COMMON THINGS CLOCKS AND WATCHES. 



back from right to left. The spring E no longer supported by 

 the fork at the end of A, readily lets the pin pass, and the piece c 

 returning to its former position, comes in the way of the suc- 

 ceeding tooth and stops it. 



At the moment that tffe balance is about to commence its swing 

 from left to right, and when the piece c is about to liberate the 

 tooth which rests against it, another tooth behind it rests against 

 the side of the notch i in the disc G, and when the escapement- 

 wheel is liberated, and the swing of the balance from left to right 

 is commencing, this tooth, pressing on the side of the notch z, 

 gives it and the balance-wheel an impulse which is sufficient to 

 restore to it all the force which it lost in the previous oscillation. 

 Except at this moment the balance-wheel in this form of escape- 

 ment is entirely free from all action of the mainspring. 



44. While a clock or watch is being wound up, the weight or 

 mainspring no longer presses upon the catch nor upon the ratchet- 

 wheel, through which the motion is imparted to the wheel work. 

 The motion of the hands is therefore suspended during the time 

 occupied in winding up, consequently if the watch or clock keep 

 regular time while it goes, it must lose just so much time as may 

 be employed in the process of winding it up. Although this, in 

 common clocks and watches, does not produce any sensible effect, 

 the errors incidental to their rates of going generally exceeding it, 

 yet in clocks used in observatories and in chronometers used for the 

 purposes of navigation, where the greatest degree of regularity is 

 required, provisions are made by which the motion of the clock or 

 watch is continued notwithstanding the process of winding up. 



Such expedients are called the MAINTAINING POWEB. 



One of the most simple arrangements by which this is accom- 

 plished in clocks moved by a weight is shown in fig. 28. The 

 weight P, which is the moving power, is connected with another 

 much smaller weight p, by means of an endless cord which 

 passes over the grooves of a series of pullies, A, B, c, and D, of 

 which A and B are moveable, and c and D fixed. The force 

 with which p descends is the excess of its weight above that of p. 



The pulley c, being prevented from revolving by the catch E 

 during the descent of the weight P, and the cord being prevented 

 from sliding upon its groove by the effects of its friction and 

 adhesion, the parts b a and c d, which descend from the pulley 

 C to the pulleys A and B, may be regarded as being virtually 

 attached to fixed points at b and c, so that they cannot descend. 

 This being the case, the weight P, descending by its prepon- 

 derance over p, and consequently the weight p being drawn up, 

 the part of the cord c d must pass over the pulley B, the part 

 e f over the pulley D, and the part g h over the pulley A. In this 

 40 



