to Heat and llhminate Ohje^s. 3 rj 



EF; their halls were hlacked with japan ink. That of No. i 

 was rather too larije for great fcnfibility. No. 2 and 3 were 

 two excellent thermometers, which rny highly eftcemed friend 

 Dr. Wilfon, late profcflbr of aflrononiy at Glafgow, had 

 lent me for the pnrpofe : their balls being very fmall, made 

 them of exquifite fenfibility. The fcalcs of all were properly 

 difcngaged from the balls, 



I now placed the ftand, with the framed pafieboard and 

 the thcrmonictcrs, upon a fmall plain boardj GH, that I 

 might be at liberty to move the whole apparatus together, 

 without deranging the relative fituation of the different parts. 



This being done, I fet a prifm, moveable qn its axis, into 

 the upper part of an open window, at right angles to the 

 folar ray, af)d turned il: about till its refracted coloured fpec- 

 trum became Ilationary upon a table placed at a proper dif- 

 tance from the window. 



The board containing the apparatus was now put qti the 

 table, and fet. in fuch a manner as to let the rays of one 

 colour pafs through the opening in the paftcboard. The 

 moveable frame was then adjuftcd to be perpendicular to the 

 rays coming from the prifm ; and the inclined planes carry- 

 ing the three thermometers, with their balls arranged ia a 

 line, were fet fo hear the opening, that any one of them 

 might eafily be advanced far enough to receive the irradi- 

 r^tlon of the colour which pafled through the opening, 

 while the reft remained clofe by, under the fhade of the' 

 paftcboard. 



By repeated trials, I found that Dr. Wihon's No. 2, and 

 mine, always agreed in (liowingthe temperature of the place 

 where I examined them, wlien the change was not very fud- 

 den ; but that mine would require tep minutes to take a 

 change, which the other woijld Hiow in five. No. 3 never 

 differed much from No. 2. 



jjl Experiment. Having arranged the thrcp thermometers 

 in the place prepared for the cxperimcntj I waited till they 

 were ftationary. Then, advancing No. i. to the red rays, 

 and leaving the other two clofe by, in the (liadc, I marked 

 ^lown what thev (liowed at different times. 



\<n.. \'II, ' S i >fp. 



