166 Mb. J. Brtce on the Recent Progress of the 



density of, and power of transmitting heat by the air vary ; and, hence, 

 we cannot yet say in wliat ratio the absorbabiUty varies as the rays 

 penetrate deeper into the atmosphere. Neither do we know in what 

 proportion the various rays are absorbable by the air. It is, however, 

 well known that a given volume of dense air is more easUy warmed than 

 the same volume of rarer air — rarified air has, in fact, a greater specific 

 heat ; so that heat becomes latent by the process of rarefaction. Hence, 

 the cold of the high regions of the atmosphere, and the explanation of 

 the fact, that within the tropics perpetual snow lies all the year round, 

 though exposed to the vertical rays of the sun. The air generally, but 

 especially in the higher regions, has so great a degree of diathermancy, 

 that the atmosphere derives almost all its heat from radiation, conduc- 

 tion, convection, and the conversion of vapours at various points 

 tliroughout its extent into rain, snow, or hail. The observations bearing 

 upon this point, made by Prof. C. Piazzi Smyth during his late resi- 

 dence on the Peak of Teneriffe, are very interesting and remarkable. 

 On the 4th August, about noon, the black bulb thermometer, exposed 

 to the direct rays of the sun, rose to 212°'4 F., the temperature of the 

 ail', in the shade, being then only 60° ; so that the direct solar radiation 

 exceeded the temperature by more than 150°! At ^\^ a.m., the direct 

 solar radiation was 180^. So rapidly, indeed, does the direct radiation 

 or heating power of the rays increase with the height, that the cochineal 

 insect is killed by the heat at 3,000 ft., though thriving well and 

 yielding a rich produce at Oratava, on the sea level, at the base of the 

 Peak. The chemical power of the rays also increases greatly with 

 the height, as shown by Saussure before photography was known. Prof. 

 Smyth found the difference remarkable on his photographic pictures ; 

 the}- were more easily taken, and much more intense at great altitudes. 

 The chemical rays are more dispersed and disturbed by the dense atmo- 

 sphere below, than the luminous ones. He conjectures that at fifty 

 miles up, usually regarded as the height of the atmosphere, the tempera- 

 ture of shade would be — 50° F., while the effect of dii'cct sunlight would 

 be increased hundi-eds of degrees. The existence of perpetual snow, 

 then, must be independent of radiation, and due to the temperature of 

 the air and the non-conducting power of ice. The decrease of tempera- 

 ture with latitude is due to a different cause, namely, the sun's 

 diminished altitude. The heating power of a beam which acts per- 

 pendicularly on a surface equal in area to a section of the beam, if 

 acting obliquely is spread over a surface greater in the ratio of the sine 

 of the obliquity to radius. The heating power on a luyrizoi^ial surface is 

 given, accoi-ding to Sir .John Hcrschel, by the expression -OlonS cos. z, 

 where z is the zenith distance of the sun, — independently, of course, of 



