In making one, the
first thing to be provided is a movement holder. Any of the three-jaw
types of such holders will answer, provided the jaws hold a movement
plate perfectly parallel with the bed of the holder. This will be better
understood by inspecting Fig. 109, which is a side view of a device of
this kind seen edgewise in elevation. In this _B_ represents the bed
plate, which supports three swing jaws, shown at _C_, Figs. 109 and 110.
The watch plate is indicated by the parallel dotted lines _A_, Fig. 109.
The seat _a_ of the swing jaws _C_ must hold the watch plate _A_ exactly
parallel with the bed plate _B_. In the cheap movement holders these
seats (_a_) are apt to be of irregular heights, and must be corrected
for our purpose. We will take it for granted that all the seats _a_ are
of precisely the same height, measured from _B_, and that a watch plate
placed in the jaws _C_ will be held exactly parallel with the said bed
_B_. We must next provide two pillars, shown at _D E_, Figs.
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