88 REPORT—1858. 
cast a hollow quadrilateral prism, g, which will be called “ the block,” provided 
with four “lugs” to receive the pivot-screws 7,7, ”,”. The table aa, sup- 
ports two similar cast-iron inclined planes ¢, 2, having for their entire length 
the trough-shaped section as shown in fig. 3. These planes are fixed to 
the table aa, by the pivot-screws ”,m, and by the adjusting-screws m, m 
beneath, so that by means of the latter, the inclination of either plane may 
be altered or fixed, being otherwise free to rotate in a vertical plane, 
within certain limits, round the pivot-screws 7,2, so as to alter the angles 
of inclination. 
Upon each of these inclined planes, is placed a large heavy ball, formed of 
a hollow sphere of hard gun-metal, of about 0°3 of an inch in thickness, 
truly spherical and polished outside, and filled up solid with lead. These 
balls are adjusted in diameter, to the breadth and form of the inclined planes 
(as in fig. 3), so as freely to roll along, with but two points of contact. _ 
When the planes 2,2 are adjusted at equal inclinations, the balls B, B, rest 
at their lowest ends, and are laterally in contact with, and supported by, the 
hard wood stops 7, 7, driven (from outside inwards) through, and well-fitted 
in, corresponding rectangular horizontal “slots” in opposite sides of the 
block g,—the end of each wood stop being curved to fit the surface of the 
balls, in a horizontal great circle, and so that the plane of the stop passes 
through the centre of gravity of the ball. Through each wood stop there 
pass the e— and e+ extremities of a galvanic conducting-circuit of thick 
copper wires, placed at about an inch apart, where they pass parallel to each 
other, through the wood stop, with their extreme ends coinciding with the 
surface of the stop next the ball, and being amalgamated ; so that while ever 
the ball reposes in contact with the wood stop, the galvanic circuit remains 
completed, through the ball, between the ends of the wires, but is broken 
the moment the ball is removed from contact with them. 
For one complete seismometer there are two such instruments as have 
been thus described,—one placed, as in fig. 2, in a N. and S., and the other 
in an E. and W. direction, as respects their length, and having thus four 
inclined planes and balls, each with its own distinct galvanic circuit from 
one common battery. A clock placed in the observatory carries round a 
cylinder with ruled paper, and each of four pencil markers continues to 
describe an unbroken line thereon so long as the balls are in contact with the 
blocks (or wood stops and galvanic poles); but (by an arrangement pre- 
cisely similar to that described for my fluid pendulum seismometer—Trans. 
Roy. Irish Acad. vol. xxi. p. 107) the moment any ball ceases to be in con- 
tact with the block, and for as long as it is so, the pencil is withdrawn, and 
leaves a break in the otherwise continuous line traced by the rotation of the 
paper. No part of this clockwork registering-arrangement is shown in the 
Plate, as several modifications of it are practicable, and no one in parti- 
cular is essential to the principle of the seismometer before us. 
To illustrate the mode of action of the instrument,—returning to fig. 1, 
suppose it to be the N. and S$. one, and adjusted so that the bar & is truly 
vertical, the parallel sides of the inclined planes ¢ and ¢ truly én directum, 
their angles of inclination to the horizon the same. Then if the arrow Q 
represent the direction of emergence of an earthquake-wave (supposed here 
to be in the plane of the meridian, and from S. to N.), at the first instant 
that the wave reaches the instrument, the bard, and table a a, with all they 
carry, will commence to descend and to compress the spring e by their inertia, 
with a velocity dependent upon the vertical component of the wave, which 
carries up the frame ec vertically. Also at the first instant of arrival of 
the wave, the ball B,, in virtue of its inertia, will move off from the block 
