34 MOVEMENT 



images, we have instituted a chronophotographic 

 method, which explains all the factors in a move- 

 ment which we want to understand. It also affords a 

 very simple experimental solution of certain very 

 complicated mechanical problems. The whole ques- 

 tion of mechanics is based on a knowledge of the 

 movement which is imparted to a mass ; for from 

 the movement the force which produces it can be 

 measured. 



To determine with accuracy the character of a 

 movement, whether it be uniform or irregular, to 

 determine its velocity and degree of acceleration 

 extremely delicate experiments are usually necessary. 



When the movement is once thoroughly under- 

 stood, its character must be expressed in a precise 

 manner. Since the time of Descartes, geometricians 

 have known how to express the characters of move- 

 ments in the form of curves with different variations. 

 But such curves, although they can express certain 

 phenomena, require, like other geometrical figures, a 

 more or less laborious construction. It was a great 

 step in advance when Poncelet and Morin showed 

 that a moving body could itself be made to trace its 

 path in the form of a curve. The first application 

 of this graphic method was made use of in the case 

 of a falling body ; it was soon, however, extended to 

 other branches of Science. Meteorology, Physics, 

 and Physiology all participated in the discovery with 

 advantage to themselves. 



In spite of the enormous development of this method, 

 it has limitations, which we can only extend by the 

 employment of chronophotography. Thus, when the 

 moving body is inaccessible, when it cannot be fastened 

 by mechanical means to the recording apparatus, this 

 new method for determining its movement must be 

 employed ; a method which demands no material link 



