ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 99 
eylinders adjusted to their places, one set running in an east and west, and the 
other in a north and south direction, so that in whatever direction the hori- 
zontal component of shock may move, the overthrown cylinders, of one or the 
other set, shall fall transversely to the lengths of either of the plank bases, and, 
lodging on the sand-bed, remain ewactly in the position as to azimuth in which 
they were overthrown. If now a shock of any horizontal velocity capable of 
overthrowing some of the cylinders, but not all of them, arrive, it will throw 
down at once all the narrower ones, and up to a certain diameter of base. 
For example, suppose a N. and S. shock, of such velocity as to overthrow 
W 6, W 5, and W 4, leaving W 3, W 2, and W 1 standing; then V will 
have been greater than the velocity due to the overthrow of W 4, and less 
than that due to the overthrow of W 3, and, within those limits, may be 
found from the preceding equation. The cylinders here overthrown, W 6, 
W 5, and W 4, will be found with their axes lying N. and S., at rest upon 
the sand-bed. The cylinders N 6, N 5, and N 4, will be also overthrown; 
but in this case they will fall in the line of their own plank bases, and may 
roll and so give no indication as to direction of shock in azimuth. Hence the 
necessity for two sets of cylinders; one set, however, will be sufficient, if 
space enough be provided between the cylinders, and if each be placed 
upon a cylindrical and separate basis of a diameter equal to its own, and in 
height equal to the depth of the sand-bed. 
This form of instrument, then, is capable of giving approximate deter- 
minations of— 
Ist. The velocity of the horizontal component of shock, neglecting the 
vertical component, which may be done where the angle of emergence is not 
great. 
2nd. The azimuthal direction of the horizontal element of shock. 
3rd. Its absolute direction of primary movement, viz. the direction of 
translation of the wave, which always coincides with the direction of mole- 
cular movement of the elastic wave itself, in the first half of its complete 
phase: e. g., if the wave show a N. S. azimuth, by the line of direction of axes 
of the overthrown cylinders, and these be thrown to the northward, then the 
wave has traversed from S. to N. 
4th. The exact time of the transit of shock may be also indicated if the 
narrowest cylinders, N 6 and W 6 be connected with a clock, so as to stop 
it at the moment of overthrow by the very simple means which I have 
pointed out in the ‘Admiralty Manual’ (art. ‘ Earthquake,” sec. vii., p. 208, 
2nd edit. ), inasmuch as, by hypothesis, the narrowest cylinders will be always 
overthrown. 
' A single cylinder or prism, however entirely distinct from either seismo- 
metrical set, and of even less stability as respects shock, may be with 
_ advantage adopted as the means for stopping the clock by the above method, 
_ which is capable of giving the time to within 0-1 of a second. 
- It is obvious that the application of the principles involved in this form 
of seismometer to observations made upon the recent overthrow of walls, 
columns, or other such objects to be found in regions which may have been 
‘visited by earthquakes, is capable of giving also approximate measures of 
‘velocity and direction of shock. This class of seismic observation will, I 
hope, be found more fully developed elsewhere. 
~ In conclusion, one other method of indirect seismometry remains to be 
explained, which does not require the aid of any seismometric instrument. 
‘The facts upon which this method depends have been alluded to in the Re- 
‘port on Earthquakes of 1850, p. 35. It has been long observed that, in 
ive surfaces of country that have been exposed to the effects of shock, 
H2 
