ocks, however small, can
be recorded with certainty by adjusting the distance between the
platinum points and the mercury.

The arrangement of Daniell's battery used for the seismograph is shown
in Fig. 4, where, for convenience of cleaning, the copper element is
made of wire (about No. 8 Birmingham wire gauge) coiled flat without the
spirals touching. Crystals of sulphate of copper are placed at the
bottom of the outer cell, into which water is poured; and the inner
cell, into which the zinc plate goes, is filled with siliceous sand.

In addition to the above some instruments of a rougher description are
employed as checks. Thus, at the foot of the pillar, _G_, there is a
wooden trough with eight holes, facing as many equidistant points of the
compass (two of them shown in section) round its inner circumference;
mercury is poured into the basin until its level is nearly up to the
lips of the holes. The effect of a shock is to throw some of the mercury
into one or more of these holes, and the greater the oscillation the
more mercury is thrown into the cells through the holes. The screws
shown outside are for drawing off the mercury from the cells, when its
quantity can be measured. The direction of the shock is shown by seeing
which cells are filled with mercury. This is the old Cacciatore
seismometer which has been long employed in Italy. (See 4 "Report of
British Association, 1858," p. 73), and Daubeny's "Volcanoes," Appendix.
The following is another contrivance. From the arm of the pillar, _G_, a
fine metal wire hangs, with a metal ball at its end, which, by its
oscillation, thrusts out one or more light glass tubes, set horizontally
in a stand, as shown in Fig. 3. The two rings are of wood, and the glass
tubes pass through holes in them; small leather washers are placed
outside the outer rings; the displacement of one or more tubes is
assumed to measure the horizontal element of the shock. By means of this
apparatus the time of the first shock is recorded, as well as the
interval between the shocks, and the duration of each; their direction,
whether vertical or horizontal, is given, as also the maximum of
intensity. Professor Palmieri has the instruments examined three times a
day, and an assistant-observer is always at hand to attend to the bell,
and put back the apparatus to its normal position for fresh observation.

It has been stated that this instrument is sensible to most of the
shocks which occur in the M