Mr. E. Foote on the Heat in the Sun's Rays. 



171 



always affects the experiment, making it somewhat difficult to 

 compare observations taken at different times. A strong light 

 obtained by reflexion or otherwise always increased the effect. 

 But the most striking results were obtained by concentrating the 

 rays with a lens. One was placed in the receiver with its focus 

 directed upon an additional thermometer ; the second and third 

 columns in the following Table contain the temperatures of the 

 air and in the sun, and the fourth the heat in the focus, while 

 the air in the receiver was" heated as before. The atmosphere at 

 the time was not entirely clear. 



The burning-glass was then so arranged, that, being within the 

 receiver, its focus was on the outside. The result was as follows : — 



Then the burning-glass was placed on the outside of the 

 receiver, and so arranged that its focus should be on the inside, 

 and the effect was the same as if both glass and focus had been 

 on the inside. 



It will be observed that the effect of the burning-glass is sim- 

 ply to increase the results before obtained. Its power depends 

 upon the temperature of the place at which the light is concen- 

 trated. That no heat triivels with the light is rendered more 

 manifest. The increased temperature of the rays on the inside 

 had no effect at their focus on the outside. 



The power of the burning-glass seems therefore to depend on 

 two considerations : — 1st, the amount of light concentrated; 

 2ndly, the amount of heat on which it acts. 



Those who have heretofore sought its best effects have, it 

 seems to me, too much neglected the latter consideration. Its 



'Ul.y'-: N 2 



