Royal Society. 223 



May 26. — A paper was in part read, entitled, " On the Transpa- 

 rency 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. — The reading of Prof. Forbes's paper 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 Tfiermometers, 

 Photometers, or 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 

 Laplace. It consists in the determination of the length of the path 

 and the mass of air which a ray of light must traverse in passing 

 through the earth's atmosphere at every different angle of obliquity. 

 The author determines the numerical value of these quantities for 

 all angles of incidence from 0° to 90°. 



The fourth section contains an account of the observations made 

 by the author in conjunction with Professor Kamtz in 1832. These 

 were conducted in 1832 at the top and bottom of the Faulhorn, a 

 mountain of the canton of Berne in Switzerland. The lower station 

 was Brientz, and the intercepted stratum of air had 6800 English 

 feet of thickness, corresponding in its weight to about one-fourth of 

 the entire atmosphere. Frequent observations were simultaneously 

 made with the actinometer and other meteorological instruments at 

 both stations, and the loss of solar heat in passing through the in- 

 tervening mass of air was thus directly determined. 



In the fifth section, the observations made from sunrise to sunset, 

 on one peculiarly favourable day (the 25th September, 1832), are 

 carefully analysed; and from the absorption at various obliquities, 

 the law of extinction in the atmosphere, within the limits of obser- 

 vation, is attempted to be deduced. 



The sixth and seventh sections include the results of similar, but 

 less perfect observations in 1832 and in 1841. 



From the facts and reasonings of this paper, the author deduces, 

 on the whole, the following conclusions : — 



1. The absorption of the solar rays by the strata of air to which 

 we have immediate access is considerable in amount for even mo- 

 derate thicknesses. 



2. The diurnal curve of solar intensity has, even in its most nor- 



