over tbe Disk of the Sun, &c. 229 
became instantly illegible. In 15 seconds, the focus of the mirror 
was shortened 2,3 inches ; in half a minute, 3,47 inches ; and, at 
the end of the minute, I found it no less than 4,59 inches 
shorter than it had been before the application of the hot iron. 
On repeating the experiment, but placing the heated iron no 
more than f of an inch from the back of the mirror, its focal 
length, in 1^- minute, became 5,33 inches shorter. 
I tried also a more moderate heat ; and, placing the iron at 
3 inches from the back, the focus of the mirror shortened in 
one minute 2,83 inches. 
A thermometer placed in contact with the reflecting surface 
of the mirror, could hardly be perceived to have risen, during 
the time in which the hot iron produced the alteration of the 
focal length. 
2 d Experiment. 
Every thing remaining as before, I suspended a small globe 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 also deranged ; so that the 
letters on the card could not be distinguished. 
I made a second trial, with the suspended iron a little more 
heated, and brought it as near the surface of the mirror as I 
judged it to be safe; since a contact would probably have 
cracked the mirror. In consequence of this arrangement, the 
locus lengthened, in one minute, 1,64 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 second minute seemed to be nearly restored. But 
the disagreement of the foci of the different parts of the re- 
flecting surface might be perceived for a long time afterwards, 
