THE PENDULUM. 123 



}ir weight. The filing must be done while the beam is held in tho 



ind ; the vice would injure it. 



Finally, the equality of the pans is tested by suspending them from 

 the beam after it has been corrected. If the pans are unequal, fix 

 a small piece of solder underneath the lighter pan, and file the piece 

 until the pans are precisely of equal weight ; or a piece of thin wire , 

 adjusted to the required weight, is twisted round one of the cords or 

 bars which support the lighter scale. 



15. The Pendulum. When a body is suspended at 

 one or two points, in a position of stable equilibrium, 

 and is drawn aside from that position, it will, if left 

 to itself, return to it, but not immediately : it will first 

 move past its former position to the opposite side, 

 return again, and repeat this motion many times. 

 Gravity compels the body to descend, until it has taken 

 up the lowest position possible ; in doing so, it gives to 

 the body a certain velocity, it stores up in it a certain 

 amount of work, and the work thus accumulated is 

 just sufficient to carry the body as far beyond its posi- 

 tion of equilibrium on the other side as it descended 

 on this side. As soon as the accumulated work is 

 expended, the body stops, and begins to move in the 

 opposite direction. This kind of motion is termed 

 vibratory or oscillating motion; a body suspended in 

 stable equilibrium and free to move is called a pendulum. 

 A pendulum solely under the influence of gravity, which 

 accelerates its motion during the descent precisely by 

 e same amount as it retards it during the ascent, 

 ould continue to move for ever, and its vibrations 

 would always be equal. But friction and the resistance 

 )f the air cause the arcs which the vibrating body 

 lescribes to become continually less, until finally the 

 )ody is brought to rest. The vibrations of a pendulum 



