COMMON THINGS CLOCKS AND WATCHES. 



and balance-wheel variously combined. 47. Watches and chrono- 

 meters. 48. Marine chronometers. 49. Stationary chronometers. 

 50. Striking apparatus. 



38. THE rate of oscillation of the balance-wheel cannot in the 

 same manner be so easily regulated by modifying its form ; but, 

 on the other hand, while the force which moves the pendulum, 

 being that of gravity, is beyond our control, that which moves 

 the balance-wheel being the force of the spiral spring, is at our 

 absolute disposition. It is accordingly by modifying this spring 

 that we are enabled to regulate the time of oscillation of this 

 regulator. 



One of the most common expedients by which this is accom- 

 plished is represented in fig. 20. 



Near the fixed point G, at the external extremity of the spiral, 

 is placed a small bar E, near the end, F, of which is a notch, or 

 o hole, through which the 



wire of the spiral passes. 

 This arrests the action of 

 the spiral, so that the 

 only part of it which 

 oscillates is that which is 

 included between F and 

 its internal extremity. 

 In a word, the point F is 

 the virtual external ex- 

 tremity of the spiral. Now 

 this point F can be moved 

 in the one direction or 

 the other, so as to increase 

 or diminish the virtual 



length of the spiral at pleasure, by means of the toothed arc AB 

 and the pinion c, which latter is turned by the index D. If 

 the index D be turned to the left, the bar E, and the point F, 

 is moved towards G, and the length of the spiral is increased. If 

 it be turned to the right, the point F is moved from G, and the 

 length of the spiral is diminished. 



In this manner the rate of vibration of the balance-wheel may 

 be adjusted by varying at will the vertical length of the spiral- 

 spring. 



39. The precision of the movement of all forms of timepieces 

 depends in a great degree on the mechanism of the escapement, and 

 accordingly much mechanical skill and ingenuity have been 

 directed to its improvement, and several varieties of form have 

 been adopted and applied. 



The recoil escapement, represented in fig. 17, consists of two 

 34 



