24 P>'of- Leslie on Heat and Climate. [July, 



Heat is circulated with augmented rapidity in summer and in 

 winter, a circumstance which, in some measure, tempers the in- 

 equality of the seasons. The quantities of light which fall in the 

 high latitudes during the long summer days is surprisingly great. 

 At the time of the aestival solstice, the luminous matter received 

 in the course of a diurnal revolution at the pole exceeds that re- 

 ceived at the equator in the proportion of 15 to 11. We might, 

 therefore, expect the summer to be excessively hot in the polar 

 regions ; and this would probably be the case, were not the 

 surplus heat expended in thawing the surface of an immense con- 

 tinent of ice. In Siberia and Lapland, the action of the sun- 

 beams is sufficient, during the spring and part of the summer, 

 to dissolve completely the snow and ice ; and nature, released 

 from her chains, hastens to pour forth, in gay luxuriance, the 

 few vegetable tribes that had withstood the severity of the 

 winter. The heat often grows oppressive, and engenders 

 myriads of winged insects, which oblige the hardy natives to 

 retire into their hovels and defend themselves with smoke. It 

 is the vicinity of the ocean that prevents the same violent heats 

 and rapid growth from being remarked on the northern extremity 

 of our own island. A body of water receives and disperses heat 

 infinitely quicker than a mass of earth.* 



But besides the great circulation of differently heated air 

 between the poles and the equator, there is in every place a 

 perpetual communication between the higher and lower regions 

 of the atmosphere. The latter motion is incomparably more 

 perfect in its effects than the former. The air is as cold at the 

 height of three miles above the equator as at the pole, which is 

 more than six thousand miles distant. And since the causes 

 which produce a circulation in the fluid are concentrated within 

 so narrow a compass, they must maintain an almost accurate 

 equilibrium. It is immaterial what portion of the air be heated 

 first, for the surplus heat will instantly be diffused through the 

 whole of the vertical column. If a portion of air ascend, an 

 equal portion must descend ; so that the same absolute heat will 

 be maintained at every altitude. Hence we may conclude, that 

 the temperatures which prevail at different heights are recipro- 

 cally as the specific attractions for heat due to air of the corre- 

 sponding densities. If d denote the rise of temperature which 

 dilated air indicates upon recovering its density, and 2000 degrees 

 be the distance of the thermometric zero, we may deduce the 



specific attraction of this rarefied air for heat to be -^ — 3 , or 



— 200Q very nearly when d is a small number. Consequently, if 



air of the usual density be allowed to expand itself into the state here 

 supposed, its temperature would sink almost the same difference, 



* The distinction between summer and winter, in the high latitudes, was pro- 

 bably more strongly marked than even at present, by reason of the greater ob- 

 liquity of the ecliptic. 



