PROCEEDINGS 



OF 



THE ROYAL SOCIETY. 



1842. No. 54. 



May 26, 1842. 



The MARQUIS OF NORTHAMPTON, President, in the Chair. 



Thomas Chapman, Esq., was balloted for, and duly elected into 

 the Society. 



Richard Quain, Esq., was also balloted for, but was not elected 

 into the Society. 



A paper was in part read, entitled, *' On the Transparency of the 

 Atmosphere, and the Law of Extinction of the Solar Rays in passing 

 through it." By James D. Forbes, Esq., F.R.S., Sec. R.S. Edinb., 

 Professor of Natural Philosophy in the University of Edinburgh. 



June 2, 1842. 



The MARQUIS OF NORTHAMPTON, President, in the Chair. 



The reading of a paper, entitled, " On the Transparency of the 

 Atmosphere, and the Law of Extinction of the Solar Rays in passing 

 through it." By James D. Forbes, Esq., F.R.S., &c., was resumed 

 and concluded. 



This paper is divided into seven sections. In the first, the qua- 

 lities of heat and light are considered in as far as they modify the 

 comparability and absolute nature of our measures of the influence 

 of the solar rays. All instruments, whether called Thermometers^ 

 Photometers^ ov Actinometers, measure but the peculiar effect to which 

 their construction renders them sensible, but are incompetent to 

 give absolute measures of either heat or light. 



The second section treats of the history of the problem of the law 

 and measure of extinction of the solar rays in passing through the 

 atmosphere of the earth in clear weather. The labours of Bouguer, 

 Lambert, De Saussure, Leslie, Herschel, Kamtz and Pouillet are 

 successively passed under review, and their instrumental methods 

 considered. 



In the third section, a mathematical problem of considerable dif- 

 ficulty and interest is investigated ; principally after the manner of 



