ON THE FACTS AND THEORY OF BARTHQUAKE PHENOMENA. 95 
B(N. and S.) and B (E. and W.) from their respective cardinal and ver- 
tical planes, will indicate the actual azimuth of the horizontal component of 
the earthquake wave—giving this indication in two ways, each controlling 
the other,—viz. by direction of throw as stated, and by distance of horizontal 
traject, which will be proportionate to sine and cosine 0. 
The stop 6, it should be remarked, is hollowed at contact with each ball, 
so as to embrace 90° of its horizontal great circle; so that in case 6=45° 
from the meridional or the E. and W. planes, the balls cannot slip aside, but 
must be thrown in the same direction, the extreme angles of the stop then 
passing through the plane of motion and centre of gravity of the balls. 
Figs. 5 and 6 show in plan the relative positions of the N.S. and E.W. 
instruments, the upper portions alone being represented, and not at the ne- 
cessary distance apart. 
These instruments singly, then, give us the velocity of the wave and its 
direction in azimuth with considerable accuracy ; but their full value would 
only be ensured by placing three such seismometers within a given district 
(as already stated for the former instrument) and connecting them all by 
galvanic wires, so that the indications of the three shall be recorded by a 
single clock register. We then have the ¢ime of arrival of the shock at each 
seismometer given with perfect accuracy, from which both its horizontal 
velocity and azimuth may be computed; and the relative positions and 
distances apart of the several seismometers being known, the true direction 
of emergence of the wave, and the point of the surface vertically over the 
origin, and the depth of the focus itself may be computed. The two following 
methods of computing these are due to Professor. Haughton, of Trinity 
College, Dublin, who communicated them to the Geological Section of the 
British Association at Dublin, on the occasion of this report being read, and 
-from whom I have received them for publication here. 
The determination of the “ coseismal line’’—a term first used by me at the 
suggestion of Sir John Herschel, to signify, the crest of the simultaneously 
emergent earth-wave upon the earth’s surface at any moment of its progress 
—is the same thing as determining the direction of its motion on the surface, 
a horizontal tangent to the coseismal line at any point being always ortho- 
gonal to the direction of motion. 
Given the Times of an Earthquake Shock at three places, to determine its 
Horizontal Velocity and Coseismal Line. 
Zz 
we Pad 
‘Let A, B, C, denote three stations at which the time of arrival of the earth- 
: quake shock is determined by the seismometers or other means, and let 
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