478 Mr. Pond on the parallax of ot, Aquila . 
sun rise, and be equally invisible. It is seldom that a star 
can be found within these very narrow limits ; but relatively 
to a Aquilae, it fortunately happens that l Pegasi is so situated 
as to be very advantageously employed. 
Not being yet perfectly satisfied with respect to the 
stability of the instrument, I have only computed those 
observations in which each star has been observed on the 
same day. In so short an interval as three hours, I cannot 
conceive any sensible change of position in the telescope can 
possibly affect the observations. Notwithstanding this pre- 
caution, they are far inferior in exactness to those of a. Cygni. 
I can only attribute this to the effect of accidental refraction. 
From the table * which accompanies this paper, it appears 
that 54 observations from the 25th July to 29th December, 
1817, divided into four equal series, give the following 
results. 
Difference in declination of 2 Aquilae and l Pegasi. 
July 25, to Aug. 25, - - i'4 9 "A 7 
Aug. 25, to Sept. 8, - - 1 49 ,20 
Sept. 8, to Nov. 1, - - - 1 49 ,61 
Nov. 1, to Dec. 29, - » 1 48 ,57 
If the first half be compared with the second half, the 
result will be, 
July 25, to Sept. 8, - - - i'49",i8 
Sept. 8, to Dec. 29, - - 1 49 ,0 Q 
I can discern no appearance of parallax in the above 
observations ; and indeed I have long considered it as a 
hopeless task to establish its existence by observations on a 
star so far from the zenith. 
* Vide page 480. 
