220 COSMOS. 



terrestrial temperature), Avhen this determination is to be 

 made from the temperature of flowing springs. Such, at any 

 rate, is the result I have arrived at from my ov^n observations 

 and those of my fellow-travelers in Northern Asia. The 

 temperature of springs, which has become the subject of such 

 continuous physical investigation during the last half century, 

 depends, like the elevation of the line of perpetual snow, on 

 very many simultaneous and deeply- involved causes. It is a 

 function of the temperature of the stratum in which they take 

 their rise, of the specific heat of the soil, and of the quantity 

 and temperature of the meteoric water,* which is itself dif- 

 ferent from the temperature of the lower strata of the atmos- 

 phere, according to the different modes of its origin in rain, 

 snow, or hail.f 



Cold springs can only indicate the mean atmospheric tem- 



xxxii., s. 270, in the Voyage dans V Oural, p. 382-398, and in the 

 Edinburgh Journal of Science, New Series, vol. iv., p. 355. See, also, 

 Kamtz, Lehrb. der Meteor., bd. ii., s. 217; and, on the ascent of the 

 chthonisothermal lines in mountainous districts, Bischof, s. 174-198. 



* Leop. V. Buch, in Pogg., Annalen, bd. xii., s. 405. 



t On the temperature of the drops of rain in Cumana, which fell to 

 72°, when the temperature of the air shortly before had been 86° and 

 88°, and during the i-ain sank to 74°, see my Relat. Hist., t. ii., p. 22. 

 The rain-drops, while falling, change the normal temperature they 

 originally possessed, which depends on the height of the clouds from 

 which they fell, and their heating on their upper surface by the solar 

 rays. The rain-drops, on their first production, have a higher tempera- 

 ture than the surrounding medium in the superior strata of our atmos- 

 phei-e, in consequence of the liberation of their latent heat ; and they 

 continue to rise in temperature, since, in falling through lower and 

 warmer strata, vapor is precipitated on them, and they thus increase in 

 size (Bischof, Wdrmelehre des imieren Erdkorpers, s. 73) ; but this ad- 

 ditional heating is compensated for by evaporation. The cooling of the 

 air by rain (putting out of the question what probably belongs to the 

 electric process in storms) is effected by the drops, which are them- 

 selves of lower temperature, in consequence of the cold situation in 

 which they were formed, and bring down w^ith them a portion of the 

 higher colder air, and which finally, by moistening the ground, give 

 rise to evaporation. These are the ordinary relations of the phenome- 

 non. When, as occasionally happens, the rain-drops are warmer than 

 the lower strata of the atmosphere (Humboldt, Rel. Hist., t. iii., p. 

 513), the cause must probably be sought in higher warmer currents, or 

 in a higher temperature of widely-extended and not very thick clouds, 

 from the action of the sun's rays. How, moreover, the phenomenon of 

 supplementary rainbows, which are explained by the interference of 

 light, is connected with the original and increasing size of the falling 

 drops, and how an optical phenomenon, if we know how to observe it 

 accurately, may enlighten us regarding a meteorological process, ac- 

 cording to diversity of zone, has been shown, with much talent and in 

 genuity, by Arago, in the Annuaire for 1836, p. 300- 



