APPLICATIONS TO MECHANICS 



99 



moving fluids will suffice to demonstrate the resources 

 of the method.* 



Oscillations and Vibrations. — When a pendulum 

 swings, the suspended mass is acted upon by gravity 

 alternately in two directions, so that the mass alter- 

 nately presents positive and negative phases of accelera- 

 tion, dependent, as is nowadays well known, on the 

 continuous action of gravity. This is true also of 



Fig. 69. — Jointed pendulum : an oscillation from right to left following on a halt 

 oscillation from left to right. 



those vibratory movements in which the elastic force 

 of a vibrating rod takes the place of gravity in an 

 oscillating pendulum ; but in some cases the conditions 

 of movement are so complicated that it is difficult to 

 foretell exactly what the oscillations will be. This 



* Hydraulic engin ers might also perhaps have recourse to chrono- 

 photography when they want to prove certain points in their theories 

 concerning waves and currents, and even when working out the effect 

 of different kinds of propellers. This they might do b\ observing the 

 movement transmitted to the molecules of the fluid by the propellers. 



