139. Steel chains. The chain used in connecting the
mainspring and fusee in watches and clocks, is composed of small
pieces of sheet steel, and it is of great importance that each of
these pieces should be of exactly the same size. The links are of
two sorts; one of them consisting of a single oblong piece of
steel with two holes in it, and the other formed by connecting
two of the same pieces of steel, placed parallel to each other,
and at a small distance apart, by two rivets. The two kinds of
links occur alternately in the chain: each end of the single
pieces being placed between the ends of two others, and connected
with them by a rivet passing through all three. If the rivet
holes in the pieces for the double links are not precisely at
equal distances, the chain will not be straight, and will,
consequently, be unfit for its purpose.
Copying with elongation
140. In this species of copying there exists but little
resemblance between the copy and the original. It is the
cross-section only of the thing produced which is similar to the
tool through which it passes. When the substances to be operated
upon are hard, they must frequently pass in succession through
several holes, and it is in some cases necessary to anneal them
at intervals.
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