420 Action of the Atmosphere upon ncwhj-dcepencd Soil. 



In Siberia ice is found even at a depth of from 300 to 400 feet. 

 Some experiments by M. Middendorf, as reported to the Aca- 

 demy of Sciences at St. Petersburg in 1844, showed that in 

 a shaft and the jjalleries of some works near the Lena, at a 

 depth of 384 English feet, the frozen crust was still not passed 

 through, though a gradual increase of temperature was observed 

 in the descent. In the same latitude (62° N.) in America the 

 frozen ground does not extend beneath 2^ feet. 



Another effect of fi'ost, not unv/orthy of notice, may be ob- 

 served frequently in gravelly ground that is not much exposed to 

 the sun. This consists in the skin of the soil being elevated 

 often about an inch or a couple of inches on the top of a set of 

 beautiful little pillars of ice. As stones, however, beyond the 

 size of a walnut are seldom elevated by this process, the result is 

 to cause these larger pieces and pebbles to sink deeper below the 

 surface. 



Even irf summer the alternations of temperature undergone by 

 soil exposed on the one hand to a full sun-heat through the day, 

 and on the other to the rapid cooling by radiation during still 

 clear nights, when it Avill be often at the freezing point, cannot be 

 without some effect. 



We now proceed to consider the conditions of liqJit to which 

 the soil is subject, and the effects resulting therefrom. 



The light of the solar rays may be considerably enhanced by 

 reflection from the clouds under favourable circumstances. This 

 indirect or reflected light is most intense when thin fleecy clouds 

 overspread the sky, and feeblest when it is covered by thick va- 

 pours, or when it is deeply azure. " The effect of the reflected 

 light of the sky," says Forbes, " is always exceedingly intense ; 

 so much so as to give rise to the most paradoxical effects with 

 regard to the intensity of solar radiation if neglected. Thus I 

 have found the whole effect of the sun and sky in a bright April 

 <lay in this country, when many white clouds were present, not 

 very inferior to that of the most piercing sunshine of the most 

 sultry day of the south of Europe unaccompanied by a single 

 cloud, M. Kiimtz found on the summit of the Faulhorn that tlie 

 direct solar effect on Leslie's photometer was equalled and often 

 exceeded by that of the diffuse atmospheric influence." [By-it. 

 Assoc. Report, 1840, p. 62.) The amount of light, being lessened 

 by the obliquity with which the sun's rays fall upon the surface 

 of the ground, will consequently vary with the altitude of the sun, 

 and with the hour and season. Tlie rays of light like those of 

 heat suffer diminution in passing through our atmosphere, and 

 Leslie computed this loss at one-fourth upon a beam darted to -the 

 ■earth under the most favourable circumstances. Hence the 

 thinner the strata of the atmosphere the stronger should be the 



