252 HEAT. 



n . s\ 



of exposure to the sun was * 2 > so that, had all the heat received from 



m 



f) 4- 



the sun been retained, the rise would have been 6 + l ~ - 



Knowing the capacity of the calorimeter, the total heat received per 

 unit of surface per unit of time could be measured. This, of course, only 

 gave the radiation reaching the earth after much had been absorbed by 

 the atmosphere. That this absorption is very considerable is sufficiently 

 shown by the fact that we receive sensibly more heat from the sun as he 

 rises higher, and thus diminishes the thickness of air through which his 

 rays must pass. 



To determine the absorption by the air, Pouillet made observations 

 when the sun was at different heights, and found that as long as the air 

 was exceedingly clear, and the observations were made on the same day, 

 the quantity of heat received per centimetre per second might be 

 represented by Aa**, where d is the thickness of air passed through on 

 the supposition that the air is a limited ocean of uniform density. 

 A is, therefore, the heat per centimetre per second which would be 

 received outside the atmosphere. By aid of this formula and the ex- 

 perimental results, A and a could be determined. Suppose, for instance, 

 that observations are made with a vertical sun, and again when it is 60 

 from the zenith. The thicknesses of air may be taken as 1 : 2. 



Hence, Q! = Aa" Q 2 = Aa**, 



Therefore, Qi 2 _ . 



or 



It was found that a changed from day to day, and with the clearness 

 and hygrometric state of the atmosphere, but the mean absorption of the 

 radiation from a vertical sun was, according to Pouillet, about 20 per 

 cent. 



He obtained 0*02939 calories per second per sq. cm., for the value of 

 A, or 1'7634 calories per minute per sq. cm. 



Multiplying by 46000, the radiation from a square cm. of the sun 

 would be about 1350 calories per second, or 1350 x 4-2 = 5650 watts = 7'6 

 horse-power. This gives about 7000 horse-power per square foot. 

 Later observers have obtained values somewhat higher than that of 

 Pouillet. His calorimeter was probably somewhat too slow in responding 

 to the heating effect of the sun. The outer surface being at a higher 

 temperature than the water as registered by the thermometer, the 

 radiation outwards was most likely greater than that allowed for. 



Again, he assumed that the air absorbed all rays in the same pro- 

 portion, whereas it absorbs or diffuses some much more than others. 

 For instance, the blue colour of the sky shows that there is greater diffusion 

 of the shorter wave-lengths, and the spectroscope gives us clear evidence 

 of marked " selective absorption." As Langley has shown, Pouillet's 

 supposition that the radiation may be represented by Aa d would always 

 lead to an underestimate of the solar constant.* This may be seen by 

 considering a simple case. Imagine that the radiation outside the atmo- 

 sphere consists of two kinds, respectively A and B in quantity. Let A 



* American Jownal of Science, September 1884 ; Phil. Mag. xviii., 1884. 



