72, we can determine with great
exactness the angular extent of lock. The diagram at Fig. 75 illustrates
how this experiment is conducted. We apply the hair broach to the end of
the fork _M_, as shown at _L_, and gently move the fork in the direction
of the arrow _i_, watching the hand _A_ and note the number of degrees,
or parts of degrees, indicated by the hand as passed over before the
tooth is unlocked and passes on to the impulse plane and the fork flies
forward to the opposite bank. Now, the quick movement of the pallet and
fork may make the hand mark more or less of an arc on the index than one
of ten degrees, as the grasp may slip on the pallet staff; but the arc
indicated by the slow movement in unlocking will be correct.

[Illustration: Fig. 75]

By taking a piece of sharpened pegwood and placing the point in the slot
of the fork, we can test the fork to see if the drop takes place much
before the lever rests against the opposite bank. As we have previously
stated, the drop from the pallet should not take place until the lever
_almost_ rests on the banking pin. What the reader should impress on his
mind is that the lever should pass through about one and a half degrees
arc to unlock, and the remainder (eight and a half degrees) of the ten
degrees are to be devoted to impulse. But, understand, if the impulse
angle is only seven and a half degrees, and the jewel pin acts in
accordance with the rules previously given, do not alter the pallet
until you know for certain you will gain by it. An observant workman
will, after a little practice, be able to determine this matter.

We will next take up the double roller and fork action, and also
consider in many ways the effect of less angles of action than ten
degrees. This matter now seems of more importance, from the fact that we
are desirous to impress on our readers that _there is no valid reason
for adopting ten degrees of fork and roller action with the table
roller, except that about this number of degrees of action are required
to secure a reliable safety action_. With the double roller, as low as
six degrees fork and pallet action can be safely employed. In fork and
pallet actions below six degrees of angular motion, side-shake in pivot
holes becomes a dangerous factor, as will be explained further on. It is
perfectly comprehending the action of the lever escapement and then
being able to remedy defects, that constitute the master workman.

HOW TO MEASURE