SOLAR HEAT. 



elaborate analysis of the results derived from them, appeared in 

 the Transactions of the Academy of Sciences of Paris for that 

 year. 



It would be incompatible with the elementary nature and the 

 consequent limits of this work, to enter into the details of these 

 researches. We shall, therefore, confine ourselves here briefly to 

 state their results. 



When the firmament is quite unclouded, the atmosphere absorbs 

 about one-fourth of the heat of those solar rays which enter it 

 vertically. A greater absorption takes place for rays which enter 

 it obliquely, and the absorption is augmented in a certain 

 ascertained proportion, with the increase of obliquity. It results 

 from the analysis of the results obtained in the researches of 

 M. Pouillet, that about forty per cent, of all the heat transmitted 

 by the sun to the earth is absorbed by the atmosphere, and that 

 consequently only sixty per cent, of this heat reaches the surface. 

 It must, however, be observed that a part of the radiant heat, 

 intercepted by the atmosphere, raising the temperature of the air, 

 is afterwards transmitted, as well by radiation as by contact, from 

 the atmosphere to the earth. 



By means of direct observation and experiment made with 

 instruments contrived by him, called pyrheliometers, by means of 

 which the heat of the solar radiation was made to affect a known 

 weight of water at a known temperature, M. Pouillet ascertained 

 the actual quantity of heat which the solar rays would impart per 

 minute to a surface of a [given magnitude, on which they would 

 fall vertically. This being determined, it was easy to calculate 

 the quantity of heat imparted by the sun in a minute to the 

 hemisphere of the earth which is presented to it, for that quantity 

 is the same which would be imparted to the surface of the great 

 circle which forms the base of that hemisphere, if the solar rays 

 were incident perpendicularly upon it. 



49. In this manner it was ascertained, that if the total quantity 

 of heat which the earth receives from the sun in a year were 

 uniformly diffused over all parts of the surface, and were com- 

 pletely absorbed in the fusion of a shell of ice encrusting the 

 globe, it would be sufficient to liquefy a depth of 100 feet of such 

 shell. 



Since a cubic foot of ice weighs 54 lb., it follows that the 

 average annual supply of heat received from the sun per square 

 foot of the earth's surface would be sufficient to dissolve 5400 lb. 

 weight of ice. 



This fact being ascertained supplies the means of calculating 

 the quantity of heat emitted from the surface of the sun, inde- 

 pendently of any hypothesis respecting its physical constitution. 



o2 83 



