of an Arc of the Meridian. 387 
and has also diminished, in a very considerable degree, the 
errors unavoidably resulting from their imperfect construction. 
The principles on which he has founded the several improve- 
ments, consist in the means of uniting the sectorial tube to its 
axis, so as to ensure the permanency of the length of its radius, 
when erected for observation ; more accurate methods of ad- 
justing the instrument vertically ; and an easy way of placing 
the face of its arch in the plane of the meridian. Another circum- 
stance of moment was, some contrivance by which the plumb- 
line should be brought precisely over the point, marking the centre 
of the circle of which the divided arch of the sector should be a 
part. The last desideratum, the ingenious artist procured, by 
applying the same contrivance which so eminently displayed 
his skill, in the construction of the quadrant belonging to his 
Grace the Duke of Marlborough ; a contrivance by which the 
plumb-line can be as readily adjusted over the required point, by 
a person standing on the ground, as any adjustment, or other 
act within his reach, can be performed. A description of this, 
as well as of the means by which the instrument is rendered 
vertical,- and otherwise correctly prepared for observation, will 
be given, with the assistance of plates. . 
Plate IX. Exhibits a general view of the sector erected for 
observation : it consists of two parts ; 1st, the frame which sup- 
ports the apparatus to which the sectorial tube is attached ; 2d, 
the work constituting that apparatus, with the tube itself. 
The external frame or stand is made of mahogany, and unites 
strength with simplicity of construction. In shape, it is an ob- 
truncated pyramid, whose base is a square of six feet in length, 
and whose vertex is half of it. This frame, although light in its 
make, is yet, when united by means of square-headed screws. 
