530 



NA TURE 



{Oct. 12, 1 8 76 



altogether, and then the tooth beyond upon the opposite 

 side of the wheel will fall upon the other pallet, and a 

 process similar will take place. By this arrangement the 

 moving body, or balance, will alternately be driven back- 

 wards and forwards. 



Prior to experiment, it is not easy to see why a con- 

 trivance such as this is should not go (in other words run 

 down) with uniformity. We have a constant weight im- 

 pelling a constant weight, and the contrivance itself 

 destroys acceleration, but the fact is, we here overlook the 

 great disturbance due to friction. 



If we could indefinitely magnify each of the surfaces 

 now in contact in this machine, we should see that 

 what we call sUding and rubbing is (especially upon 

 the pallets) in reality tearing and grinding, and the 

 wonder would be, not that the motion produced is not 

 equal and regular, but that it should have any tendency 

 whatever in this direction. 



No doubt the first steps towards equalising the motion 



of such apparatus were in the direction of a general im- 

 provement in their workmanship and mechanical arrange- 

 ment. Then came the fundamental ones of the pendu- 

 lum for clocks and the pendulum spring for watches, and 

 lastly, those in the arrangement of that mechanism 

 (called the escapement) which modifies the manner in 

 which the power of the clock weight is finally adminis- 

 tered to the pendulum or balance. 



It will be convenient to discuss these improvements not 

 strictly in historical sequence : we shall begin with the 

 machinery itself, or clock-train. 



Jrains. 



Fig. 4 shows the general arrangement of a modem 



clock-irain. g is the " great wheel" connected with the 



"barrel" B, around which the line carrying the weight W 



is wrapped. This great wheel drives a pinion, p^, fastened 



upon the spindle of the centre-wheel c, and the centre- 

 wheel in turn drives another pinion, fastened to the spindle 

 of the third wheel T, and the third wheel again another 

 upon the spindle of the escape-wheel s. The escape-wheel 

 operates upon two arms or " pallets," A A, and by their 

 means passes on impulse to the pendulum. For a clock 

 with a seconds' pendulum there are generally thirty teeth 

 in the escape-wheel, and as one tooth passes either pallet 

 at every other vibration of the pendulum, you will see 

 that it turns once in a minute, and its spindle carries 

 the seconds' hand. The numbers of teeth in the escape- 

 pinion, third wheel, third pinion, and centre wheel are so 

 arranged that the centre-wheel turns once for every sixty 

 turns of the escape-wheel, that is, once in an hour. The 

 great wheel which engages the centre pinion turns once 

 in twelve hours, and for an eight-day clock there are, of 

 course, sixteen turns of the line upon the barrel. 



Fig. 5 shows the apparatus for obtaining the relative 

 motions of the hour and minute-hands. Upon the spindle 

 S S of the centre- wheel (which you recollect turns once in an 

 hour) is placed, friction-tight (that is, so stiff that it clings 

 to the spindle, and yet loose enough to be movable by 



MJI 



Fig. s- 



hand), the wheel M, with a long socket reaching through 

 to the left which carries the minute-hand M H at its 

 extremity. This wheel gears with another, i, which it 

 moves round in twice its time, i.e., in two hours. Con- 

 nected with this second wheel is a pinion, P, and the 

 wheel H (which rolls upon the socket of the wheel m), 

 gears into it. This wheel is arranged to move round six 

 times as slowly as the pinion P, that is to say, in twelve 

 hours, and it carries a socket to which the hour-hand 

 H H is attached. The socket-wheel, M, being on the 

 spindle of the centre-wheel, only friction-tight, you can, 

 of course, shift the combination without disturbing the 

 clock- train. 



The barrel, B, is connected with the great wheel by 

 means of a ratchet-wheel and click (see Fig. 4). The 

 ratchet-wheel, %, is fastened to the barrel, and when you 

 wind up the weight by turning the barrel, its teeth being 

 pointed backwards, pass under the click L. When you 

 cease winding, the square face of the tooth meets the 

 click, and communicates pressure through it to the grtac 

 wheel. 



