58 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 30. 

 A 



How do the 

 times of the vi- 

 brations of a 

 pcTiduliim com- 

 pare with each 

 other ? 



Explain the 

 reason of tliis 

 law. 



will once more bring it down to A ; it will then rise again to C, and so con- 

 tinue to oscillate backward and forward. 



If we now suspend the ball by a string, or 

 wire, in such a manner that it will swing 

 freely, its motions will be the same as that 

 of the ball rolling upon the curve. A body 

 thus suspended is called a Pendulum. In 

 Fig. 30, D C, the part of the circle through 

 which the pendulum moves, is called its arc, 

 and the whole movement of the ball from D 

 to C is called an oscillation. 



115. The times of the 

 vibrations of a pen- 

 dulum, are very nearly 

 equal, whether it 



moves much or little ; or, in 



other words, through a greater, or less part of its arc. 



The reason that a large vibration is performed in the same 

 time as a small one, or, in other words, the reason the pendu- 

 lum alwaj'S moves faster in proportion as its journey is longer, 

 is, that in proportion as the arc described is more extended, the steeper are 

 the declivities through which it falls, and the more its motion is accelerated. 

 Thus, if a pendulum, Fig. 30, begins its motion .at D, the accelerating force is 

 twice as great as when it is set free at b ; and if we take two pendulums of 

 equal lengths, and liberate one at D and another at b at the same time, they 

 will arrive at the same moment at E. 



116. This remarkable property of the pendulu^ enables us 

 to employ it as a register, or keeper of time. A pendulum of 

 invariable length, and in the same location, will always make 

 the same number of oscillations in the same time. Thus, if 

 we arrange it so that it will oscillate once in a second, sixty 



of these oscillations will mark the lapse of a minute, and 3,600 an hour. 



A common clock is, therefore, merely an arrangement for 

 molfclockT™" registering the number of oscillations which a pendulum 

 makes, and at tlie same time of communicating to the pendu- 

 lum, by means of a weight, an amount of motion sufficient to make up for 

 what it is continually losing by friction on its points of support, and by the 

 resistance of the air. 



The wheels of the clock turn round by the action of the weight, but they 

 are so connected with the pendulum, that with every double oscillation a tooth 

 of the last wheel is allowed to pass. If, now, tin's wheel has thirty teeth, as 

 is common in clocks, it will turn round once for every sixty vibrations. And, 

 if the axis of this wheel project tlirough the dial-plate or face of a clock, with 

 a hand fastened on it, tliis hand will be the second hand of the clock. The 

 other wheels arc so connected with the lirst, and the number of teeth so pro- 



HoTT- does this 

 property of the 

 pendulum en- 

 able us to reg- 

 ister time? 



