[ l8 5 ] 
eight-day clock, having a fecond-hand, an iron pen- 
dulum, and an adjufting ferew at its bob. 
Altitude inftruments of wood and brafs, adjufted 
by fpirit levels, had engaged my attention j but having 
found feveral of thefe defe&ive, by comparing their 
refults with meridian altitudes, and the time by the 
clock ; and having many times examined the clock by 
double altitudes of the fun, taken with a Hadley’s qua- 
drant, having a nonius to minutes, and an artificial ho- 
rizon of fweet oil in a tea-faucer, I determined to de- 
pend on fuch an inftrument and horizon, for afeer- 
taining the error of the clock, and corredtnefs of my 
meridian. 
An artificial horizon of water, and even of quick- 
filver, I had found to be too eafily difturbed, and 
therefore had, fome time before, introduced oil, and 
found it vaftly preferable. And in taking altitudes, 
I always obferve, when the fun, or other celeftial 
body, is as near the prime vertical, or eaft and 
weft azimuth, as poflible ; and generally take either 
five double altitudes, half a minute of time afunder 
each, or three double altitudes, a minute of time 
afunder, dividing the fum by either ten or fix, as the 
cafe is, for a mean fingle altitude, correfponding to 
the mean time of thofe obfervations by the clock. 
And in taking the fun’s tranfit acrofs the meridian, 
I take a mean of the times of appulfe to feveral par- 
allel and equidiftant lines on each fide of the meri- 
dian, and it generally gives the tranfit to lefs than a 
fecond of time. 
The daily tables of the fun’s declination, equation 
of time, &c. which I ufe, are thofe in the epheme- 
rides of the Abbe de la Caille; and the latitude of 
Vol. LII. B b m? 
