228 



NA TURE 



U^ 



an. 1 1 . 



1S77 



of temperature in the pendulum, and the mode of its sup- 

 port, in a clock which I have recently made with an 

 escapement on a new principle, in which the simplicity of 

 the dead-beat escapement of Graham is retained, while j 

 its great defect, the stopping of the whole train of wheels 

 by pressure of a tooth upon a surface moving with the 

 pendulum, is remedied. 



I'magine the escapement-wheel of a common dead-beat 

 clock to be mounted on a collar fitting easily upon a shaft, 

 instead of being rigidly attached to it. Let friction be 

 properly applied between the shaft and the collar, so that 

 the wheel shall be carried round by the shaft unless 

 resisted by a force exceeding some small definite amount ; 

 and let a governor giving uniform motion be applied to 

 the train of wheel-work connected with this shaft, and so 

 adjusted that, when the escapement-wheel is unresisted, 

 it will move faster by a small percentage than it must 

 move to keep time properly. Now let the escape- 

 ment wheel, thus mounted and carried round, act 

 upon the escapement, just as it does in the ordinary 

 clock. It will keep the pendulum vibrating, and will. 



just as in the ordinary clock, be held back every time it 

 touches the escapement during the interval required to 

 set it right again from having gone too fast during the 

 preceding interval of motion. But in the ordinary clock 

 the interval of rest is considerable, generally greater 

 than the interval of motion. In the new clock it is 

 equal to a small fraction of the interval of motion : -^-^ in 

 the clock as now working, but to be reduced probably to 

 something much smaller yet. The simplest appliance to 

 count the turns of this escapement-wheel (a worm, for 

 instance, working upon a wheel with thirty teeth, carrying 

 a hand round, which will correspond to the seconds' hand 

 of the clock) completes the instrument ; for minute and 

 hour-hands are a superfluity in an astronomical clock. 



In various trials which I have made since the year 

 1865, when this plan of escapement first occurred to me, 

 I have used several different forms, all answering to the 

 preceding description, although differing widely in their 

 geometrical and mechanical characters. In all of them 

 the escapement-wheel is reduced to a single tooth or arm. 



to diminish as much as possible the moment of inertia 

 of the mass stopped by the pendulum. This arm revolves 

 in the period of the pendulum (two seconds for a one 

 second's pendulum), or in some odd multiple of it. Thus 

 the pendulum may execute one or more complete periods 

 of vibration without being touched by the escapement. In 

 all my trials the pallets have been attached to the bottom 

 of the pendulum, projecting below it, in order that satis- 

 factory action with a very small arc of vibration (not more 

 on each side than -^}^-q of the radius, or i centimetre for 

 the seconds' pendulum) may be secured. 



In the clock in my house the seconds' pendulum of the 

 fine movement, vibrates with great constancy through 

 half a millimetre, that is to say, through an arc of 77,70 of 

 the radian, on each side of the vertical. This, I believe, 

 is the smallest range that has hitherto been realised in 

 any seconds' pendulum of an astronomical or other clock. 

 In the drawing s represents the vertical escapement 

 shaft, round which is fitted loosely the collar c, carrying 

 the worm v. The small wheel, d, is worked by v, and 

 carries round the seconds' hand of the clock, a repre- 

 sents a piece of fine steel wire, being the single arm 

 to which the teeth of the escapement-wheel are reduced 

 in the clock described in this paper; pp the pallets 

 attached to bars projecting, downwards from the bob, B, 

 of the pendulum ; /, a foot bearing the weight of the 

 collar-worm and escapement tooth. The bar connect- 

 ing / with the collar is of such a length as to give a 

 proper moment to the frictional force by which the collar 

 is carried round. The shaft j- carries a wheel, represented 

 in section by ww, which is driven by a train of wheel- 

 work (not shown in the drawing) from the governor. This 

 wheel is made to go J per cent, faster than once round in 

 two seconds, while the pendulum prevents the collar from 

 going round more than once in two seconds. 



My trials were rendered practically abortive from 1865 

 until a few months ago by the difficulty of obtaining a 

 satisfactory governor for the uniform motion of the 

 escapement-shaft ; this difficulty is quite overcome in the 

 pendulum governor, which I now proceed to describe. 



Imagine a pendulum with single-tooth escapement 

 mounted on a collar loose on the escapement shaft just as 

 described above — the shaft being vertical in this case 

 also. A square-threaded screw is cut on the upper cjuarter 

 of the length of the shaft, this being the part of it on which 

 the escapement-collar works ; and a pin fixed to the collar 

 projects inwards to the furrow of the screw, so that, if the 

 collar is turned relatively to the shaft, it will be carried 

 along, as the nut of a screw, but with less friction than an 

 ordinary nut. Below the screw and long nut-collar, three- 

 quarters of the length of the escapement-shaft is surrounded 

 by a tube which, by wheel- work, is carried round about 5 per 

 cent, faster than the central shaft. This outer shaft, by 

 means of friction produced by the pressure of proper 

 springs, carries the nut collar round along with it, except 

 when the escapement-tooth is stopped by either of the 

 pallets attached to the pendulum. A stiff cross-piece (like 

 the head of a T), projecting each way from the top of the 

 tubular shaft, carries, hanging down from it, the governing 

 masses of a centrifugal friction governor. These masses 

 are drawn towards the axis by springs, the inner ends of 

 which are acted on by the nut collar, so that the lower 

 or the higher the latter is in its range, the springs pull the 

 masses inwards with less or more force. A fixed metal 

 ring coaxial with the main shaft holds the governing 

 masses in when their centrifugal forces exceed the forces 

 of the springs, and resists the motion by forces of friction 

 increasing approximately in simple proportion to the 

 excess of the speed above that which just balances the 

 forces of the springs. As long as the escapement-tooth is 

 unresisted, the nut collar is carried round with the quicker 

 motion of the outer tubular shaft, and so it screws up- 

 wards, increasing the force of the springs. Once every 

 semiperiod of the pendulum it is held back by either 



