EARTHQUAKES. 361 



is written is kept moving uniformly by clock-work, we obtain also a 

 register in time as well as space. 



But in an earthquake the surface of the earth undergoes also a ver- 

 tical movement which has to be recorded. The principle by which an 

 instrument may be constructed to attain this end is as follows : If a 

 weight hangs by a long, elastic cord, so that when set dancing up and 

 down it oscillates very slowly, then, when a sudden jerk is given to the 

 point of support, the weight will for the moment stand almost station- 

 ary, and a pencil attached to it may write its record on a surface fast- 

 ened to the part jerked. This idea has been utilized in the construction 

 of a vertical seismograph, but various important modifications have 

 been introduced for the purpose of annulling the spontaneous dance 

 of the weight after the shock has occurred.* 



It will undoubtedly serve to give an impulse to this science that 

 henceforth the intending observer need not waste time in devising and 

 constructing instruments, but can purchase the complete equipment of 

 a seismological observatory, as recommended by Ewing, and may begin 

 work at once. 



Many other instruments besides these have been used for the obser- 

 vation of earthquakes, and among the best are those of Bertelli, Rossi, 

 and Palmieri. An instrument which tells only that there has been a 

 shock, without giving a record of the nature of the movement, is called 

 a seismoscope. Some of the Italian instruments are seismoscopes, 

 which, however, give an approximate idea of the severity and direc- 

 tion of the vibration, and others claim to be accurate seismographs or 

 seismometers. But I do not think that any of them can compete with 

 the instruments described in outline above. 



And what do recording instruments tell us of the actual occurrences 

 during an earthquake ? 



" They show," writes Ewing,f " that, as observed at a station on 

 the surface of the earth, an earthquake consists of a very large num- 

 ber of successive vibrations in some cases as many as three hundred 

 have been distinctly registered. They are irregular both in period and 

 amplitude, and the amplitude does not exceed a few millimetres " (a 

 millimetre is one twenty-fifth of an inch), " even when the earthquake 

 is of sufficient severity to throw down chimneys and crack walls, while 

 in many instances the greatest motion is no more than the fraction of 

 a millimetre. The periods of the principal motions are usually from 

 half a second to a second, but . . . the early part of the disturbance 

 often contains vibrations of much greater frequency. The earth- 

 quake generally begins and always ends very gradually, and it is a 



* I make no attempt to apportion the credit among the several inventors of these in- 

 struments. The men mentioned have played the leading parts, and the work of all seems 

 to be thorough and sound. 



f " Memoirs of the Science Department of the University of Tokio," No. 9, 1883, 

 p. 13. 



