2 3 1 
over the Disk of the Sun, &c. 
of their focal length ; and also prove, that the figure of the 
reflecting surface is considerably injured, during the time that 
such a change takes place. . We are consequently authorised to 
believe, that the small alteration in the focus of a mirror exposed 
to the rays of the sun, arises from the same cause. For, since a 
thermometer, when the sun is shining upon it, will show that 
its temperature is altered, the action of the solar rays upon a 
mirror must be attended with a similar effect in its temperature. 
See obs. 47, 48, 49, and 50. 
The same experiments will now also explain why the obser- 
vations of the sun, related in our transit of Mercury, between 
io h 32' and n h 28', were not attended with success; for we 
have seen that heat occasions a derangement in the action of the 
reflecting surface ; and it follows that, under such circumstances, 
high magnifying powders cannot be expected to show objects 
very distinctly. 
If it should be remarked, that I have not explained why the 
focus of a glass mirror should shorten by the same rays of the 
sun which lengthen that of a metalline speculum, I confess that 
this at present does not appear ; and, as it is not material to our 
purpose, I might pass it over in silence. We are however pretty 
well assured, that the alterations of the focal length must be 
owing to a dilatation of the glass or metal of which mirrors are 
made, and must be greatest where most heat is applied. Our 
experiments therefore cannot agree perfectly with solar obser- 
vations; for, in the glass mirror, the application of partial heat 
in front, must undoubtedly have been much stronger about the 
middle of the mirror (though the centre of it was sometimes 
guarded by a brass plate equal to the size of the small speculum) 
than at the circumference. But when, on the contrary* a mirror 
