SEISMOMETEY. 31 



Ball and Plate Seismograph. — Another form of 

 tjeismograph, which is closely related to the two forms of 

 apparatus just described, is Verbeck's ball and plate 

 seismograph. This consists of a surface plate resting on 

 three hard spheres, which in turn rests upon a second 

 surface plate. When the lower plate is moved, the 

 upper one tends to remain at rest, and thus may be used 

 as a steady mass to move an index. 



The Principle of Perry and Ayrton. — An instru- 

 ment which is of interest from the scientific principle it 

 involves is a seismograph suggested by Professors Perry 

 and Ayrton, who propose to support a heavy ball on 

 three springs, which shall be sufficiently stiff to have an 

 exceedingly quick period of vibration. By means of 

 pencils attached to the ball by levers, the motions of the 

 ball are to be recorded on a moving band of paper. The 

 result would be a record compounded of the small vibra- 

 tions of the springs superimposed on the larger, slower, 

 wave-like motions of the earthquake, and, knowing the 

 former of these, the latter might be separated by analysis. 

 Although our present knowledge of earthquake motion 

 indicates that the analysis of such a record would often 

 present us with insuperable difficulties, this instrument 

 is worthy of notice on account of the novelty of the 

 principle it involves, which, the authors truly remark, has 

 in seismometry been a ' neglected' one. 



Instruments to record Vertical Motion. — The instru- 

 ments which have been devised to record vertical motion 

 are almost as numerous as those which have been de- 

 vised to record horizontal motion. The earliest form of 

 instrument employed for this purpose was a spiral 

 spring stretched by weight, which, on account of its 

 inertia, was suj^posed at the time of a shock to remain 

 steady. No satisfactory results have ever been obtained 



