390 



Laplace. It consists in tlie 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- 

 mal state, several inflections ; and its character depends materially 

 on the elevation of the point of observation. 



3. The approximations to the value of extra- atmospheric radia- 

 tion, on the hypothesis of a geometrical diminution of intensity, are 

 inaccurate. 



4. The tendency to absorption through increasing thicknesses of 

 air is a diminishing one ; and in point of fact, the absorption almost 

 certainly reaches a limit beyond which no further loss will take 

 place by an increased thickness of similar atmospheric ingredients. 

 The residual heat, tested by the absorption into a blue liquor, may 

 amount to between half and a third of that which reaches the sur- 

 face of the earth after a vertical transmission through a clear at- 

 mosphere. 



5. The law of absorption in a clear and dry atmosphere, equiva- 

 lent to between one and four thicknesses of the mass of air traversed 

 vertically, may be represented, within those limits, by an intensity 

 diminishing in a geometrical progression, having for its limit the 

 value already mentioned. Hence the amount of vertical transmis- 

 sion has always, hitherto, been greatly overrated ; or the value of 

 extra-atmospheric solar radiation greatly underrated. 



6. The value of extra-atmospheric solar radiation, on the hypo- 

 thesis of the above law being generally true, is 73° of the actino- 



