472 ON IMPRESSIONS OP COLD 



centrating the heat or cold projected from any remote body. 

 Hence also the construction of the Pyroscope, a delicate instru- 

 ment, adapted to distinguish and measure those peculiar impres- 

 sions of heat or cold. It consists of a differential thermometer, 

 having one ball naked, and the other coated with gold or silver 

 leaf.; this metallic surface reflecting the greatest part of the 

 projections, while the vitreous surface absorbs almost die whole 

 effect. If a broad plate of glass be interposed before a cubical 

 canister holding hot or cold water, the action on the pyroscope, 

 though concentrated by a reflector, will yet be very much redu- 

 ced. But, on removing the plate farther from the canister, and 

 therefore nearer to the reflector, this action will be still more 

 diminished ; so as at last to become almost extinguished. Hence 

 the projected influence of heat or cold consists not in any 

 streaming matter, for the same proportion of the warm or fri- 

 gid rays would evidently be intercepted by the glass screen in 

 whatever part of the route between the canister and the re- 

 flector it was planted. 



But the question is completely decided, by varying the ex- 

 periment. Let two glass plates be covered, each on one 

 side with tin-foil ; applying the metallic surfaces now together, 

 place the double screen immediately before the hot -canister, 

 and the jeffect on the pyroscope will be greatly reduced ; re- 

 verse the position of the plates, by exposing the coated sides, 

 and the action on the sentient ball will be entirely extinguish- 

 ed. The interposition of a screen would in every case detain 

 the whole impressions, if it did not itself become affected by 

 them, and therefore come to act as a secondary but feeble pro- 

 jecting surface. Hence the different influence of a metallic 

 screen, in comparison with one of glass or paper. 



The impressions of heat or cold emitted from a body are 

 not sent equally in all directions. Those projected perpendi- 

 cular 



