COMMON THINGS CLOCKS AND WATCHES. 



the latter half swing, and is again arrested when the swing is 

 completed. 



20. The motion which is imparted to the hands upon the dial 

 necessarily corresponds with this intermitting motion of the 

 escapement-wheel. If the clock be provided with a seconds- 

 hand, the circumference of the dial being divided into sixty equal 

 parts by dots, the point of the seconds-hand moves from dot to dot 

 during the second half of each swing of the pendulum, having 

 rested upon the dot during the first half swing. 



The whole train of wheel-work being affected with the same 

 intermitting motion, the minute and hour hands must move, like 

 the second hand, by intervals, being alternately moved and stopped 

 for half a second. This intermission, however, is not so observable 

 in them as in the seconds-hand, owing to their comparatively slow 

 motion. Thus, the minute-hand moving sixty times slower than 

 the seconds-hand, moves during each half swing of the pendulum 

 through only the sixtieth part of the space between the dots, and 

 the hour-hand moving twelve times slower than the minute-hand 

 moves in each half swing of the pendulum, through the 360th part 

 of the space between the dots. It is easy, therefore, to comprehend 

 how changes of position so minute are not perceptible. 



21. If the pendulum vibrated upon its axis of suspension uncon- 

 nected with the clockwork, the range of its oscillation would be 

 gradually diminished by the combined effects of the friction upon 

 its axis and the resistance of the air, and this range thus becoming 

 less and less, the oscillation would at length cease altogether, and 

 the pendulum would come to rest. Now this not being the case 

 when the pendulum is in connection with the wheelwork, but on 

 the contrary, its oscillations having always the same range, it is 

 evident that it must receive from the escapement- wheel some force 

 of lateral impulsion, by which the loss of force caused by friction 

 and the resistance of the air is repaired. 



It is easy to show how the effect is produced. It has been 

 shown that during the first half of each swing, a tooth of the 

 escapement-wheel rests upon one or other pallet of the anchor. 

 The pallet re-acts upon it with a certain force, arresting the motion 

 of the wheelwork, and receives from it a corresponding pressure. 

 This pressure has a tendency to accelerate the motion of the pen- 

 dulum, and this continues until the tooth slips off, and is liberated 

 from the pallet. It is this force which repairs the loss of motion 

 sustained by the pendulum by friction and atmospheric resistance. 



Thus we see, that while on the one hand the pendulum regu- 

 lates and equalises the motion imparted to the wheelwork by the 

 weight or mainspring, its own range is equalised by the reaction 

 of the weight or mainspring upon it. 

 16 



