102 T. C. Mendenhall — Seismoscopes and 



the simple seismoscope, with a time-taking attachment, is far 

 more likely to furnish valuable information. 



Seismoscopes. 



A good seismoscope should possess some if not all of the 

 following characteristics : — 



It should be simple, inexpensive and not liable to become 

 inoperative through long periods of rest ; 



It should be capable of adjustment to varying degrees of 

 sensitiveness ; 



The adjustment of different instruments to nearly the same 

 degree of sensitiveness should be possible ; 



Its equilibrium should be unstable ; that is, when once dis- 

 turbed it should not " reset " itself ; 



It should not be liable to register phenomena other than 

 actual movements of the earth upon which it rests. 



While much has been done and notable advances have been 

 made in the construction of seismographs with a view to the 

 determination of the character of the motion of the earth 

 particle, it does not appear that a seismoscope satisfying these 

 conditions has yet been described or extensively used, although 

 an infinite variety of instruments bearing the name have been 

 devised. If whatever is done in the near future is likely to be 

 done through the use of the time-registering apparatus, and 

 this, I believe, is the opinion of the majority of the members 

 of the earthquake Commission assembled upon the invitation 

 of the Director of the Geological Survey, the subject becomes 

 one of considerable importance, and a recognition of this fact 

 has resulted in the suggestion of several new forms of seismo- 

 scopes within the last two or three years, a few of which have 

 been actually constructed and tested. The first of them, and 

 the only one as far as I know which has been used to register 

 the occurrence of an earthquake in this country, was designed 

 in the Physical Laboratory of the Signal Office at Washington. 

 While others contributed suggestions as to certain details, the 

 general form of the instrument is due to Junior Professor C. 

 F. Marvin, of the office of the Chief Signal Officer.* 



It is shown in figure 1 and a little explanation will make its 

 operation clear. 



An iron cylinder weighing three or four pounds has a 

 cylindrical hole of about 2 cm in diameter, bored through 

 concentric with its axis. At a point a little distant from the 



*The first practical use of an instrument of this type, was at the Flood Rock 

 Explosion in October, 1885. It was placed by direction of the writer at the 

 nearest point of observation, on Ward's Island. Its performance was entirely 

 satisfactory. See Science, Oct. 16, 1885. An instrument somewhat similar in 

 design with a very imperfect contact-making device was suggested by Milne. 



See Trans. Seis. Soc. Japan, vol. hi. 



