488 ON IMPRESSIONS GF COLD 



the liquor will indicate merely the difference of opposite ef- 

 fects, and will rise or fall according as the impressions of cold 

 or heat sent from the sky chance to predominate. If the light 

 reflected down from the heavens be profuse, it will excite 

 more heat than the simultaneous frigorific impressions can de- 

 stroy. Under a canopy of fleecy clouds, a considerable excess 

 of heat is hence excited on the black ball ; but when the sky is 

 clear, the influence of cold generally prevails, increasing as 

 the sun declines. 



The question is now to discover the cause of the phenomena 

 which have been thus revealed. I have already stated, that 

 different experiments appeared to concur in indicating the cold- 

 ness of the superior atmosphere as the source of those effects. 

 Since pulses are darted from such various surfaces, and since 

 the softness of the external coat, and its tendency to fluidity, 

 seem vastly to augment their power ; may they not likewise be 

 excited from a boundary of air itself? This extension of a 

 oreat principle in the economy of Nature, has never yet been 

 surmised j nor can it be readily brought to the test of direct 

 experiment, since a body of air, whether hotter or colder 

 than the general medium, would evidently not remain station- 

 ary, but continually rise or fall. 



I sought accordingly to examine the effect of directing the 

 sethrioscope to a hot stream of ascending air. I placed on bricks 

 before that instrument, the lower part being screened, a large 

 mass of iron carried from the fire, at almost a red heat. 

 The sethrioscope then gave impressions of heat or cold, ac- 

 cording as its aperture was without or within the warm cur- 

 rent, or was affected by the anterior or the posterior boundary. 

 But this experiment proved very troublesome, and occasionally 

 turned out a little unsatisfactory. Another experiment per- 

 formed out of doors, to try the action of hot smoke raised from 



a 



