gg6 An Account of the Measurement 
parallel to the meridional one, and at equal distances from it ; 
they were placed there with a view of rendering an adjustment 
of the horizontal wire sufficiently easy. These are illuminated 
by means of the lamp which carries the concave reflector be- 
fore spoken of. There is a hole, with a lens, in the side of the 
telescope, directly opposite to the lamp, having behind it a dia- 
phragm of brass, coated with plaister of Paris, and inclined to 
the vertical axis of the tube, at an angle of 45 degrees. The 
quantity of light, suited to the circumstances of the observation, 
is regulated by coloured glasses, placed over the hole in the side 
of the tube. 
The plummet, suspended at the wire, falls into a cylindrical 
cup, swinging by two pins on its edge, on the extremity of a 
brass frame annexed to the interior stand ; which frame is ca- 
pable of being raised or lowered at pleasure by a milled-headed 
screw; so that the wire can, at any time, be released from the 
weight of the plummet, by screwing up the vessel containing it. 
There are two arches attached to the end of the tube, one on 
each side of it, and firmly united together by means of brass 
pillars ; which arrangement effectually secures the divided arch 
from injury. The total extent of the arch is about 15 0 , having 
half of its subtense on each side zero. It is divided into every 5 
minutes; the micrometer-screw' measuring any supplementary 
quantity. Golden pins were let into the arch, by the advice of 
the Astronomer Royal, on which the divisions were laid off by 
Mr. Berge, in a very masterly and accurate manner, as will be 
seen hereafter. A magnifier, whose focal distance is about half 
an inch, is placed under the bottom of the cross piece opposite 
to the arch, and is furnished with a horizontal adjustment for 
bringing it directly over the plumb-line. 
