and Longitude of the Royal Obfervatory at Greenwich. 1 7 1 
1755, p. 578, 579, where M. de la Caille takes the dif- 
ferences of zenith diftances of 14 ftars obferved by Dr. Brad- 
ley (in correfpondence to the fame obferved by himfelf at 
the Cape of Good Hope, for determining the moon’s parallax in 
declination) publifhed in the Memoires of tlie Royal Academy 
of Sciences for 1752, and the fame obferved by himfelf at 
Paris, after his return from the Cape, and correcting them for the 
difference of the refractions at the refpective zenith diltances, 
according to his own table of refractions, and the known apparent 
motions of the (tars, finds the mean 2 0 37' 23^,9, which added 
to 48° 51' 29^,3, his latitude at the College of Mazarine, gave 
him 51 0 28" 5 3 ",2 for the latitude of Greenwich, exceeding 
Dr. Bradley’s latitude by 13 or i4 /7 . 
Now the legitimacy of this conclufion depends upon a fup- 
pofition that both inftruments meafured the true angle, or that 
their total arcs were juftly laid off, and that the Abbe de la 
Caille’s table of refractions is juft. The firft indeed has been 
proved with refpeCt to Dr. Bradley’s quadrant, but never has 
been attempted with refpeCt to the Abbe de la Caille’s 
fextant; for the examination which the Abbe made of his in- 
ftrument by parts for every y°l (fee Memoires of the Royal 
Academy of Sciences for 1751 p. 405.), could not determine 
the error of the whole arc, as the difference from the truth 
might be infenfible upon fuch fmall arcs, and the examination 
feems to have been intended to find the differences of thefe 
fmall arcs from one another rather than from the true arc 
which they reprefent. We may therefore be allowed to doubt 
of the truth of this circumftance. This doubt will be further 
ftrengthened by feveral particulars which I ftiall adduce. 
1. The apparent altitude of the pole at the Royal Obferva- 
tory 48° 51' 12", refulting from the Abbe de la Caille’s 
Z 2 obfer- 
