PEW. 



DEW. 



appear in UM sequel of this article, U really an element of the complex 

 pWomenoo under discuamion). it might be expected that the surface 

 oo which UM dew U deposited, and still more that on which the hoar 

 frost is formed, would have it* temperature raised, and no cause wa 

 apparent why it should be depraved. 



Nrly contemporary with Six's last published observations, were 

 OHM experiment, made and described, though not explained, by U. 

 Benedict Prevost, of Beaancun. But Dr. Thomas Young, in his work 

 entitled "Lectures on Natural Philosophy," published in 1807,-* 

 mine of physical truth, which the labours of half a century have not 

 exhausted. completely accounted for the effects witnessed in those 

 experiments, giving an explanation of them, of which it may be said, in 

 ooordance with a remark of his biographer the late Dr. Peacock, that 

 it embraced the basis of the true theory of the cooling of the sur- 

 face of the earth by radiation, and the interruption of that process 

 by clouds, Ac. and by implication, that of the formation of dew 

 itself. This, however, had remained unnoticed, and, indeed, until 

 the discoveries of Leolie on radiant heat, hut little further progress 

 could be made in the investigation of this subject; and even the 

 consequence* of those discoveries remained for some time unperceived. 

 But an obscure, because neglected, physician, giving his < 

 leisure to physical inquiries, the late Dr. William Charles W, 11*. F . I! .S . 

 now discovered and established the true theory of dew, which Sir 

 John F. W. Herschel (' PreL Disc, on the Study of Nat Phil.,' 1C8), 

 h.. characterised " as one of the most beautiful specimens of inductive 

 experimental inquiry," earnestly recommending the essay in which it 

 U developed " for perusal to the student of natural philosophy, as a 

 model with which he will do well to become familiar." 



Dr. Wells, after verifying the facts previously observed, ascertained 

 by numerous observations and experiment*, that the cold at the sur- 

 face, compared with that of the air above, precedes the formation of 

 dew, and often exists without dew being formed ; and that bodies 

 become colder than the neighbouring air before they are dewed. The 

 cold, therefore, which Mr. Wilson and Mr. Six supposed to be the 

 effect of dew was found by Dr. Wells to be the cause of it. 



The question, then, naturally arises, what ore the causes which 

 produce this low temperature of the earth, while the incumbent air is 

 at a higher temperature F This is shown by the experiments and 

 masoning of Dr. Wells to be caused by the radiation of heat without 

 an equivalent return. The surface of the ground allows a portion of 

 the heat which it receives from the solar rays to escape by radiation 

 when their action is withdrawn ; hence its temperature falls ; and if 

 the air, holding watery vapour, rest upon it without much agitation 

 (a circumstance by which, as the constant renewal of warm air 

 impart* heat, the effect would be counteracted), a portion of the 

 vapour will be condensed on the surface, and if the temperature is 

 still lower, will be congealed : thus it is that dew and hoar frost are 

 : 1 only when the atmosphere is clear or the clouds not dense, 



for in the latter state they return an equivalent portion of radiant 

 heat 



Dew forms in very different quantities on different substances under 

 the same circumstances ; thus, on metals it U sparingly deposited ; on 

 glass it forms abundantly, as it does also on straw, grass, cloth, paper, 

 ud other similar substances. Now as the metals radiate heat to a 

 miall amount only, and the other bodies mentioned in a much greater 

 degree, they become consequently colder than the metals, and hence 

 f^mdm*" more vapour into dew. 



Animal substances are among those which acquire dew in the 

 greatest quantity; among these Dr. Wells found that swan's-down 

 exhibited the greatest degree of cold in general, and was also most 

 easily managed in experiment, as it was used while adhering to the 

 skin of the bird. On other occasions wool was employed, and the 

 following statements are the results of Dr. Wells's experiments with 

 respect to the influence which several differences in the situation, 

 mechanical state, and intrinsic nature of bodies have upon the pro- 

 duction of dew. 



A general fact is, that whatever diminishes the view of the sky, as 

 ecu 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 ; two parcels of wool, each weighing ten 

 grains, were placed, one on the middle of a board, and the other to the 

 middle of the underside ; the two parcels were an inch asunder, and 

 equally exposed to the action of the air : on weighing the two portions, 

 it was always found that the upper portion had acquired most dew, 

 the greatest difference being twenty grains to four grains. 



The following is a tabular view of observations with respect to 

 temperature made by Dr. Wells on the evening of the 19th of August, 

 1818: 



8h. 45m. 7h. 7h. 20m. 7h. 40m. 8h. 45m. 



