C 20 4 1 
XI. Extract of a Letter from the Rev. John Brinkley, D. D. 
F. R. S. Andrew’s Professor of Astronomy in the University of 
'Dublin , to the Rev. Nevil Maskelyne, D. D. F. R. S. Astro- 
nomer Royal, on the annual Parallax of a. Lyrce. 
Read April 12, 1810. 
I have now had sufficient experience of my eight feet circle, 
to be highly satisfied with it, and have arrived at one con- 
clusion, that it is of importance in astronomy. 
My observations on a. Lyrae for the purpose of discovering 
an annual parallax now amount to 47 in number, viz. 22 near 
opposition, and 25 near conjunction, and the mean of these 
gives a result of 2,^52 as the parallax of the annual orbit for 
that star, and I have no doubt that it exceeds 2". 
My observations of different circumpolar stars, and of the 
same star in different states of the thermometer, seem to re- 
quire a small alteration in the numbers of Dr. Bradley’s 
formula for refraction. 
The formula so altered is 
Refraction ~ $6, "9 x tang. I Zen. dis. *— 3,2 Refr. | x- "' sh " ? f ^ arotn, x 5°° 
L j 29,6 450-fther. 
By means of this formula, the observations of circumpolar stars 
considerably distant, give the same co-latitude to a great de- 
gree of exactness. 
