on Earthquakes in Japan. 359 



tioD. This friction is applied by allowing a vertically placed 

 sliding rod to rest with its lower end, which is pointed, on a 

 plate of glass placed on the bob of the pendulum. This rod 

 is weighted until, for displacements equal to that of the 

 largest earthquake likely to occur, the pendulum is " dead 

 beat."" We have found that when the rod is thus adjusted the 

 error in amplitude due to friction is very small. 



(b) Pendulum Seismometers y with apparatus for registering 

 two or three components of the motion. — These machines are 

 almost identical in principle with the tremor-indicator de- 

 scribed above. For a detailed description see Phil. Mag. for 

 September 1881. 



(c) Torsion-Pendulum Seismograph. — In this instrument 

 the bob of the pendulum is in the form of a flat ring, on the 

 top surface of which a flat disk of smoked glass is placed. 

 The suspending wire of the pendulum is so proportioned that 

 the torsional period is about one minute. Above the glass 

 plate two light connecting rods radiate from the suspending 

 wire in directions at right angles to each other. The outside 

 ends of these rods are hinged to the short arms of two light 

 indices having one axis in rigid connexion with the earth, and 

 the points of their long arms resting on the same radius of 

 the smoked plate. When this instrument is arranged for use, 

 the bob is twisted through rather more than 180° and held by 

 a catch, which at the time of an earthquake can be released 

 by an electromagnet in connexion with a circuit-closer. The 

 bob and glass plate then twist slowly round, and the indices 

 at the same time write two rectangular components of each 

 motion of the earth on the smoked glass. The bob may be 

 arranged so that it is again caught after one turn if thought 

 desirable ; and for very small earthquakes it is well to do 

 so, in order to prevent the subsequent motion of the pendulum 

 from confusing the record. The peculiarity of this instrument 

 is that the bob of the pendulum is used both as a receiving- 

 plate for the record, and as a steady mass for the purpose of 

 working the levers which write it. This gives simplicity and 

 compactness to the instrument; but it is evident that it can only 

 be used in conjunction with an automatic starting-apparatus. 



(d) Conical-Pendulum Seismograph. — For a detailed de- 

 scription, with drawings, of this instrument see Phil. Mag. for 

 September 1881. 



(e) Bracket Seismograph. — This consists essentially of a 

 heavy weight carried at one end of a horizontal bracket which 

 is free to turn on a vertical axis at its other end. When 

 the frame carrying this axis is moved in any direction, except 

 that parallel to the direction of the length of the bracket, 



2E 2 



