Dr. Brinkley on the 
278 
Greenwich circle is subject to a similar cause of error (sup- 
posing the discordance should not arise from parallax), and 
that this cause has been diminished, if not entirely done 
away, by reducing the internal air to the same temperature 
as the external. 
However, from all the consideration that I have been 
enabled to give the subject, I am led to entertain doubts of 
the fitness of an instrument similar to the Greenwich mural 
circle, for this delicate enquiry. I do not allude to the objec- 
tion stated by Mr. Pond, since, as he justly observes, that is 
obviated by keeping the telescope fixed to the same place on 
the circle during a period of observations, as v* as the case in 
the observations of 1813, and as to those mentioned in the 
Appendix. And in respect to Mr. Pond's paper, and its 
Appendix, as given in the first part of the Transactions for 
1817, it appears to me doubtful, whether the results, if they 
could be exactly obtained (that is, if the elements from which 
they are deduced were exact), may not be such as to furnish 
a discordance explained by a parallax nearly equal to mine, 
or whether the results might not be entirely against parallax. 
My reasons for entertaining these doubts, will appear in the 
following remarks respecting the elements used in computing 
the index error, in instruments similar to the Greenwich 
mural circle. 
The polar distance of a star, as observed by a mural circle, 
requires, besides the corrections for refraction, aberration, 
annual variation, &c. also the application of the index error. 
This index error is determined by the mean of results 
deduced from observations of stars of the standard catalogue. 
