosphere and of swansdown lying upon grass. I had 
 this evening placed no swansdown upon grass. 
 
 These experiments were not made till nearly the whole of the present 
 edition of my Essay was printed, and could not, therefore, be mentioned in 
 their proper place. 
 
APPENDIX 
 
APPENDIX. 
 
 THE ACTION OF AQUEOUS VAPOUE UPON 
 KADIANT HEAT. 
 
 IF the doctrine propounded by Professor John Tyndall, 
 F.R.S., regarding the aqueous vapour of the atmosphere in 
 relation to solar and terrestrial radiation, receives confirmation 
 from the experiments of other investigators, considerable 
 modification will be necessary in the received explanations of 
 various meteorological phenomena, and of dew among them. 
 In order to present the reader with the most complete infor- 
 mation respecting the subject of dew, it is desirable to throw 
 into an Appendix matter which would have been inappropriate 
 as notes, but without a knowledge of which the Essay on 
 Dew could not be studied to full advantage at the present 
 day. For the greater portion of this article we are indebted 
 to the writings of Professor Tyndall, and we here publicly 
 acknowledge and thank him for his courteous assent to our 
 request to be permitted to quote them. The reader will 
 please to remember that quotations not otherwise acknow- 
 ledged are from Professor TyndalPs pen. 
 
 " Aqueous vapour is always diffused through the atmos- 
 phere. The clearest day is not exempt from it ; indeed, in 
 the Alps, the purest skies are often the most treacherous, the 
 blue deepening with the amount of aqueous vapour in the 
 air. Aqueous vapour is not visible ; it is not fog ; it is not 
 cloud ; it is not mist of any kind. These are formed of vapour 
 which has been condensed to water ; but the true vapour is 
 an impalpable transparent gas. It is diffused everywhere 
 throughout the atmosphere, though in very different propor- 
 tions." 
 
124 APPENDIX. 
 
 "The quantity of this vapour is small. Oxygen and 
 nitrogen constitute about 99 J per cent, of our atmosphere ; 
 of the remaining 0-5, about 0-45 is aqueous vapour, the rest 
 is carbonic acid." 
 
 Professor Tyndall has experimentally demonstrated : that 
 aqueous vapour is a powerful absorbent of radiant heat ; that 
 dry air is perfectly transparent, or acts as a vacuum, to radiant 
 heat ; that the more humid the air the more radiant heat it 
 absorbs ; that, for the same per centage of moisture, the 
 rate of absorption varies as the pressure. 
 
 " Compared atom for atom, the absorption of an atom of 
 aqueous vapour is 1600 times that of air. Now, the power to 
 absorb and the power to radiate are perfectly reciprocal and 
 proportional. The atom of aqueous vapour will therefore 
 radiate with 1600 times the energy of an atom of air." 
 
 " Regarding the earth as a source of heat, no doubt at least 
 10 per cent, of its heat is intercepted within ten feet of the sur- 
 face. This single facb suggests the enormous influence which 
 this newly developed property of aqueous vapour must have 
 in the phenomena of meteorology." 
 
 The sun pours its heat upon the surface of the earth 
 during the day ; and the earth at night emits its acquired 
 heat into space. When the day is longer than the night the 
 earth has a balance in its favour, which goes on accumulating 
 during the summer season ; but a proportionate daily loss by 
 emission occurring in winter, the annual temperature of the 
 earth remains constant. Such is the grand result; but 
 highly important functions are exercised upon these pro- 
 cesses by the variable conditions and amounts of aqueous 
 vapour present in the atmosphere from day to day. 
 
 Fog or mist causes a sensation of cold, and interferes 
 with the normal healthy action of the skin and the lungs. 
 The cause is now apparent in its property of absorbing heat 
 from the living body. 
 

 
 APPENDIX. 125 
 
 The action of aqueous vapour in arresting the emission of 
 heat into space renders it a kind of blanket to the surface of 
 the earth, and its presence must always tend to maintain a 
 medium, temperature equally warding off the fierce heat of 
 the day, and maintaining a higher temperature at night. 
 Thus vapour whether transparent and invisible, or visible 
 as clouds, fog, or mist is intimately connected with the im- 
 portant operations of solar and terrestrial radiation. Cloudy 
 or humid days diminish the effect upon the soil of solar 
 radiation ; similar nights retard the radiation from the earth. 
 A dry atmosphere must be the most favourable for the direct 
 transmission of the sun's rays ; while equally the withdrawal 
 of the sun from any region over which the air is dry must be 
 followed by very rapid cooling of the soil. 
 
 The formation of dew, attendant upon nocturnal radiation, 
 Professor Tyndall explains as follows : 
 
 " The aqueous vapour of our atmosphere is a powerful j 
 radiant, but it is diffused through air which usually exceeds 
 its own mass more than one hundred times. Not only, then, 
 it own heat, but the heat of the large quantity of air which 
 surrounds it must be discharged by the vapour, before it can 
 sink to its point of condensation. The retardation of chilling 
 due to this cause enables good solid radiators, at the earth's 
 surface, to outstrip the vapour in their speed of refrigeration ; 
 and hence upon these bodies aqueous vapour may be con- 
 densed to liquid, or even congealed to hoar-frost, while at 
 a few feet above the surface it still maintains its gaseous 
 state." 
 
 The amount of condensation will depend upon the existing 
 atmospheric conditions. In an overcast state. of the sky the 
 radiation from the earth and the counter-radiation from the 
 clouds will be equal, or nearly so ; there will not be sufficient 
 cooling for the formation of dew. In clear air with trans- 
 parent vapour present, and therefore comparatively small in 
 
126 APPENDIX. 
 
 amount, the counter-radiation will be less, the earth will 
 cool, and dew may be formed. In the clearest and driest 
 state of atmosphere, vapour never being absent altogether in 
 the lowest stratum, counter-radiation will be minimised, the 
 earth's surface will cool rapidly, the deposit of dew will be 
 copious ; a gentle wind being favourable for the full effect. 
 
 66 The difference between a thermometer, which, properly 
 confined, gives the true temperature of the night air, and 
 one which is permitted to radiate freely towards space, must 
 be greater at high elevations than at low ones;" because, 
 there being a less thickness of vapour present over high 
 elevations, the passage of radiant heat from the earth will be 
 less impeded. 
 
 " The following remarkable passage from Hooker's 
 Himalayan Journals, 1st edit. vol. II., p. 407, bears upon the 
 present subject : c From a multitude of desultory observations 
 I conclude that, at 7,400 feet, 125.7, or 67 above the 
 temperature of the air, is the average effect of the sun's rays 
 
 on a black bulb thermometer These results, 
 
 though greatly over those obtained at Calcutta, are not much, 
 if at all, above what may be observed on the plains of India. 
 The effect is much increased by elevation. At 10,000 feet, in 
 December, at 9 a.m., I saw the mercury mount to 132, while 
 the temperature of shaded snow hard by was 22. At 13,000 
 feet, in January, at 9 a.m., it has stood at 98, with a differ- 
 ence of 68'2, and at 10 a.m. at 114, with a difference of 81-4, 
 whilst the radiating thermometer on the snow had fallen at 
 sunrise to 0'7. J 
 
 (( These enormous differences between the shaded and un- 
 shaded air, and between the air and the snow, are, no doubt, 
 due to the comparative absence of aqueous vapour at these 
 elevations. The air is incompetent to check either the solar 
 or the terrestrial radiation, and hence the maximum heat in 
 the sun and the maximum cold in the shade must stand very 
 
> 
 
 APPENDIX. 127 
 
 wide apart. The difference between Calcutta and the plains 
 of India is accounted for in the same way." 
 
 Mons. C. Martins found on the summit of the Pic du Midi 
 the heat of the soil exposed to the sun to be twice as great as 
 in the valley at the base of the mountain ; and he remarks, 
 " The immense heating of the soil, compared with that of the 
 air on high mountains, is the more remarkable, since, during 
 the nights, the cooling by radiation is there much greater 
 than in the plain." 
 
 " Wherever the air is dry we are liable to daily extremes 
 of temperature. By day, in such places, the sun's heat 
 reaches the earth unimpeded and renders the maximum 
 high ; by night, on the other hand, the earth's heat escapes , 
 unhindered into space and renders the minimum low. Hence j 
 the difference between the maximum and minimum is 
 greatest where the air is driest." 
 
 When air is rarefied, a partial condensation of its vapour 
 takes place. Aqueous vapour, ascending into the atmosphere, 
 gradually attains regions more and more rarefied, and the 
 accompanying chilling must have a certain condensing effect. 
 The surface of the vapour presented to the clear firmament 
 will likewise impart its heat to space by radiation; and 
 must, as a secondary cause, produce condensation. The like 
 result will accompany the deflection upwards of moist winds 
 by mountains. 
 
 " Cumuli are the heads of columns of vapour, which rise 
 from the earth's surface, and are precipitated as soon as they 
 reach a certain elevation. Thus, the visible cloud forms the 
 capital of an invisible pillar of saturated air. Certainly the 
 top of such a column, raised above the lower vapour- 
 screen which clasps the earth, and offering itself to space, 
 must be chilled by radiation ; in this action alone we have a 
 physical cause for the generation of clouds." 
 
 " A freedom of escape, similar to that from bodies of 
 
128 APPENDIX. 
 
 vapour at great elevations, would occur at the earth's sur- 
 face generally, were the aqueous vapour removed from the 
 air above it; for the body of the atmosphere is a perfect 
 vacuum as regards the transmission of radiant heat. The 
 withdrawal of the sun from any region over which the 
 atmosphere is dry, must be followed by quick refrigeration. 
 . . . . The winters of Thibet are almost unendurable 
 from this cause. . . The removal, for a single summer 
 night, of the aqueous vapour from the atmosphere which 
 covers England, would be attended by the destruction of 
 every plant which a freezing temperature could kill. In 
 Sahara, ' where the soil is fire and the wind is flame,' the 
 refrigeration at night is often painful to bear. Ice has been 
 formed in this region at night. In Australia, also, the 
 diurnal range of temperature is very great, amounting, 
 commonly, to between 40 and 50 degrees. In short, it may 
 be safely predicted, that wherever the air is dry, the daily 
 thermometric range will be great. This, however, is quite 
 different from saying that where the air is clear, the ther- 
 mometer range will be great. Great clearness to light is 
 perfectly compatible with great opacity to heat ; the atmos- 
 phere may be charged with aqueous vapour while a deep blue 
 sky is overhead, and on such occasions the terrestrial radiation 
 would, notwithstanding the clearness, be intercepted." 
 
 The important discovery of Dr. Tyndall, that the deport- 
 
 i' ment of aqueous vapour and that of dry air are totally 
 dissimilar towards radiant heat, opens out to meteorologists 
 
 " a fresh arena for thought and investigation. Hitherto the 
 deportment of the atmospheric air in its aggregate con- 
 dition of dry gases and diffused vapour in relation to 
 radiation has been alone considered; and the neutral 
 character of dry air was certainly never suspected. It would 
 seem, therefore, that the meteorologist has in future mainly 
 to confine his attention to aqueous vapour as the prime 
 
APPENDIX. 129 
 
 agent in the phenomena attending solar and terrestrial 
 radiation, while equally the action of radiation must be 
 kept in contemplation in investigating the phenomena in 
 which aqueous vapour is chiefly concerned. The new 
 doctrine will probably modify the explanations given in 
 existing treatises and memoirs of numerous physical pheno- 
 mena. For instance, how can the following statement from 
 Herschel's Meteorology confront the fact that dry air is as 
 transparent to radiant heat as a vacuum ? " It is a property 
 of caloric radiant from bodies heated below incandescence to 
 be eminently absorbable by transparent media." 
 
