704 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



iron, when cooled so far as to be no longer visible, has but a feeble stock of heat 



remaining, and loses it very fast. A contrivance to renew and keep this heat might 



certainly be made, and I should indeed have attempted to carry some method or 



other of this kind into execution, had not the following trials appeared to me 



sufficiently conclusive to render it unnecessary. Admitting, as has been proved i 



the 15th experiment, that the alternate rising and Min ^ ^ 



falling of a thermometer placed in the focus of a Screened 55 Very red-hot. 



lens, when the ball of it is successively exposed to, or g^ J »l ™; hot . 



screened from, its effects, is owing to the refraction Open 6 6o§ Still red. 



of the lens, and cannot be ascribed to a mere alter- Screened 8 57| A little red. 



' , Open 10 59% Doubtful, 



nate transmission and stoppage of heat, I proceeded Screened 12 57k Not visible. 



as follows, fig. 9. My lens, 1.4 focus, and 1.1 gpen^ u 58| 



diameter, being placed 2.8 inches from the face of open 18 584 



the heated cylinder of iron, the thermometer N° 2, Screened 20 57* 



1 1111 ii Open 22 58 



in its focus, was alternately guarded by a small paste- screened 24 57\ 



board screen put before it, and exposed to the effects Open 26 53 



11 1 • -N.T J.L t- Screened 28 5/j 



of condensed heat by removing it. JNow, the be- 

 ginning of this experiment being exactly like that of the 1 5th, with the ther- 

 mometer N° 3 left out, the arguments that have before proved the refraction of 

 heat in one state, will now hold good for the whole. For here we have a regular 

 alternate rising and falling of the thermometer, from a bright red heat of the cy- 

 linder, down to its weakest state of black heat; when the effect of the rays, 

 condensed by the lens, exceeded but half a degree the loss of those that were 

 stopped by it, 



Exper. 20. Confirmation of the 1 Qth. — In order to have some additional proof, 

 besides the uniform and uninterrupted N o 2 N o 3 



operation of the lens in the foregoing ex- Advanced sideways. 



T , ,, •.! Min. In the Focus. Always open. 



penment, I repeated the same, with an Screened g 2 £ (,- 3 r 



assistant thermometer, N° 3, placed first Open 

 of all at 4 of an inch from N° 2, and Q c p r ^ ned 

 more towards the lens, but so as to be Screened 

 out of the converging pencil of its rays, ^^ ned 

 and also to allow room for the little Open 

 screen between the 2 thermometers, that Screened 

 N° 3 might not be covered by it. Here N° 3, being out of the reach of re- 

 fraction, gradually acquired its maximum of heat, in consequence of an uniform 

 exposure to the influence of the hot cylinder; after which it began to decline. 

 N° 2, on the contrary, came to its maximum by alternate great elevations, and 

 small depressions; and afterwards lost its heat by great depressions, and smallgele- 

 vations. After the first 8 m , I changed the place of the assistant thermometer, by 

 putting it into a still more decisive situation ; for it was now placed by the side of 



