20 TRANSIT OF MERCURY. 



4rth Experiment. 



Experiments on When the fmall heated globe of the 2d experiment was 



focaMen?h°in ^"^pended in front of the mirror, the focus lengthened ,27 inch 



mirrors by heat, in one minute; nor would the lengthening increafe by leaving 



the hot iron longer in its poiition. The foci in this, as well as 



in the 3d experiment, were fo much injured that they could 



not be meafured with any precition ; and it was evident, that 



high magnifying powers ought not to be uled with a mirror of 



which the temperature is undergoing a continual ctiange. 



1 repeated the experiment with the iron nearly red hot; and 

 found the focus lengthened 1,1-8 inch in 30 feconds. Five 

 minutes after the removal of the iron, the regularity of the 

 figure of the mirror was pretty well reftored. 



With a moderate heat, I had, in 30 feconds, a lengthening 

 of the focus, of ,57 inch; and, in about li minule after the 

 removal of the heated iron, diftinft vition was nearly redorecl. 



Thefe four experiments (how, that a change in the tempera- 

 ture of mirrors, occationed by heat, is attended with an altera- 

 tion of their focal length ; and alfo prove, that the figure of the 

 ' refleding furface is contiderably injured, during the time that 



fuch a change takes place. We areconfequently authorifed to 

 believe, that the fmall alteration in the focus of a mirror ex- 

 pofed to the rays of the fun, arifes from the fame caufe. For, 

 fince a thermometer, when the fun is fliining upon it, will fliow 

 that its temperature is altered, the adtion of the folar rays upon 

 a mirror niuft be attended with a iimiiar effect in its tempera- 

 ture. See obtervation 47, 48, 49, and 50. 



The fame experiments will now alfo explain why the obfer- 

 vations of the fun, related in our tranfit of Mercury, between 

 lO** 32' and ll'' 28', were not attended with fuccefs; for we 

 have feen that heat occafions a derangement in the a6tion of 

 the reflecting furface ; and it follows that, under fuch circum- 

 ftances, high magnifying powers cannot be expeded to (liow 

 objeds very difiindly. 



If it fliould be remarked, that I have not explained why the 

 focus of a glafs mirror fliould fliorten by the fame rays of the 

 fun which lengthen diat of a metalline fpeculum, I confefs that 

 this at prefent does not appear; and, as it is not material to 

 our purpole, I might pafs it over in filence. We are however 

 2 pretty 



