416 



EARTHQUAKE PHENOMENA. 



of four tubes — say, from south to north — that the marker in the 

 southern tube will be pushed southward a certain space by the 

 movement of the mercury, and will remain to point out the space 

 when the mercury has returned to rest. If the motion be in some 

 direction between two adjacent tubes — say, from southeast to north- 

 west — the markers in the south and east tubes will both show a cer- 

 tain motion, equal in this case, but in others with a certain ratio to 

 each other, by which the direction between the cardinal points may 

 be calculated. 



For the vertical element: let the barometer tube, Fig. 5, be filled 



with mercury so that about six inches shall stand in the 



open end a, into which thrust a marker, as in Fig. 4, and 



about twelve inches in the sealed limb; place this vertically, 



5, and secure it to a fixed mass of rock, a heavy low building, 



or large tree; from the amount co which the marker is found 



moved up in the tube the altitude of the wave may be found; 



and it is obvious that, by the conjoint indications of the four 



a horizontal tube-markers, and this vertical one, the direction 



I of emergence of the wave is determinable. 



tS These instruments are of the nature of fluid pendulums, 



» their use assumes the velocity of the earth -wave constant, 



i and, in common with all pendulums, they have certain dis- 



i advantages as seismometers. — (See Mallet, 4th Report Brit. 



Ass.) If 



'=•7 



be the time of oscillation of any solid pendulum whose length is I, 

 then 



77 V 9 (sin- a 



-}- sin. a') 



will be the time of oscillation of any such fluid pendulum, a and a' 

 being the angles of inclination of the limbs of the tube to the hori- 

 Where these are parallel and vertical, sin. a = sin. a' =z 1 and 



zon. 



t =*V 



0-5 X I 



9 



They are much superior to common solid pendulums, where the 

 dimensions of the shocks are small; but where these are great and 

 very violent, heavy solid suspended pendulums will be found more 

 applicable; the length of the seconds pendulum for latitude Green- 

 wich will always be desirable. Where fluid pendulums are not at- 

 tainable, a solid pendulum to answer some of the purposes may be 

 thus prepared: Fix a heavy ball, such as a four-pound shot, at one 

 end of an elastic stick, whose direction passes through the centre of 

 gravity of the ball; a stout rattan will do. Fix the stick vertically 

 in a socket in a heavy block of wood or stone, and adjust the length 

 above the block as near as may be to that of the seconds pendulum 

 for Greenwich. Prepare a hoop of wood, or other convenient mate- 



