352 WRITINGS OF JOSEPH HENRY. [1855- 



and this was determined to be 231,675 units falling on each 

 square centimetre of the whole surface. Calculating the 

 amount of ice which this quantity of heat would melt* 

 Pouillet obtained a thickness of 30"89 metres, or a little more 

 than 101 feet; that is, if the total quantity of heat which the 

 earth receives from the sun in the course of a year were uni- 

 formly distributed over all points of the globe, and were 

 employed without loss in dissolving ice, it would melt a 

 stratum having the above thickness. 



The data given by these experiments enabled him to 

 solve another problem which would appear even of a more 

 transcendental character: that is, the amount of heat given 

 off by the whole surface of the sun in a given time. For this 

 purpose it is only necessary to consider the sun as the centre 

 of a spherical enclosure, the radius of which is the distance 

 from the earth to the sun ; * and it must be evident that on 

 each square centimetre of the concave surface of this vast 

 sphere as much heat is received as on a square centimetre 

 at the surface of the earth. If then the number 1'7633, 

 before obtained, is multiplied by the number of square centi- 

 metres in this spherical surface the absolute quantity of 

 heat given off by the sun during a given time will be ascer- 

 tained. Or by reference to the visual angle subtended by 

 the sun, the number expressing this quantity for each minute 

 of time may be stated as 84,888 thermal units for each square 

 centimetre of the solar surface. 



If this quantity of heat emitted by the sun were exclusively 

 employed in dissolving a stratum of ice, applied to the solar 

 surface, and enveloping it on every side, it would melt in 

 one minute a stratum of 11*8 metres thick ; and in one day 

 a stratum of 16,992 metres, or about lOh miles. 



These results cannot be considered more than approxi- 

 mations, though in the progress of science, they may be 

 rendered much more precise, and may be applied to solve 

 many problems relative to the physical phenomena of the 

 earth and our solar system. 



*The proportional amount of the entire solar radiation intercepted by tha 

 terrestrial hemisphere is 1 -^ 2 300,000 000. 



