88 Mr. Cecit on a Machine for Grinding 
about twelve hours. The mould is formed of two iron plates of 
proper curvature, (the concave plate having a hole in the centre), 
and an iron ring, all accurately turned. The mirror may be cast 
in a horizontal position, with its face downwards. Some of the 
best workmen cast mirrors vertically, in an iron mould, having 
a circuitous git entering at the bottom. In this case great caution 
is necessary to confine the melted metal from escaping through 
the joints of the mould. 
The gages are first to be made perfect by rubbing them 
length-ways, one upon the other, with fine emery: whereby the 
prominent parts, undergoing most friction, will be worn away, 
and the receding parts will advance, till the contact becomes 
uninterrupted in all positions, that is, till both gages become truly 
circular with the same curvature. 
The leaden tools also may be fitted more perfectly to the 
gages in a common lathe; after which, the grinding may com- 
mence, by rubbing the concave leaden tool upon the convex 
tool, with a rectilineal motion across the centres. Considering the 
whole spherical surface as made up of parallel circular ares, each 
are will thus be rubbed length-ways, upon a similar arc, great 
circles upon great circles, and small circles upon small; so that 
each are will become truly circular, as m the gages: and, by 
changing the motion in every possible direction, the whole surface 
will become truly spherical. The concave leaden tool and the 
mirror are ground alternately upon the convex surface of the larger 
tool, till all three surfaces become spherical with the same cur- 
vature. 
The substances employed for grinding upon lead, are first 
sand, when the surface requires to be much reduced; then emery 
of different degrees of fineness; the coarsest kind used being su- 
perfine corn emery; then a spherical surface of hone or Turkey 
stone; which is intersected with several trenches, at right angles 