 The reader has already perceived how Tyndall's new 
 teaching modifies the received explanations of the formation 
 of dew, of cloud, of rain, of the cause of wide ranges of 
 temperature. In our note to Wells's Essay on Dew it will be 
 seen that it gives an addition to the explanation of the arti- 
 ficial mode of obtaining ice practised in Bengal. We give yet 
 another interesting point on which this subject has a bearing, 
 the theory of serein. "'Most authors,' writes Melloni, 
 ( attribute to the cold, resulting from the radiation of the air, 
 the excessively fine rain which sometimes falls in a clear sky, 
 during the fine season, a few moments after sunset. But,' 
 he continues, f as no fact is yet known which directly proves 
 the emissive power of pure and transparent elastic fluids, it 
 appears to me more conformable,' &c. &c. If the difficulty 
 here urged against the theory of serein, be its only one, the 
 theory will stand, for transparent elastic fluids are now 
 proved to possess the power of radiation which the theory 
 assumes. It is not, however, to radiation from the air that 
 the chilling can be ascribed, but to radiation from the body 
 itself, whose condensation produces the serein. )9 
 
 Professor Tyndall has shown that water is opaque to the 
 radiation from aqueous vapour which, indeed, is composed of 
 the same constituents in a different state of aggregation, 
 
 K 
 
130 APPENDIX. 
 
 and when in contact must, as in a steam boiler, preserve the 
 same temperature. The opacity of aqueous vapour to the 
 radiation from water may be inferred, and hence may be con- 
 cluded that " the very act of nocturnal refrigeration, which 
 causes the condensation of water on the earth's surface, gives 
 to terrestrial radiation that particular character which renders 
 it most liable to be intercepted by our atmosphere, and thus 
 prevented from wasting itself in space." 
 
 (e The extraordinary energy of water as a radiant, in all 
 its states of aggregation, must play a powerful part in a moun- 
 tain region. As vapour, it pours its heat into space, and 
 promotes condensation ; as liquid, it pours its heat into space, 
 and promotes congelation ; as snow, it pours its heat into 
 space, and thus converts the surfaces on which it falls into 
 more powerful condensers than they otherwise would be." 
 
 The meteorological instruments employed for measuring 
 the intensity of both solar and terrestrial radiation, and the 
 temperature of air and of evaporation, ought to furnish obser- 
 vations from which the effect on radiant heat of the aqueous 
 vapour actually in the atmosphere could be made apparent. 
 Attempts, indeed, have been made recently to exhibit this 
 effect. The Philosophical Magazine, No. 212, contains the 
 results, from observations made at Melbourne, bearing on the 
 absorption of heat by aqueous vapour, and its relation to 
 terrestrial radiation, by M. G. Neumayer. His deductions 
 cannot be accepted per se y for his mode of observation was 
 certainly open to objection. The thermometer employed to 
 measure the radiation was placed in a parabolic reflector, 
 that in an open box filled with cotton, raised one foot-and- 
 a-half above the ground, and the whole placed inside a close 
 fence, so that the zenith over the instrument was only free 
 for the space of 38 across* The neutralising effect of 
 partial screens has been so well demonstrated by Wells, that, 
 confirmed as it has been by others, it is surprising that a 
 
APPKNDIX. 
 
 131 
 
 meteorologist should expect to detect small differences by 
 endeavouring to restrict the essential conditions for the pro- 
 duction of the full effect. The position of the instrument 
 was arbitrary and novel the usual plan being to place the 
 reflector on the soil or grass. 
 
 An altogether unobjectionable, and a more successful 
 attempt to exhibit the action of aqueous vapour on terrestrial 
 radiation, has been made by Colonel Strachey, F.R.S., an 
 account of which will be found in the Philosophical Magazine, 
 No. 213. From the Madras observations, for the years 
 1841-2-3-4, those days were selected on which the sky was 
 almost free from clouds ; then the fall of temperature from 
 6 h. 40 m. p.m., to 5 h. 40 m. a.m., and the mean tension of 
 vapour during the night were calculated, and the temperature at 
 6 h. 40 m. noted. The following is a statement deduced from 
 the Colonel's tables for each year; the mean of all the 
 observations when the tension of vapour was between 1*00 
 inch and *90, between *89 and *80, and so on, being taken : 
 
 Tension 
 of vapour. 
 
 Fall of thermome- 
 ter 6h. 40m. p.m.. 
 to 5h. 40m. a.m. 
 
 Thermometer at 
 6 h. 40 m. p.m. 
 
 Number of 
 observations. 
 
 INCH. 
 
 
 
 o 
 
 
 0-93 
 
 4-9 
 
 85-8 
 
 27 
 
 85 
 
 6-1 
 
 e4-6 
 
 13 
 
 72 
 
 7-1 
 
 &0-5 
 
 23 
 
 65 
 
 8-3 
 
 78-8 
 
 32 
 
 58 
 
 87 
 
 76-8 
 
 8 
 
 " The general conclusion, that the loss of heat by radiation 
 from the earth at night is directly affected in a very con- 
 siderable degree by the quantity of vapour in the air, seems 
 to be unmistakeably indicated by these figures. Just the 
 same results are shown in all other cases in which I have 
 been yet able to examine such observations of night tem- 
 peratures under a clear sky ; and the following figures are 
 
 K2 
 
132 
 
 APPENDIX. 
 
 got from observations made between the 4th and the 25th of 
 March, 1850 (also taken from the Madras records), during, 
 which period the sky remained clear, and great variations 
 of the quantity of vapour took place. 
 
 Tension of Vapour 
 
 888 
 
 849 
 
 805 
 
 749 
 
 708 
 
 659 
 
 605 
 
 554 
 
 435 
 
 Fall of Temperature 
 
 
 
 
 
 
 
 
 
 
 from 6 h. 40 m. P.M. 
 
 6-7 
 
 7-1 
 
 8 n -3 
 
 8-5 
 
 10-3 
 
 12-6 
 
 12-1 
 
 13-1 
 
 16-5 
 
 to 5 h. 40 m. A.M. 
 
 
 
 
 
 
 
 
 
 
 " The temperature of the air at 6 h. 40 m. p.m., varied 
 from 78-9 to 84- 7." 
 
 " The variation of the heating power of the sun prevents 
 useful comparisons between the increase of heat during the 
 day, at different times of the year. But the fact comes out 
 clearly enough, that the air, when dry, is more freely tra- 
 versed by the sun's heat than when damp. The observations 
 of March, 1850, arranged so as to bring out this point, give 
 the following results : " 
 
 Tension of Vapour. 
 
 824 
 
 737 
 
 670 
 
 576 
 
 511 
 
 394 
 
 Eise of Temperature 
 
 o 
 
 
 
 
 
 o 
 
 o 
 
 o 
 
 from 5 h. 40 m. a.m. 
 
 12-4 
 
 15-1 
 
 19-3 
 
 22 '2 
 
 24-3 
 
 27-0 
 
 to 1 h. 40 m. p.m. 
 
 
 
 
 
 
 
 Colonel Strachey's investigation tends to confirm the 
 views entertained by Professor Tyndal. 
 
 Experiments made by Professor Magnus* lead him to 
 assert, that aqueous vapour does not possess the great 
 absorptive power attributed to it. " The radiation of trans- 
 parent aqueous vapour (that is, of aqueous vapour in the 
 
 Vide Philosophical Magazine, No. 214. 
 
APPENDIX. 133 
 
 proper sense of the term)/' he says, " is but little more than 
 that of dry atmospheric air. Hence it follows that the 
 absorptive power of air which contains, or is saturated with, 
 transparent vapours, differs little from that of dry, and that 
 it is only if air is misty (that is, contains condensed vapour) 
 that it radiates and equally well absorbs heat." 
 
 "But I think these experiments were not needed. A 
 well-known phenomenon, which depends on the radiation oi 
 heat, furnishes a more striking proof of the small absorptive 
 capacity of aqueous vapour than all experiments in the 
 laboratory. If aqueous vapour were in fact so good an 
 absorbent, as Professor Tyndall maintains, dew could never be 
 formed ; for the aqueous vapour, which is indispensable for 
 dew, would at the same time form a covering over the sur- 
 face of the earth and prevent its radiation. But just where 
 the atmosphere is particularly rich in water, in the tropics, 
 is dew principally formed ; and those regions would be devoid 
 of all fertility were it not that moisture is imparted to the 
 plants by dew. Should it be urged that the vapour absorbs 
 heat indeed, but radiates a portion only back to the earth 
 while the greater part goes to the higher regions of the 
 atmosphere, this process of partial radiation would repeat 
 itself from layer to layer, and hence the temperature in 
 different layers must diminish with the height. This, how- 
 ever, is notoriously not the case in the formation of dew : the 
 temperature merely sinks near the good radiating surface of 
 the earth; and, a few feet about, it is not lower than over abadly- 
 radiating place which is not covered with dew. Further, a 
 cooling would be impossible, since all layers of the atmosphere, 
 just as they radiate away part of their heat from the earth, 
 send another portion back to it. If aqueous vapour possessed 
 so great an absorptive capacity as Professor Tyndall ascribes 
 to it, extremely little of the radiating heat could reach the 
 clouds, since the enormous layer of vapour extending to them 
 
134 APPENDIX. 
 
 would entirely absorb it. It would then be inexplicable that 
 clouds prevent dew. Since Wells's memorable investigations 
 it is generally assumed that thermal rays reach the clouds 
 almost undiminished, and are thence reflected back to the 
 earth ; if there were no such almost unhindered transmission 
 through moist air the clouds at their great distance could 
 not hinder the radiation as does a board or any other solid 
 body, at a small distance from the earth." 
 
 "The conclusions which Professor Tyndall deduces for 
 certain climatic phenomena remain unchanged if nebulous be 
 substituted for actual vapour. For this it is which contributes 
 to the maintenance of the beautiful green of the British 
 Islands; for it moderates the burning rays of the sun, as well 
 as prevents the great colds, which only occur with a clear sky 
 and a copious radiation." 
 
 Professor Tyndall in " Remarks on the Paper by Professor 
 Magnus"* meets objections by reflections suggested by 
 Professor Magnus's description of his own experiments, and 
 adds : " With regard to the formation of dew, the amount 
 deposited depends on the quantity of vapour present in the 
 air; and where that quantity is great, a small lowering of 
 temperature will cause copious precipitation. Supposing 
 50, or even 70 per cent, of the terrestrial radiation 
 to be absorbed by the aqueous vapour of the air, the 
 uncompensated loss of the remaining 30 would still produce 
 dew, and produce it copiously where the vapour is abundant. 
 Attenuated as aqueous vapour is, it takes a good length of it 
 to effect large absorption. I have already risked the opinion 
 that at least 10 per cent, of the earth's radiation is inter- 
 cepted within 10 feet of the earth's surface; but there is 
 nothing in this opinion incompatible with the observed for- 
 mation of dew. A surface circumstanced like that of the 
 
 * Vide Philosophical Magazine, No. 214. 
 
APPENDIX. 135 
 
 earth, and capable of sending unabsorbed 80 or 90 per cent. 
 of its emission to a distance of 10 feet from itself, must of 
 necessity become chilled, and must, if aqueous vapour in 
 sufficient quantity be at hand, produce precipitation. 
 
 "I should willingly leave to others the further develop- 
 ment of this question, feeling assured that, once fairly 
 recognised by field meteorologists, the evidence in favour of 
 the action of aqueous vapour on solar and terrestrial radiation 
 will soon be overwhelming." 
 
 An independent experimentalist, H. Wild, has followed 
 precisely the method pursued by Professor Tyndall, and that 
 by Professor Magnus. He did not succeed in obtaining any 
 results from the latter method, which he was compelled to 
 abandon. He concludes his memoir * with these words : "I 
 feel bound, from my own experience, to give a decided pre- 
 ference to Tyndall's method, not only on account of the 
 greater facility with which it furnishes qualitative results^ 
 but also in consequence of its greater delicacy. It is princi- 
 pally in consequence of this greater delicacy that, notwith- 
 standing the negative results furnished by Magnus's method, 
 I maintain that the greater absorptive power of moist air, as 
 compared with dry, has been fully established by the experi- 
 ments made according to Tyndall's method ; and I am of 
 opinion that meteorologists may, without hesitation, accept 
 this new fact in their endeavours to explain phenomena, 
 which hitherto have remained more or less enigmatical." 
 
