and Longitude of the Royal Obfervatory at Greenwich. 167 
from the apparent zenith diftance of the equator obferved with 
the iron quadrant compared with the apparent zenith diftance 
of the pole obferved with the brafs quadrant, according to the 
extract from the manufcript in my pofleffion before cited. 
The mean apparent zenith distance of the equator, by my 
oblervations of fix years from 1 765 to 1771, was related before 
51 0 zf 29", 8. The mean apparent zenith diftance of the 
pole was found by Dr. Bradley, from 1750 to 1752 to be 
38° 30 7 35 "• Their fum 89° 58' 4", 8 taken from 90° leaves 
1' 55^2, the fum of the refradtions at the two zenith diftances.. 
Saying then, as 1/ 5 6 7/ , 7 the fum of the refractions by Dr.. 
Bradley’s rule, to i 7 5 5 // , 2 the fum by obfervation, fo are 
1 7 ii / 7 ,4 and 45 7/ ,3 the refpeCtive refraCtions at the two appa- 
rent zenith diftances of the equator and pole by Dr. Brad- 
ley’s rule, to i 7 io /7 ,5 and 44", 7 the two refractions at thofe 
zenith diftances, which added to them give the co-latitude 
38® 3 1 7 i9 /7 ,7, and’ the latitude 51 0 z 3 7 4c/ 7 , 3. And as> 
i 7 5 ^' ’7 • l ' 55 "$' :: is- 57 " refraCtion at the apparent 
zenith difiance 45 0 f by Dr. Bradley : 56^27 the true re- 
fraCtion at that zenith difiance, or not half a fecotid differing 
fiom Dr, Bradley’s, but more to be depended on as deduced 
from obfervations made with the brafs quadrant only, and cal- 
culated from a parallax of the fun nearer to the truth. 
But if the apparent zenith diftance of the pole be made ufe 
of, refulting from a mean of 310 obfervations made with the 
brafs quadrant, according to Dr. Bradley’s manufcript, com- 
municated, by Dr. Hornsby, from the whole of his obferva- 
tions from 1750 to 1 753, viz. 38° 30 7 3b 77 , the fum of this 
and 51 0 zf 29 7/ ,8, the apparent zenith diftance of the equator- 
found by myfelf with the fame infirument, or 89° 58 ' 5' 7 8. 
taken, from 90° leaves V 54 // J 2, the fum of the two refractions-. 
at 
