12 THE POPULAR SCIENCE MONTHLY. 



of water, carefully weighed, was placed on a light wooden support, 

 touching it at only three points. This was put inside of a considera- 

 bly larger cylinder, also of tinned iron ; this outer cylinder having a 

 double cover with a hole in it the cover large enough to shade the 

 sides of the vessel, and the hole a little less than three inches in diam- 

 eter. A delicate thermometer was immersed in the water, with a sort 

 of dasher of mica for the purpose of stirring it and keeping the tem- 

 perature uniform throughout the mass. The ajij^aratus was so placed 

 and adjusted that the whole of the light and heat passing through 

 the hole in the cover would fall upon the surface of the water, the 

 sun at that time (December 31st) being within 12 of the zenith at 

 noon. 



This apparatus was placed in the sunshine and allowed to stand for 

 ten minutes, shaded by an umbrella, and the slight rise in the tempera- 

 ture of the water was noted. Then the umbrella was removed and. the 

 solar rays were allowed to fall upon the water for the same length of 

 time, and the much larger rise of temperature was noted ; finally, the 

 apparatus was again shaded and the change for ten minutes again ob- 

 served. The mean between the effects in the first and last ten-minute 

 intervals can be taken as the measure of the influence of other causes 

 besides the sun, and, deducting this from the rise during the ten min- 

 utes' insolation, we have the effect of the simple sunshine. 



Herschel's figures for his first experiment run as follows : 



Eise of temperature in first ten minutes 0*25 



" " " " second ten minutes (sun) 3-90 



" " " " third ten minutes OlO 



The mean of the first and third is 0*17, and this deducted from the 

 second gives 3*73 Fahr. as the rise of temperature produced by a sun- 

 beam three inches in diameter, absorbed by a mass of matter equiva- 

 lent to 4,638 grains of water. (We do not indicate the minutiae of the 

 process by which the weight of the tin vessel, thermometer, stirrer, 

 etc., are allowed for.) Nothing more is now necessary to enable us to 

 compute just how much heat is received by the earth in a day or a year, 

 except, indeed, the determination of the very troublesome and some- 

 what uncertain correction for the absorption of heat by the earth's 

 atmosphere ; a correction deduced by means of observations made at 

 varying heights of the sun above the horizon. 



Herschel preferred to express his results in terms of melting ice, 

 and put it in this way : 



The amount of heat received on the earth's surface, with the sun 

 in the zenith, would melt an inch thickness of ice in two hours thirteen 

 minutes, nearly. 



Since there is every reason to believe that the sun's radiation is 

 equal in all directions, it follows that, if the sun were surrounded by a 



