472 
Dr. Brinkley's remarks on 
From the facts that I shall produce, I think a more correct 
estimate may be formed of the relative merits of the Dublin 
and Greenwich circles. 
The subject divides itself into two parts. 
1. On the difference of parallax between 7 Draconis and 
« Lyras. 
2. On the absolute parallax of a. Lyrse. 
I shall remark on each separately. 
Mr. Pond observes “ It is impossible to conceive a more 
44 simple process that that of determining with the mural 
44 circle the difference of polar distance between these stars 
41 (7 Draconis and a Lyrse). From their proximity in right 
44 ascension, the operation is the same as that of measuring 
44 the angular distance of two terrestrial objects about 12 0 
44 asunder, with a theodolite surrounded by six microscopes : 
“ for the mural circle, in principle, exactly resembles a ver- 
14 tical theodolite ; with this difference, that its microscopes, 
44 instead of being placed on a frame-work of brass, are se- 
44 curely fixed on a stone pier. Now I find that the angular 
44 distance thus measured in winter, does not differ one-tenth 
44 of a second from the same angular distance measured in 
44 summer ; and therefore,, that the difference of parallax 
04 between the two stars is absolutely a quantity too small 
44 to be measured." 
With this passage I shall also take the following from the 
Philosophical Transactions, 1817, Part I, page 166. 44 — — 
44 a Lyras and y Draconis have been observed together for 
44 five successive years. Above three hundred observations 
44 of each star have been made in opposition , and as many in 
44 conjunction , and I find the difference of parallax, from the 
