frame provided with two teeth or pallets A and B on opposite sides of the center of motion of the driver.* The latter revolves in the direction of the arrow, and its wiper a is shown in the act of urging the follower to the right by pressing against the side of the tooth A. Revolving a little farther in the same direction, a will, by its circular motion, escape from A, and at the same instant b will encounter B, and will urge it in the opposite direction, until b in like manner escapes from it, when c will act upon A. In this way the rotation of abc will produce the reciprocation of the frame.

241. But the frame may also be made the driver; for if it be moved to the left, A will push a and make the wheel revolve in the contrary direction to the arrow, and c will pass B. When this has happened, let the frame be moved back again; then, after moving a short space, B will meet c, and move the wheel still farther round, until b has passed A, when the return of the frame will enable A to push b. Thus the reciprocation of the frame will cause the wheel to revolve in the opposite direction to that in which itself would produce the reciprocation of the frame. But when the frame is the driver, there will always be a short motion at the beginning of each oscillation, during which no motion will be given to the wheel.

242. Fig. 156 is another method by which a revolving wheel A gives a reciprocating motion to a sliding

bar bk.†

B

Fig. 156.

k

04

At the moment the circular motion, the

The wheel has six pins projecting from its face. The pin 1 is shown in the act of driving the bar to the right by acting upon the tooth at k. The pin 3 also moves a bell-crank lever, the upper arm ƒ of which travels in the contrary direction to the bar. first pin 1 escapes from the side of k by its pin b will have reached the arm f, and this will, by acting upon b, push the bar in the reverse direction. Again, when the pin 3 escapes from the arm of the bell-crank, the pin 2 will begin to act upon k, exactly as the pin 1 had previously done, while the pin 4 will in like manner replace the pin 3, and raise the bellcrank. This action will go on continually, producing a short, alternate, but very abrupt and jerking, motion in the bar.

243. In these two contrivances the teeth of the wheel are made to act upon two distinct pieces attached to the reciprocating

* This contrivance is taken from De la Hire, Traité de Mécanique, prop. 114. From Thiout, Traité d'Horlogerie, t. i. p. 85.

N

*

piece, and so arranged that as one tooth escapes from the reciprocating piece, the other shall begin its action, whence this group of combinations receives the term of escapements. Escapements are most largely employed in clock and watch-work to communicate the action of the moving power to the pendulum or balance; but when so employed they receive many delicate arrangements, which have for their object the distribution of the power in such a manner as will the least interfere with the due action of the pendulum. Such arrangements being governed by dynamical principles, are excluded from our present plan. Escapements are, however, employed in Pure Mechanism to convert rotation into reciprocation, as for example, in the bell of an alarum-clock. In the two forms already given the reciprocation is communicated to a sliding bar; in those which follow it is given to an axis, which may be either perpendicular or parallel to the revolving wheel.

244. When the axes are at right angles the crown-wheel escapement, fig. 157, is commonly employed.

A is the revolving axis, to the extremity of which is fixed a crown-wheel with large saw-shaped teeth; Ce the vibrating axis or verge. This carries the two pieces or pallets b and a, which are set in planes making an angle with each other to allow of the escaping action. When the wheel revolves in the direction Fig. 157.

of the arrow, one of its teeth pressing against the pallet a will turn the verge in the same direction, until, by the circular motion of a, its extremity is lifted so high that the crownwheel tooth passes under it, or, in other words, this tooth escapes from the pallet. By the same motion of the verge the pallet b is brought into a vertical plane, and the tooth c now presses it in the contrary direction, and turns the verge back again until e escapes from under b, when a new tooth begins to act upon a, and so on. Thus the rotation of the crown-wheel produces the vibration of the verge, the crown-wheel being the driver.

245. The anchor-escapement, fig. 158, is adapted to parallel

axes.

The revolving wheel has pins 1, 2, 3, ... and turns in the direction of the arrow. The vibrating axis B has a two-armed piece carrying the pallets at its extremities, and resembling somewhat the form of an anchor; whence the name of the combination.

