The alteration is made in the shape of the pallets of the anchor. The wheel is much the same. Each pallet consists of two faces: a driving face a b and a sliding face b c.
When the tooth b has done its work by pressing on the driving face, and thus driving the anchor over, say, to the left, then the tooth on the opposite side falls on the sliding face of the other pallet. This being an arc of a circle, has no effect in driving the anchor one way or the other; hence the pendulum is free to swing to the left as far as it likes and return when it feels inclined, always with the exception of a little friction of the tooth on the faces of the pallets, but when it returns and begins to move towards the right, the tooth slides back along the face of the pallet till the pendulum is almost at the middle of its swing; then an impulse is given by the pressure of the tooth upon the inclined plane a´ b´. As soon, however, as the tooth leaves b´, another tooth on the other side at once engages the sliding face b c of the other pallet, and so the motion goes on.
This beautiful escapement is at present used for astronomical clocks; the pallets are made of agate or sapphire, and therefore do not grind away the teeth of the wheel perceptibly, and the loss by friction on the sliding surfaces is exceedingly small.
There are several other ways even better than this for securing a free pendulum movement. We have now to return to our clock.
The centre arbor moves round once in an hour, and carries the minute hand. In order to provide an hour hand, which shall turn round once in twelve hours, we fasten a cogwheel and tube N on to the minute hand arbor by means of a small spring, which keeps it rather tight, but allows it to slip if turned round hard (see [Fig. 45]). This spring is a little bent plate slipped in behind the cogwheel on which its ends rest; its centre presses on a shoulder on the minute hand arbor; it is a sort of small carriage spring. The cogwheel n has thirty teeth. This cogwheel engages another cogwheel o with thirty teeth, on a separate arbor, which carries a third cogwheel, p, with six teeth, and this again engages a fourth cogwheel, q, with seventy-two teeth, mounted on a tube which slips over the tube to which the cogwheel a is attached. It is now easy to see that for each turn of the minute hand arbor the arbor p makes one turn, and for each turn of the arbor p the cogwheel d, makes one-twelfth of a turn. From which it follows that for each turn of the minute hand arbor the cogwheel d with its tube, or, as it is sometimes called, its “slieve,” makes one-twelfth of a turn, and thus makes a hand fastened to it show one hour for every complete turn of the minute hand.
The minute hand is attached to the tube or slieve which carries the cogwheel N. The hour hand is attached to the tube or slieve which carries the cogwheel Q, and one goes twelve times as slowly as the other.
But if you want to set the clock it is easy to do so by reason of the fact that the minute hand is not fixed to the arbor, but only to the slieve on the cogwheel that fits on the arbor, and is held somewhat tight to the arbor by means of the spring. The hands can thus be turned, but they are a little stiff. A washer on the minute hand arbor keeps the slieve on the cogwheel pressed tight against the spring, being secured in its turn by a very small lynch-pin driven through a hole in the minute hand arbor.
It remains to explain a few subsidiary arrangements, not always found upon all clocks, but which are useful.
In order to prevent the overwinding of the clock (see [Fig. 43]), which would cause the cord to overrun the drum, an arm is provided, fitted with a spring. As the weight is wound up the free part of the cord travels along the drum or the fusee; and the cord, when it is near the end of the winding, comes up against the arm and pushes it a little aside. This causes the end of the arm to be pushed against a stop on the axis of the fusee, and thus prevents the clock being further wound up. The stop, being ratchet-shaped, does not prevent the weight from pulling the ratchet wheel round the other way, and thus driving the clock; it only prevents the rotation of that wheel when the string is near it, and the winding is finished.