SEISMOMETEY. 13 



and in amplitude, and they have a duration of from a few 

 seconds to several minutes. We will illustrate the records 

 of actual earthquakes in a future chapter, but in the 

 meantime the idea that an earthquake consists of a single 

 shock must be dismissed from the imagination. 



To construct an instrument which at the time of an 

 earthquake shall move and leave a record of its motion, 

 there is but little difficulty. Contrivances of this order 

 are called seismoscopes. If, however, we wish to know 

 the period, extent, and direction of each of the vibrations 

 which constitutes an earthquake, we have considerable 

 difficulty. Instruments which will in this way measure 

 or write down the earth's motions are called seismometers 

 or seismographs. 



Many of the elaborate instruments supplemented with 

 electro-magnetic and clockwork arrangements are, when 

 we examine them, nothing more than elaborate seismo- 

 scopes which have been erroneously termed seismographs. 



The only approximations to true seismographs which 

 have yet been invented are without doubt those which 

 during the past few years have been used in Japan. It 

 would be a somew^hat arbitrary proceeding, however, to 

 classify the different instruments as seismoscopes, seismo- 

 meters, and seismographs, as the character of the record 

 given by certain instruments is sometimes only seismo- 

 scopic, whilst at other times it is seismometric, depending 

 on the nature of the disturbance. Many instruments, for 

 instance, w^ould record with considerable accuracy a single 

 sudden movement, but would give no reliable information 

 regarding a continued shaking. 



Eastern Seismoscopes. — The earliest seismoscope of 

 which we find any historical record is one which owes its 

 origin to a Chinese called Choko. It was invented in the 

 year A.D. 136. A description is given in the Chinese 