* Vide Chapter on Trains below.

Fig. 158.

OB

The pin 1 is shown in the act of pressing against the pallet surface ab. Now as the normal of the point of contact passes on the same side of the two axes A and B, the pin, which acts upon the pallet by sliding contact, will tend to turn the pallet in the same direction as the wheel (Art. 31). aB will therefore revolve upwards, and the pin will slide towards b and there escape from the pallet. At this instant the pin 3 will reach the second pallet-surface cd, of which the normal passes between the two axes; the action of this pin will therefore turn the axis B in the reverse direction; the second pallet-arm Bd will rise, and the pin 3 escape from the pallet at d, when a new pin will act upon ab as before; and thus the vibration be maintained.

.A

750

D

O

246. This escapement has received a great variety of forms. The teeth of the wheel are more commonly long and slenderpointed spur-teeth, of which many examples may be found in the treatises of Horology.

A very simple arrangement is shown at the lower part of fig. 158, in which D is the verge, pn, nm, the pallets; these are fixed against the face of an arm which lies parallel to the plane of the wheel, and so far from it as to clear the tops of the pins. The pin 6 is shown in the act of pressing the pallet mn, and therefore of depressing the arm; when this pin reaches n it escapes from mn, and begins to act upon pn, by which it raises the arm and escapes at the lower end of the second pallet, when 5 begins to touch and depress the first pallet mn, and so on.

247. In all these escapements the verge may be made the driver, and thus a reciprocating motion be made to produce a rotation. The wheel will always revolve the contrary way to that in which it turns when itself drives (Art. 241).

Thus in fig. 158, let the arm Ba be depressed, the pallet ab will then drive the pin 1 backwards (that is, contrary to the arrow), until pin 4 has passed under the point of d. If the arm Ed be now depressed de will act upon pin 4, and continue the backward rotation until 2 has passed under the point b. Ba being again depressed will repeat the former action upon 2, and so on. But the rotation of the wheel will be necessarily intermittent, for at each change of direction in the pallet-arm the pallet must pass through a short space before it begins to touch the pin, above which it must have been previously raised to allow the same pin

to pass under it. escapement.

This will also be true of the crown-wheel

248. In fig. 159 the axes are parallel, but the action is more direct than in the common anchor-escapement.

Fig. 159,

As in the former contrivance, either the wheel or the pallets may drive. I will describe it under the latter action.*

C is the axis of the pallets G and F. If the pallet-arm be moved to the left, F will encounter a, and at the same moment G will have passed beyond b, therefore F continuing its motion will turn the wheel, in the direction of the arrow, so that when G returns it will enter the next space cb, and striking the tooth b will thus continue the rotation of the wheel, and so on.

* This contrivance, by Meynier, is to be found in the Machines Approuvées, 1724.

Fig. 160.

Fig. 161.

249. ANY two curves revolving in the same plane whose wrapping connector (vide p. 24) cuts the line of centers in a constant point, will preserve a constant angular velocity ratio. In practice, however, circles or rather cylinders only are employed, which are fixed to revolving axes, and manifestly possess the required property. To enable the rotation to proceed in the same direction indefinitely, the band which serves as a wrapping connector has its two ends joined so as to form an endless band, which embraces a portion of the circumference of each circle or pully, and is stretched sufficiently tight to enable it to adhere to and communicate its motion to the edge.

The band may be direct, that is, with

parallel sides, as in fig. 160, or it may be crossed, as in fig. 161. In the first case the axes or pullies will both revolve in the same direction, in the latter case in opposite directions.

250. Motion communicated in this manner is remarkably smooth, and free from noise and vibration, and on this account, as well as from the extreme simplicity of the method, it is always preferred to every other, unless the motions require to be conveyed in an exact ratio.

For, as the communication of motion between the wheels and band is entirely maintained by the frictional adhesion between them,