Sciences of Meteorology and Terrestrial Magnetism. 165 



incapable of being condensed into a liquid by any degree yet known 

 of cold or pressure ; tbe other readily and by natural means reducible to 

 a liquid. The mixture possesses the highest degree of mobility ; so that 

 a change of condition, originating in a particular tract, is propagated into 

 portions widely remote, and masks the conditions there operating. 

 Movements in the fluid mass of a most complicated kind are thus gene- 

 rated, which, though originating in the action of a known force, cannot 

 be brought under the dominion of any calculus, or mathematical analysis. 

 Indeed, such movements in elastic fluids are about the most difficult 

 to grapple with, and bring into subjection to dynamic laws. The 

 assumption that such laws and certain definite ratios existed, has led to 

 many errors, now happily given up. Such are the so-called laws at one 

 time attempted to be established for the dependence of temperature on 

 latitude, and by a simple ratio on the altitude. — Then, further, the floor 

 of the atmosphere varies everywhere ; its seat on a fluid base, as that of 

 the great ocean, must produce very different effects upon it through its 

 whole mass, from those which would prevail if it were seated on a conti- 

 nental mass ; and even continental masses of land vary in their physical 

 conditions. Hence the vast difficulties attendant on the subject of 

 atmospheric temperature, and the establishment of laws Ln this branch 

 of meteorology. 



(9.) Abstracting all consideration of the mixed aqueous vapour, it is 

 clear that the atmosphere receives its heat in three ways, Jirst, by direct 

 radiation from the sun ; second, by reflected rays of heat emanating 

 from the heated ground ; and third, by contact with the heated ground 

 itself. Various recent experiments have been made on the direct heating 

 power of a vertical sun. Sir J. Herschel found it, at the Cape of Good 

 Hope, to be such as would melt 'OOTS-i in. in thickness from a sheet 

 of ice in 1", when the ice was exposed to it perpendicularly ; or, which 

 is the same thing, and perhaps more intelligible, f in. in P 40". Experi- 

 ments at Paris have made it 0070.3. The mean of the two, Sir J. 

 Herschel thinks very near the truth, namely, "007285 in. in 1'", or 

 •7285 in l*" 40". In a cloudy atmosphere nearly the whole is absorbed, 

 going to heat the air or evaporate the clouds. With oblique sunshine 

 the heating power would of course be much less ; depending, in fact, 

 upon the cosine of the obliquity, or zenith distance, of the ray, — the 

 zenith distance being supposed under 80°. From several recent experi- 

 ments, it has been concluded that about 33 per cent., or g of a vertical 

 sunbeam, is absorbed in its passage through a cloudless atmosphere, 

 before reaching the sea level. This is expended in heating the air 

 through its wliole vertical extent, or in evaporating the clouds. As yet, 

 I believe, we have no UnowhA^o of any law, aopordinp to wJiIcli, the 



