192 Mr. Groombridge’s Observations 
1000. On the quantity of refraction to be assumed at 45 0 , 
astronomers not having been agreed, and Dr. Maskelyne 
having suggested in the precepts to his folio tables, that Dr. 
Bradley having supposed the sun’s parallax io-§-",from which 
he inferred the refraction at 45 0 to be 57" ; if he had used the 
true parallax 8£", lie would have found the refraction at the 
altitude of 45 0 to be 56^'. I therefore assumed, and have con- 
stantly applied that quantity, correcting the same for the ba- 
rometer and thermometer: the whole by the formula of 
Dr. Bradley. 
In the above table, the third column is the mean of the 
observed zenith distance corrected by the equations to 1st 
jan. 1807 ; the fourth is the mean of the computed refraction, 
for each observation ; the sixth is the sum of the third and 
fourth colums, which gives the mean double zenith distance 
of the Pole, according to the assumed refraction in the 
seventh column ; the eighth is the correction of the assumed 
refraction by the factor found ,02845, which is applied 
to the seventh column, and gives the true double zenith dis- 
tance of the Pole in the ninth, the half of which is the cor- 
rected co-latitude in the tenth column. It appears that so far as 
jj Ursae majoris, the sum or difference of the zenith distances in 
the seventh column are sufficiently uniform to be used for the 
correction of the refraction. I then proceed to compare the first 
thirteen stars, where the zenith distance below the Pole is less 
than 6o°, with the twenty-one following. From the former 
thirteen, the mean of the seventh column is 77 0 3' 53", 0908 ; 
and the mean sum of the refractions in the fifth column is 
94/9377 ; from the latter twenty-one the means of the same 
columns are 77 0 3' 5o/ / 5248 and i 85/ , 1357. The difference 