 The observations registered from the solar radiation 
 thermometer, and the terrestrial radiation thermometer, 
 in connection with a record of the state of the sky, and the 
 amount of cloud, enable us to test the action of aqueous 
 vapour upon radiant heat. Thus, taking as an illustration, 
 
 * Vide Philosophical Mayazine, No. 216. 
 
^ 
 
 136 
 
 APPENDIX. 
 
 the observations made at the observatory at Adelaide, 
 Australia, in January 1863, we get the following result : 
 
 Sky during the day 
 of 24 hours. 
 
 Mean difference be- 
 tween diurnal and 
 nocturnal radiation. 
 
 Number of days, 
 observations. 
 
 Clear 
 Cloudy 
 
 Overcast 
 
 88 -3 
 80-2 
 64-3 
 
 15 
 10 
 6 
 
 This example, taken at random from a voluminous record, 
 is completely in accordance with all that Tyndall has written. 
 It may be added that the solar thermometer used had a black 
 bulb, and was enclosed in an exhausted tube, having a large 
 ball, in the centre of which the bulb was placed. It was 
 held in a horizontal position by two light wooden brackets, 
 screwed on to a stout post, five feet above the ground. The 
 terrestrial thermometer was a spirit minimum, and was placed 
 on short grass. 
 
 For an account of the mode of experimenting on this 
 subject, of the instruments employed, and of the method of 
 reduction, reference must be made to Tyndall' s work entitled 
 " Heat considered as a mode of Motion," an exceedingly 
 entertaining and instructive book. 
 
 When experimentalists are satisfied with the conclusive- 
 ness of their results, with the validity of their methods of pro- 
 cedure, and with the certainty of elimination of all disturbing 
 agents in their operations, when, in short, they agree among 
 themselves as to what to do and the effects resulting, then it 
 will be advisable, if not necessary, to endeavour to bring the 
 facts under the regime of condensed, cogent, and logical 
 statements and arguments. At present the subject is open to 
 attack, The original experimentalist, not foreseeing the 
 importance of his enquiries into aqueous vapour, has not 
 
APPBNDIX. 
 
 137 
 
 laboured to give his results coherency and conclusiveness. 
 We can only pretend to having studied the subject attentively, 
 but we trust that the account we have given of the new 
 doctrine will tend to concentrate attention to it. 
 
 SOLAR RADIATION. 
 
 The influence of radiant heat in the economy of nature is 
 still a subject about which very little is positively known, 
 notwithstanding the researches of Rumford, Leslie, Davy, 
 Prevost, Dulong^and Petit, Daniell, Melloni, Glaisher, 
 Herschel and others, not forgetting Tyndall. Dr. Wells, in 
 his "JEksoy on Dew," has indeed treated one particular 
 branch of the subject, the relation between nocturnal radia- 
 tion and the formation of dew, with the concomitant effects 
 upon vegetation, with singular ability. The maximum 
 heating effect of radiation, to which Wells had given no 
 attention, Daniell made a subject of enquiry, and he 
 attempted to bring together our knowledge of solar radiation, 
 in an Essay which was first published in 1823, and which 
 contains probably the best information which can be had even 
 at the present time. Now, as then, a satisfactory answer 
 cannot be given to the question : " What is the maximum 
 calorific impression which plants are subject to in any 
 latitude ?" Influenced as the atmosphere is by the varying 
 temperatures of the terrestrial substances acted upon by the 
 solar power, it becomes an interesting branch of meteoro- 
 logical enquiry to examine, in all its generality, the varying 
 conditions of this problem. The subject is replete with 
 difficulties ; there is not yet even a recognised standard 
 instrument which will furnish absolute indications of its 
 effects, and properly measure them. Various means have 
 
138 APPENDIX. 
 
 been employed for the purpose. Thermometers have been 
 used with the bulbs naked, blackened bright, blackened dull,, 
 or covered with black wool ; sometimes enclosed in vacuo in 
 glass jackets, but more frequently not; placed either on the 
 soil itself, in the foci of metallic reflectors, or in blackened 
 recesses, resting on the soil and exposed to the sun's rays. 
 Few of the rays impinge directly upon the bulb of the 
 thermometer so placed ; the reflection from it must be con- 
 siderable, varying with size and shape, so that no two instru- 
 ments agree probably in their indications. 
 
 " Different kinds of black glass differ notably as to their 
 power of transmitting radiant heat. In thin plates some 
 descriptions tint the sun with a greenish hue, others make it 
 appear a glowing red without any trace of green. The latter 
 are by far more diathermic than the former. In fact, carbon, 
 when perfectly dissolved and incorporated with a good white 
 glass, is highly transparent to the calorific rays." . " The 
 black glass chosen for thermometers, and intended to 
 absorb completely the solar heat, may entirely fail in this 
 object, if the glass in which the carbon is incorporated be 
 colourless. To render the bulb of a thermometer a perfect 
 absorbent, the glass with which the carbon is incorporated 
 ought, in the first instance, to be green." Tyndall on 
 " Radiation," Eede Lecture, 1865. 
 
 (t It is well known to the agriculturist and the gardener 
 that without the direct influence of the sun, whatever may 
 be the temperature of the air, the fruits of the earth seldom 
 come to perfection." The direct heating power of the 
 sun requires to be investigated with reference to the 
 hour of the day, the season, the latitude, and the elevation 
 above the sea. Daniell has shown that it follows the course 
 of the sun's declination. The maximum intensity and effect 
 occur in June, while the greatest mean temperature of the 
 air does not take place till July. In the month of June, a 
 
APPENDIX. 139 
 
 plant may be so circumstanced in this country as to undergo 
 all the changes of heat from 154 to 30. " This arrange- 
 ment, no doubt, has an important influence upon the 
 processes of fructification in the vegetable kingdom. Agri- 
 culturists are well aware of the advantage of direct solar 
 heat in the flowering of wheat, and other corn crops, an 
 advantage which is never compensated by any elevation of 
 temperature under a clouded sky."* According to the same 
 authority, the force of the sun's direct radiation decreases in 
 approaching the equator, and increases in advancing towards 
 the poles. It also increases, from below, upwards. The same 
 cause which obstructs the passage of radiant heat in the 
 atmosphere from the sun, opposes also its transmission from 
 the earth into space. 
 
 These results of the researches made by Daniell are 
 sufficiently at variance with our common notions of heat and 
 cold; and, though physical considerations will account for 
 them, still they must serve to increase the general interest 
 which the subject of radiant heat elicits. 
 
 From experiments made at the Cape of Good Hope, Sir 
 John Herschel calculated the direct heating effect of a vertical 
 sun, at the sea-level, to be capable of melting 0*00754 of an 
 inch, per minute, from a sheet of ice exposed perpendicularly 
 to its rays. M. Pouillet concluded, from similar experi- 
 ments, that the effect is expressed by 0*00703 of an inch. 
 A mean of these two numbers, 0*007285 in., may, therefore, 
 be regarded as a measure of the sun's vertical heating 
 power, at the sea-level, in a perfectly cloudless sky. The 
 effect of the invisible vapour, always present in variable 
 quantities in the clearest state of the weather, in inter- 
 cepting the passage of heat to the earth, has not been taken 
 into account in the experiments which gave these deter- 
 
 * Daniell'a Meteorological Essays. 
 
140 APPENDIX. 
 
 urinations. Could it be taken into calculation, different 
 and more consistent results might doubtless be expected. 
 
 "The direct heating power of the sun's rays may be 
 measured in two different ways statically and dynamically. 
 The statical method consists in equilibrating the heating 
 power of sunshine on some body (as a blackened thermo- 
 meter) with some external cooling influence which is itself 
 measurable or which we have reason to believe invariable. 
 It has been usual to suppose this accomplished by simply 
 noting the degree marked by such a blackened thermometer 
 in the sun (exposed till it ceases to rise), in excess of that 
 marked by a similar one in the shade. This is much as if a man 
 should measure his strength by the depth to which he could 
 thrust a pole into the ground, in the absence of any knowledge 
 of its sharpness, or the resistance of the soil. The cooling 
 influences (conduction and radiation) are dependent for their 
 effects on local and temporary circumstances too numerous 
 and too variable to estimate. The measure itself requires to 
 be measured. The objection is palliated, but not removed, by 
 enclosing the thermometer in an exhausted glass tube, which 
 eliminates conduction by cutting off the contact of air, and 
 by other methods designed to deaden or equalize external 
 influences. It is, however, insuperable in principle. 
 
 " The dynamical method consists in ascertaining the amount 
 of physical change of a nature susceptible of definite mea- 
 surement, effected on some object by a given sectional area of 
 sunbeam in a given time ; such, for instance, as the dilatation 
 of a liquid, the melting of ice, or the raising of the temper- 
 ature of a given quantity of water a certain number of 
 degrees." * Herschel was the first to attempt such a 
 measure of the sun's heating effect; and his experiments 
 eventually led to the invention of the actinometer. Pouillet's 
 
 * " Meteorology," by Sir J. F. W. Herschel. 
 

 APPENDIX. 141 
 
 pyrheliometer is an" instrument for giving similar measure- 
 ments. 
 
 The actiiiometer consists of a glass cylinder containing 
 
 ammonio-sulphate of copper. One end of the cylinder is 
 closed by a screw working in a liquid-tight collar. This 
 screw must be made of silver, the only metal not acted upon 
 by the liquid enclosed, and its object is to admit of a change 
 of capacity when the liquid is too much dilated with heat. 
 The other end is drawn out into a tube of narrow bore, upon 
 which the dilatation of the liquid is measurable as in a 
 thermometer. A spirit thermometer is inserted in the axis of 
 the cylinder, and its stem passes through the axis of the 
 adjusting screw. " This instrument being several times 
 alternately exposed for one minute in the sun and shade, and 
 the changes of volume in each case read off on the scale, the 
 differences or sums of the mean changes, according as the 
 action has been in the same or in a contrary direction, gives 
 the dilatation produced by the sunshine alone (freed from 
 the disturbing influences), corresponding to the actual tem- 
 perature of the liquid, which being reduced by an appropriate 
 table to give the temperature acquired, affords a measure of 
 the effect of a given sectional area of sunbeam in heating a 
 a definite volume of liquid. Finally the result is reduced to 
 actines, or units of solar radiation, each actine denoting that 
 amount of radiation which would suffice to melt one- 
 millionth part of a metre from the thickness of a sheet of ice 
 perpendicularly exposed, in one minute, supposing it wholly 
 absorbed. * * * The portability and facility of use and 
 reduction of this instrument, as well as the consistent results 
 it affords, leave nothing to desire, and afford a perfect 
 measure of solar radiation." Unfortunately it is very costly, 
 the price being from 5 to 8. Instructions for using it 
 and for reducing the observations would take too much 
 space. The very few persons who may require them are 
 
142 APPENDIX. 
 
 recommended to consult the "Admiralty Manual of Scientific 
 Enquiry " where, in an article on meteorology, the inventor 
 has himself given all the necessary information. 
 
