66 



NATURE 



\_Nov. 15, I ! 



perfect pendulum of that time. Nowadays, the cycloidal pendulum 

 has been replaced by one which swings through a very small arc, 

 and the continual shortening during the oscillation in the cycloidal 

 pendulum is by this means dispensed with, whilst the friction 

 also being much reduced, tliere is less interference from that 

 source. With this very small swing the difference between the 

 arc of the circle described and the cycloid in which the cycloidal 

 pendulum swung is practically indistinguishable. 



The great difference between the modern clock and the 

 ancient one. is that in the former the pendulum is inter- 

 fered with as little as possible whilst swinging, and makes 

 each SM ing under precisely similar conditions. To attain this 

 is to have done much. In the first place, if the clock has a 

 heavy iveight, that weight will probably interfere a good deal 

 with the swinging of the pendulum. The clockweight, there- 

 fore, must be as light as possible. Secondly, if the wheelwork 

 is always in contact with the pendulum, this also will interfere 

 with its free and natural movement. There must be, then, 

 such an arrangement that the wheelwork shall be brought into 

 contact with the pendulum only for the shortest possible time. 

 Thirdly, it must be remembered that the different substances which 

 it is most convenient to use in the construction of pendulums, 

 vary their dimensions with the variations of the teniperature and 

 moisture of the air in which they are placed, and great care must 

 be taken to eliminate any errors \\ hich might arise from such a 



Fig. 19. — Cycloidal Pendulum. 



source. How are these various conditions complied with ? 

 The first, that the clockweight must be small, is not diffi- 

 cult to adhere to ; but it will be well to consider the way in 

 which the second condition, that the action between wheelwork 

 and pendulum shall be the least possible, is met. This is 

 done by employing what is called an escapement. It is so named 

 because the pendulum in its swing is allowed to escape from the 

 wheelwork, and thus retain a perfect freedom. The particular 

 form of escapement about to be described is that which, for a 

 reason that will appear immediately, is called the dead-beat 

 escapement (see Fig. 20). 



The escape wheel is the modern representative of the 

 toothed wheel of the old clock, whilst the projections w and 

 D are modifications of the pallets on the swinging bar in 

 that instrument. Let the pendulum move in the direction 

 of the arrow. The toolh T has just been released, thus per- 

 mitting the tooth V to engage the other pallet D. Now 

 whilst the tooth remains on the p.iUet, the escape wheel re- 

 mains locked, while the pendulum is quite free to swing, 

 there being nothing to retard it save the very slight friction 

 between the tooth .and the surface of the pallet. The rota- 

 tion of the escape wheels, however, brings the tooth on to 

 the oWique edge of the pallet, and with it in this position the 

 pendulum is aided in its forward swing. Then the pallet 



escapes, receiving an impulse, but since this is received almost 

 as much before the pendulum has reached its vertical position as 

 after it has passed that point, no increase or diminution in the 

 time of its oscillation takes place. Jt is in this way that the 

 second of our conditions is complied wiih, the wheelwork being 

 effectually prevented from interfering v\ ith the regularity of the 

 pendulum's swing. It is called the dead-beat escapement, be- 

 cause when the tooth falls on the circular portion of the pallet 

 and locks the escape wheel, the seconds-hand fitted to it stops 

 dead without recoil, because the arc of the surface of the pallet 

 is struck from the centre of motion. In an astronomical clock a 

 still more modern form of escapement, called the gravity escape- 

 ment, is sometimes employed. 



It will perhaps be convenient at this stage to compare the 

 fineness of the division of time given by a clock of this descrip- 

 tion with the fineness of the division of the second of arc 

 w e have already discussed. There is, however, a little difficulty 

 about this, because at present there seems to be no special reason 

 why any particular unit of time should be selected. Ordinarily 

 a day is divided into twenty-four hours, each of these twenty- 



FlG. 20. — Dead-beat Escapement. 



four hours is subdivided into sixty minutes, these again being 

 each divided into as many seconds. The origin of this division 

 of time will be seen later on ; for the present let the fact remaia 

 that it is so. 



Now a modern clock beats practically true seconds, and 

 astronomers after a little practice gain the power of mentally 

 breaking that second up into ten divisions, each of which is of 

 course one-tenth of a second, so that we can say that a day 

 may be divided into 864,000 parts, and in this way institute a 

 com]iarison of the fineness of the division of time with those 

 minute measurements of angular space with which we so recently 

 dealt. 



It is a familiar fact that the length of a pendulum which 

 vibrates seconds is some thirty-nine inches, and it is easy to 

 understand that there are many conditions in which a clock of 

 this kind, with its pendulum of more than a yard long, cannot be 

 used. Not only indeed is there this inconvenient length of the 

 pendulum, but it is necessary that the clock to which it belongs 



