268 



THE HUMAN MOTOR 



as the tuning fork, and attracts an armature, D, of soft iron, 

 carrying a style S. This armature oscillates, therefore, at each 

 vibration of "the tuning-fork, being brought back to its initial 

 positions by a small spiral spring R. The indicator, mounted on 

 a supporting pedestal, is placed against the registering cylinder, 



which allows the 



. 

 Diapason 



15G. 



transmission of 

 a time record 

 from a long dis- 

 tance (see fig. 

 137). As has 

 been said, Ja- 

 quet's chrono- 

 graph, reading 

 to J of a second, 

 is quite Sufficient 



Registration of time by an electro-tuning fork 

 in practice, the probable error being rarely 



When the time has to be read by a watch, the error must be 

 reduced by increasing the duration of the phenomenon. Thus 

 we observe n, similar movements in a time t ; the duration of 



one movement is -, with a probable error of -. 

 n t 



199. Measurement of Speed. In uniform movement, speed is 

 the relation of the distance to the time, the distance covered in 

 a second. 



When a movement is periodic, such that in each period it is 

 uniform, the distance divided by the duration of the period gives 

 the average speed. The walk of a man obeying a determined 

 cadence is an example. The average speed of walking is there- 

 fore easy to find. More often it is expressed in steps, the size 

 of the step being estimated in centimetres. 



In the case of a bicycle it suffices to count the numbers of strokes 

 of the pedal. A tachometer or speedometer can also be used. 



There are also acoustic timekeepers analogous to the metro- 

 nome, which can be placed on the handle-bars, to beat the 

 measure of the stroke of the pedal, thus regulating the speed. 

 Also we can register electrically the revolutions of the wheel, 

 especially if the bicycle is mounted on a fixed framework, as in 

 certain experiments ( 221). 



Marey (*) evolved various arrangements for counting the 

 number of steps and measuring their length in human locomotion. 

 He utilised the old pedometer in conjunction with air transmission, 

 A full description of this odograph (from the Greek 080?, distance) 

 is not necessary. 



( l ) Marey, La Mtthode Graphique, p. 183, 1878.) 