 The instrument termed by Pouillet pyrheliometer may 
 be constructed with water, mercury, or any other fluid ; 
 the preference, however, must be given to mercury as 
 the more stable and manageable fluid. A shallow steel 
 cylinder is filled with mercury. Into the cylinder the 
 bulb of a thermometer is introduced, the stem of which, 
 standing at right angles to the flat end of the cylinder, is 
 protected by a piece of brass tubing. The opposite end of the 
 cylinder is coated with lamp-black, and is the part to be 
 exposed towards the sun. A collar and screw is fitted to the 
 tubing, by means of which the instrument may be attached 
 to a stake driven into the ground. " It is necessary that the 
 surface which receives the sun's rays should be perpendicular 
 to them, and this is secured by appending to the brass tube 
 which shields the stem of the thermometer a disk of pre- 
 cisely the same diameter as the steel cylinder. When the 
 shadow of the cylinder accurately covers the disk, we are sure 
 that the rays fall, as perpendiculars, on the upturned surface 
 of the cylinder. The observations are made in the following 
 manner : First, the instrument is permitted, not to receive 
 the sun's rays, but to radiate its own heat for five minutes 
 against an unclouded part of the firmament ; the decrease of 
 the temperature of the mercury consequent on this radiation 
 being noted. Next, the instrument is turned towards the 
 sun, so that the solar rays fall perpendicularly upon it for 
 five minutes the augmentation of temperature is now noted. 
 Finally the instrument is turned again towards the firmament, 
 away from the sun, and allowed to radiate for another five 
 minutes, the sinking of the thermometer being noted as before. 
 You might, perhaps, suppose that exposure to the sun alone 
 would be sufficient to determine its heating power; but we must 
 
APPENDIX. 143 
 
 not forget, that during the whole time of exposure to the sun's 
 action, the blackened surface of the cylinder is also radiating 
 into space ; it is not therefore a case of pure gain ; the heat re- 
 received from the sun is, in part, thus wasted, even while the 
 experiment is going on ; and to find the quantity thus lost, the 
 first and last experiments are needed. In order to obtain the 
 whole heating power of the sun, we must add to his observed 
 heating power the quantity lost during the time of exposure, 
 and this quantity is the mean of the first and last observations. 
 Supposing the letter R to represent the augmentation of 
 temperature by five minutes' exposure to the sun, and that 
 t and t ' represent the reductions of temperature observed 
 before and after, then the whole force of the sun, which we 
 may call T, would be thus expressed : 
 
 T = E + i (t + tO 
 
 " The surface on which the sun's rays here fall is known ; 
 the quantity of mercury within the cylinder is also known ; 
 hence, we can express the effect of the sun's heat upon a 
 given area, by stating that it is competent, in five minutes, 
 to raise so much mercury, or so much water, so many degrees 
 in temperature. Water, indeed, instead of mercury, was 
 used in M. Pouillet's pyrheliometer." Tyndall's Heat con- 
 sidered as a Mode of Motion, 2nd Ed. 
 
 The makers of this instrument have to take scrupulous 
 care to measure exactly the interior diameter and depth of 
 the steel cylinder. If the instruments could be made 
 generally to a uniform pattern in every particular there is 
 no doubt that extended employment of them, in various parts 
 of the world and at different seasons, would yield valuable 
 results. 
 
 "A thermometer with the bulb blackened affords the 
 only means the traveller can compass of measuring the 
 power of the sun's rays. It should be screened or put in a 
 blackened box or laid on black wool." Such is the indefinite 
 
144 APPENDIX. 
 
 language used by the author of the "Himalayan Journals," 
 a traveller than whom none have made better meteorological 
 observations, or better use of them. The pyrheliometer 
 appears to be an instrument of precision peculiarly suited to 
 the requirements of the scientific traveller. 
 
 " Experiments made by Pouillet, at different altitudes, 
 with the pyrheliometer, show that, everything else being 
 equal, the generation of heat by the solar rays is more 
 powerful in higher altitudes than near the surface of our 
 globe, and that consequently a portion of these rays is 
 absorbed in their passage through the atmosphere. Why, 
 in spite of this partial absorption, the mean temperature of 
 low altitudes is nevertheless higher than it is in more 
 elevated positions, is explained by the fact that the atmos- 
 phere stops to a far greater degree the calorific rays 
 emanating from the earth, than it does those from the 
 sun." J. E. Mayer, on Celestial Dynamics. 
 
 To trace in their fullest extent all the conditions con- 
 nected with solar radiation, it is necessary to measure its 
 effects in different latitudes ; to follow its changing influence 
 through the different months of the year; to estimate its 
 progress during the several hours of the day; to trace its 
 power on the varied tribes of vegetation ; and to discover, 
 under all the diversified circumstances, its maximum force. 
 In like manner, in order to trace the laws which regulate 
 terrestrial radiation, the extent of its power must be traced 
 in different latitudes ; its influence upon plains and moun- 
 tains compared; and its effects in the several months 
 measured. 
 
 TEEEESTEIAL EADIATON. 
 
 In the absence of the sun the heat acquired by the earth 
 during the day, is radiated away through the atmosphere 
 
APPENDIX. 145 
 
 into space. Nebulous and invisible aqueous vapour equally 
 exert a retarding influence upon this process ; whence, in a 
 great measure, result the equable climates of insular and 
 maritime regions, since these are favourably situated as 
 regards humidity. On the contrary, inland countries, espe- 
 cially those characterised by the absence of great rivers 
 or lakes, are naturally dry ; hence, having the conditions for 
 freest action of radiation, their climates are extreme. 
 
 Diurnal radiation, when powerful, scorches the skin and 
 withers tender plants. Nocturnal radiation kills tender 
 plants by the rapid abstraction of heat from all their surfaces 
 which are exposed to a clear sky ; and causes the formation 
 of dew and hoar-frost. 
 
 In the British Isles, vegetation, says Daniell, is liable to 
 be affected at night, from the influence of radiation, by a 
 temperature below the freezing point of water, ten months 
 in the year ; and even in the two months July and August, 
 the only exceptions, the radiant thermometer sometimes falls 
 to 35. Mr. Lowe observed, near Nottingham, on 2nd 
 June, 1864, the . temperature on the grass as low as 23 0< 3. 
 Many potatoes, which were two feet high, were killed, as 
 well as french and kidney beans ; even the young shoots of 
 the ash were destroyed. 
 
 The instrument usually employed for measuring the 
 effect of nocturnal radiation is a registering alcohol ther- 
 mometer. It should be protected by a glass shield through 
 which the graduations, etched upon its stem, may be read ; 
 instead of being mounted in a wooden or metallic frame. 
 The thermometric spirit should be quite colourless. This 
 instrument is usually placed, with its bulb exposed to the 
 sky, just above the points of grass, the stem being supported 
 horizontally. Whether or not a better instrument, or a 
 better plan of using this, can be devised, is a matter which 
 
 L 
 
146 APPENDIX. 
 
 should engage the serious attention of meteorologists. For 
 without doubt observations indicative of the radiation from 
 grass are not strictly comparable. Not only does the radi- 
 ating power of grass vary with the species, it varies even 
 with the luxuriance and length of the blades while the situ- 
 ation, whether on a plane or sloping surface, and the nature 
 of the subjacent soil must also influence the process differ- 
 ently in different places. 
 
 The apparatus used by Daniell for observations on noc- 
 turnal radiation consisted of a spirit thermometer placed in 
 the focus of a parabolic reflector, resting on the grass, or 
 soil, and exposed to the sky. The reflector was made of cop- 
 per, and plated with silver ; its diameter was six inches, and 
 the length of its focus 1 inches. The thermometer was 
 passed through a collar in its side, the scale being kept on 
 the outside. The reflector was placed upon a foot with a ball 
 and socket joint, that enabled it to turn in any direction. 
 
 6 ( Dr. Wollaston was the first to expose a concave metallic 
 mirror turned upwards to the free air, with a thermometer 
 placed in its focus; and he found that the thermometer indicated 
 a lower temperature after being thus exposed for a short time." 
 
 Daniell considered this the most accurate mode of mea- 
 suring terrestrial radiation. " The radiant thermometer is so 
 completely insulated by the reflector from the counter 
 radiation of surrounding bodies that it may be applied with 
 equal effect in any situation, where the aspect of the sky is 
 very limited. Even in the streets of London, where the 
 radiation of an exposed thermometer is nearly neutralized, 
 and the utmost effect never exceeds two or three degrees, 
 that of the thermometer, guarded by the reflector, is wholly 
 unimpeded. Experiments that are thus made, in whatever 
 situation, are strictly comparable, provided they are screened 
 from any strong action of the wind." 
 
APPENDIX. 
 
 147 
 
 Mr. Glaisher 's experiments on nocturnal radiation did 
 not lead him to speak so favourably of the parabolic reflector. 
 Thus to quote his memoir on this subject in the Philosophical 
 Transactions, 1 847, he writes : 
 
 " The thermometer whose bulb is in the focus of the 
 mirror when the clouds are low remains stationary, and fre- 
 quently reads higher than any other thermometer however 
 placed ; the reason seems to be that the heat radiated from 
 the cloud, being received on the reflector and reflected to the 
 bulb in the focus, exceeds that radiated from the thermo- 
 meter." 
 
 Mr. Glaisher has shown that, under the same state of 
 sky, the excess of the reading of the thermometer in air 
 above that in the reflector is the same very nearly at all 
 times of the year, but that the amount of the excess varies 
 with every variation of the quantity of cloud. 
 
 The following table expresses the average excess, accord- 
 ing to the state of the sky, of the readings of a self- register- 
 ing thermometer placed in air at the height of 4 feet, and 
 protected from radiation, above those of a similar thermo- 
 meter placed in the focus of a metallic parabolic reflector, 
 fully exposed to the sky. 
 
 Amount of cloud. 
 (Scale to 10). 
 
 Number 
 of nights. 
 
 Mean 
 excess. 
 
 10 
 8 
 5 
 2 
 
 
 321 
 
 198 
 282 
 257 
 331 
 
 2-74, 
 4-13 
 5-52 
 7-06 
 8-33 
 
 In all simultaneous observations it was found by Mr. 
 Glaisher that the readings of the thermometer in the re- 
 flector had no advantage over the readings of one placed on 
 lead. Thermometers which were from one to three inches 
 from the top of grass generally read the same as that in the 
 reflector. Sufficient consideration has, apparently, not been 
 
148 
 
 APPENDIX. 
 
 given by meteorologists as to the best substance to be employed 
 as a radiating surface with a view of obtaining observations 
 at different places which may be comparable. It would seem 
 that glass or lead might be found to possess advantages over 
 grass. 
 
 The relative depressing power of various substances laid 
 on grass, to that of long grass, assigned by Mr. Glaisher, 
 from a mean of all his observations, are given below, long 
 grass being taken as a standard : 
 
 SUBSTANCE. 
 
 From simultaneous 
 observations. 
 
 From registering 
 thermometer. 
 
 Kadiating 
 power. 
 
 No. of ob- 
 servations. 
 
 Radiating 
 power. 
 
 No. of ob- 
 servations. 
 
 Hare skin 
 
 1316 
 
 70 
 
 
 ... 
 
 Rabbit skin - 
 
 1240 
 
 59 
 
 ... 
 
 
 Raw white wool - - - - 
 
 1222 
 
 357 
 
 1280 
 
 276 
 
 Unwrought white cotton wool - 
 
 1085 
 
 228 
 
 1167 
 
 28 
 
 Long grass - 
 
 1000 
 
 479 
 
 1000 
 
 467 
 
 Flannel 
 
 871 
 
 29 
 
 
 
 In focus of parabolic reflector - 
 
 858 
 
 464 
 
 962 
 
 716 
 
 Glass ..... 
 
 864 
 
 163 
 
 ... 
 
 ... 
 
 Sheet copper - ... 
 
 839 
 
 172 
 
 
 ... 
 
 Blackened tin - 
 
 770 
 
 104 
 
 844 
 
 84 
 
 Sheet lead 
 
 757 
 
 224 
 
 1000 
 
 72 
 
 Jet black lamb's wool 
 
 741 
 
 33 
 
 
 ... 
 
 Sheet zinc 
 
 681 
 
 213 
 
 
 ... 
 
 White tin 
 
 657 
 
 112 
 
 677 
 
 94 
 
 Snow - 
 
 657 
 
 Few. 
 
 ... 
 
 
 Sheet iron - 
 
 642 
 
 161 
 
 
 
 Slate - 
 
 573 
 
 100 
 
 
 ... 
 
 The following table gives the results obtained by Mr. 
 Glaisher by exposing glass thermometers at different heights 
 above the surface of a grass field : 
 
 Eadiating power. No. of observations 
 
 Long grass 1,000 479 
 Above the points of the grass, 
 
 One inch 671 226 
 
 Two inches 570 9 
 
 Three 477 89 
 
 Six 282 165 
 
 One foot 129 212 
 
 Two feet 86 199 
 
 Four 69 87 
 
 Six 52 18 
 
 Eight 17 96 
 
APPENDIX. 149 
 
 To the experience of Daniell and Glaisher we will 
 add the opinion of Sir J. Herschel, as expressed in his 
 " Meteorology." 
 
