the cylinder and escape wheel were not adapted for each other, but they
were the best he had. Chapter II, on the cylinder escapement, will
enable our readers to master the subject and hence be better able to
judge of allowances to be made in order to permit imperfect material to
be used.

In illustration, let us imagine that we have to put in a new cylinder,
and we have none of precisely the proper size, but we have them both a
mere trifle too large and too small, and the question is which to use.
Our advice is to use the smaller one if it does not require the
escape-wheel teeth to be "dressed," that is, made smaller. Why we make
this choice is based on the fact that the smaller cylinder shell gives
less friction, and the loss from "drop"--that is, side play between the
escape-wheel teeth and the cylinder--will be the same in both instances
except to change the lost motion from inside to outside drop.

In devising a system to be applied to selecting a new cylinder, we meet
the same troubles encountered throughout all watchmakers' repair work,
and chief among these are good and convenient measuring tools. But even
with perfect measuring tools we would have to exercise good judgment, as
just explained. In Chapter II we gave a rule for determining the outside
diameter of a cylinder from the diameter of the escape wheel; but such
rules and tables will, in nine instances out of ten, have to be modified
by attendant circumstances--as, for instance, the thickness of the shell
of the cylinder, which should be one-tenth of the outer diameter of the
shell, but the shell is usually thicker. A tolerably safe practical rule
and one also depending very much on the workman's good judgment is, when
the escape-wheel teeth have been shortened, to select a cylinder giving
ample clearance inside the shell to the tooth, but by no means large
enough to fill the space between the teeth. After studying carefully
the instructions just given we think the workman will have no difficulty
in selecting a cylinder of the right diameter.

MEASURING THE HEIGHTS.

The next thing is to get the proper heights. This is much more easily
arrived at: the main measurement being to have the teeth of the escape
wheel clear the upper face of the lower plug. In order to talk
intelligently we will make a drawing of a cylinder and agree on the
proper names for the several parts to be used in this chapter. Such
drawing is shown at Fig. 171. The names are: The