TRANSIT OF MERCURY, 19 



On repeating the experiment, but placing the heated iron no Experiments oa 



naore than | of an inch from the back of the mirror, its focal the change of 



length, in l| minute, became 5,33 inches fliorter. ^°^^"^"fv\?at 



mirrors ^j iiv«t« 



I tried alfo a more moderate heat ; and, placing the iron at 

 3 inches from the back, the focus of the mirror fliortened in 

 one minute 2,83 inches. 



A thermometer placed in con tad with the refleding furface 

 of the mirror, could hardly be perceived to have rifen, during 

 the time in which the hot iron produced the alteration of the 

 focal length. 



2d Experiment. 



Every thing remaining as before, I fufpended a fmall gtobe 

 of heated iron in front of the mirror, at one inch and a half 

 from its vertex ; and, in two minutes, the focus was lengthened 

 5,3 inches. The figure of the mirror was alfo deranged j fo 

 that the letters on the card could not be diflinguiflied. 



I made a fecond trial, with the fufpended iron a little more 

 heated, and brought it as near the furface of the mirror as I 

 judged it to be fafe ; fince a conta6t would probably have 

 cracked the mirror. In confequence of this arrangement, the 

 focus lengthened, in one minute, ] ,64 inch. 



On removing the heated iron, the mirror returned, in one 

 minute, to within ,18 inch of its former focal length ; and, at 

 the end of the fecond minute feemed to be nearly reflored. But 

 the difagreement of the foci of the different parts of the re- 

 fleding furface might be perceived for a long time after wards, 

 and caufed an indiftindnefs of vifion, which plainly indicated 

 that, under fuch circumftances, the magnifying power of the 

 telefcope, 225, was more than it ought to be, in order to fee 

 well. 



3d Experiment, 



I now changed the glafs mirror for a metalline one; and, on 

 placing the heater near the back of it, the focus of the fpecu- 

 lura, in 30 feconds, became ,77 inch fliorter. But, conti- 

 nuing the obfervation, inftead of fliortening ftill farther in the 

 next 30 feconds, it became ,3 inch longer, fo that, at the end 

 of a minute, it was only ,47 fliorter than before the approach 

 of the hot iron. 



C 2 4thExps^ 



