Chemical and Physical Notes 95 



instrument would furnish any trustworthy information ; the 

 external glass envelope introduces so much disturbance. 



When a thermometer, whether its bulb be blackened or 

 not, is exposed to the direct rays of the sun, its temperature 

 rises at first rapidly, then more and more slowly until finally 

 it reaches a temperature at or about which it remains steady, 

 provided that it is in a place where the air is still and its 

 temperature constant. While its temperature remains steady 

 the thermometer is continually receiving heat from the sun's 

 rays and giving it off to its surroundings, and the meaning 

 of the constancy of its temperature is that the receipt and 

 expenditure of heat per unit of time are equal. 



If we know the thermal mass of the bulb of the thermo- 

 meter and its term of cooling and the difference between its 

 temperature and that of the air, we can calculate the rate at 

 which it is losing heat, and this, at the stationary temperature, 

 is the rate at which it is receiving heat. 



If the thermometer is exposed with its axis perpendicular 

 to the direction of the sun's rays, then it receives the rays 

 which fall on a surface equal to the axial sectional area of 

 the bulb. Consequently, the heating power of the bundle of 

 solar rays having this sectional area is equal to the heating 

 power of the thermometer when the excess of its temperature 

 above that of the medium has become constant. This is 

 rigorously true of that portion of the pencil of rays which 

 penetrates the bulb of the thermometer. If the rays have 

 had to traverse the windows of a room, some of the rays 

 are absorbed by the glass, and a very large portion is 

 dissipated by reflection from it. Further, even a blackened 

 bulb does not transmit and absorb all the heat rays that 

 strike it. 



If two thermometers of different pattern are exposed side 

 by side to the direct rays of the sun, they usually assume 

 very different stationary temperatures, even although their 

 graduation may be perfectly exact. This is to be expected, 

 because thermometers of different pattern are sure to differ 

 considerably both in term of cooling and in thermal mass. 

 The nearest approach to equality in these particulars is found 



