266 



THE HUMAN MOTOR 



197. Measurement of Time. The unit of time is the second. It 

 is rarely necessary, in the phenomena under consideration, to 

 measure small fractions of a second. However, in walking at 

 100 or 150 steps a minute, or in operating the keys of a piano, 

 or a typewriter, etc., the duration of each movement may be from, 

 say. i to | of a second. It suffices in most cases to use a metro- 

 nome, which will give the cadence of the work, will regulate it 

 invariably, and will indicate the duration of any isolated, 

 mechanical act. Generally a metro- 

 nome can hardly measure a quarter of 

 second, as it makes 40 to 208 beats a 

 .minute (fig. 152.) The various 

 cadences are obtained by moving the 

 weight B on the pendulum and fixing 

 it at the desired position (see 26). 



The metronome is wound up, and 

 goes for about three- quarters of an 

 hour, but it can also be actuated by 

 electricity. 



To record the time, on a registering 

 cylinder, at the same time as the 

 graphs of the movement, the chrono- 

 graplnsu^ed. In Jacquet's chronograph 

 a beat takes place every second or 

 fifth of a second, and displaces the 

 style, which is usually made of alu- 

 minium (fig. 153). It has two dials, of 

 which one marks the seconds and the 

 other the minutes, and is mounted on ? support like the tambours. 

 The style makes a horizontal mark at each fifth of a second. To 

 measure time, without registering it, the stop watch is used. 

 It is started or stopped by pressing a button in one direction, 



Fro. 152. 



Metronome. 



Fio. 153. 



Jacquet's Chronograph. 



