224 Dr. Osborne Reynolds on 



having a time movement. In this way the complex motions 

 of the points are beautifully recorded, as in Prof Ewing's 

 experiments on the Tay Bridge, and Prof J. Milne's numer- 

 ous experiments in railway carriages, &c. 



Such curves represent the complex movements of the 

 point of the structure examined ; and any analysis of the 

 motion into its simple periodic components remains to be 

 accomplished by mathematical reduction — or by such 

 instrumental synthesis as that which may be effected in Sir 

 William Thomson's " Harmonic Analyzer." 



The Harmonic Analyzers about to be described differ 

 essentially from the seismometer in that they do not 

 measure or record the actual motions of the structure, 

 while they single out and exaggerate any component 

 periodic motion according to its period and direction, which 

 are defined in the instruments. The principle of these 

 Harmonic Analyzers is that of the accumulation of motion 

 which takes place] when a weight is subject to a periodic 

 disturbance which coincides in period and direction with 

 that of free vibration of which the weight is susceptible. 



If a small weight w be elasticall}- attached to a much 

 heavier weight so that it requires a definite force (E/) to 

 disturb the weight {w) through a distance /, the large 

 weight remaining at rest ; then, if released after any dis- 

 turbance, the small weight w will vibrate in the direction of 

 disturbance, and with a constant period 



ev.i 



in seconds) 



i.e. in the period of free vibration of the small weight. 



If the small weight be at rest and the large weight be 



subject to a periodic disturbance having a period f — J ; then, 



if this period is larger than the period of free vibration of 

 the small weight, ^>., if 



- is smaller than 27r , / — 

 n \' ge 



