FROM THE HIGHER ATMOSPHERE. 477 



the accumulation of heat would sometimes hardly reach to 

 three degrees. Under like circumstances, but especially 

 when the air was still, the differential thermometer indicated 

 more than twice as much effect on fresh ploughed land as 

 on fine pasture. Nor was this inferiority of a surface of turf 

 owing to the waste of heat produced by a more copious exha- 

 lation of moisture ; for, on spreading a layer of dry hay, or 

 even wool,, over a part of the naked soil, the temperature of it 

 was in a few minutes reduced to the same degree as that of the 

 grassy sod. 



The influence of reflex light, from the clouds and the sky, in 

 heating the ground, is often very considerable. The rays sent 

 from a fleecy canopy may sometimes amount to the half or the 

 third part of those which would be received directly from the sun. 

 But a stratum of dense black clouds intercepts almost the whole 

 of the scattered light. A similar effect is produced by another 

 sort of screen. Thus, when the sky was overcast, but the wea- 

 ther calm, the pendant differential thermometer intimated 

 scarcely two degrees of heat at the surface, in a small fir wood; 

 but marked eight or ten degrees, when carried to a neighbour- 

 ing glade. 



In this climate, about two hours after sunrise, the ground has 

 the same temperature as the incumbent mass of air, but grows 

 commonly warmer than it, till nearly two hours after noontide; 

 from which time, it again declines, and becomes relatively cold- 

 er than the air, perhaps two hours before sunset ; sinking still 

 lower during the night. The differences are thus comparative- 

 ly very small between the temperature of the ground and that 

 of the conterminous air, seldom exceeding the fifth, or perhaps 

 even the tenth part of the whole diurnal change. The hot or 

 cold pulses discharged from the ground, must, therefore, in all 

 cases, be only trifling, and quite insufficient to produce that rapid 

 approximation to an equilibrium which is actually observed. I 



was 



