22 5 
over the Disk of the Sun , &c. 
tenths of a second ; but was perhaps less than one. This leads 
to the discovery of one of the causes of the apparent magnitude 
of the fixt stars. 
Focal Length. 
(47.) Nov. 14, 1801 . The focal length of my 10-feet mirror 
increases by the heat of the sun. I have often observed this 
before ; the difference, by several trials, amounts to 8 hundredths 
of an inch. 
(48.) Dec. 13, 1801. The focal length of my 10-feet mirror, 
while I was looking at the sun, became shorter, contrary to 
what it used to do ; but, there being a strong frost, I guess that 
the object metal grows colder, notwithstanding its exposure to 
the sun's rays. 
(49.) Nov. 9, 1802. io h 50'. The focus of my 7-feet glass 
mirror became 18 hundredths of an inch shorter, on being ex- 
posed for about a minute to the sun. The figure of the speculum 
was also distorted ; the foci of the inside and outside rays dif- 
fering considerably, though its curvature, by observations on the 
stars, has been ascertained to be strictly parabolical. 
i2 h o'. The same mirror, exposed one minute to the action 
of the sun, became 21 hundredths shorter in focal length. 
The focus of a 10-feet metalline mirror, when exposed one 
minute to the sun's rays, became 15 hundredths of an inch 
longer than it was before. 
(50.) January 9, 1803. When I looked with the glass 7- 
feet mirror, several times, a minute or two at the sun, it shortened 
generally ,24, ,2 6, and ,30 of an inch, in focal length. 
The observations which are now before us, appear to be suffi- 
cient to establish the following principle ; namely, 
MDCCCIII. G g 
