ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 79 
2(a). 
3 (a). 
4. (a). 
5(d)a 
of the earth-wave. Small transversal vibrations, arriving almost 
along with the earth-wave, as well as the effects of the form of the 
dish, and of its delivering-spouts or adjutages, disturb the initial 
simple surge of the mercury across the diameter of the dish, and pro- 
duce reflected and other secondary surge movements of the mercury, 
which traverse round the circumference of the dish, and spill out 
more mercury in irregular gulps. The final result is, that no reliance 
whatever can be placed upon its final indication, as to the plane of 
the earth-wave transit having passed through the centre of gravity of 
that semicircle of cups which are found to contain the most mercury. 
The result is not materially different if the line of transit of the earth- 
wave be perfectly horizontal. This instrument gives no information 
whatever beyond a most uncertain approximation to the direction 
of the horizontal component of the earth-wave transit. 
The same objections generally apply to this form of instrument, and 
one in addition, viz. that a viscid liquid like molasses must always 
give indications short of the truth as to excursion in the dish due 
to any given shock, and the more so as it is more tenacious and 
approaches nearer to a solid; and as we have no correct means of 
measuring viscidity, even assuming it constant for the same liquid, 
nor any certainty that the specific gravity of such liquids remains 
constant (it is certain molasses will not remain of the same density 
in any climate for any considerable length of time), so observations 
made through their means at different times and places can never 
be comparable. 
The same objections that apply to 1 (a) apply to the tub of coloured 
water, but in a mitigated degree, the diameter being large, the 
volume and depth of the liquid great, and the cylindrical sides of the 
tub free from any apertures or inequalities. The initial surge gives 
a much more distinct indication of direction than in either of the 
preceding instruments; and it does not very frequently happen that 
a diameter may not be found approximating, with tolerable certainty, 
to the plane of earth-wave transit. But in cases where the normal 
wave is preceded or accompanied by very appreciable transversals, 
those violent tremors that are now known as the frequent ac- 
companiments of the actual shock—the water-tub seismometer will 
give no indication, or an uncertain one, unless watched and re- 
marked as to transit-direction at the instant of the occurrence of the 
shock. 
Tubes partially filled with mercury give almost unobjectionable 
indicatious as to direction of transit. Their evils are too great 
delicacy or sensitiveness, for the observation of that class of earth- 
quakes of mean power, which are the most important to be studied, and 
by which they are completely deranged occasionally, while they are 
continually being disturbed in such a seismic region by small tremulous 
movements that are unimportant to notice. As respects their indi- 
cations of velocity and dimensions of the wave, they are liable to the 
objections already noticed as applicable to all pendula. 
nd 6 (6). The main disadvantages of these constructions, viz. the 
suspended and the inverted solid pendulum have been already 
pointed out; it may be added here, however, that with the inverted 
pendulum of Forbes, the supporting spring is more or less crippled 
down, by asharp vertically (or nearly vertically) emergent shock, which 
gives a lateral movement (greater or less) to the pendulum, as though 
