FROM THE HIGHER ATMOSPHERE. 477 
the accumulation of heat would sometimes hardly reach to 
three degrees. Under like cireumstances, 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 kas 
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 relativel y cold- 
er than the air, perhaps two hours: before sunset ; sinking still 
Jower 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 
