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 Wells, 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 

 oreat 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 

 around. In the month of August last, I carried it, with some 



other 



