with an Equatorial Micrometer.' 321 
the di [Terence ot a degree^ either north or fouth of the objeft, 
in declination *. 
Confidering, however, that the approaching elongation 
would be in the morning ; and that the belt chance of 
feeing Mercury with this inftrument would be fome time in 
the twilight, between Mercury’s riling and the riling of the 
Sun ; yet, on fuppofition of catching the planet in his paflage 
over the wires, there would be no chance of feeing any ftar 
pals over the field-, wherewith to compare him, till the follow- 
ing evening, which being at leaft fourteen hours, the certain 
polition of the telefcope for fo great a length of time was 
almojl more than I could reafonably hope for. To judge how 
far I might form an expeflation, by way of t previous trial, 
I compared Saturn with y Capricorni f, and found the return 
V » 
* The field of the telefcope of Mr. Aubert’s inftrument is two degrees j 
but that of the original, wherewith this obfervation was made, is 1° ij': tq 
gain which, the eye-glafs being immoveable (and achromatic to prevent the 
indiftin&nefs that would othervvife have taken place near the border) the magni- 
fying power was obliged to be confiderably reduced, in refpeft of what has been 
ufual for micrometers, that is, fo as not to exceed 20 times: in confequence, 
there is, therefore, no need for fo long a telefcope, this being but 341- 
inches focal length of obje£t-glafs; but being a double achromatic, made by 
the late Mr. John Dollond, ft is capable of as great an aperture as could bs 
given to the fimple objedt-glafles of twelve or fifteen feet telefcopes, that were 
then generally given to micrometers ; but the pencil of light being greateft in 
this, is attended with this advantage, that the fmall ftars can be feen very diftinct 
and in great abundance, like the modern night-glafles : and there is in reality no 
need of great magnifying powers for the prefent purpofe; for the place of the 
wire being viewed by an eye-glafs, of about if inches focus, its place may be 
diftinguifticd to lefs than a 2300th part of an inch, which, on the radius of 34 f-. 
inches, is fcarce 2"f of a degree; and which, as I apprehend, is nearer the 
truth than can reafonably be expe&ed from inftruments out of the meridian. 
f According to this obfervation h preceded y y? the 2d of Sept, at 9 h. 15' 
P.M. mean time by 30' 9". 7 MT, and with greater declination fouth than y by 
4 *' 23". 
A a a 2 
of 
