479 
the parallax of a Lyra. 
arc, and the change may take place to a much greater extent 
between winter and summer. 
Mr. Pond mentions the precautions he took to avoid errors 
from the effects of unequal temperature. That the utmost 
pains were taken to reduce the temperature of the Observa- 
tory to that of the open air, the difference throughout the 
year not exceeding one degree. This latter part is not quite 
plain. It can scarcely be meant that there was never through- 
out the year a greater difference between the internal and 
external thermometers than one degree. It appears nearly 
impossible that this would generally take place on clear 
nights, after sun-set, from the beginning of July to the be- 
ginning of October, when these stars pass the meridian. 
Mr. Pond, indeed, expressly mentions, that the weather 
was so mild and uniform on the winter nights of 1822-23, 
that he was enabled to reduce the external and internal tem- 
peratures to the greatest uniformity. But this has nothing 
to do with the observations in question. In winter, y Dra- 
conis and a, Lyras pass in the middle of the day ; and then, 
except in rare cases of extreme cold, here, and also at Green- 
wich, as will be seen by a reference to the observations, the 
internal and external temperatures are generally nearly the 
same. 
In the last paragraph of this part of Mr. Pond's paper, in 
alluding to my instrument, he seems to consider it as only 
having two microscopes instead of three, which is a differ- 
ence of great importance. 
