214 Mr. J. Croll on the Diminution of Difference between \ 



surface, and we find that they differ by a very considerable num- 

 ber of degrees. We now ascend some miles into the air, and on 

 again comparing the thermometers we find that the difference 

 between them has greatly diminished. It has been often proved, 

 by direct observation, that the intensity of the sun's rays increases 

 as we rise in the atmosphere. How then does the exposed ther- 

 mometer sink more rapidly than the shaded one as we ascend ? 

 The reason is obviously this. The temperature of the thermo- 

 meters depends as much upon the rate at which they are losing 

 their heat as upon the rate at which they are gaining it. The 

 higher temperature of the exposed thermometer is the result of 

 direct radiation from the sun. Now, although this thermometer 

 receives by radiation more heat from the sun at the upper posi- 

 tion than at the lower, it does not necessarily follow on this ac- 

 count that its temperature ought to be higher. Suppose that 

 at the upper position it should receive one-fourth more heat from 

 the sun than at the lower, yet if the rate at which it loses its 

 heat by radiation into space be, say, one-third greater at the 

 upper position than at the lower, the temperature of the bulb 

 would sink to a considerable extent, notwithstanding the extra 

 amount of heat received. Let us now reflect on how matters 

 stand in this respect in regard to the actual case under our 

 consideration. When the exposed thermometer is at the higher 

 position, it receives more heat from the sun than at the lower, 

 but it receives less from the earth ; for a considerable part of 

 the radiation from the earth is cut off by the screen of aqueous 

 vapour intervening between the thermometer and the earth. 

 But, on the whole, it is probable that the total quantity of radiant 

 heat reaching the thermometer is greater in the higher position 

 than in the lower. Compare now the two positions in regard to 

 the rate at which the thermometer loses its heat by radiation . 

 When the thermometer is at the lower position, it has the warm 

 surface of the ground against which to radiate its heat down- 

 wards. The high temperature of the ground thus tends to di- 

 minish the rate of radiation. Above, there is a screen of aqueous 

 vapour throwing back upon the thermometer a very considerable 

 part of the heat which the instrument is radiating upwards. 

 This, of course, tends greatly to diminish the loss from radiation. 

 But at the upper position this very screen, which prevented the 

 thermometer from throwing off its heat into the cold space above, 

 now affects the instrument in an opposite manner ; for the ther- 

 mometer has now to radiate its heat downwards, not upon the 

 warm surface of the ground as before, but upon the cold upper 

 surface of the aqueous screen intervening between the instru- 

 ment and the earth. This of course tends to lower the mercury. 

 We are now in a great measure above the aqueous screen, with 



