Recoil that show the above feature are called "recoil " escapements, which have the tendency to gain by an increase of the impelling power and to lose when it becomes less—a condition which is reversed in escapements to be described hereafter. The reason of this is that the increase of power offers a greater resistance to the recoiling action of the pallet, thereby exhausting the momentum of the balance earlier and reducing the arc of the vibrations, while by a decrease of power the tooth of the crown-wheel becomes a less impeding factor to the pallet, and the balance is allowed to move in a greater arc.
12 shows the Graham escapement (invented by George Gralthin), whose prototype evidently is the anchor escapement (invented by Hook about 1675), which is also a recoil escape ment. In Graham's escapement the escape-wheel stands still while the pallets are moving, and therefore it has received the name of "dead-beat" escapement. The Graham escapement consists of the escape-wheel A and the anchor Dxmn. The anchor is supported at D, from which point it is kept in oscillating motion by a pendulum not indicated in the drawing. The escape-wheel moves in the direction of the arrow, one of its teeth c pressing against the inclined end-surface of the pallet r; if now the oscil lating pendulum moves the anchor to the left, the tooth c will glide over the inclined surface and leave the pallet; at that moment the tooth y will drop at a on the pallet 2 and rest on it until the anchor moves to the right, when it will glide over the inclined surface a, b of the pallet and escape, and the tooth 0 will drop to the pallet I. While a tooth is gliding over the inclined surface of a pallet it exercises against the anchor a pres sure which is transmitted to the pendulum by an arm fixed to the arbor D. This pressure gives the impulse to the pendulum for the continuation of its oscillations. The anchor thereby serves its double purpose, first by trans mitting the impulses to the pendulum, and secondly by causing the step like movement of the escape-wheel by interrupting its progress at such intervals as are determined by the length of the pendulum.
It is the form of the pallets which makes the Graham escapement a "dead-beat" escapement. The curve of the pallets is that of a circle having its centre at D; a tooth, if it drops on a pallet, strikes it beyond the inclined surface and rests on its curved portion, this being a section of a circle with its centre at the support of the anchor, and will keep the escapement-wheel at a standstill until the tooth glides over the incline.
We have observed that in the verge escapement an increase of the impel ling power causes the balance to increase its oscillations; this effect is reversed on the Graham and all other " dead-beat " escapements. While resting on the curve of the pallet the pressure of the escape-wheel does not materially oppose the progress of the anchor; if, therefore, the impel ling power is increased, it will act upon the driving surface of the pallet with a greater force and give a greater impulse to the pendulum, thereby enlarging the arc of its oscillations. This will diminish the number of its oscillations, and of course will make the clock go slower, while obvi ously a decrease of the impelling power will have the opposite effect, and make it go faster. The Graham escapement gives good results when all parts of the clock can be well protected and inclosed. For tower clocks, however, it does not give the desired result. The air-currents, striking under changing force or direction the large and exposed bands, will either retard or accelerate the movements of the clock. In one case it will react upon the pendulum by decreasing the arc of oscillation and make the clock go slower; in the other case it will diminish the arc and make it go faster. The gravity escapement, invented by E. 13. Denison (Lord Grimthorpe) in 1854, removes this evil very successfully.
Double Three-legged Gravity Escapement. —In the double three-legged gravity escapement exhibited in Figure 5 (pi 124), the gravity impulse pallets aa' are two bent plates pivoted as nearly as possible to the suspen sion point of the pendulum; bb' is the locking wheel, made up of two thin plates, each having three long teeth or "legs," and fastened on the escape wheel arbor a little distance apart; between them, near to the arbor, at equal distances are placed three pins, called the " lifting" pins. Each of the impulse pallets has at its bend an arm (ill), the tip of which approaches the pins above and below the escape-wheel arbor; at the lower extremity the impulse are prevented from going beyond a fixed point by checking pins (ggi). Both of the impulse pallets are placed inside of the space between the locking-wheel plates b and b'. At their bend each of the pallets has a block (ss'). The pallet to the right has the block on its front, the other pallet has the block (indicated in the Figure by dotted lines) on its opposite or rear side. In order to show the locking wheel, part of the pendulum is cut away.