2 Mr. J. (Jroll 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 onagain comparing the thermometers we find that the differencebetween them has greatly diminished. It has been often proved,by direct observation, that the intensity of the sun’s rays increasesas 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 losingtheir heat as upon the rate at which they are gaining it. Thehigher temperature of the exposed thermometer is the result ofdirect radiation from the sun. Now, although this thermometerreceives 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 thatat the upper position it should receive one-fourth more heat fromthe sun than at the lower, yet if the rate at which it loses itsheat by radiation into space be, say, one-third greater at theupper position than at the lower, the temperature of the bulbwould sink to a considerable extent, notwithstanding the extraamount of heat received. Let us now reflect on how mattersstand in this respect in regard to the actual case under ourconsideration. When the exposed thermometer is at the higherposition, it receives more heat from the sun than at the lower,but it receives less from the earth; for a considerable part ofthe radiation from the earth is cut off by the screen of aqueousvapour intervening between the thermometer and the earth.But, on the whole, it is probable that the total quantity of radiantheat reaching the thermometer is greater in the higher positionthan in the lower. Compare now the two positions in regard tothe rate at which the thermometer loses its heat by radiation.When the thermometer is at the lower position, it has the warmsurface 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 aqueousvapour throwing back upon the thermometer a very considerablepart 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 thethermometer 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 thewarm surface of the ground as before, but upon the cold uppersurface 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