126 



THE PENDULUM. 



The arcs of \ibration are now of different lengths, but the 

 pendulum a has in the beginning of its path a steepei 

 descent than the pendulum a l ; hence it acquires a 

 greater velocity, and performs its vibration in the same 

 time as that in which the pendulum a l describes its 

 shorter path. The magnitude of the arc of vibration of a 

 pendulum is called the amplitude of the vibration, and 



FIG. 93 (an. proj. ^ real 



we observe thus that the time of vibration of a pendulum 

 is independent of the amplitude. This is called the 

 law of isochronism. Strictly speaking, it is not quite i 

 correct. The pendulum requires a somewhat longer 

 time for a greater arc of vibration than for a shorter ; 

 but the error committed in assuming the law to be 

 strictly true, is very small. If we note the time 

 required by a given pendulum to make 1,000 vibra- 