 " When a thermometer is exposed with its bulb in the focus 
 of a concave silver hemisphere* or paraboloid, highly polished 
 both internally and externally, and deep enough, when exposed 
 with the concavity upwards, to cut off from the thermometer 
 all view of the earth, and as it were to continue the sky 
 beneath it, it can only receive heat by condensationf and radia- 
 tion from the air, and from the condensation of moisture. A 
 thermometer so exposed under a clear sky, and shaded from 
 direct sunshine, always marks several degrees below the 
 temperature of the air, and its depression affords a rude 
 measure (of the statical kind, and open to all the objections 
 to which that kind of measure is openj) of the facility for the 
 escape of heat by radiation afforded under the circumstances 
 of exposure." 
 
 Terrestrial radiation is even a more difficult phenomenon 
 for the traveller to estimate than solar radiation, the danger 
 of exposing instruments at night being always great in wild 
 countries. Avoiding radiation from surrounding objects is 
 very difficult, especially in wooded countries. The ther- 
 mometer and metallic reflector appear to be suitable for the 
 varying conditions in which a traveller would make observa- 
 tions on noctural radiation. 
 
 THE SPECTKUM OF AQUEOUS VAPOUK. 
 
 The optical study of aqueous vapour shows that it pos- 
 sesses an elective power of absorption for light, or in other 
 words, that this vapour produces dark lines and bands in the 
 
 * " The figure is absolutely of no importance. A paraboloid is generally 
 used, for no reason that we can perceive but to increase the cost." 
 f Query, conduction ? J See page 140. 
 
150 APPENDIX. 
 
 spectrum of a luminous ray which traverses a sufficient 
 thickness of it. 
 
 Certain constant lines in the solar spectrum were found to 
 be incessantly variable in their intensity with the height of 
 the sun, that is to say, with the thickness of the earth's 
 atmosphere traversed by the sun's rays. An investigation of 
 the spectrum on a high mountain, the Faulhorn, proved that 
 these lines grew weaker in proportion to the height ascended, 
 that is, in proportion as the solar rays traversed a smaller 
 thickness of the terrestrial atmosphere. Thus it became 
 obvious that the earth's atmosphere affected the characteristics 
 of the spectrum. It was found that the earth's atmosphere 
 produces in the red, orange, and yellow of the spectrum, a 
 system of lines ten times as numerous as the solar lines of 
 these regions ; on the contrary, in the green, the blue, and 
 the violet, the lines of solar origin predominate. 
 
 An attentive study of the solar spectrum led to the idea 
 that this action of the atmosphere is attributable to the 
 aqueous vapour dissolved in the earth's atmosphere, if not 
 wholly, at least to a very important part. Comparisons, 
 followed for a long time, on the solar light, during various 
 seasons of the year, showed very clearly that for the same 
 heights of the sun certain lines of the spectrum were more 
 pronounced as the dew-point was higher. On the Faulhorn, 
 on extremely dry days, the lines in question disappear almost 
 entirely from the spectrum. 
 
 There could thus be little doubt as to the action of aqueous 
 vapour; yet it was necessary, seeing the importance of the 
 result, to submit this point of theory to a direct verification, 
 by investigating the modifications which a well-defined beam 
 of light of known composition experienced by the fact of its 
 passage through a tube of sufficient length containing only 
 aqueous vapour. 
 
 An iron tube 37 metres in length was mounted; it was 
 
APPENDIX. 151 
 
 placed in a wooden box of the same length, filled with well 
 dried wooden shavings an arrangement which avoids all 
 appreciable loss of heat. The vapour was furnished by a 
 moveable steam-engine of six-horse power, and the light 
 by a lamp of six jets, placed in the direction of the axis of 
 the tube. The experiments confirmed in the completest 
 manner, what the study of the solar spectrum had already 
 indicated. In one experiment (August 3rd, 1866), in which 
 the tube, well freed from air, was full of vapour at the 
 pressure of seven atmospheres, the spectrum was observed 
 with five dark bands, two of which, well marked, spread from 
 D to A (Fraunhofer), and recalled the solar spectrum seen 
 in the same instrument towards sunset. 
 
 From the first comparisons made between the spectrum 
 of aqueous vapour and that of solar light, the group A of 
 Fraunhofer, B (in great part at least), the group C, and two 
 groups between C and D are due to the action of the aqueous 
 vapour of the atmosphere. 
 
 This experiment has further given an interesting result. 
 The spectrum of the transmitted light was seen to be very 
 dark in the most refrangible part, while it was brilliant 
 in the regions of red and of yellow. Thus, although aqueous 
 vapour energetically absorbs certain red and yellow rays, it is 
 very transparent for most of these rays, while it acts in a 
 general manner on the more refrangible rays. It follows 
 from this that aqueous vapour would be of an orange colour 
 by transmission, and the redder the greater the thickness 
 through which it acts. 
 
 The results will have to be verified and established with 
 the greatest care. If it be finally demonstrated, we shall 
 find in it an explanation of the red colour, so variable in its 
 tints, but always observed at sunset as well as at arm-rise. 
 
 The consequences of this discovery of the spectrum of 
 
152 APPENDIX. 
 
 aqueous vapour will be overlooked by no one. We are finally 
 agreed as to the origin of a considerable portion of the rays 
 of the solar spectrum ; and the knowledge of these rays will 
 allow us to investigate the higher regions of our atmosphere 
 as regards their humidity regions which at present are 
 inaccessible to our means of investigation. But it is more 
 especially in astronomy that the results will be interesting 
 to develope. 
 
 This article has been abridged from a memoir by the dis- 
 coverer M. JANSSEN, printed in Comptes Rendus, August 13th, 
 1866, a translation of which is given in the Philosophical 
 Magazine, No. 216. 
 
 [We have endeavoured in this appendix to place before the 
 reader, in a popular manner, the leading features and general 
 conclusions of the latest researches into the functions and 
 properties of the aqueous vapour of the earth's atmosphere. 
 We have been careful to use, as nearly as possible, the words 
 of the illustrious experimentalists and writers themselves, 
 and our object throughout has been to give only such 
 general information as may well lead the reader to the 
 original sources, the works and memoirs which we have 
 always distinctly indicated. The paths of physical science, in 
 which Tyndall and Janssen have been the pioneers, will 
 assuredly lead to important and fertile fields of inquiry. 
 The applications of our knowledge regarding the relation of 
 aqueous vapour and radiant heat, and the spectrum of 
 aqueous vapour, will most probably be of the utmost impor- 
 tance in giving an impetus and a reform to the pursuit of 
 meteorology. Day by day new lights and new ideas will be 
 evolved, and what is here recounted will soon need sup- 
 plementing.] 
 
 D. LANE, PBINTKB, STBAND, LONDOK, W.C. 
 
International Exhibition, 1862. 
 
 AWARDED FOR CASELLA's PATENT 
 MERCURIAL MINIMUM THERMOMETERS, 
 
 COMBINED WITH ACCURACY AND EXCELLENCE 
 IW METEOROLOGICAL INSTRUMENTS. 
 
 ILLUSTRATED LIST OP 
 
 METEOROLOGICAL 
 
 I i| INSTRUMENTS, 
 
 WITH CONCISE 
 
 DESCRIPTION AND PRACTICAL INFORMATION 
 
 RELATING TO 
 
 SUCH INSTEUMENTS AS AEE BEST ADAPTED FOE PUBLIC INSTITUTIONS 
 OE FOE PEIVATE USE. 
 
 SECOND EDITION. 
 
 BY 
 
 L. CA^S 
 
 ical, ptical, JSurfceginu anfc Scientific Instrument 
 
 MAKER TO THE ADMIRALTY. 
 
 To the Boards of Trade and Ordnance, the Royal Observatories at Kew, Victoria, Toronto, 
 
 and Calcutta, ike British Meteorological Society, the Royal Geographical Society, 
 
 and the Governments of India, Spain, Portugal, Russia, America, 4.c. 
 
 23, HATTON GARDEN, LONDON, E.G. 
 
ADDRESS. 
 
 THE Meteorological information and instruction contained in the first 
 Edition of this List having been much approved of, has induced the publi- 
 cation of this second, and, it is hoped, improved edition. 
 
 To Directors and Managers of large Observatories, or other public Institu- 
 tions, it presents a practical selection of STANDARD INSTRUMENTS, embracing 
 every recent improvement, whilst to Amateurs desirous of taking reliable 
 observations, it gives a faithful means of enabling them to select, and thus 
 commence at the outset with suitable Instruments, the indications of which 
 will bear the strictest test. 
 
 Besides this List, L. OASELLA has published another, descriptive of such port- 
 able and pocket Instruments as he has expressly designed for travellers, includ- 
 ing Livingstone, Burton, Speke, and other distinguished Fellows of the E/oyal 
 Geographical Society, as well as the Members of the Alpine Club, for whose 
 wants reduction in size, adaptation to every climate, with perfect precision, 
 were especially requisite ; among these are hie Pocket Apparatus for 
 measuring heights by the vapour of water, his new ALTAZIMUTH, or Pocket 
 Theodolite, &c., &c. The success of these efforts is best shown by the 
 extended popularity of the Instruments, the constant attempts made to 
 imitate them,, as well as the public testimonies in their favour by the 
 Gentlemen themselves. 
 
 For Clinical and general Medical Investigations, Thermometers and other 
 Instruments have also been arranged by L. C., including an accurate means of 
 measuring the ventilation of Hospitals, an economic Microscope for Medical 
 Students, &o., &c. 
 
 A brief List has also been prepared of L. CASELLA'S Economic Series of 
 METEOROLOGICAL INSTRUMENTS, for Beginners, Gardening and Horticultural 
 purposes, for which he has also been honoured with a First Class Certificate 
 from the Royal Horticultural Society. The great object being to adopt each 
 Instrument in the best manner, to the particular purpose for which it is required. 
 
 23, HATTON GARDEN, LONDON, E.C. 
 January \st, 1865. 
 
Fig. 1. 
 
 STANDARD 
 
 METEOEOLOGICAL INSTRUMENTS, 
 
 MANUFACTUBHD BY 
 
 L. CASELLA. 
 
 BAROMETERS. 
 
 1. Standard Barometer (jig. 1), on Fortm's principle, 
 
 with brass frame, glass cistern, and fixed ivory 
 point. The mercury boiled in the tube, which is 
 0-4 in. internal diameter. The mercury in the 
 cistern is adjustable to the zero, or ivory point, 
 by means of a thumb-screw acting upon a flexible 
 base. The vernier, with rack adjustment, reads to 
 the 1 -500th part of an inch, or, by estimation, to 
 001 inch. In front of the barometer a thermometer 
 is attached, in contact with the tube, with divisions 
 etched on the stem. For facility of reading, a 
 moveable jointed reflector is attached to, and moves 
 with, the vernier. The instrument is suspended 
 from a bracket at the top of the mahogany board, 
 so as to ensure perfect perpendicularity. At the 
 bottom of the board is a socket, with clamping 
 screws for steadying the barometer when an obser- 
 vation is made. The instrument is so mounted, 
 that it can be turned at pleasure to any source of 
 
 light 8 10 
 
 (If with Millimetre Scale, 10s. 6d. extra.) 
 
 2. Standard Barometer of extra large size, tube 075 in. 
 
 internal diameter, arranged for observatories, 
 
 20 
 
 Instructions. The Barometer may be placed in any convenient 
 room, where it is not near a fire, or exposed to the sun's rays. It 
 should be in a good light, with the scale about five feet from the 
 ground, so that the zero point in the cistern, and the vernier on the 
 scale may bo easily seen. First, hang the board on the wall, then 
 insert the lower part of the cistern through the bottom bracket, and 
 suspend the instrument as in Fig 1. Next turn the small thumb 
 screws of the lower bracket to fix it in the perfectly vertical position in 
 which it hangs. When the Barometer is thus placed unturn the 
 thumb screw till the mercury falls in the cistern to the level of 
 the ivory point, and then unturn the small iron screw on the top of 
 the cistern to admit the air more freely. 
 
