38 EAKTHQUAKES. 



contrivance which will withdraw a catch. In this way an 

 impression in the form of three dots is received on the disc, 

 and the time known without either stopping or sensibly 

 retarding the clock. 



For ordinary observers, if a time-taker is not used in 

 conjunction with a record-receiver, as good results as 

 those obtained by ordinary clock- stopping apparatus are 

 obtainable by glancing at an ordinary watch. Subse- 

 quently the watch by which the observation was made 

 should be compared with some good time-keeper, and the 

 local time at which the shock took place is then approxi- 

 mately known. 



From what has now been said it will be seen that for 

 a complete seismograph we require three distinct sets of 

 apparatus — an apparatus to record horizontal motion, an 

 apparatus to record vertical motion, and an apparatus to 

 record time. The horizontal and vertical motions must 

 be written on the same receiver, and if possible side by 

 side, whilst the instant at which the time record is made 

 a mark must be made on the edge of the diagram which 

 is being drawn by the seismograph. Such a seismograph 

 has been constructed and is now erected in Japan. It is 

 illustrated in the accompanying diagram. 



The Gray and Milne Seismograph. — In this apparatus 

 two mutually rectangular components of the horizontal 

 motion of the earth are recorded on a sheet of smoked 

 paper wound round a drum, D, kept continuously in motion 

 by clockwork, w, by means of two conical pendulum- seis- 

 mographs, C. The vertical motion is recorded on the 

 same sheet of paper by means of a compensated-spring 

 seismograph, s L M B. 



The time of occurrence of an earthquake is deter- 

 mined by causing the circuit of two electro-magnets to be 

 closed by the shaking. One of these magnets relieves a 



