on the terrestrial Rays that occasion Heat. 309 



port of the lens had an eccentric pivot, on which it might be 

 turned away from its place, and returned to the same situation 

 again, at pleasure. This arrangement being made, the thermo- 

 meter was for a few moments exposed to the rays of the candle, 

 till it had assumed the temperature of its situation. Then the 

 lens was turned on its pivot, so as to intercept the direct rays, 

 which passed through the opening in the pasteboard screen, 

 and to refract them to the focus, in which the thermometer was 

 situated. 



No. 2. 



°' 53i 



1 55i 



3 5 6 



Here, in three minutes, the thermometer received 2-f- degrees 

 of heat, by the refraction of the lens. The lens was now turned 

 away. 



o' 5 6 



1 54f 

 a 54>i 

 3 53¥ 



Here, in three minutes, the thermometer lost 2-^ degrees of 

 heat. The lens was now returned to its situation. 



Q' 53i 



2 . 55f 



3 5<> 



And, in three minutes, the thermometer regained the 2| 

 degrees of heat. A greater effect may be obtained by a different 



