on Earthquakes in Japan. 361 



spring sufficiently long to allow of enough elongation to give 

 a long period of vibration. 



(b) Flexible-bottomed- Cylinder Seismograph. — This consists 

 of a tin can with an indiarubber bottom, which rests on a thin 

 flat spring. Against the lower side of this spring, opposite 

 the centre of the rubber bottom, the end of the short arm of 

 a lever presses. The can is partly filled with water, which 

 by its inertia causes the bottom to be deflected if the can be 

 suddenly raised or lowered. The motions of the bottom are 

 communicated to the lever, a point at the end of the long 

 arm of which is caused to register the motion on a suitable 

 receiving-plate. 



(c) Compensated-Spring-Eever Seismograph ; 



(d) Hydrometer Seismograph. 



For descriptions, with drawings, of these instruments, see 

 Phil. Mag. for September 1881. 



4. Apparatus on which to record Earthquake Motions. 



(a) Carnage Receiver. — A simple and convenient instru- 

 ment is a strip of smoked glass resting on a three-wheeled 

 carriage. This carriage is pulled along by means of a small 

 weight, while a thread from the opposite end of the carriage 

 is wound round the axle of a large fan and causes it to turn 

 in a vessel of oil, thus regulating the motion. This fan is held 

 by a pin, which at the time of an earthquake is drawn back 

 by means of an electromagnet in connexion with a circuit- 

 closer. When this occurs, the weight as it falls pulls the 

 carriage slowly along, while the pointers of a seismograph 

 write the undulations of the earth on its surface. 



A check on the regularity of the plate's movement is ob- 

 tained by causing a small pendulum, to which is attached a 

 flexible spring, to mark time upon it. This pendulum is set 

 free during the first turn of the fan, which knocks out a 

 prop used for holding the pendulum in a deflected position. 



The plate is subsequently coated with photographer's var- 

 nish, and then photographed by the "blue process" so well 

 known to engineers. 



(b) Drum-Clock Receiver. — Another very convenient me- 

 thod is to take an ordinary clock, and place a drum on an axis 

 formed by a prolongation of the key-pin of the striking appa- 

 ratus. Fasten back the detents governing the striking, and 

 attach the starting-apparatus to the fan. If the clock is 

 governed by a balance-wheel, it is easy to arrange the starting- 

 lever so that, by starting the drum, it will stop the clock. If 

 the clock is governed by a pendulum, the best method is to 

 catch the pendulum. 



The drum on the axis of the striking-spring is covered with 



