PHILOSOPHICAL SOCIETY OF WASHINGTON. 95 



tion of 7,400 feet, he concluded that the average effect of the sun's 

 rays on a black-bulb thermometer was 125.7° or 67° (37.2° C.) 

 above the temperature of the air. The shade temperature was, 

 therefore, 14.8° C. With this value of r, and the value p = £ for 

 the spherical bulb, we get r — t? = 41.6° at the top of the atmos- 

 phere where p = 1. The value of p for that altitude, and also the 

 value of s for the observations, are not accurately known. At the 

 elevation of 7,400 feet, Pouillet's value of p = .75 would have to 

 be considerably increased, but the effect of the exponent « would 

 perhaps bring the value of p E equal to about .75. With this value 

 of p £ the formula gives t — r' = 32.4°, five degrees too small for 

 the observed value. 



Again, at the height of 13,100 feet, he found in January, at 9 

 a. m., the temperature of the black bulb 98° with a difference of 

 68.2°, and at 10 a. m., 114° with a difference of 81.4°. From the 

 average of these we get r' = — 0.4° C. and r — -' = 41.6° C. 

 The preceding formula gives r — r = 45.7° C. at the top of the 

 atmosphere where p = 1. At the elevation of 13,100 feet the value 

 of p£ should not be very much less than unity — perhaps about as 

 much less as would reduce the theoretical value 45.7° down to the 

 observed value 41.6°. 



It should be remarked here that the theory requires that the two 

 thermometers should have exactly the same surroundings. If the 

 one thermometer is in a vacuum surrounded by a glass bulb and 

 the other outside, this condition is not perfectly fulfilled, and the 

 indication of the thermometer outside in the shade might vary a 

 little from one in the shade within the bulb, unless this -bulb is so 

 situated as to have the same temperature as the external shade 

 thermometer. 



If, in place of a black-bulb thermometer, we had a thin disk with 

 a blackened side exposed perpendicularly to the sun's rays, and 

 the opposite side of polished silver of which the radiating power is 

 extremely small, we should have in this case the value of p = 1 

 very nearly, and with this value of p the formula would give, in 

 the first of the examples above, for the top of the atmosphere, 

 t — r' = 106.6° C, which, added to the shade temperature, 14.8°, 

 would give r = 121.4° C. This enormously high temperature is 

 not inconsistent with observation, for water has been made to boil 

 from the effect of the direct rays of the sun at the earth's surface, 



