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. Astronomer Royal, on the Annual Parallax of a Lyrae

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 α Lyrae. 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 α 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 \[ \text{Refraction} = 56,"9 \times \text{tang.} \left\{ \frac{\text{Zen. dis.} - 3.2 \text{ Refr.}}{29.6} \right\} \times \frac{500}{450 + \text{ther.}} \] By means of this formula, the observations of circumpolar stars considerably distant, give the same co-latitude to a great de- gree of exactness.