170 Dr. Maskelyne’s Obfervations on the Latitude 
from what has been faid of the obfervations on which the re* 
fpeCtive latitudes were founded, cannot he fuppofed to differ 
above 3 or 4" from the truth. What then becomes of the 
uncertainty of 1 fuppofed by the late M. Cassini ? 
The fame difference of latitude I find nearly from a compa- 
rifon of my own obfervations of y and /3 Draconis, taken with 
the zenith fe&or in 1768, with thofe of the Abbe de la 
Caille in 1750 and 1756, given in his Fund ament a Aftro- 
nomine, after making the proper allowances for aberration, pre- 
cellion, and nutation, and correcting my obfervations by Dr. 
Bradley’s refraction, and the Abbe de la Caille’s by his 
table, and making allowance for the diftance of the Abbe de 
la Caille’s Obfervatory from their Royal Obfervatory; viz. 
2 0 38' 25", 4 from y Draconis, and 2 0 3s 7 26", 1 from /3 Dra- 
conis ; the mean being 2 0 38' 2 5", 7, differing only 0^,3 from 
that Rated above; but from Dr. Bradley’s obfervations 
2 0 38' 24^9, and 2 0 38' 27", 2, mean 2 0 38' 26"' o. It 
is too well known to aftronomers to need my pointing out, 
that the beft method of determining the difference of lati- 
tude of places, differing but little in latitude, is by fuch 
differences of zenith diffances of ftars paffing near the zeniths, 
as the two above cited, obferved at both places, in the fame 
manner as the amplitude of the celeftial arc is obferved for 
finding the length of a degree of the meridian by comparifon 
with geometrical meafures. 
The queftion now will be, upon what foundation was the 
late M. Cassini’s fuppofition of an uncertainty of 15" in the 
latitude of Greenwich built ? This appears evidently to have 
been upon a paffage in the Abbe de la Caille’s refearches 
into the agronomical refractions and latitude of Paris, con- 
tained in the Memoires of the Royal Academy of Sciences for 
l 7 S 5 > 