 To Read the Barometer. First read the attached thermometer, 
 and adjust the mercury by means of the thumb screw so that it 
 barely touches the ivory point in the cistern, which, with its reflec- 
 tion, will then appear as a double cone this point is the zero of the 
 scale ; the height of the column is then taken by adjusting the lower 
 edge of the vernier, so that it shall exactly form a tangent to the 
 convex surface of the mercury in the tube, just excluding the light 
 by keeping the eye in the same plane with the back and front lower 
 edges of the vernier. Every care should also be used to avoid in- 
 fluencing the temperature whilst making the observation. 
 
 3. Standard Barometer on the Kew principle, in which 
 
 the graduations of scale are arranged to com- 
 pensate for the lise and fall of mercury in the 
 cistern, whereby the reading from a point in the cis- 
 tern is obviated. The mounting, &c., being the same 
 in every respect as No. 1. . . . 5 15 
 
L. CASELLA'S LIST OF 
 
 than any other. . . . The most convenient form of all 
 
 , 
 
 Maximum Thermometers." 
 
 In "Practical Meteorology," by John Drew, Ph. D., F.R.A.S., &c., we find " Its construction is 
 simple and its indications sure, as those can testify who have used it." 
 
 An arrangement of Phillip's Maximum Thermometer has just been effected by L. Casella for 
 clinical and physiological investigations. See Aitkins's " Science and Practice of Medicine ;" 3rd 
 Edition. London : 1864. 
 
 Directions for Using the Maximum Thermometer. Suspend the instrument by means of 
 the brass loop attached to the back, let the other end rest upon a projecting nail (a) to keep it 
 horizontal, as shown in fig. 11 ; disengage and lower this end to allow the detached portion of mer- 
 cury to approach the rest, which it will do within about one quarter of a degree. On an increase 
 of temperature, the mercury will rise as in an ordinary thermometer, and continue to do so as long 
 as the heat increases, propelling the detached portion to whatever extreme the heat may attain, 
 On a decrease of temperature, the mercury will contract and recede in the usual manner, leaving 
 the detached portion to indicate the highest temperature, which it does at the end furthest from the 
 bulb. 
 
 CASELLA'S PATENT MERCURIAL MINIMUM THERMOMETERS. 
 
 15 . Casella's Mercurial Minimum Thermometer, on porcelain scale, with hard- 
 
 wood back, and divided on the stem (fig. 15) . . ; > * * v ' 1 15 
 
 16. Casella's mercurial minimum Thermometer, for Terrestrial Radiation, 
 divided and figured on the stem, and enclosed in glass cylinder for protec- 
 tion, like fig. 13 1 15 
 
 This is the only practical Mercurial Minimum Thermometer hitherto invented. 
 Mercury is the only fluid employed in this instrument. The bulb and column are 
 made of the same size as in the Maximum Thermometers; and cold is thus 
 registered under precisely the same conditions as heat ; no steel or other index 
 
STANDARD METEOROLOGICAL INSTRUMENTS. 7 
 
 is employed ; whilst the annoyance arising from sluggishness, evaporisation, and 
 breakage of the column in the spirit minimum, is entirely avoided. The general form 
 is shown in Jiff. 15 ; d being a tube with large bore, at the end of which a flat glass dia- 
 phragm is formed by the abrupt junction of the small chamber (a 6), the inlet to which 
 at b is larger than the bore of the indicating tube. The result of this is that, having 
 set the thermometer, the contracting force of the mercury in cooling withdraws 
 the fluid in the indicating stem only, whilst on its expanding with heat, the long 
 column does not move, the increased bulk of mercury finding an easier passage 
 through the larger bore into the small pear-shaped chamber attached. 
 
 Directions for Using the Mercurial Minimum Thermometer. Place the instrument in 
 a horizontal position, with the back plate (e) suspended on a nail, and the lower part supported on 
 a hook (/). The bulb end may now be raised or lowered, causing the mercury to flow slowly until 
 the bent part (d) is full and the chamber (a b) QUITE EMPTY. At this point the flow of mercury 
 in the long stem of the tube is arrested by adhesion to the diaphragm (6), and indicates the exact 
 temperature of the bulb, or air, at the time. On an increase of heat the mercury will expand into 
 the small chamber (a b) ; and a return of cold will cause its recession from this chamber only, until 
 it reaches the diaphragm (6) to which it adheres. Any further diminution of heat withdraws the 
 mercury down the bore to whatever degree the cold may attain, where it remains until further 
 withdrawn by increased cold, or till re-set for future observation. When out of use, or after transit, it 
 may be that raising the bulb may not at first cause the mercury to flow from the small chamber 
 as above ; in such case a slight tap or jerk with the hand on the opposite end with the bulb up, will 
 readily cause it to do so. 
 
 THE VALUE OF THIS INSTRUMENT IS BEST SHOWN BY THE FOLLOWING 
 
 TESTIMONIALS : 
 
 From Sis HENRY JAMES, R.E., F.R.S., Director of the Ordnance Survey and Topographical 
 Depot of the War Department, Author of" Instructions for taking Meterological Observations," 
 with Tables, Notes, fyc. : " I have great pleasure in stating that, after having had one of your 
 mercurial minimum thermometers carefully observed and registered at this office, and one at South- 
 ampton for the last three months, and during a period in which we have had a great range of 
 temperature, I have found it to act perfectly, and never once to get out of order. I therefore think 
 you have achieved a great success, and hope you will receive its reward." 
 
 B. STEWABT, Esq., Director of the Kew Observatory, in his Description of the instrument 
 before the meeting of the British Association for 1862, said : "Before bringing this instrument 
 to the notice of this association I have carefully tested its action at the Observatory, and find its in- 
 dications in every way satisfactory." 
 
 In the proceedings of the BBITISH METEOROLOGICAL SOCIETY for November, 1862, "Dr. 
 Thompson, Vice President, F.R.S., again directed the attention of the members to this instrument, 
 in the use of which he had derived the highest satisfaction ever since its invention." 
 
 From H. STORKS EATON, Esq., M.A., of the British Meteorological Society: " Little Bridy, 
 Dorset. Sir, I have found your Minimum Thermometer much more sensitive than the ordinary 
 alcoholic minimum. It is particularly well adapted for determining the amount of Terrestrial 
 RADIATION, and has not once failed during the .twelve months I have had it in use." 
 
 From F. F. TUCKETT, Esq., of Bristol :" The idea really is a beautiful one, and admirably 
 carried out. The Meteorological world are indeed indebted to you and your son for this excellent 
 invention." 
 
 From T. LAWRENCE, Esq., Medical Staff, Mooltan, Punjaub, India : " Your Mercurial 
 Minimum Thermometer continues to work admirably. It seems to me the only instrument adapted 
 for minimum registration in this climate." 
 
 STANDARD MINIMUM THERMOMETERS. 
 
 17. minimum Thermometer, filled with pure alcohol, for ordinary registration, 
 engine divided on the stem, and mounted to correspond with the Standard 
 Maximum No. 10, (fig. VI) . . , 18 6 
 
 18. MINIMUM THERMOMETER, on porcelain scale, divided and fitted as above, and 
 mounted to correspond with Maximum No. 11 ^ .-. . 18 6 
 
 19. MINIMTJM THERMOMETER, for terrestrial radiation, divided and figured on the 
 stem, which is enclosed in a glass cylinder for protection, (fig. 19) 17 6 
 
 20. Maximum and Minimum Thermometer combined, in neat pocket mahogany 
 case, for travellers, as expressly designed for the Livingstone Expedition. 
 The tubes are divided on the stem, and may also be used as strictly standard 
 instruments , 200 
 
 21. Ditto, ditto, of smaller size, as designed for the Alpine Club 150 
 
L. CASELLA'S LIST OF 
 
 Fig. 19, 
 
 Directions for Using the Spirit Minimum Thermometer. Suspend the top end by a 
 hook (A.) fig. 10, the bulb end resting on a projecting nail (B), and about half an inch lower than 
 the opposite end. To set the index, press gently on the suspended end, which will raise the other 
 and bring the index to its place, i.e., the edge of the fluid inside. As the temperature decreases the 
 spirit will recede and take the index back with it ; but, on an increase of temperature, the spirit 
 will advance, leaving the index to mark whatever extreme of cold may have occurred. If, in transit, 
 the index is shaken out of the spirits, or the spirits separated, all is easily rectified by a swing or two 
 of the arm, holding the bulb downwards, and then letting the thermometer hang with the bulb 
 down about ten minutes, to allow the fluid to settle and become perfectly united. 
 
 H YG ROMETERS. 
 
 22. Mason's Hygrometer (wet and dry bulb), metal scales, 
 mounted on mahogany board for suspension. The 
 Thermometers are divided on the stem, and the 
 figures burnt in on Casella's indelible porcelain slips 
 at the side, as supplied by L. Casella to the Members 
 of the British Meteorological Society (Fig. 22) 220 
 
 23. Ditto ditto, as above, with porcelain scales 1 15 
 
 24. Mason's Hygrometer, extra size, with expanded 
 graduations, the tubes being fifteen inches long, and 
 divided to 0.2, mounted, &c., as Fig 22 330 
 
 The wet and dry bulb, or MASON'S HYGROMETER, 
 consists of two thermometers placed parallel, about four 
 inches apart, with their graduations as nearly as possible 
 identical. The bulb of one is covered with thin muslin, 
 from which trail a few threads of lamp cotton; these, 
 being first wetted, are passed into a small attached 
 vessel of water, two or three inches distant, and the 
 bulb thus kept continually moist, causes this thermo- 
 meter to indicate the temperature of evaporation, whilst 
 the dry bulb thermometer shows the temperature of the 
 air. From the readings of the dry and wet thermo- 
 meters, the dew-point may be obtained by means of 
 the accompanying tables. During frost, however, 
 jwhen the capillary action of the cotton is stopped, 
 it is best to cut off the projecting threads, leaving 
 the bulb covered ; and when an observation is required 
 the bulb should be wetted, by means of a camels-hair brush, with water 
 as near 32 as possible. In such cases it is not necessary to remove the ice from 
 
 Fig 22. 
 
STANDARD METEOROLOGICAL INSTRUMENTS. 
 
 9 
 the 
 
 the bulb, but merely remove the drop which first forms from the water, tl 
 temperature will then speedily settle so as to indicate the point of evaporation. 
 
 The following table will greatly assist in familiarizing the inexperienced with 
 the value of the Hygrometer indications : 
 
 DRY 
 
 Rill R 
 
 DIPFBBBirCKS OF DBY AND^WBT BULBS. 
 
 DRY 
 
 RULR 
 
 DItlEBBJTCES OP DBT AffD WBT BULBS. 
 
