Mr. Pond on the parallax of a. Lyra. 55 
computed by the outer or inner thermometer ; and it is to 
this circumstance, in a great measure, that I attribute the 
perfect coincidence between the observations at different 
seasons. 
It has been objected, however, that perhaps some unex- 
pected effect of temperature deranges the instrument by the 
exact quantity of the difference of parallax attributed to these 
stars by Dr. Brinkley ; if we suppose a derangement from 
temperature so considerable as to give a sensible error, 
even after being diminished by the effect of six microscopes, 
we should expect the error to he much greater when the 
experiment is tried with two microscopes only ; for to sup- 
pose the contrary, would be to deny the tendency of six 
microscopes to correct the errors of two. Now I find the same 
difference of polar distance whether I employ two micro- 
scopes or six ; temperature, therefore, cannot materially have 
vitiated the results by causing derangement in the form of 
the instrument. 
In the whole of the above process I do not see one objec- 
tionable point, and if called upon to invent an instrument for 
this particular experiment, I could not devise one more per- 
fect in principle than the mural circle. 
Whoever will compare the above simple process with the 
more complicated one necessarily employed in using an in- 
strument with two microscopes, turning freely in azimuth, 
will not hesitate, I think, in deciding upon which of the two 
instruments temperature is likely to produce the greatest 
error. 
On the absolute parallax of a. Lyra. 
The preceding observations only indicate that y Draconis 
