274 NITROGEN. 



temperature of the different strata of the atmosphere is indeed 

 believed to be that due to their dilatation, supposing that they 

 had all the same original temperature and density as the lowest 

 stratum. 2o. To the circumstance that the atmosphere de- 

 rives its heat principally from contact with the earth's surface. 

 The sun's rays appear to suffer little absorption in passing 

 through the atmosphere ; but there are some observations on 

 the force of solar radiation which are not easily reconciled with 

 that circumstance. A thermometer, of which the bulb is 

 blackened, rises a certain number of degrees above the tempe- 

 rature of the air, when exposed to sun, but the rise is decidedly 

 greater on high mountains than near the level of the sea, and in 

 temperate, or even arctic climates, which is more remarkable, 

 than within the tropics. It is a question how solar radiation is 

 obstructed in the hotter climates. (Daniell's Meteorological 

 Essays, 2nd ed.) 



The blue colour of the sky has been found by Brewster to be 

 due to light that has suffered polarization, which is therefore 

 reflected light, like the white light of clouds. The air of the 

 atmosphere must therefore have a disposition to absorb the 

 red and yellow solar rays and to reflect the blue rays. At great 

 heights, the blue colour of the sky was observed by Theodore 

 de Saussure to become deeper and deeper, being mixed 

 with black, owing to the absence of white reflecting vesicular 

 vapour or clouds. The red and golden tints of clouds appear 

 to be connected with a remarkable property of steam lately 

 discovered by Professor Forbes. A light seen at night through 

 steam issuing into the atmosphere from under a pressure of from 

 5 to 30 pounds on the inch, is found to appear of a deep 

 orange red colour, exactly as if observed through a bottle con- 

 taining nitrous acid vapour. The steam, when it possesses this 

 colour, is mixed with air, -and on the verge of condensation ; 

 and it is known that the golden hues of sunset depend upon a 

 large proportion of vapour in the air, and are indeed a popular 

 prognostic of rain.* 



The movement of masses of air, or wind, is always produced 

 by inequality of temperature of the atmosphere at different 

 points of the earth's surface, or in different regions of the at- 

 mosphere of equal elevation. The primary movement is always 



* Phil. Mag. 3rd Series, vol. 14, pp. 121 and 425, and vol. 15, pp. 25, and 419. 



