Terrejlrial Rays that occajion Heat. 25$ 



inches diameter, and %\ inches long*. This, being made 

 red-hot, was ftuck upon an iron handle fixed on a ftand fo 

 as to prefent one of its circular faces to a lens placed at 2.8 

 inches diftance; its focus being 1.4 inch, and diameter 1.1. 

 Before the lens, at fome diftance, was placed a fcrcen of wood, 

 with a hole of an inch diameter in it, by way of limiting the 

 object, that its image in the focus might not be larger than 

 necefiary. The fcreen alfo ferved to keep the heat from the 

 thermometers. No. 2 was fituated in the fecondary focus of 

 the lens ; and No. 3 was placed within T % of an inch of it, 

 and -at the fame diftance from the lens as No. 2. By this 

 arrangement, both thermometers were equally within the 

 reach of transmitted heat; or, if there was any difference, it 

 could only be in favour of No. 3, as being behind a part of 

 the lens which, on account of its thinnefs, would ftop lefs 

 heat than the middle. Now, as the experiment gives a refult 

 which differs from what would have arifen from the fituation 

 of the thermometers, on a fuppofition of tranfmitted heat, we 

 can only afcribe it to a condenfation of it by the refraction of 

 the lens; and, in this cafe, the thermometer No. 3, by its 

 fituation, muft have been partly within the reach of the heat- 

 image formed in the focus. During the experiment, the ther- 

 mometers were alternately fcreened two minutes from the 

 effe&s of the lens, and expofed to it for the fame length of 

 time, and the refult was as follows : 



No. 2. No. 3. ~\ Here, in the firfi: 



In the Focus. Near the tocus. j and fecond mi- 



Screened o' 56" 56 nutes, No. 2 



Open 3 62 60 I gained two de- 



Scrcencd 4 59 5 s \ g rees of heat 



Open 6 61 59 morethanNo.3. 



Screened 8 58^ 57!- In the third and 



Open 10 59V 58|- J fourth, it loft one 



more than No. 3. In the fifth and fixth, it gained one more. 

 In the feventh and eighth, it loft i[ more; and in the ninth 

 and tenth, it gained \ more than the other thermometer. 

 This plainly indicates its being acted upon by refracted heat. 

 Left there (hould remain a doubt upon the fubject, I now rc- 



* Set Tiatc VIII. fig. 1. 



moved 



