CHRONOMETERS. 87 



The showing the time is contrived by the motion 

 of the indices or hands on the dial-plate; one of 

 which, called the minute-hand, goes completely 

 round the circle in an hour ; and the other, called 

 the hour-hand, once in twelve hours. To obtain 

 exactly this degree of velocity in the hands, they 

 are fixed on the ends of axes which turn round by 

 means of wheels attached to them, having teeth 

 on their circumferences; these teeth are connected 

 with that fixed on the axis of the crownwheel; and 

 it is by proportioning the number of these teeth, 

 that the proper degree of velocity is obtained. 

 The minute-hand is fixed on the axis of the wheel 

 E; consequently it must revolve once in an hour. 

 The pivot C of this wheel passes through the plate 

 S T, and continues to r: fixed upon the pivot C is 

 a wheel N, which acts upon the wheel O, the pinion 

 of which p carries round another wheel g that has 

 a hollow axis which goes on the axis of the wheel 

 N and turns on it: this wheel g turns round once 

 in twelve hours, and has the hour-hand fixed on it. 



The wheel E is driven by the pinion c belonging 

 to the axis of the wheel F F, which is moved by 

 the pinion f of the crown or scape wheel G H. 

 Supposing, then, that the pendulum makes two vi- 

 brations in a second, (which will be what is called 

 a half-second pendulum,) it will make 7200 vi- 

 brations in an hour. If the balance wheel consists 

 of 30 teeth, it will turn once in the time that the 

 pendulum makes sixty vibrations; for at every turn 

 of the wheel the same tooth acts once on the 

 pallet I, and once on the pallet K, which occasions 

 two vibrations in the pendulum; and the wheel 

 having 30 teeth, it occasions twice 30 or 60 vi- 

 brations; consequently this wheel must perform 

 120 revolutions in an hour, because 60 vibrations, 



g 4 



