CERTAIN MOVEMENTS IN MAN 147 



an elastic body is distorted in proportion to the degree 

 of force applied. 



We have endeavoured to use dynamometers of uni- 

 form pattern throughout our researches on animal 

 movements. And for that purpose we have always 

 employed coils of indiarabber tubing, which were 

 more or less compressed according to the external 

 force applied. In consequence of this pressure, the 

 contained air was more or less squeezed out into the 

 chamber of a recording tambour. 



The coils of tubing of which the dynamometer is 

 formed are wound concentrically like the spring 

 of a watch. The central end is closed, and the peri- 

 pheral or free end communicates with the chamber of 

 a recording tambour. The coil itself is glued on to 

 a disc of cardboard. 



This instrument goes by the name of " The spiral 

 dynamometer." The tube used has a very fine bore 

 with very thick walls, so that it can resist strong 

 pressure without bursting. The distortion or com- 

 pression is very regular, so that the lever of the 

 registering tambour is practically raised to a height 

 proportional to the force applied.* 



By modifying this spiral form, we were able to con- 

 struct a traction dynamograph (Fig. 99). The latter 

 apparatus was used for measuring the force exercised by 

 horses when differently harnessed. One of the ends was 

 securely fastened to the vehicle and the other to the 

 swing-bar. Traction on the part of the horses tended 

 to approximate two discs, which compressed between 

 them the turns of the spiral coil.f The air expelled 



* To obtain greater accuracy the instrument may be empirically 

 graduated by applying a series of regularly increasing weights, and 

 marking off on a scale the corresponding discursions of the lever. 



t We proved by these experiments that, if the traction was applied 

 through the medium of elastic tracts, the gradual distribution of 

 the shocks thereby occasioned effected a great economy in the force 



