On Solar Radiation. 



199 



"Neither should each observer neglect to determine for 

 himself the heat stopped by each of his glasses. This may 

 be done also by alternating triplets of observation made 

 with the glass on and off: thus, — 



Glass off. 



©■ 

 X- 



©• 



Glass on. 



©■ 



x. 



Glass off, 



0. 

 X- 



© 



&c. 



beginning and ending with the glass off, and (as in all cases) 

 beginning and ending each triplet with a sun observation. 

 For the purpose now in question, a very calm day must be 

 chosen, and a great many triplets must be taken in succes- 

 sion. It will be found that a single thickness of the ordinary 

 bluish or greenish plate glass stops about 0*20 (=-1) of the 

 incident calorific rays; a second glass about 016 (or a ma- 

 terially less proportion) of those which have escaped the 

 action of the first. No two glasses, however, are precisely 

 alike in this respect. 



" Very interesting observations may be made by two obser- 

 vers furnished with well-compared actinometers, the one 

 stationed at the summit, the other at the foot of some great 

 elevation ; especially if the stations can be so selected, that 

 the observers shall be nearly in the line of the incident sun- 

 beam at the time of observation, so as both to lie in the 

 atmospheric column traversed by the rays. Many convenient 

 stations of this kind might be found in mountainous coun- 

 tries ; and by repeating the observation two or three times 

 under favourable circumstances, interchanging observers 

 and instruments, &c. and accompanying the observations 

 with all circumstantial and local elements of precision ; there 

 is no doubt that the co-efficient of extinction of solar heat 

 in traversing at least the lower strata of our atmosphere 



