t 95 ] 
tlie plane at different diftances from the zenith. By 
diftributing thefe errors in the beft manner I could, 
I am of opinion, that the error of the plane of his in- 
ffrument may be fuppofed to decreafe uniformly at 
the rate of half a fecond in time for every degree of 
zenith diftance from 28° to 6o°, the error being ^ rr 
at the former, and 23 // at the latter, by which quan- 
tity ftars paffed the horary wire, in his inftrument, before 
they came to the true meridian. It fhould feem alfo 
that the error continued nearly the fame from 60 to 
75 degrees, being at the latter only 22": but that 
it decreafed irregularly from 75 0 to 85°, viz. 1" in 
time for each degree from 75 0 to 8o°, and 0^,4 for 
each degree from 8o° to 85 degrees. The mural arc 
was fixed upon a ftone pier, the fouthern part of 
which was found to fettle yearly, from whence the 
error of the line of collimation to the fouth neceffa- 
rily became every fucceflive year greater and greater. 
As Mr. Flamfieed feems not to have had any method 
of adjufting his inftrument by a plumb-line, thefe er- 
rors muff have been irregular at different feafons of 
the fame year, and were perhaps never truly deter- 
mined. But as the obfervations here referred to were 
made on the fame day, and within the compafs of 
an hour, they are probably not affedted with this lat- 
ter error. We are at prefent concerned with the dif- 
ference of two zenith diftances, and -not with the ab-. 
folute quantity of thofe zenith diftances. The con- 
clufions may indeed be affedted with an error in the 
divifions 3 and from the examination which I 
have been able to make, I am of opinion that the 
arc of Mr. Flamfteed’s inftrument was not of the 
proper quantity 5 and that, though the obfervations 
generally 
