The peculiarity of the chronometer is its escapement.
Let A B be the scape wheel, and C D a small lever attached to C, the pivot on which the balance wheel and spring is fastened. Let E G be a lever, with a tooth F which engages the teeth of the scape wheel and prevents it moving round. Let H be a spring holding the lever E G up to its work.
Fig. 66.
The lever has a spring K E fastened to it at the point K. This spring is very delicate. If the lever C D is turned so that the little projection M on it strikes the spring E from left to right, then, as the spring rests on the lever, the whole lever is pushed over, and the teeth of the scape wheel set free. At that instant, however, the escapement is so arranged that the arm C D is just opposite the tooth D of the scape wheel, so that the scape wheel, instead of running away, leaps with its tooth D on to the lever C D and swings the balance wheel round. The balance wheel is free to twist as much as it pleases, but the moment it has twisted so much that the projection M passes the spring E, then the lever G E flies back to its place, and the scape wheel is again checked. Meanwhile the balance wheel flies round till at last it is brought to rest by the balance spring. It then recoils and sets out on its return path. This time, however, the projection M merely flips aside the spring E and the balance wheel goes back, till again it is brought to rest and returns. As soon as the lever comes opposite D the projection M then again hits the spring E, and releases the catch at F, and another tooth of the scape wheel goes by.
There then you have a completely free escapement, and consequently an accurate one. Many watches are made with these escapements, but they are more expensive than those in common use.
There is but little remaining in a watch that is not in a clock, for the wheel-trains and general arrangements are very similar.
It is possible to apply the chronometer’s detached escapement to a clock. This was done by several clock-makers in the eighteenth and early part of the nineteenth century. One method of doing it is as follows:
A is a block of metal fitted to the bottom of the pendulum, B a light lever pivoted on it. C is the scape wheel, with four teeth; D a tooth of the scape wheel, which hops on to the projection of the pendulum the moment that the impact of the point E of the lever B E has pushed aside the lever G F, and thus released the scape wheel. The advantage is that it is a very easy escapement to make. But it is in reality a detached (that is to say, a completely free) chronometer escapement, as can easily be seen.