COMMON THINGS CLOCKS AND WATCHES. 



upon which the hours, minutes, and sometimes the seconds, 

 are marked by equal divisions, over which the point of the hand 

 moves. The mechanism is a train of wheelwork, so constructed 

 that the rato of rotation of the last wheel upon the axle of which 

 the hand is fixed, shall have a certain proportion to the rate of 

 rotation of the first wheel, upon the axle of which the weight is 

 suspended. And if, as is generally the case, there be two or 

 three hands, then the wheel-work is so constructed, that while 

 one of the hands makes one revolution, another shall make twelve 

 revolutions, and the third shall make sixty revolutions during a 

 single revolution of the latter, and therefore seven hundred and 

 twenty during a single revolution of the former. 



If no other appendage were provided, the weight would, 

 in such an apparatus, descend with a continually increasing 

 velocity, and would therefore impart to the hands a motion of 

 rotation more and more rapid, which would not consequently 

 serve as a measure of time. This defect is removed by the 

 addition of a pendulum, combined with a wheel upon which it 

 acts called the escapement. It is the property of the pendulum 

 that its oscillations are necessarily made always in equal times, 

 and its connection with the escapement-wheel is such, that one 

 tooth of that wheel, and no more, is allowed to pass the upper 

 part of the pendulum during each oscillation right and left. But 

 this escapement-wheel itself forms part of the train of wheel- 

 work by which the first wheel, moved by the descending weight, 

 is connected with the wheels which move the hands, and con- 

 sequently, by regulating and rendering uniform the motion of this 

 escapement- wheel^ the pendulum necessarily regulates and renders 

 uniform the motion of the entire apparatus. 



The instrument thus arranged, therefore, imparts an uniform 

 motion of rotation to each of the hands, but this is not enough to 

 render it a convenient time measurer. It is necessary that the 

 motion of the hands should have some definite and simple relation 

 to the natural and conventional division of time into days, hours, 

 minutes, and seconds. For this purpose it is required not only 

 that the hands should move uniformly, but that the first, or 

 slowest of them, should make two complete revolutions in a day, 

 or a single revolution in twelve hours ; and, as a necessary con- 

 sequence of this, that the second should make a single revolution 

 in an hour, and the third in a minute. 



11. From what has been stated, it will be apparent that the 

 actual rate of motion imparted to the hands will be determined by 

 the rate of oscillation of the pendulum. It has been shown that 

 for each oscillation, right and left, of the pendulum, one tooth of 

 the escapement- wheel passes, and if the escapement-wheel have 



