April 24, 19 19] 



NATURE 



57 



with which Mudge's escapement would have been 

 very considerably improved. 



Mudge's escapement was followed by Bloxam's, 

 the action of which will be obvious from Fig. 2. 

 It is still to be seen in action in Bloxam's own 

 clock, which is now, by his nephew's permission, at 

 the Science Museum. The noteworthy feature in it 

 is that the locking arms are much longer than the 

 lifting teeth, so that the frictitn of unlocking is much 

 reduced. 



It was on Bloxam's design that Lord Grimthorpe 



improved in the construction of his well-known 



"double three-legged gravity escapement," used for 



' the first time in the great clock in the Houses of 



i Parliament. The principal feature in this escapement 



! is the long wind-fly, which moderates the shock of 



i impact of the teeth on the pallets, and which the 



large angular movement of the scape-wheel (60° at 



each tick as against 20° in Bloxam's) rendered 



effective. 



A new principle was introduced into the gravity 

 escapement by Capt. Katep about the year 1840, and 

 is described in vol. cxxx. of the Phil. Trans. Fig. 3 

 is taken from Kater's paper, and shows clearly the 

 design. The gravity pieces are lifted alternately as in 

 Mudge's and Bloxam's constructions, but they do not 

 themselves unlock the escapement, merelv serving 

 to upset the equilibrium of a heavy piece (seen in the 

 figure above the wheel), which does the unlocking, 

 but, owing to its high moment of inertia, gets slowly 

 under way and so unlocks the wheel only when the 

 gravity piece then in contact with the pendulum is 

 no longer touching it. 



V^rit6 produced a gravity escapement in which 

 pivot friction was got rid of, but this escapement had 

 four little balls hanging from four silk threads, and 

 was somewhat delicate and complicated. 



It occurred to me some time ago that Kater's 

 principle might be applied in such a way that the 

 pendulum should be entirely freed from all friction 

 whatever, while the impulses given to the pendulum 

 were exactly uniform. A full description of this escape- 

 ment will be found in Patent Office Specification 

 No. 113,501, but it may be said, very briefly, to consist 

 in two little weights which rest alternately on the two 

 ends of a rocking frame having considerable moment of 

 inertia, and on two little upright stems at the ends 

 of arms fastened to the pendulum near its point of 

 support. When the rocker is horizontal, and the 

 pendulum at rest and vertical, things are so adjusted 

 that the weights are resting indifferently on both 

 the pendulum arms and the ends of the rocker. If, 

 then, the pendulum is pushed to one side, say the 

 right, it carries the right-hand little weight upwards, 

 relieving the rocker of its weight, and deposits on the 

 opposite end of the rocker the other little weight. 

 This upsets the equilibrium of the rocker, which 

 commences to turn over, and so releases the scape- 

 wheel, which turns the rocker back rather beyond 

 the horizontal in the sense opposite to that of its 

 last motion, so that when on its return the pendulum 

 again exchanges weights w-ith the rocker, it deposits 

 the right-hand weight at a lower level than that at 

 which it was picked up. The esftipement is simple, 

 and a clock fitted with it has given results which are 

 encouraging. 



Before concluding. I must refer to a remarkable 

 series of papers which commenced last year to appear 

 in the Proceedings of the Royal Society of Edinburgh 

 bv Prof. R. .A. Sampson, the Astronomer Royal for 

 Scotland. Prof. Sampson is, as all astronomers must 

 be, much interested in accurate timekeeping, and has 

 experimented with three different clocks, having es- 

 capements which I must very briefly describe. One is 



NO. 2582, VOL. 103] 



by Mr. Cottingham, and is essentially the same as an 

 escapement which the late Sir David Gill, then 

 Astronomer Royal at the Cape, had imagined. The 

 pendulum is driven by a gravity piece which, so long 

 as it is in contact with the pendulum, by that very 

 contact completes an electric circuit which holds up 

 an armature against the poles of an electromagnet. 

 This armature is itself the stop which limits the 

 travel of the gravity piece. The latter, therefore, goes, 

 on impelling the pendulum until it is brought up 

 against the armature. When this happens the 

 gravity piece is left behind by the pendulum and the 

 circuit is broken. At once the armature falls against 

 a stop, and the gravity piece is lifted, so that the 

 pendulum takes it up again at a higher level than 

 that at which they parted company. Sir David Gill 

 found trouble from the slight adhesion which exists 

 between two metallic surfaces when a current is 

 broken between them, and gave much attention to 

 experiments designed to avoid this. I do not know 

 how far he succeeded, but it seems clear from Prof. 

 Sampson's paper that the escapement is very success- 

 ful now. The idea has probably occurred to many 

 people. I began making a clock about thirty years 

 ago on what was practically the same principle, but 

 gave it up because at that time it did not seem prac- 

 ticable to find a battery capable of giving a current 

 lasting nearly half a second for each second that 

 passes. 



Another of Prof. Sampson's clocks is driven by an 

 escapement invented by Riefler, of Munich, which is 

 unlike any of those we have been considering, and 

 in which the necessary energy is communicated to the 

 pendulum by bending the suspension spring. The 

 block from which the suspension spring hangs, in- 

 stead of being fixed as immovably as possible, which 

 it generally is, is supported on knife-edges, and the 

 suspension spring, which, of course, always tries to 

 keep straight, causes the block to turn on these 

 edges, and so unlock the scape-wheel, which bends 

 the spring back against the motion of the pendulum 

 and thus keeps it going. 



The third escapement which is being observed at 

 Edinburgh, and the last I propose to refer to, is that 

 adopted by the Synchronome Co., and belongs to the 

 class where the action takes place at the bottom of 

 the pendulum or of the crutch instead of the top. This 

 is fuUv described in the specification of a patent 

 granted to Mr. Shortt, and numbered 9527 of 1915. 



So much for escapements. 



We may, in conclusion, for a moment review the 

 difliculties' attending the accurate measurement of 

 time and note how they have been attacked. ^ 



If ever a perfect clock is constructed it will cer- 

 tainly be a pendulum clock, and it will have to fulfil 

 two conditions, necessary and sufficient. They are 

 these : — First, the moment of inertia of the pendulum 

 must be invariable; and, secondly, the forces which 

 act on it must be invariable. If these two conditions 

 could be fulfilled, the last word in horology would 

 have been said. So far, of course, neither condition 

 has been fulfilled, but surprisingly £jood work has 

 been done. As for the first condition, that the 

 moment of inertia must be invariable, the chief diffi- 

 culty is to avoid change by change of temperature. 

 There are two ways of diminishing this change. 

 The pendulum must be compensated in one of the 

 well-known ways — by Harrison's gridiron construction; 

 or that of Graham by the expansion of mercury in 

 the bob; or, again, bv the zinc and iron combination 

 used in many turret-clocks; or. best of all. bv availing 

 ourselves of the low expansion nickel-steel recently 

 introduced bv Guillaume. Also, for added security, 

 the whole dock must be enclosed in a thermostatic 



