EARTHQUAKE PHENOMENA. 423 



of an inch square; adjust the length and position of the log, so that 

 it shall form a support for the end of the lath c, as in the figure. 



It is obvious that the moment the log f is overthrown by a shock 

 the lath will drop at the end c, (which should be slightly weighted,) 

 and the teeth or rack nicks catching the pin b of the pendulum-bob 

 will stop the clock: on examining which, the dial will show the time 

 to a second when the shock took place, and the tooth in the rack will 

 show at what part of the arc of vibration the pendulum was arrested, 

 which will obviously give the time of the shock to a fraction of a 

 second. 



This method may be applied to any form of clock, and with any 

 length of pendulum. Observation should be accurately made by a 

 seconds watch, or still better, with a Breguet chronoscope, which 

 readily reads to T V of a second, of the total duration of the shock in 

 passing the observer's station; and the observer should endeavor to 

 record the number of small, rapidly recurrent shocks, and their total 

 duration at each epoch. 



Returning now to observations to be made upon the earth-wave, 

 indirectly or by its effects, consisting principally of — 1. Observations 

 on buildings and other objects, fissured, dislocated, or thrown down. 

 2. On bodies bent, projected, displaced, or inverted. 3. Bodies 

 twisted on a vertical axis, with more or less diplacement. Some of 

 the most precious data are to be obtained, by the observation, after 

 the earthquake, of the fissures and dislocations of buildings. Choice 

 should be made of buildings rectangular in plan, of tolerably good 

 masonry, and but one-story in height, such as churches, &c. ; and as 

 often as possible such should be chosen as have their principal Avails 

 running north and south and east and west. These may be advan- 

 tageously described as Cardinal buildings. With a given force of 

 shock, and in buildings of generally similar form, the extent of fis- 

 sure depends chiefly upon the character and "bond" of the masonry. 

 The direction of fissures is nearly vertical when due to nearly hori- 

 zontal shocks; but those of steep emergence produce highly-inclined 

 fissures, often crossing each other. Cardinal buildings exposed to 

 shocks, the horizontal component of which is either N.S. or E.W., 

 are fissured chiefly near the quoins, and through the walls whose 

 planes are in the line of shock. But irregularities in the mass of the 

 walls, due to apertures, the brittleness of masonry, and slight devia- 

 tions from cardinal direction in the shock itself, frequently produce 

 subordinate fissures in the walls transverse to its line of movement, 

 when these are not overthrown. 



"When the direction of shock is diagonal to the plan of the walls, a 

 triangular mass is dislodged from the upper part of each of the adja- 

 cent walls, at the quoin from which the wave comes. With steep 

 emergence such masses may be dislodged from both quoins at the 

 same end of a rectangular building, which is that towards which the 

 wave moves. Heavy roofs and tiled or arched floors suffer most from 

 shocks of steep emergence. Buildings situated near about vertically 

 over the centre of disturbance present evidence of dislocation in 

 every direction, i. e.. by the vertical, or nearly vertical, emergence 



