Seismological Investigations. Ill ' 



cussion, are sufficient to establish the claim. Seismoscopes, 

 with the accompanying clocks, should be the only instrument 

 used, and these should be, as far as possible, of the same type. 

 Before the adoption of any one of several different species 

 available, a careful study of them should be made, all being 

 tested under similar conditions and these conditions should 

 resemble as closely as possible those under which the instru- 

 ment is expected to work. As seismic phenomena are here 

 too infrequent to admit of such preliminary test or calibration, 

 means must be provided for imitating at will the motion of a 

 point upon the surface of the earth. Such an arrangement 

 would also be useful in testing seismographs and the validity 

 of their records. 



For this purpose what may be called a -.' seismic table " has 

 been designed. It consists essentially of a horizontal surface, 

 large enough to furnish room for two or three seismoscopes or 

 seismographs, and which by suitable mechanism may be given 

 a vibratory motion in either one or all of three directions at 

 right angles to each other, two of these motions being in a 

 horizontal plane. The vibrations are all derived from a single 

 piece rotating with a uniform angular velocity, and all are on 

 a close approximation to simple harmonic motion. By a 

 simple device their amplitude may be varied at will and 

 without arresting the movement, from zero to a prescribed 

 limit. Another mechanism enables the operator to vary the 

 frequency of vibration from zero to any desired number per 

 second. The machine is to rest upon a solid foundation and 

 power is to be drawn from a steam or gas engine. At any 

 moment, the amplitude and frequency of vibrations along any 

 of the three components will be shown by suitably arranged 

 indices. The disturbance to which a seism oscope is subjected 

 can generally be resolved into approximately simple harmonic 

 motions along these axes, and as the period and amplitude of 

 vibration for earthquakes of moderate intensity are now toler- 

 ably well-known, through the investigations of Japanese earth- 

 quakes by Ewing, Milne, Gray and Sekiya, it will be possible 

 to reproduce their movements with considerable accuracy and 

 with such variations as to intensity as to satisfy every demand 

 in testing seismic instruments. In this way it can be deter- 

 mined to what particular species of vibration a seismoscope is 

 sensitive ; whether the same seismoscope can be repeatedly set 

 to the same degree of sensitiveness, and whether several differ 

 ent instruments can be made to agree in this respect. In 

 addition to its great valucas affording a means of testing and 

 comparing instruments, it is believed that such an apparatus 

 may be useful in studying certain observed effects of earth- 

 quakes upon simple structures. By submitting small but 



