ble that of the other. These
pairs, with their supporting columns, are shown in plan, where one pair
lies N. and S., another E. and W., a third N.E. and S.W., and the other
N.W. and S.E. It will be observed that metallic bars pass from the
pillar, _P_, over the ends of all the long branches, and similar bars
pass from _R_, over the ends of the short branches; the pillars
themselves, as in the case of the other instruments, are each connected
with one pole of a Daniell's battery, the connections including the
electro-magnets, _C_ and _D_. The description of one U tube, _n_, will
apply to all the others; _n_ is partly filled with mercury, and an iron
or platinum wire, _o_, suspended from the bar above the short branch,
dips into the mercury therein, while another platinum wire hung from the
bar over the mouth of the longer branch, has its end very close to the
surface of the mercury in that branch. Any shock which is not
perpendicular in direction to the plane of the branches of the U will
cause the mercury to oscillate in the tubes, and more sensibly in that
with the smaller diameter; when it rises up in the latter, so as to
touch the platinum point, the connection between _P_ and _R_ is made and
the circuit completed, starting the action of the electro-magnets _C_
and _D_, which record the shock, as already described. By having the
planes of the tubes set in the different azimuths, already mentioned,
one or more of the pairs is sure to be acted upon, and by observing in
which the oscillation takes place the direction of the shock is supposed
to be ascertained. Besides this, each long branch of the U, viz., that
of smaller diameter, has a small ivory pulley, _q_, fixed above it, over
which passes a single fibre of silk, with an iron float at one end,
resting on the surface of the mercury; at the other end of the fibre
hangs a counterpoise; fixed to the pulley is a fine index hand, capable
of moving along a graduated arc. When the shock takes place the mercury,
rising in the long branch, raises the float on its surface, the silk
fibre at the same time makes the pulley revolve with its index hand,
which afterwards remains stationary, as the counterpoise prevents the
float from sinking again with the mercury. The reading on the graduated
arc is thus a measure of the movements produced in the instrument by the
horizontal element of the shock, and is supposed to measure that shock.
It is assumed that in all these instruments sh