94 BULLETIN OF THE 



of the constant. Pouillet's value of B for the minute-unit was 

 1.146, and this reduced to the second-unit is .01910. The value 

 (i = 1.0077 required no change to satisfy the results of Mr. NichoFs 

 experiments. 



The value of A, deduced from the experiments of Pouillet and 

 Herschel with the actinometer, is .03046 for the mean distance of 

 the sun, both sets of experiments, when reduced to the sun's mean 

 distance, giving very nearly the same value. At the time of the 

 earth's perihelion this is about one-thirtieth greater, and at aphelion 

 as much less. 



Pouillet's value of p for clear weather is about 0.75, but others 

 make it considerably less. It can hardly be regarded as a constant, 

 but only as a sort of average of values for clear weather, which 

 may differ very much at different times. According to Tyndal, 

 who maintains that the absorption power of the atmosphere in clear 

 weather depends almost entirely upon the amount of aqueous vapor 

 in it, the value of this constant, even in clear weather, must depend 

 very much upon the hygrometric state of the atmosphere. 



With the preceding numerical values of the constants of A and 

 B, the preceding equation gives 



,a\ " i 1.685 ope , . 

 (4) p?—* = -/^- + 1 



for determining the value of r — r', for any zenith distance of the 

 sun, of which the secant is s, where the value of p and the shade 

 temperature t' are known. But since the value of B was deter- 

 mined for a vacuum, this formula is only applicable where the radi- 

 ating body is in a vacuum, and cannot be applied in cases where 

 the body receives or loses heat by conduction or convection. 



The first term of the second number of the preceding equation 

 depends upon K, the heat received from the sun, and, therefore, 

 vanishes where the body is in the shade, and we then have r — r' 

 — 0. Hence the temperature of all bodies having the same sur- 

 roundings must cool down to the same temperature, r'. This is a 

 necessary consequence of the equality of the absorbing and radiat- 

 ing powers of bodies. 



The author had been able to find but few observations of the 

 value of t — t' to compare with the theoretical value given by the 

 preceding formula. Hooker states that from a multitude of de- 

 sultory observations made on the Himalaya Mountains at an eleva- 



