of the Equatorial Instrument . So 
great wheel moves on friction wheels ; and I believe every 
care has been taken, that the experience of one of the first 
artists of this kingdom * could suggest, to render it a com- 
plete piece of mechanism. It goes five months with once 
winding up ; and, from the experience I have hitherto had, 
does not seem to vary its rate between winter and summer 
more than equal to three seconds per day, sidereal time. 
(19.) All, I believe, that remains for me to mention, in 
this part of the account, is the meridian mark. (See Tab. 
XIII.) A,B,C,D,E, is a solid brick pier, erected at the dis- 
tance of 2970 feec from the centre of the observatory ; 
8 feet high, 9 feet wide at bottom from A to E, and 4 feet 
wide at top from B to C ; 1 foot 6 inches thick at top from 
C to D, and 2 feet 3 inches at bottom. F is an iron box, 
8 inches square, and 1 foot high, exclusive of the top, or 
chimney ; within this box is placed one of Argand's patent 
lamps, which shines through a circular aperture, of about 
inch, in the front of the box, and exhibits the appearance 
of a fixed star to the naked eye by night ; but the aperture 
being covered by a semitransparent piece of glass, ground 
rough on one side, transmits a steady uniform light, that 
through the telescope resembles the disc of a small planet 7"^ 
in diameter. By making the wire in the telescope, which is 
only in diameter, bisect this circular light, the in- 
strument may be very nicely adjusted to the meridian, indeed 
with so great precision, that I think an error of one second 
cannot be committed. Around this glass aperture, which by 
clay appears perfectly black, is painted a circle of black, (on a 
ground of white) whose external diameter is 3 inches, and 
# Mr. John Arnold. 
N 
MDCCXCIII. 
