1883.] 



On Terrestrial Radiation. 



23 



was from the north-east. The great differences of temperature be- 

 tween earth and air which both his observations and mine reveal, are 

 due to a common cause, namely, the withdrawal of the check to 

 terrestrial radiation which is imposed by the presence of aqueous 

 vapour. 



Let us now compare these results with others obtained at a time of 

 extreme atmospheric serenity, when the air was almost a dead calm, 

 and the sky without a cloud. At 3.30 p.m., January 16th, the 

 thermometers were placed in position, and observed afterwards with 

 the following results 



Air 43° Wool 37° 



3.40 p.m. . 

 3.50 

 4 



4.15 

 4.30 

 5 



5.30 

 6 



42 

 41 



40 

 38 

 37 

 37 

 36 



35 

 35 

 34 

 32 

 28 

 30 

 32 



These observations, and especially the last of them, merit our 

 attention. There was do visible impediment to terrestrial radiation. 

 The sky was extremely clear, the moon was shining, Orion, the 

 Pleiades, Charles' Wain, including the small companion star at the 

 bend of the shaft, the north star, and many others, were clearly 

 visible. On no previous occasion during these observations had I 

 seen the firmament purer; and still, under these favourable conditions, 

 the difference between air and wool at 6 p.m. was only 4°, or less than 

 one-fourth of that observed on the morning of the 10th of December. 



We have here, I submit, a very striking illustration of the action of 

 that invisible constituent of the atmosphere, to the influence of which 

 I. drew attention more than twenty- two years ago. On the 10th of 

 December the wind was light from the north-east, with a low tem- 

 perature. On the 16th of January it was very light from the south- 

 west, with a higher temperature. The one was a dry air, the other 

 was a humid air ; the latter, therefore, though of great optical trans- 

 parency, proved competent to arrest the invisible heat of the earth. 



The variations in the temperatures of the wool recorded in the last 

 column of figures are, moreover, not without a cause. The advance of 

 temperature from 28° at 5 p.m. to 32° at 6 p.m., is not to be accounted 

 for by any visible change in the atmosphere, or by any alteration in 

 the motion of the air. The advance was due to the intrusion at 6 p.m. 

 of an invisible screen between the earth and firmament. 



As the night advanced, the serenity of the air, became, if possible^ 

 more perfect, and the observations were continued with the following 

 results : — 



