of an Arc of the Meridian. 401 
the telescope be subsequently moved. Above ahh , is a small 
frame-work of brass, from which the plumb-line depends: it is 
attached to two of the four horizontal tubes on which the Y 
frames are fastened, cd is the pipe fastened to the end of the 
rod ; the end of this pipe has a screw, which passes through a 
nut, and acts against the end of the lever d, whose centre of 
motion is g, and whose other extremity is f, where there is a 
small piece of hard steel with a notch, for the reception of the 
plumb-line fh, suspended from^. Against the upper surface of 
the arm gd, a helical spring continually presses downwards ; it 
is fastened above the end of the lever, at e ; by which means, the 
arm gd is constantly pressed against the end of the pipe, ob- 
viating the possibility of any play or shake of the lever, round 
its centre g. 
The same figure contains an elevation of the frame-work just 
described, as seen by an eye in the plane of the diaphragm pro- 
duced. It is necessary that it should be closely inspected, for 
the purpose of obtaining an adequate idea of its construction. 
In this figure, xv is a small cylinder, with a screw and loose 
collar at the end v, for fastening the plumb-line, which goes 
over the notch n, and passes through a hole in the upper part 
of the axis at f, and out again at h, almost touching the mother- 
of-pearl at p. sru is a strong spring, fixed at s, through the 
middle of which, at r, passes a screw, which is, in fact, an ad- 
justing screw, for bringing the plumb-line close to the dot on 
the surface of the diaphragm ; and here it is necessary to ob- 
serve, that the plane of the divided arch and that of this dia- 
phragm, are one and the same when produced. There is no 
part of the instrument more complete than the apparatus for 
suspending the plumb-line, and that which regards the dot, I 
