12 



THE HUMAN MOTOR 



also blackened, and of which the the background was hung 

 with black velvet. L. Soret placed little incandescent lamps on 

 the head and feet and photographed by these means the 

 chorographic movements. The American, Fred Colvin, attached 

 a ring with an electric torch to the index finger of a workman. 

 The trajectory of a moving body may thus be obtained and 

 a general view of the movement realized. 



This method of points and lines constitutes geometrical 

 chronophotography. It has been employed in the analytical 

 study of animal locomotion, the movement of projectiles, etc. 

 The employment of films permits the exhibition of various 

 pictures at a speed which, if they are projected, synthetises 

 them to the eye, provided that the retained impression lasts 

 ^y of a second : this is the principle of cinematography (*) 



7. Movements of Bodies. Natural bodies are systems not 

 material points ; they are assumed, for simplification, to be 

 indeformable, and their dimensions to remain invariable ; in 

 reality, perfect solids do not exist, as they become more or 

 less self -deformed. However, the movements of a solid body 

 are as follows : 



1. MOVEMENTS OF TRANSLATION. The body is moved without 

 turning, each of the straight lines thereof remaining parallel. 

 Such a movement is generally attained by the use of guides. 



2. MOVEMENTS OF ROTATION. Here the body revolves on a 

 straight line, called the axis of rotation, and each point of the 

 body has the same angular speed of rotation. The movements 

 of rotation take place around horizontal or vertical axes sup- 

 ported by pivots or bearings ; if a complete revolution is not 

 required, hinges are used as with doors, lids of boxes, etc. 



3. HELICOIDAL MOVEMENTS.- -in this case the body turns 

 round the axis, at the same time being 



displaced along the length of the axis ; 

 there is rotation and translation. The 

 rotation may be in either direction (vide 

 fig. 18). Either from left to right 

 (dextrorsum helix] or from right to left 

 (sinistrorsum helix] . 



O 



>0b 



o 



The first type of helix, the right- .. j. 



handed, is the more common. The ver- 

 tical distance ab between two revolutions de*trorsum 'nstror>8i/> 

 of a helix is called the pitch. 



(*) See J. L. Breton, La Chronophotographie (in Rev. Scient. et Indust., 

 1897, p. 179) ; Marey, La Chronophotographie, 1899 ; Karl Marbe, Theori* 

 der Kinemaiographischen Projectionen, Leipzig, 1910. 



