400 An Account of the Measurement 
itself, to which part the hand is applied. In this section, the 
top v is not furnished with the pipe connecting it with the bent 
lever above ; but the representation of it, together with the lever 
itself, and accompanying spring, will be understood by referring 
to the plate which contains a representation of those parts. 
In Plate XII. is a section of the axis passing through the 
pivots, and one exhibiting the face of the several united planes 
constituting the diaphragm, which adjust, in every direction, 
the slice of mother-of-pearl. Above this latter, is a view of the 
lever which gives motion to the plumb-line, with the pipe, 
spring, &c. ; these are represented as seen from an eye at one 
of the pivots ; and, above the other section, is a view of the same 
apparatus, seen in a direction at right angles to the former one. 
In the latter of the above-mentioned sections, hah is the 
diaphragm, having in the middle, as at p , a circular piece of 
mother-of-pearl, extremely thin, with a small dot in the centre. 
This brass work is annexed to the large end of a hollow conical 
piece of brass, which exactly fits the axis at its proper place. 
It is there screwed fast, and may be considered as of one piece 
with the axis. In the adjoining figure, hfkl , is a section of the 
cone, p being the place in the opening of the brass work which 
receives the mother-of-pearl. In the representation of the dia- 
phragm, a and b are two screws, at right angles to each other, 
and respectively attached to flat pieces of metal which slide 
on each other. If fh represent the plumb-line, the direction of 
each screw is that of an angle of 45 degrees with it. By means 
of these adjusting screws, the dot or little circle at^> was placed 
in its proper position, or in the centre of the circle ahb ; so that, 
on an adjustment of the plumb-line, in any one position of the 
instrument, the dot still remains accurately bisected, however 
