FROM THE HIGHER ATMOSPHERE. 4*73 



cular to the surface of a cubical canister containing either warm 

 or iced water, are the most intense, and the rest appear to di- 

 minish in the ratio of the sine of obliquity. Hence the action 

 exerted on the sentient ball of the pyroscope, is always pro- 

 portional to the visual angle subtended by the propellent sur- 

 face. 



Nor are those impressions necessarily propagated in straight 

 lines. If connected rings of pasteboard be fashioned into a 

 sort of cornucopia, its mouth being directed towards the fire, 

 notwithstanding the twisted form of the passage, a very consi- 

 derable action will be indicated on the ball of the pyroscope, 

 presented at the narrow end. 



From all these combined observations it follows, that the 

 portion of heat or cold, of which the discharge depends on the 

 quality of the surface, is propagated by the vehicle of its gaseous 

 medium, though not by any actual streaming of fluid matter. 

 No alternative then remains, but to admit that the impressions 

 of heat or cold are conveyed through the air with a spreading 

 and progressive tendency, in the same manner as the pulses of 

 sound. The aerial medium, by a series of internal oscillations, 

 successively transfers its charge, and delivers an impression at 

 the end of the chain of communication, of the same kind pre- 

 cisely as it had received at the beginning. 



This rapid transmission of Heat to a distance from its source 

 has been hastily termed radiant , and various inaccurate concep- 

 tions are entertained concerning its mode and extent of action. 

 1. In the first place, then, the process now described never ob- 

 tains, unless where a difference of temperature occurs ; and 

 it only contributes, along with other active causes, to restore 

 the equilibrium of heat. 2. But, in the next place, it has in 

 every case a subordinate share only in the diffusion of heat. 

 When the air is perfectly still, and of the ordinary density, the 

 tide of heat vibrated from a vitreous surface amounts scarcely 

 Vol. VIII. P. II. 3 to 