Heat of air 4 feet abore the grass COl ' 80l 59' 58 ' 54' 



wool on a raised boatd . *M 54) 51} 481 

 swan's down on UM same 54| 5J 51 47 j 



fUlf.ce of the raised board 58 57 55 J 



4t 



fnuplat 







51 



49 



42 



Sir Robert Barker and Mr. Williams have both given accounts of 

 the process by which ice is formed in Bengal, while the temperature of 

 the air U above 82* ; and its production was attributed, by the former 



altogether, and by the latter in great measure, to cold produced by 

 evaporation. Dr. Wells's experiments, however, show not only that 

 the evaporation which occurs is insufficient to account for the effect, 

 but that the cold produced by radiation alone will satisfactorily explain 

 it. On the subject of such cold. 1 >r. Wells make* the following curious 

 statement : " I had often," he says, " in UM pride of half knowledge, 

 uniled 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 tram attaining 

 the temperature of the atmosphere, by whkh alone 1 thought them 

 liable to be injured. But, when I had learned that bodies on the sur- 

 face 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 H***"*'* 

 useless." And he follows up this part of the subject by relating some 

 extremely simple experiments in proof of the efficacy of the practice, 

 and in explanation of the effect by the processes of the r ... 

 reflexion of heat. 



It had been remarked by Aristotle * and by other writers that dew 

 appears only on calm and serene nights. This observation, according 

 to Dr. Wi-lU, U not to be received in its strictest sense, as he has fre- 

 quently found a small quantity of dew on grass, both on windy nighte, 

 if the sky was clear or nearly so, and on cloudy nighU. if there was no 

 wind. If, indeed, the clouds were high, and the weather calm, he has 

 sometimes seen on grass, though the sky was entirely hidden, n 

 inconsiderable quantity of dew. Again, stillness of the atinos|>i 

 so far from being necessary for the formation of this fluid, that ite 

 quantity has seemed to Dr. Wells to be increased by a very gentle 

 motion in the air : but he says, " I can aver, after much experience, 

 that I never knew dew to be abundant, except in serene weather," 

 ami ' dew bos never been seen by me on nighte both cloudy and 

 windy." t 



Dr. Wells remarks that, during nights which ore equally cl< 

 calm, dew often appears in very unequal quantities, even after due 

 allowance has been made for any difference in their lengths : thus it 

 is more abundant shortly after rain than during a long season of dry 

 weather. It U generally more abundant during southerly and westerly 

 winds than those which blow from the north and the east ; but thin 

 remark, of course, only applies to particular localities. To the greater 

 or less quantity of moisture in the atmosphere, at the time of tin- 

 action of the immediate cause of dew, are likewise to be referred 

 several other facts respecting iU copiousness; these are thus stated by 

 Dr. Wells (' Essay,' p. 10) : 



" 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 temperatures of the day and the ni^ht in 

 the former seasons of the year than in the latter. In spring this cir- 

 cumstance is prevented often from having a considerable effect by the 

 opposite influence of northerly and easterly winds; Imt ilmii 

 and serene nights in autumn dew is almost always highly abundant. 



" 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 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 arc 

 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 < 

 faster, while its temperature is rising, than in proportion to the heat 

 acquired, a decrease of its heat, in any small given quantity, during 

 the night, must bring it, if the temperature be high, much in 

 the point of repletion before it be acted upon by the immediate cause 

 of dew, than if the temperature were low. 



" In. the last place, I always found, when the clearness and stillness 

 of the a|iM"-|ili' i .-..!. 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 loss in the 

 former than in the latter time, in consequence of a previous pr< 

 tion 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." 



The experiments of Dr. Wells were repeated in Fiance by if. 

 Pouillet ; but two seta of experiments of some value, undertaken for 

 special purposes by the late Professor J. F. Danie.ll, appear to have 



* Meteor, lib. I, c. x. et De Mumlo, c. lit. 



f ' An Essay on Dew, and on several appearances connected with It,' Lend. 

 1814, p. 65. It may be well to record, as the late Dr. T. Thomson pointed out 

 at the time, that the word conduction, as applied to a certain process of the pro- 

 pagation of brat (and subsequently to that of electricity) was first introduced 

 Into the English linguagc by Dr. Wells in this work. A second editiun or the 

 Essay appeared in 1818, which was republishcd shortly after the author's 

 death, In a now scarce volume, containing other writings by him, including his 

 autobiography, Lond. and Edln. 1818, 8vo, reissued in 1820. 



