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^Xpfind itself laterally, and become heaped up suddenly, 

 luound the tract of the mirror's pressure; which must, 

 theiefore, to obviate this, be so conducted, as to traverse, 

 ill quick and regular succession, every part of the polisher, 

 in order to recover the regularity of its figure as fast as it 

 becomes vitiated. And this is efl'ected in two ways: either 

 by enlarged circular strokes of the metal, brought consider- 

 ably beyond the edges of the polisher, in order to repress, 

 towards the center, the pitch, which had become raised 

 near its edges; or by strait diametral strokes, across its 

 surface, in every direction, successively: either of which 

 will tend to preserve the iigure of the polisher, and, con- 

 sequently, of the mirror, nearly spherical. As, however, a 

 spherical figure is not that which is ultimately intended, 

 so these modes of conducting the process are to be pur- 

 sued only till the mirror has acquired a sufficient polish, 

 and a figure nearly spherical: and then, in order to give 

 it a parabolic or hyperbolic shape, the motion of the mir- 

 ror, on the polisher, should be such, as that the center 

 of it may describe a spiral line round the center of the 

 polisher, by enlarging the circular strokes, till the edge of 

 the mirror arrives at the edge of the polisher; and then 

 contracting the motion gradually, till the mirror returns to 

 the center, in the same spiral course. By which means, 

 any sudden and irregular elevation of t;he pitch, beyond 

 the place of the mirror, will be prevented; while, at the 

 same time, it will become regularly elevated, from the 



center 



