SEISMOMETRY. 35 



trical or a mechanical contrivance called a 'starter,' of 

 which many forms have been contrived, the earthquake 

 is caused to release a detent and thus set in motion the 

 mechanism which moves the record-receiver. 



The great advantage of continuously-moving machines 

 is that the beginning and end of the shock can usually 

 be got with certainty, while all the uncertainty as to the 

 action of the ' starter ' is avoided. Self-starting machines 

 have, of course, the advantage of simplicity and cheap- 

 ness, while there is no danger of the record getting ob- 

 literated by the subsequent motion of the plate under the 

 index. 



Time-recording A'pjparatus, — Of equal importance 

 with the instruments which record the motion of the 

 ground, are those instruments which record the time at 

 which such motion took place. The great value of time- 

 records, when determining the origin from which an earth- 

 quake originates, will be shown farther on. The most 

 important result which is required in connection with 

 time observations, is to determine the interval of time 

 taken by a disturbance in travelling from one point to 

 another. On account of the great velocity with which 

 these disturbances sometimes travel, it is necessary that 

 these observations should be made with considerable ac- 

 curacy. The old methods of adapting an apparatus to a 

 clock which, when shaken, shall cause the clock to stop, 

 are of little valae unless the stations at which the obser- 

 vations are made are at considerable distances apart. This 

 will be appreciated when we remember that the disturb- 

 ance may possibly travel at the rate of a mile per second, 

 that its duration at any station may often extend over a 

 minute, and that one set of apparatus at one station may 

 stop, perhaps, at the commencement of the disturbance, 

 and the other near the end. A satisfactory time-taking 



