DEPTH OF EARTHQUAKE-FOCUS. 



129 



and opened, and the recording stile worked by the shaking of a metallic 

 bob, hung by a delicate spiral spring above a mercury-cup ; the shak- 

 ing of the bob being determined by the tremor of the earth. Such an 

 instrument records the exact moment of occurrence of earthquake- 

 shocks, however slight ; also, the moment of passage of every wave and 

 its time of oscillation ; and if there be' more than one such instrument, 

 the moment of occurrence at different places gives the velocity of the 

 surface- wave v'. 



If we desire to record not only the time but also the character of 

 the earth-movement, then a different kind of seismograph is neces- 

 sary. The principle of all these is the principle of a pendulum. If 

 we have a pendulum with a heavy bob swinging freely, when an earth- 

 quake arrives, the 

 bob will stand still, 

 while the earth 

 moves beneath it. 

 This relative move- 

 ment of the pend- 

 ulum may be re- 

 corded by suitable 

 device. But an 

 ordinary freely 

 swinging pendu- 

 lum moves often 

 too largely, and 

 continues its move- 

 ment after the ces- 

 sation of the cause. 

 What we want is a 

 pendulum which 

 will stand indiffer- 

 ently in any posi- 

 tion (astatic). One 

 of the best forms 

 of instrument yet 

 devised is that of 

 Prof. Ewing (Fig. 

 107). It consists 

 essentially of three pendulums swinging in the manner of a bracket or 

 a gate, and placed in three rectangular planes ; (1) vertical north and 

 south, a j (2) vertical, east and west, b; and, (3) horizontal, c. The 

 horizontal one is retained in position by sensitive spiral springs. Stiles 

 are fixed to these pendulums in such wise as to record on a circular 

 smoked glass plate rotating in a horizontal plane. No. 1 records the 

 9 



Fig. 107.— Ewing's Seismograph : a and b. horizontally oscillating 

 pendulum; c, vertically oscillating pendulum: d. driving clock; 

 e, time-recording clock. (Taken from a photograph of one at the 

 University of California.) 



