418 EARTHQUAKE PHENOMENA. 



transit over the shaken country remains to be observed, and wherever 

 it may be possible to connect three or more such instruments as have 

 been described at moderately distant stations, say 15 to 30 miles 

 apart, by galvanic wires, so as to register at one point the moment of 

 time at which each instrument was affected, the best and most complete 

 ascertainment of transit rate may be expected. Galvano-telegraphic 

 arrangements of the simple character required are become familiar, 

 and are easily setf. to work. The best seismometer to which they can 

 be applied (for voyagers) is that described in Mallet's 4th Report, 

 &c. , page 87, and plate xv; and no surveying ship proceeding to 

 earthquake regions should be unprovided with three such seismom- 

 eters and the requisite time-recording apparatus. 



A still simpler form of rough seismometer suited to the resources of 

 distant and isolated observers remains to be described. It depends 

 upon principles altogether different from those already mentioned, and 

 is most applicable to seismic districts where the angle of wave emer- 

 gence is not steep — i. e., where shocks are usually nearly horizontal. 



Every body overthrown by an earthquake shock is upset by its own 

 inertia causing it to move in the opposite direction to that in which the 

 ground has moved under it. Thus a Avail falls towards the south if 

 the shock passes across its length from south to north, and if any 

 such homogeneous parallelepiped or right rectangular prism, standing 

 on end upon a level surface, be so upset by its own inertia, the sup- 

 porting surface being suddenly moved beneath it, in the direction of 

 its own plane, (as by the horizontal component of an earthquake 

 shock,) it may be shown that the velocity of the surface must be 



v* = i q I^Tw x n-™-°\ 



where a is the altitude of the solid, L its diameter of base or thick- 

 ness, and 6 the angle formed by the side, and a line drawn through 

 the centre of gravity to the extremity of the base and V = 2 g h. 

 This velocity is independent of the density or material of the solid, 

 because the oversetting force, being its own inertia, is always pro- 

 portional to the density. 



This is the foundation of all accurate and useful observation of dis- 

 located and overthrown buildings in countries that have suffered by 

 earthquake, and by which not only may the direction of (the horizon- 

 tal component of) the earthquake shock be obtained, but a close 

 approximation made to its velocity. 



With a given velocity, V, therefore, it is possible to assign the 

 dimensions a and b, such that the solid shall just overset, and with 

 this velocity a similar solid, but having 6 greater, shall remain un- 

 moved: assuming always, that friction against the supporting surface 

 gives sufficient adhesion to prevent sliding. 



If, in place of a square prism, such as a wall, the solid be a right 

 cylinder like a column, the diameter of its base being b, then 



•xro 15 b- -f- 16 a 2 w .-„-— — — ... a . 



V- = —J—^ X g a 2 -+- b~ (1 - cos 6). 



1 -I (If A / 



This gives the means of constructing a seismometer of great simpli- 



