lifted the gravity arms were brought as near to the axis of the scape-wheel
as possible, while the locking arms were brought as far from
the axis as possible so that the pressure should be light. The pallet
arbors were cranked, to embrace the pendulum-spring, so that their
centres of motion might coincide with that of the pendulum as nearly
as possible—perhaps an unnecessary refinement; at least the three-legged
and four-legged gravity escapements answer very well with
the pallet arbors set on each side of the top of the spring. The size
of the wheel determines the length of the pallets, as they must be at
such an angle to each other that the radii of the wheel when in contact
with each stop may be at right angles to the pallet arm; and
therefore, for a wheel of this size, the depth of locking can only be
very small. The pinion in Bloxam’s clock only raises the pallet
through 40′ at each beat; i.e. the angle which we call γ, viz. the
amplitude of the pendulum when it begins to lift the pallet, is only
20′; and probably, if it were increased to anything like α/ √2, where
α is the semiarc of swing, the escapement would trip immediately.
The two broad pins marked E, F, are the fork-pins, and A and B are
the stops. The clock which Bloxam had went very well; but it
had an extremely fine train, with pinions of 18; and nobody else
appears to have been able to make one to answer.
| Fig. 19.—Four-legged Gravity Escapement. |
Fig. 20.—Double Three-legged Escapement. |
Bloxam’s escapement was modified in form by Lord Grimthorpe, his chief improvement being the addition of a fly vane, which, however, had previously been used for remontoires to steady the motion. He tried various modifications of construction, but finally adopted the “four-legged” and “double-three-legged” forms as being the most satisfactory, the former for regulators and the latter for large clocks. Fig. 19 is a back view of the escapement part of an astronomical clock with the four-legged wheel; seen from the front the wheel would turn the other way. The long locking teeth are made about 2 in. long from the centre, and the lifting pins, of which four point forwards while four other intermediate ones point backwards, are at not more than 130 of the distance between the centres EC, of the scape-wheel and pallets; or rather C is the top of the pendulum spring to which the pallets Cs, Cs′ converge, though the resultant of their action is a little below C. It is not worth while to crank them as Bloxam did, in order to make them coincide exactly with the top of the pendulum, as the friction of the beat pins on the pendulum is insignificant, and even then would not be quite destroyed. The pallets are not in the same plane, but one is behind and the other in front of the wheel, with one stop pointing backwards and the other forwards to receive the teeth alternately—it does not matter which; in this figure the stop s is behind and the stop s′ forward. The pendulum is now going to the right, and just beginning to lift the right pallet and free the stop s′; then the wheel will begin to turn and lift the other pallet by one of the pins which is now lowest, and which moves through 45° across the line of centres, and therefore lifts with very little friction. It goes on till the tooth now below s reaches s and is stopped there. Meanwhile the pallet Cs′ goes on with the pendulum as far as it may go, to the end of the arc which we have called α, starting from γ; but it falls with the pendulum again, not only to γ but to −γ on the other side of 0, so that the impulse is due to the weight of each pallet alternately falling through 2γ; and the magnitude of the impulse also depends on the obliqueness of the pallet on the whole, i.e. on the distance of its centre of gravity from the vertical through C. The fly KK′ is set on with a friction spring like the common striking-part fly, and should be as long as there is room for, length being much more effective than width.
The double three-legged gravity escapement, which was first used in the Westminster clock, is shown in fig. 20. The principle of it is the same as of the four-legs; but instead of the pallets being one behind and the other in front of the wheel, with two sets of lifting pins, there are two wheels ABC, abc, with the three lifting pins and the two pallets between them like a lantern pinion. One stop B points forward and the other A backward. The two wheels have their teeth set intermediately or 60° apart, though that is not essential, and the angle of 120° may be divided between them in any other proportions, as 70° and 50°, and in that way the pallets may be still more oblique than 30° from the vertical, which, however, is found enough to prevent tripping even if the fly gets loose, which is more likely to happen from carelessness in large clocks than in astronomical ones.
Of course the fly for those escapements in large clocks, with weights heavy enough to drive the hands in all weather, must be much larger than in small ones. For average church clocks with 114 sec. pendulum the legs of the scape-wheels are generally made 4 in. long and the fly from 6 to 7 in. long in each vane by 114 or 112 wide. For 112 sec. pendulums the scape-wheels are generally made 412 radius. At Westminster they are 6 in.
Lord Grimthorpe considered that these escapements act better, especially in regulators, if the pallets do not fall quite on the lifting pins, but on a banking, or stop at any convenient place, so as to leave the wheel free at the moment of starting; just as the striking of a common house clock will sometimes fail to start unless the wheel with the pins has a little run before a pin begins to lift the hammer. The best way to manage the banking is to make the beat-pins long enough to reach a little way behind the pendulum, and let the banking be a thin plate of any metal screwed adjustably to the back of the case. This plate cannot well be shown in the drawings together with the pendulum, which, it may be added, should take up one pallet just when it leaves the other.
In chronometer spring remontoires the pendulum, as it goes by, flips a delicate spring and releases a small weight or spring which has been wound up in readiness by the action of the scape-wheel and which by leaping on to the pendulum gives it a push. One on this principle made about the middle Chronometer spring remontoire. of the 19th century by Robert Houdin is to be seen at the Conservatoire des Arts et Métiers. It is very complicated. The following is more simple. In fig. 21 a scape-wheel AB has 30 pins and 360 teeth. It is engaged with a fly vane EP mounted on a pinion of 12 teeth. Each pin as it passes raises an impulse arm CD which is hooked upon a detent K. A pall NM then engages the fly vane and prevents the scape-wheel from moving farther. The impulse arm being now set, as the plate F attached to the lower end of the pendulum flies past from left to right a pall G knocks aside the detent K, and allows a pin O projecting from the end of the impulse arm to fall upon an inclined pallet h, which is thus urged forward. As soon as the pallet has left the pin, the impulse arm in its further fall strikes N, which disengages the pall at P and allows the scape-wheel to move on and again wind up the impulse arm CD, which is then again locked by the detent K. On the return journey of the pendulum the light pall G, which acts the part of a chronometer spring, flips over the detent. The pallet is double sided, h and h′, so that if by chance the clock runs down while the pendulum swings from left to right the impulse arm will be simply raised and not smashed. It has a flat apex, on which the pin falls before descending. The impulse given depends on the weight of the impulse arm and may be varied at pleasure. The work done in unlocking the detent is invariable, as it depends on the pressure of the fly vane at P and is independent of the clock-train. The duration of the impulse is very short—only about 110 of the arc of swing. It is given exactly at the centre of the swing, and when not under impulse the pendulum is detached.
Clock Wheels.—Since, as we have seen, any increase in the arc of a pendulum is accompanied by a change in its going rate,
