78 i REPORT—1858. 
of the direction of surface-transit (horizontal component) of the earth-wave. 
However finely suspended the pendulum—if acted on by gravity only, or, 
however constructed if by elasticity or by elasticity and gravity, it is found 
impracticable to produce an instrument that shall make even the second half 
of its very first complete vibration strictly in the plane of the original dis- 
turbance, é. e. in that of the wave’s transit. If, for example, any one of the 
Fig. 4. 
instruments 5 (6), 6 (b), or 7(), be caused to make a semivibration by a 
movement of the nature of one horizontal jerk, and strictly in one vertical 
plane a 6 (fig. 4), the trace made will in most instances be found thus; ed, the 
first semivibration, is made sensibly in the plane of movement, but the re- 
turning complete vibration de, is found diverging from it through a sensible 
anglecde. If the vibration of the instrument be suffered to continue, its trace 
rapidly becomes an extremely elongated ellipse, whose excentricity constantly 
diminishes, as well as the actual dimensions of both its axes, until the in- 
strument comes to rest, after tracing thus a mass of elliptic spirals, from 
which nothing certain can be gathered as to direction in some instances— 
in which, at best, it is only possible to arrive at a probable direction of 
originating impulse, by drawing a mean major axis through all these closed 
curves. 
Constructively, this evil arises not only from the nature of the suspension, 
if a pendulum of gravity, or, if one of elasticity, from the form, material, 
&c. of the suspending or supporting spring ; but also, in both sorts, from the 
fact that it is practically impossible that the point of suspension (or, in the 
spring, its centre of resistance), the centre of oscillation, and the resultant 
of the various opposing forces of the stile or tracing-point, shall lie in one 
vertical plane, and that that plane shall always coincide with that of the wave’s 
movement; and hence lateral divergence of the pendulum and elliptic spiral 
oscillation. But it is also partly due to the nature of the earth-wave motion 
itself, which is never a purely normal one, but always more or less disturbed 
by small transversals ; so that the initial movement impressed upon the pen- 
dulum is really not exactly that of the wave’s transit. Before entering fur- 
ther, however, upon the subject of the actual perturbations of the superficial 
earth-wave, as now known, and their effects in relation to seismometers, some 
remarks may be advisable as to the special objections which I have either 
observed or experimentally ascertained in respect to each particular arrange- 
ment of the seismometers already described. 
1(a). The Cacciatore mercurial dish.—If the earth-wave emerge with a 
considerable angle from the horizon, and large velocity, the mercury 
first surges up at the side of the dish towards which the earth-wave 
is in transit, and in the direction opposite to its motion; it then, 
after spilling out some of the mercury, commences its return oscil- 
lation, moving in the same direction as the earth-wave, and spills out 
another portion at the opposite side of the dish. The sum of the weights 
so spilled out, taken at either side of a diameter transverse to the 
earth-wave’s vertical plane of transit, will vary with every change 
in the angle of emergence, or in the velocity or in the dimensions 
