METEOROLOGY. 



171 



Meteorolo- 

 gy- 



The indica- 

 tion! of the 

 .Kthrio- 

 teope may 

 beei- 

 pUtard on 

 he prrnri- 

 f>U 

 t . - 



Other feet* 

 tated by 

 Dr. Well. 

 IB mippon 

 < I . 



theory 



tion of caloric from each indefinitely thin film or stra- 

 tum, so that the degree of cold which ultimately reaches 

 fte surface of the ground, can be nothing more than 

 . which the intermediate strata of the atmosphere 

 bad been previously reduced. If it be said, that these 

 reach the earth without expending their 

 cooling energy on the atmosphere, then we are at a loss 

 to see any thing else in this than the " vulgar and un- 

 philosophical" principle of radiation, or in what re- 

 spects Mr. Leslie's theory differs from that of Dr. Well-;, 

 except that the latter ascribes the cold induced on the 

 ground to the radiation of heat upwards, while the 

 former explain* it by the radiation of cold downwards. 

 In whatever light Mr. Leslie's theory be viewed, it seems 

 to be liable to much stronger objections than the one 

 on which he has passed so severe a censure, and to 

 which of the two the epithets of " cloudy and m\ 



.ay be justly applied, it is not very difficult we 

 think to decide. 



But it may perhaps be alleged, that the reality of 

 frigorilM- pulse* from a clear sky is established by the 

 .tions of an instrument lately constructed by Mr. 

 Leslie, and to which he has given the name of .F.thrio- 

 scope. This instrument is nothing more than the dif- 

 ferential thermometer, placed in the hollow of an ob- 

 long spheroidal cup of bras* or silver, having the sen* 

 l>all in the axis, and the other, which i covered 

 with silver leaf, lodged in the cavity, and nearly in 

 contact with the side of the cup. When the instru- 

 :i this state is freely exposed to the sky, the boll 

 in the focus of the cup suffers a diminution of tempera- 

 ture, the enclosed air is contracted, and the liquid rise* 

 in the tube towards the ball, indicating a difference of 

 temperature between the two bsJls which in some cases 

 amoi. im.il degree*. This effect is as- 



cribed by Mr. Ix*lie to " an imprwiim of cold shot 

 downwards from the higher regions ;" but the pheno- 

 menon is just as easily accour/ , the principle 

 adopted by Dr. Wells. The radiation of heat to the 

 sky, from the sentient ball of the .Ethrioscopc, is aug- 

 mented by that ball being plared in the focus of a 

 metallic cup, which is itself radiating heat towards the 

 same quarter, exactly as in the case of a heated body 

 placed in the focus of a metallic reflector. Were the 

 ball of the jEthrioscope exposed by itself, it would 

 radiate beat only from the side directed towards the 

 but by being placed in the focus of the cup, ra- 

 diation goes on from every point of its surface. This 

 insti Mini nl thtitfute funmhcs no additional nupport to 

 Mr. Leslie's theory, nor does it at all militate against 

 the commonly received opinion regarding the radiation 

 of caloric. See our article COLD. 



Various circumstance* beside* those already men- 

 tioned, occurred to Dr. Will* in the course of his ex- 

 periments, which tend to confirm his theory of the 

 production of dew. Thus he found that substances, 

 a* metals for example, which readily receive heat by 

 conduction, but which radiate slowly, were, crlrrit pa- 

 rihitt, lea*) dewed than bodies possessing different qua- 

 lities. He found also that grass on a clear and still 

 night was uniformly colder than a gravel walk, evi- 



y owing to the different quantities of heat v 

 they received by conduction from the interior of the 

 ground ; and that on the same principle filamentous 

 and downy substances become colder than all others. 

 The influence of wind in preventing a reduction of 



temperature on the surface of the earth, even when the 

 sky is clear, he ascribes not to its diminishing the 

 quantity of radiation, but to its bringing successive 

 portions of warmer air in contact with the ground, so 

 that no dew can be formed till the whole of the air 

 thus agitated is cooled down to the point of deposition. 

 The same cause will account for less dew being depo- 

 sited on mountains than on plains. 



Some philosophers have ascribed the cold observed 

 on die surface of the ground to evaporation, and there 

 can be no doubt that, in certain circumstances, a con- 

 siderable reduction of temperature may in this way be 

 produced. We have frequently observed congelation 

 going on in moist places during a brisk wind, while the 

 temperature of the atmosphere at the height of a few 

 'i as 40, and have more than once seen 

 an icicle forming at the bulb of a moistened thermo- 

 meter, while a contiguous dry one indicated a tempe- 

 rature of 38 or 3y*. It is impossible, however, upon 

 this principle,- to account for many of the phenomena 

 observed by Dr. Wells. If the cold were produced 

 by evaporation alone, the difference betv en the tem- 

 perature of the ground and that of the atmosphere- 

 near it, would diminish as the air became moist, and 

 would disappear altogether when the latter was com- 

 pletely satunted. Hut the cold observed by L)r. Wells 

 .- after a deposition of moisture had 

 taken place, and when not only evaporation must have 

 ceased, but caloric must have been disengaged or be- 

 come sensible, by the vapour passing into a liquid 

 state. Besides, evaporation could have nothing to do 

 with the reduction of temperature observed on sub- 

 stance* exposed in a state ot dryness, and not in con- 

 tact with the earth. 



When the temperature of the ground, after the de- 

 position of moisture has taken place, is at or below 32, 

 the dew is congealed and becomes hoar frail. Accord- 

 ing to the experiments detailed above, this may happen 

 when the temperature of the atmosphere, a few feet 

 above the surface, i* considerably above 40. We have 

 often observed a copious hoar-frost after a clear and 

 calm : the minimum temperature, as in- 



dicated by a self-registering thermometer, had not 

 been lower than 41 The theory advanced by 



Dr. Wells satisfactorily explains this curious pheno- 

 menon, as well as the practical utility of the method 

 frequently adopted by gardeners for preserving tender 

 plants, by laying or suspending over them any cover- 

 ing, however thin, that serves to screen them from 

 exposure to a clear sky. 



We cannot conclude this article without expressing 

 our regret, that the application of the principles de- 

 veloped by the researches of Leslie, Anderson, and 

 Wells, has hitherto so seldom found a place in meteor- 

 ological journals. To the laborious experiments and 

 profound investigations of these eminent individuals 

 the science has been deeply indebted ; but it still re- 

 mains for meteorologists to bring these discoveries to 

 the test of daily observation, in order to confirm or 

 correct the principles on which they are founded. In 

 illustration of this remark, as well as of some other 

 observations that have been made in the course of this 

 article, we submit the following plan of a meteorolo- 

 gical journal, which, though as extensive as any we 

 nave yet seen, still admits of some important addition* 

 and improvement*. 



Evapora- 

 tion not 

 sufficient to 

 account for 

 the pheno- 

 mena stat- 

 ed above. 



Formation 

 of hoar 

 frost. 



PUn or* 

 meteorolo- 

 gical jour- 

 nal. 



