Tzvo Harmonic Analyzers. 225 



the small weight will follow essentially the movements of 

 the larger weight as if rigidly attached, while if the period 

 •of motion of the larger weight is smaller than that of the 

 period of free vibration of the small weight, the small 

 weight will remain virtually at rest. But when the period 

 of motion of the large weight coincides with the period of 

 free vibration of the small weight, the small weight will 

 take and accumulate the disturbance, oscillating with in- 

 creasing amplitude until it reaches such an extent that the 

 energy dissipated is equal to that received from the dis- 

 turbance. If the elasticity of the connections be fairly 

 perfect, the amplitude of the small weight will be very 

 considerable, although the disturbing motion is otherwise 

 insensible. 



If the small weight {w) has only one degree of freedom, 

 i.e.y if the elasticity of the connections is not equal in all 

 directions, there will be three axes of elasticity, and if the 

 elasticities along two of these directions are much greater 

 than the third this is the direction of freedom ; then, 

 when the period of free vibration along the third axis, i.e.^ 

 in the direction of freedom, coincides with the period of 

 disturbance, the small weight will only take up the dis- 

 turbance when this has a component in the direction of 

 freedom; that is, if the direction of the disturbance is at 

 right angles to the direction of freedom, there will be no 

 vibration. So that in this way the direction of the dis- 

 turbance may be ascertained, or vice versa. 



Similar results follow if, instead of the disturbance 

 coming through the elastic supports, the body be subject 

 to a synchronous periodic force. If the period of the force 

 were not synchronous with any of the three periods of free 

 vibration corresponding respectively to the three axes of 

 elasticity, the resulting vibration would, as before, merely 

 correspond with the time effect of the force, but on 

 '<:oincidences with any one of these, unless the direction of 



