54 
Mr. Pond on the parallax of a Lyra. 
On the difference of parallax between <y Draconis and a Lyra. 
It is impossible to conceive a more simple process than that 
of determining with the mural circle the difference of polar 
distance between these stars. From their proximity in right 
ascension, the operation is the same as that of measuring the 
angular distance of two terrestrial objects, about 12 0 asunder, 
with a theodolite surrounded by six microscopes : for the 
mural circle, in principle, exactly resembles a vertical theodo- 
lite ; with this difference, that its microscopes, instead of being 
placed on a frame-work of brass, are securely fixed on a 
stone pier. Now I find that the angular distance thus mea- 
sured in winter does not differ one-tenth of a second from 
the same angular distance measured in summer ; and there- 
fore, that the difference of parallax between the two stars is 
absolutely a quantity too small to be measured. In this in- 
vestigation, it is to be considered that any constant error in 
the determination of the absolute polar distances has nothing 
to do with the question, it being the difference only of those 
distances at opposite seasons that is required. To render all 
errors throughout the whole course of observation as constant 
as possible, the telescope remained fixed to the same part of 
the limb of the instrument, and the utmost pains were taken 
to reduce the temperature in the Observatory to that of the 
outer air ; the difference throughout the year not exceeding 
one degree. The winter of 1821-1822 was extremely favour- 
able for astronomical observation ; there were an unusual 
number of fine nights, and the weather was so mild and uni- 
form, that we were enabled to equalize the temperature, so as 
to make it of no importance whether the observations were 
