HOT SPRING*. 217 



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 different from the temperature of the 

 lower strata of the atmosphere, according to the different 

 modes of its origin in rain, snow, or hail.f 



Cold springs can only indicate the mean atmospheric tem- 

 perature when they are unmixed with the waters rising from 

 great depths, or descending from considerable mountain eleva- 

 tions, and when they have passed through a long course at a 

 depth from the surface of the earth which is equal in our lati- 

 tudes to 40 or 60 feet, and, according to Boussingault, to about 



* 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 rain 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 temperature than the 

 surrounding medium in the superior strata of our atmosphere, in conse- 

 quence of the liberation of their latent heat ; and they continue to rise 

 in temperature, since in falling through lower and warmer strata vapour 

 is precipitated on them, and they thus increase in size (Bischof, 

 Wdrmelehre des inneren Erdlcorpers, s. 73) ; but this additional heat- 

 ing is compensated for by evaporation. The cooling of the air by rain 

 (putting out of the question what probably belongs to the electric pro- 

 cess in storms) is effected by the drops, which are themselves of lower 

 temperature, in consequence of the cold situation in which they were 

 formed, and bring down with 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 phenomenon. When, as occasionally 

 happens, the rain-drops are warmer than the lower strata of the atmo- 

 sphere, (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, according to diversity of zone, has 

 been shown, with much talent and ingenuity, by Arago, in 

 for 1836, p. 300. 



