[Illustration:  Fig. 100]

[Illustration:  Fig. 101]

Suppose, for illustration, we should make such a change in the pallet stone of the entrance pallet; we have increased the angle between the lines k l by (say) one and a half degrees; by so doing we would increase the lock on the exit pallet to three degrees, provided we were working on a basis of one and a half degrees lock; and if we pushed back the exit pallet so as to have the proper degree of lock (one and a half) on it, the tooth which would next engage the entrance pallet would not lock at all, but would strike the pallet on the impulse instead of on the locking face.  Again, such a change might cause the jewel pin to strike the horn of the fork, as indicated at the dotted line m, Fig. 99.

Dealing with such and similar abstractions by mental process requires the closest kind of reasoning; and if we attempt to delineate all the complications which follow even such a small change, we will find the job a lengthy one.  But with a large model having adjustable parts we provide ourselves with the means for the very best practical solution, and the workman who makes and manipulates such a model will soon master the lever escapement.

QUIZ PROBLEMS IN THE DETACHED LEVER ESCAPEMENT.

Some years ago a young watchmaker friend of the writer made, at his suggestion, a model of the lever escapement similar to the one described, which he used to “play with,” as he termed it—­that is, he would set the fork and pallets (which were adjustable) in all sorts of ways, right ways and wrong ways, so he could watch the results.  A favorite pastime was to set every part for the best results, which was determined by the arc of vibration of the balance.  By this sort of training he soon reached that degree of proficiency where one could no more puzzle him with a bad lever escapement than you could spoil a meal for him by disarranging his knife, fork and spoon.

Let us, as a practical example, take up the consideration of a short fork.  To represent this in our model we take a lever as shown at Fig. 99, with the elongated slot for the pallet staff at g.  To facilitate the description we reproduce at Fig. 102 the figure just mentioned, and also employ the same letters of reference.  We fancy everybody who has any knowledge of the lever escapement has an idea of exactly what a “short fork” is, and at the same time it would perhaps puzzle them a good deal to explain the difference between a short fork and a roller too small.

[Illustration:  Fig. 102]

[Illustration:  Fig. 103]