1 8o Dr. Maskelyne’s Obfervations on the Latitude * 
1 fhall now dole my enquiry into the latitudes of Green- 
wich and Paris, and Dr. Bradley’s and the Abbe de la 
Caille’s refractions, by a remark naturally arifing from my 
comparifon of, and endeavours to reconcile, their obfervations, 
which I defire to fubmit to the confideration of aftronomers, it 
not having, that I know of, been made before ; that a table of 
refractions fhould be made for every vertical inftrument from 
obfervations made with itfelf turned alternately north and 
fouth ; and that the table, fo made, applied to obfervations 
made with it, will give the true zenith diftances, whether the 
total arc of the inftrument be accurately juft, or affefted with 
a fmall error, or however unequally it be divided below the 
pole, provided the divifions are equal between themfelves in 
the part of the inftrument lying between the equator, the 
zenith, and the pole. 
It remains to give fome account of the longitude of Green- 
wich, or rather of the difference of meridians of Greenwich 
and Paris, in reply to the late M. Cassini’s doubts on the fub- 
je£t. This had been fettled by Dr. Bradley at f 20", as ho 
informed me himfelf, and that he had deduced it from eclipies 
of Jupiter’s firft fatellite obferved at both places, and that he 
had found it come out the fame both from the immerfions and 
emerftons. This quantity had been inferted in the table of 
latitudes and longitudes of places, prefixed to Dr. Halley’s 
tables, on the authority of Dr. Bradley, fo long ago as the 
year 1749, the date of the publication of thofe tables, and 
was generally admitted by aftronomers till the year 1763, 
when the late Mr. James Short, F. R. S. computed it from 
the four tranfits of Mercury over the fun in 1723, 1736, 1743, 
and 1753, obferved at Paris, London, and Greenwich, to be 
9' 1 6 // , See Philofophical Tranfadions, Vol.LIII. p. 158. In 
5 the 
