Two Harmonic Analyzers, 223 



On Two Harmonic Analyzers. By Osborne Reynolds, 

 LL.D., F.R.S., M. Inst. C.E., Professor of Engineering, 

 the Owens College. 



[Received April 2nd^ i8gi.) 



The object of these instruments is to afford a ready 

 means of ascertaining the periods of free vibration of struc- 

 tures or members of structures. If any portion of a material 

 structure (?>., an elastic structure) is disturbed from its 

 normal position of equilibrium and suddenly released, the 

 structure is thrown into a complex state of vibration, which 

 gradually subsides. While the vibration lasts each point in 

 the structure goes through movements which may be very 

 complex, but which are, nevertheless, compounded of simple 

 periodic or harmonic movements, each simple movement 

 taking place in a definite direction as well as having a definite 

 period. 



The art of measuring and recording the complex move- 

 ments at a point of the earth during an earthquake has 

 long been a study, and the seismometer of Professor Ewing 

 has been applied to record the movements of points of 

 various structures when subjected to disturbances. The prin- 

 ciple of these seismometers consists in attaching a weight 

 to the point of the structure to be examined, by attachments 

 of such slight elasticity, that the disturbances communicated 

 to the weight are insensibly small, and the weight remains 

 sensibly steady amid the surrounding vibrations, and forms 

 a steady observatory from which the vibrations may be 

 measured. This measurement is effected by causing pencils 

 vibrating with the structure to describe lines on cards attached 

 to the steady weight, or vice versa, the cards being fixed, or 



