92 Mr. Cecit on a Machine for Grinding 
jerking. But it is necessary continually to take up and examine 
the mirror during the polishing, and to wipe off whatever is found 
adhering to its surface. 
The centre of the mirror is confined to move in a given hori- 
zontal line, or small circular are, by a lever (k, J, m), having at 
one extremity (k) the same centre of motion as the triangle, the 
other extremity (m) resting, by an iron point, upon a brass socket 
in the centre of the box containing the mirror. Any degree of 
pressure may be produced on the centre of the mirror, without 
the inertia of a moving weight, by extending a string from any 
point (2) of the lever (k, J, m), to the extremity (nm) of the lever 
(n, 0), moveable about the point (0), and bearing a weight (W). 
When it is wanted to remove or examine the mirror, the lever 
(k, l, m), which sustains the weight (W) may be raised by the 
point (m) and laid on one side; the mirror may then be taken 
up, or turned upon its back: the string remains in the groove as 
before, and is kept stretched by the moveable weight (W’). An 
alteration has been suggested with respect to this lever which may 
seem an improvement, though in practice it is less expeditious 
for exchanging the mirrer. The extremity (m) of the lever (k, m), 
is attached, by a joint, toa small iron upright, fixed into the middle 
of the iron bar (g,h). The other extremity (k) is left at liberty, 
and bears a small weight: which presses, at a mechanical advan- 
tage, on the centre of the mirror. This weight, being near the 
centre of motion remains nearly stationary while the mirror is 
moved horizontally, and therefore takes effect by pressure without 
inertia. The vertical pressure, in the former case, does not any 
way interfere with the triangular frame; but on this latter con- 
struction, there will be some increase of friction at the holes (g’) 
and (hk), by a constant pressure upwards. 
The rotatory motion is communicated in the following manner. 
The point (p) of the triangular frame is connected, by the cross bar 