 
 2 
 
 4 
 
 6 
 
 8 
 
 10 
 
 12 
 
 
 2 
 
 4 
 
 6 
 
 8 
 
 10 
 
 12 
 
 34 
 
 79 
 
 63 
 
 
 
 , 
 
 
 
 __ 
 
 58 
 
 87 
 
 76 
 
 66 
 
 57 
 
 49 
 
 
 
 36 
 
 82 
 
 66 
 
 53 
 
 _ 
 
 __ 
 
 _ 
 
 60 
 
 88 
 
 76 
 
 68 
 
 68 
 
 60 
 
 43 
 
 38 
 
 83 
 
 63 
 
 66 
 
 45 
 
 _ 
 
 
 62 
 
 88 
 
 77 
 
 67 
 
 68 
 
 60 
 
 44 
 
 40 
 
 84 
 
 70 
 
 58 
 
 47 
 
 
 
 __ 
 
 64 
 
 88 
 
 77 
 
 67 
 
 69 
 
 61 
 
 45 
 
 42 
 
 84 
 
 71 
 
 69 
 
 49 
 
 _ 
 
 
 
 66 
 
 88 
 
 78 
 
 68 
 
 60 
 
 62 
 
 45 
 
 44 
 
 85 
 
 72 
 
 60 
 
 60 
 
 _ 
 
 __ 
 
 68 
 
 88 
 
 78 
 
 68 
 
 60 
 
 52 
 
 46 
 
 46 
 
 86 
 
 73 
 
 61 
 
 51 
 
 _ 
 
 ,__ 
 
 70 
 
 88 
 
 78 
 
 69 
 
 61 
 
 63 
 
 47 
 
 48 
 
 86 
 
 73 
 
 62 
 
 62 
 
 44 
 
 _ 
 
 72 
 
 89 
 
 79 
 
 69 
 
 61 
 
 64 
 
 48 
 
 50 
 
 86 
 
 74 
 
 63 
 
 63 
 
 45 
 
 
 
 74 
 
 89 
 
 79 
 
 7P 
 
 62 
 
 55 
 
 48 
 
 52 
 
 86 
 
 74 
 
 64 
 
 54 
 
 46 
 
 _ 
 
 76 
 
 89 
 
 79 
 
 71 
 
 63 
 
 65 
 
 49 
 
 54 
 
 86 
 
 74 
 
 64 
 
 65 
 
 47 
 
 _ 
 
 78 
 
 89 
 
 79 
 
 71 
 
 63 
 
 56 
 
 60 
 
 66 
 
 87 
 
 75 
 
 65 
 
 56 
 
 43 
 
 
 
 80 
 
 90 
 
 80 
 
 71 
 
 63 
 
 66 
 
 50 
 
 Representing by 100 the total quantity of vapour which the air at any tempera- 
 ture can hold in suspension, the percentage of vapor actually present at any time 
 may be found in the table, opposite the temperature of the dry bulb thermometer, 
 and under the difference of the dry and wet bulb readings. The percentage cor- 
 responding to intermediate degrees and differences, not given in the table, can be 
 readily inferred. Thus, air temperature being 63, and difference 5, the humidity 
 of the air would be about 72. 
 
 In the British Isles the Hygrometer very seldom shows a greater difference than 
 12, hence it is presumed that the table will be of much practical utility. Below 
 freezing, 32, the observations of the Hygrometer are uncertain ; but then they are 
 not of so much importance, since the state of the air is known to be dry. 
 
 If the difference between the dry and the wet bulb readings be taken from the 
 reading of the wet bulb, the remainder will be the temperature of the dew-point, 
 very nearly Glaisher's factors should be employed for greater accuracy. They 
 are given in most works on meteorology. 
 
 25. Daniel's Hygrometer 5 the thermometers engine divided on the stems, with 
 ether test, complete in mahogany case (fig. 25) . . . . 330 
 
 This elegant instrument consists chiefly of a bent glass 
 tube, with two balls a black one containing ether, the 
 stem of which encloses a sensitive thermometer with the 
 bulb placed in the centre of the ball, and a white one 
 covered with thin muslin, the interior of the tube being 
 thoroughly deprived of air. 
 
 Directions for Using Daniel's Hygrometer. Turn the instru- 
 ment so that by applying the warm hand to the covered bulb all the 
 ether goes into the black bulb, then place it upright as shown in 
 the sketch, and pour ether upon the muslin enveloping the white 
 ball (a) Jig. 13, and when sufficient cold is produced by evaporation 
 of the ether from the black ball to condense the moisture of the atmos- 
 phere upon its surface, in the form of a ring just below the centre, the 
 internal thermometer will show the exact temperature at which the 
 deposition of dew takes place, which is called the dew-point. 
 
 26. Regnanlt's Condensing Dew- Point Hygrometer 
 
 (Casella's Improved), with ether bottle, &c, com- 
 plete in mahogany case (jig. 26) . 440 
 
 *** Agreeably to the suggestions of Colonel Sykes, F.R.S., and Dr. MiUer, F.B.S., 
 L. Casella has adapted a black glass bottle, with silver neck and tube, which may 
 be had instead of the silver bottle, or extra, at an additional charge of 20s. 6d. 
 
10 
 
 L. CASELLA'S LIST OF 
 
 Although Mason's Hygrometer has for some time "been in general use, yet 
 Regnault's is now much employed for taking direct observations of the dew-point. 
 In its present portable form it consists essentially of two 
 sensitive thermometers one exposed to the action of 
 the atmosphere, and the other to the influence of a 
 current of air passing through ether. An important 
 part of this instrument is the small polished silver bottle 
 into which, through an ivory stopper, one of the ther- 
 mometers is inserted. On one side, within the bottle, a 
 small silver tube descends nearly to the bottom, the 
 other end, passing outwards, is connected with a small 
 flexible aspirating tube. Supporting the bottle is a 
 hollow bent neck, connecting it with a stand, which is 
 also hollow, by which the air freely escapes at the base. 
 
 Directions for Using Regnault's Hygrometer Pour just 
 
 as much ether into the silver cup as will cover the bulb, and insert 
 the thermometer as shown in the drawing. On causing the air to 
 bubble slowly through the ether, by breathing through the tube, the 
 immersed thermometer will show a decline in the temperature ; and 
 when a film of moisture forms on the larger part of the silver bottle, 
 the temperature at that instant indicates the dew-point. The 
 observer should stand so as not to allow the breath or heat of the 
 person to affect the instrument. 
 
 RAIN GAUGES. 
 
 27. Rain Gauge, as made for the Observatories of the 
 War Department, of stout copper, height 10 inches, 
 receiving surface 10 inches square, and funnel 
 formed inside to prevent evaporation, with jar 
 graduated to hundreths of an inch (fig. 27J 1 5 
 
 28. Rain Gauge, Casella's, pedestal form, 3 feet high, 
 receiving surface 9 inches in diameter, made of 
 stout copper japanned, with strong glass tube 
 graduated to show 3 inches of rain in tenths and 
 hundreths, with extra stop-cock for frosty weather. 
 In this arrangement the rain is measured as it falls, 
 being visible at all times in the glass tube, and is 
 poured off by simply turning the stop-cock, without 
 removing the gauge from its place f%. 28) 3 10 
 
 29. RAIN GAUGE (Dr. Livingstone's), as expressly arranged by Casella for the 
 
 Zambesi Expedition ; receiving surface 3 inches diameter, with small stout 
 glass measure divided to hundredths of an inch, in leather case for the 
 pocket . 16 6 
 
 30. RAIN GAUGE, cylindrical form, of stout copper, 7-inch, with deep brass rim, 
 and inside receiving pan and bottle, by which large or small quantities are 
 measured without disturbing the gauge, and efficient protection secured 
 against evaporation or frost, or of overflow during the heaviest rains of the 
 tropics . 3 10 
 
 31. RATN GAUGE (Fig. 31), 5 -inch diameter, with receiving-bottle, &c., as arranged 
 by G. J. Symons, Esq., to meet the desire of the British Association in their 
 recent grants to him for the purpose of obtaining an accurate knowledge of 
 the rainfall of the British Isles 15 6 
 
 32. Ditto, ditto, of Copper, for durability . . ""' * . . . 110 
 
 33. Evaporating Dish or Gauge, of copper, with wire gauze cover. The receiving 
 surface of same diameter as the gauge with which it is used, say five inches, 
 with graduated glass measure . .",'., . . 15 6 
 
 34. EVAPORATING GAUGE, as above, seven inches . . 18 6 
 
 Ditto, ditto, seven inches . . .' ' * -* 1 5 
 
STANDARD METEOROLOGICAL INSTRUMENTS. 
 
 11 
 
 The increasing importance attached to a knowledge of the quantity of water 
 which falls on, or evaporates from, various localities, has caused the exercise of 
 considerable judgment in the construction of 
 instruments the best suited for this purpose. 
 Extended experiments, with areas or receiving 
 surfaces, varying from three to twenty inches 
 in diameter, have led to the adoption of medium 
 sizes for standard and general instruments. 
 
 Instructions for Evaporating Gauge. 
 
 Nearly fill it with water, measured by the graduated glass measure, and 
 place it out of doors freely exposed to the air. After exposure, again mea- 
 sure the water, and the difference between the first and second measure- 
 ment shows the amount that has evaporated. Should rain have fallen, 
 however, during the interval, the quantity equal to that collected in the 
 adjoining rain gauge must first be deducted from the evaporator, the 
 remainder compared with the measured quantity put in showing the 
 amount that has evaporated. For districts which are subject to very heavy 
 rainfall, an evaporating gauge with overflow pipe to meet any exigency 
 may be had at a slight increase in the price these, however, are not so 
 easily used, and are not recommended except when obliged by such necessity. 
 
 ANEMOM ETE RS. 
 
 35. Anemometer (Robinson's) Small size, with four Index 
 Wheels, registering successively 100, 1,000, 10,000, and 
 100,000 revolutions. In this arrangement the cups travel 
 at the rate of one-third the wind's velocity, and each revo- 
 lution represents 3.14 feet; thus 3.14x3.=9.42 feet, being 
 the distance travelled by the wind for each revolution. 
 This again multiplied by the number of revolutions indi- 
 cated on the dial, shows the distance the wind has travelled 
 between one observation and another. The dials are read 
 from right to left, and the amount indicated at the last 
 observation is to be deducted from that shown on the dials 
 at the time of the current observation . 330 
 
 ,36. Robinson's Anemometer, for Registering the Telocity of 
 the Wind in miles and tenths of miles, greatly improved 
 and modified, for registering to 500 miles, (fig. 33) described by Sir H. 
 James, F.R.S., in his Book of Instructions for Meteorological Observations, 
 page 29 . . . . . . . 440 
 
 37. ROBINSON'S ANEMOMETER, as above, to register to 5000 miles 550 
 This gauge has an extra dial to record to 5,000 miles, each division representing 600 miles, as shown 
 
 by the first dial. 
 
 38. LIND'S ANEMOMETER . . , 7 . V . . . . 22 
 
 This instrument though ingenious in theory and improved by Sir Snow Harris is not recom- 
 mended, and must fall into total disuse as the self-recording principal of Dr. Robinson becomes 
 more known. 
 
12 
 
 L. CASELLA'S LIST OF 
 
 39. Anemometer for Sanitary Investigations, for showing the exact amount of 
 draft or current of air passing in any part of a room down to one foot per 
 second . 400 
 
 Robinson's Anemometer con- 
 sists essentially of f our hemisph er- 
 ical cups, having their diametral 
 planes exposed to a passing cur- 
 rent of air ; they are carried by 
 four horizontal arms attached 
 to a vertical shaft, which is 
 caused to rotate by the velocity 
 of the wind. Dr. Robinson found 
 that the cups, and, consequently, 
 the axis to which they are at- 
 tached, revolve with one third the 
 wind's velocity, which is measured 
 by a simple arrangement of two 
 wheels working in endless screws, 
 and, by means of two indices, 
 shows, on inspection of the dial, 
 the velocity of the wind. The 
 outer or front wheel, which re- 
 volves once for every five miles, 
 is furnished with two graduated 
 circles, the interior circle being 
 subdivided to miles and tenths of 
 miles, whilst the outer circle is 
 divided into one hundred parts, 
 each being equivalent to five miles, 
 so that it measures 500 miles of 
 wind. The stationary index at 
 the top of the dial marks on the 
 inner circle the number of miles 
 (UNDER FIVE), and tenths, that 
 the wind may have traversed, in addition to the miles shown by the traversing 
 index, which revolves with the dial and indicates on the outer circle the transit of 
 every five miles. This anemometer is rendered extremely portable by the arms, 
 which carry the cups, being made to take off. When in use, it may be screwed on 
 a shaft, or ordinary piece of iron pipe, which accompanies it, and fixed in any 
 desirable position. 
 
