478 ON IMPRESSIONS OF COLD 
was hence still inclined to think, that the statement which I had’ 
formerly made, of the existence of an ascending warm current 
in the atmosphere, combined with a descending cold current, 
was adequate to the explication of all the phenomena. It is: 
obvious, indeed, that the lower portions of air, becoming heat- 
ed, in the progress of the day, by their contact with the ground, 
must rise upwards, and their place will consequently be sup- 
plied, by like portions of cold air which descend from the high- 
er regions. These opposite currents, though set in motion du- 
ring the flood of light, will continue their play long afterwards, 
perhaps through the greater part of the night, till again vigo- 
rously excited by the presence of the sun. Hence the ground 
is only heated to a certain limit during the day, and grows al- 
ways colder as the night advances. 
But my late ingenious and learned friend Dr Wexts, by the. 
publication of his Essay on Dew, which contains some acute ob- 
servations, conjoined with a few striking experiments, though 
performed in the gross way, had contributed to revive the no- 
tion of a copious radiation, or pulsatory discharge of heat from 
the earth’s surface. It was, therefore, desirable to ascertain 
the existence and real extent of such pulsations. The accu- 
rate means of determination were within my reach, though I 
had hitherto, in a great measure, neglected their application ; 
for though the pyroscope measures those impressions with 
great delicacy in a close apartment, it is liable out of doors to 
some derangement from the influence of light, and has its ac- 
tion diminished by the sweep of violent winds. To avoid as 
much as possible these disturbing influences, I constructed 
a small pendant pyroscope, the lower ball being left naked, 
and the upper ball gilt with silver-leaf, to reflect almost the 
whole of the incident light. This instrument I fixed to a short 
arm, made to slide along a staff which could be stuck in the 
ground, In the month of August last, I carried it, with some 
other 
