34 EAETHQUAKES. 



tending to cause motion in the assumed steady points of 

 the seismographs. One of the most perfect instruments 

 would be obtained by registering photographically the 

 motions of the recording index by the reflection of a ray 

 of light. Such an instrument would, however, be difficult 

 to construct and difficult to manipulate. One of the best 

 practical forms of registering apparatus is one in which 

 the record is written on a surface of smoked glass. This 

 can afterwards be covered with a coat of photographer's 

 varnish, and subsequently photographed by the ' blue pro- 

 cess' so well known to engineers. 



To obtain a record of all the vibrations of an earth- 

 quake it is necessary that the surface on which the seis- 

 mograph writes should at the time of an earthquake be in 

 motion. Of record-receiving machines there are three 

 types. First, there are those which move continuously. 

 The common form of these is a circular glass plate like 

 an old form of chronograph, driven continuously by clock- 

 work. On this the pointers of the seismograph rest and 

 trace over and over again the same circles. At the time 

 of an earthquake they move back and forth across the 

 circles, which are theoretically fine lines, and leave a 

 record of the earthquake. Instead of a circular plate, a 

 drum covered with smoked paper may be used, which, 

 after the earthquake, possesses the advantage, after un- 

 rolling, of presenting the record in a straight line, instead 

 of a record written round the periphery of a circle, as is 

 the case with the circular glass plates. Such records are 

 easily preserved, but they are more difficult to photo- 

 graph. 



The second form of apparatus is one which is set in 

 motion at the time of a shock. This may be a con- 

 trivance like one of those just described, or a straight 

 smoked glass plate on a carriage. By means of an elec- 



