114 On Solar Radiation. 



seem, at first sight, an impossible task to determine the com- 

 parative measure of the sun's heat, in the state in which it 

 arrives at the earth's surface, and that which it would have at- 

 tained were the atmosphere wholly removed. Some approxi- 

 mation to such a result has, however, been obtained by a very 

 simple though indirect method. The thickness of air tra- 

 versed by a sunbeam is, of course, least when the sun is ver- 

 tical, and greatest when he is near the horizon ; at inter- 

 mediate elevations the heat is intermediate. Now, by com- 

 paring the thermometric effect of the sun's rays (which is the 

 object of the actinometer), at several different thicknesses of 

 atmosphere, the law of extinction is approximately found, and 

 an inference is made as to what the intensity would be when 

 the thickness of the atmosphere is nothing. This inference 

 will be proportionally more accurate as the observations are 

 pushed to a less thickness of interposed air ; and I have shewn 

 in the paper already referred to,* that the previous estimates 

 had greatly underrated the intensity of the unimpaired so- 

 lar-beam, and had also underrated the absorptive power of the 

 atmosphere, owing to the observations on which they were 

 founded having been made only when the sun-beam had al- 

 ready traversed a great thickness of air, when the law of absorp- 

 tion is very different from the law at small thicknesses. 



Now, to obtain observations of solar heat at small thick- 

 nesses, we must, in the first place, ascend in the atmosphere, and 

 also use the sun's rays when his elevation is greatest, that is, 

 near the solstice. I mounted the Cramont in hopes of pro- 

 secuting these experiments, when the sun had still 21° of 

 northern declination, and after having left below me a thick- 

 ness of 9000 feet of the densest part of the atmosphere. Un- 

 fortunately, as we have seen, these delicate experiments were 

 prevented by indifferent weather. 



It will probably surprise many persons to be told, that even 

 when the sun's rays shoot vertically through a pure atmo- 

 sphere, as between the tropics, they lose in their passage 

 (owing to the opacity of the air) very nearly half their 



♦ Phiosophical Transactions for 1 842, page 225. 