 The mode of reading the instrument is best explained by example. Suppose the stationary pointer 
 reads 1 mile 2| tenths, i.e., 1*25 miles, and the traversing index reads 25 on the outer circle, the read- 
 ing is the sum, or 26*25 miles. This reading must be subtracted from the subsequent reading to 
 ascertain the velocity of the wind in the interval. Suppose the second reading, say next morning, 
 to be 24*05 miles ; subtracting the former reading, 26*25, the difference, 214*25 miles, is the distance 
 traversed by the wind in the interval. The observations should be recorded as read off, and a 
 second column of the register should be provided for daily differences, or other interval as the 
 observer may select. When the traversing index has passed the 500, the observer should make 
 a * in the register before the next reading. Thus, suppose the reading yesterday was 387 miles, 
 and to-day * 154 miles : here it is evident that the index has passed beyond the 500 ; so we place 
 the * before it, and, in taking the difference, treat it as 654, which gives 267 miles of wind. 
 Every star in the register counts as 600 miles of wind. Suppose the mean daily velocity of the 
 wind for any interval, say one month, be required, all that is necessary is to take the reading of 
 the instrument on the last day, add to it 500 for every * which appears in the register during the 
 interval, subtract the reading on the first day, and divide the remainder by the number of days. 
 
 40. Enlarged Anemometer, for Harbours and Public Observatories, on the Kew 
 principle, with clock-work arrangement for registering the time of the velo- 
 city and direction of the wind. The action of the Kew Anemometer on this 
 principle has not once ceased during the nine years that it has been in use. 
 
 65 
 
 41. Ditto ditto if made to register in a lower room of the building 75 Q 
 
 Fig. 38 
 
STANDARD METEOROLOGICAL INSTRUMENTS. 
 
 13 
 
 42. Hypsometrical Apparatus, for measuring moun- 
 tain heights by the vapour of boiling water. The 
 extended improvements effected in this instru- 
 ment of late by L. Casella are such as to render 
 it by far the most portable means we have of 
 measuring great elevations. The thermometer, 
 strong, with small bulb, is divided and figured on 
 the stem, and sheltered when in use by a double 
 telescope chamber into which it is placed to any 
 required depth through a loose piece of india- 
 rubber which rests on the top ; the chamber 
 being filled with vapour from the boiling water 
 beneath. The inner chamber, and tube thus 
 completely enveloped, the vapour descends in the 
 outer chamber and escapes by the outlet. By 
 this means the mercury both in the bulb and 
 stem is immersed in pure vapour, whatever kind 
 of water may be employed. (Jig. 42.) The port- 
 able leather case (jig, 42*) contains the whole 
 when packed for travelling. Price, with one 
 thermometer, divided to 0.l . 4 15 
 
 43. 11 ypso metrical Apparatus. The complete success 
 attending the above arrangement has induced 
 L. Casella to arrange a still smaller apparatus on 
 the same principle, which has been much 
 used by the members of the Alpine Club. 
 It may be carried with perfect ease in the trow- 
 sers' pocket, and, by those a little experienced in 
 its use, is often preferred, for its simplicity and 
 certainty, to the Mountain Barometer. With one 
 thermometer divided to 0.5 - 2 10 
 
 Casella' s Tables, with instructions for using the Hypsometrical Apparatus 010 
 
 ANEROID BAROMETERS. 
 
 Fig. 50 
 
 The Aneroid, though not an inde- 
 pendent instrument, or capable of the 
 same permanent precision as a stand- 
 ard mercurial barometer, is yet pro- 
 bably the most portable and sensitive 
 means we possess of measuring or- 
 dinary variations in atmospheric 
 pressure and foretelling changes in 
 the weather, either on land or at 
 sea, whilst for measuring moderate 
 heights it is most valuable and 
 convenient. A well made ordinary- 
 instrument will measure approxi- 
 mately the heights of mountains up 
 
14 L. CASELLA'S LIST OF 
 
 to 5,000 or 6,000 feet, the height of a house, or even the difference in height 
 between one room and another, and are now made by L. Casella to measure 
 as much as 15,000 feet. This ingenious and elegant instrument depends 
 essentially for its action on the compressibility by the atmosphere of a thin 
 flat circular corrugated metallic box, from the interior of which the air has been 
 removed, and the flat surfaces of which are held in a state of tension or separation 
 from each other by means of springs. The varying weight of the air gains or 
 loses on this resistance, and is indicated by means or the hand upon the dial. 
 
 44. Aneroid Barometer, card dial, ordinary size, in case . . 2 10 
 
 45. Ditto ditto, metal dial . . . /. . . .' .300 
 
 46. Ditto, ditto, with thermometer . -. . . . . 330 
 
 47. Ditto, ditto, finely divided, with extra compensation for tem- 
 perature, with or without thermometer . ... . 4 
 
 48. Ditto, ditto, ditto, with jewelled movements, highly finished; 
 the scale expressly adapted for measuring heights up to 15,000 feet, 
 according to order, with improved revolving index 6 to 8 8 
 
 49. Small Aneroid 2^ by 1 inches, for the pocket, graduated from 
 
 24 to 31 300 
 
 50. Watch Size Aneroid, to show up to 10,000 feet of elevation (%. 56) 400 
 
 51. Ditto, to 15,000 feet, with thermometer . -..'' 4 10 
 
 52. Ditto, to 3,000 feet, with expanded scale of graduation . . 500 
 
 53. Ditto, ditto, in silver . . .; .-<..; , . . , 5 10 
 
 * # * The Watch Size and Altitude Aneroids above-named are thoroughly compensated for tem- 
 perature, have the latest improved works, and improved revolving indices. 
 
 SPECIAL INSTRUMENTS MADE TO OEDER. 
 
 51. King's Barograph, or self-registering Barometer, as erected in the Liverpool 
 Observatory. In this construction the barometric column may be caused to 
 range from five to ten inches for each inch in the ordinary barometer, and to 
 record continuously the smallest as well as the largest fluctuations which 
 may occur in the pressure of the atmosphere. 
 
 55. Earth Thermometer, for ascertaining the temperature below the soil, or the 
 heat developed in hay-stacks, pine and melon pits, &c., with pointed copper 
 tube ls. 6d., and upwards. 
 
 56. ^Bthrioscope, the invention of Sir John Leslie, for ascertaining the absolute 
 intensity of terrestrial radiation, with which instrument time is an element 
 of an observation. 
 
 57. Pyrheliometer, the invention of M. Poullet, for ascertaining the absolute 
 heating effect of the sun's rays, or " solar radiation," time being an element 
 of observation. 
 
 58. Actinometer, the invention of Sir J. Herschell, for a like purpose to the 
 above. 
 
 59. Anemoscope, or Portable Wind Yane, for travellers, with compass, bar needle, 
 &c showing the direct course of the Wind to half a point of the compass 
 (fig. 59) - , 220 
 
 60. Thermometer Stands, Of the various forms in use, that designed by Sir 
 Henry James, for the use of the Royal Engineers in all climates, seems in 
 general to be the most convenient. It is easily made and fixed, and shelters 
 the instruments alike from the sun and rain, whilst it admits of a perfectly 
 free circulation of air. See " Instructions for Taking Meteorological 
 Observations," by Sir H. James, RE., F.R.S., &c. . . . 220 
 
STANDARD METEOROLOGICAL INSTRUMENTS. 
 
 MISCELLANEOUS INSTRUMENTS. 
 
 15 
 
 Fig. 66. 
 
 61. Ozouomcter, consisting of strips of paper pre- 
 pared with iodide of potassium and starch. The 
 papers are to be suspended so as to be exposed 
 to the free access of air, but sheltered from wet 
 and the direct rays of the sun : when affected by 
 ozone, they become tinged, the intensity of whicn 
 is measured by a gradu- 
 ated scale of twelve tints, 
 which accompanies the 
 ozonometer 066 
 
 Fig. 59 
 
 G2. Sedan's Ozonometer^ latest construction, with scale to 21 . 086 
 
 63. OZONE CAGE of fine wire gauze, as recommended by Sir James Clark 15 6 
 
 64. OZONE CAGE, small size, for travellers . . . '*.''. 15 6 
 
 65. Casella's Forms for Registering Ozonometer Indications for one year 016 
 
 66. Electrometer (Gold Leaf), for atmospheric electricity, with 2 ft. brass collect- 
 ing rod, clip for lighted fusee, and improved mode of insulation, with stand 
 and book containing spare gold leaves (fig. 66) . , . . 18 6 
 
 To replace the gold leaves, unscrew the top brass plate which supports them, and moisten with 
 the breath the flat brass piece to which they are to be attached ; press this gently down on one end 
 of a gold leaf, and the opposite side on another in the same way. 
 
 67. Casella's Forms for Registering Meteorological Observations, with concise 
 
 remarks and instructions, in twelve monthly forms for one year 4 
 
 68. Portable Meteorological Register and Note Book, with Weather Diagrams, 
 
 Tables, and Instructions. (Post Free) 030 
 
 STANDARD WORKS ON METEOROLOGY. 
 
 s. 
 
 'Meteorology," by Sir J. Herschell . " ." * - . . . . 06 
 
 1 The Laws of Storms," by Professor Dove . . 010 
 
 1 The Weather Book," by Admiral FitzEoy . . .... . . . 016 
 
 ' Practical Meteorology, by John Drew . . * - * '-". . . . 06 
 
 ' Meteorological and Physical Tables," by Professor Guyot . . . . . 16 
 
 ' Hygrometrical Tables, by J. Glaisher' 02 
 
 ' Instructions for taking Meteorological Observations," by Colonel Sir H. James . 7 
 
 8. Board of Trade " Barometer Manual :" or, How to Foretell Weather, by Admiral FitzRoy, 
 
 F.R.S. Post free ~ 010 
 
 9. Thompson's " Introduction to Meteorology," . . * .' t . . . . 12 6 
 10. Piddington's " Horn Book of Storms," . ."""'.'"-,.".'. . . 16 6 
 
ASTRONOMICAL AND METEOROLOGICAL INSTRUMENTS 
 
 ABBAffGED IN SETS FOB 
 
 COLLEGES, SCHOOLS, AND PRIVATE OBSERVATORIES, 
 
 AS WELL AS THE BEQUISITE 
 
 WEATHEB INDICATORS FOB GARDENERS, FARMERS, FISHERMEN, ETC. 
 
 POCKET INSTRUMENTS FOR TRAVELLERS, 
 
 INCLUDING- 
 
 CASELLA'S NEW POCKET ALTAZIMUTH, 
 
 FOB ALTITUDES, AZIMUTHS, COMPASS BEABING, CLINOMETBIC DEGBEES, AND LEVELS; 
 
 and other 
 
 SURVEYING AND REGISTERING POCKET INSTRUMENTS, 
 
 Especially designed by L. CASELLA, for the Members of 
 THE ALPINE CLUB. 
 
 .AJXTID 
 
 Of the Purest Crystal, and in every tint, and Brazilian Pebbles, in every Form of Mounting. 
 
 Binocular f Opera, and Tourists' Glasses and Pocket Telescopes, of the Finest 
 English and Foreign Manufacture. 
 
 DR. AITKINS' CLINICAL THERMOMETERS. 
 
 See Aitkins' " Science and Practice of Medicine." 
 
 MEDICAL ANEMOMETER, 
 
 FOE MEASUEING DEAFTS OE CUEEENTS OF AIE IN EOOMS AND HOSPITALS. 
 
 CASELLA'S COMPLETE ILLUSTRATED CATALOGUE, TWO SHILLINGS. 
 LISTS OF INSTEUMENTS FOE TEAVELLEES, ON APPLICATION. 
 
 International Exhibition, 1862. 
 
 THE 
 
 ONLY PRIZE MEDAL 
 
 AWABDED FOB 
 
 Instruments, 
 
 23, HATTON GARDEN, LONDON, E.G. 
 
 /* 
 
14 DAY USE 
 
 RETURN TO DESK FROM WHICH BORROWED 
 
 LOAN DEPT. 
 
 This book is due on the last date stamped below, or 
 
 on the date to which renewed. 
 Renewed books are subject to immediate recall 
 
 . ' 
 
 NOV27 
 
 
 LD 21A-50m-3,'62 
 
 General Library m 
 University of California