gg REPORT—1858. 
mass of the mercury is in unequal columns in each tube, so that 
its displacement is dependent solely on inertia; it therefore sympa- 
thizes with the movement of the earth-wave, emergent in whatever 
way; in the latter, the correctness of indication of the instrument de- 
pends not at all'on the inertia of the mercury, but simply upon the 
alteration of relative surface-level in the opposite legs of the U-tubes, 
when the latter are thrown more or less out of plumb by the sup- 
posed undulation of the earth’s surface at the transit of the shock. 
3 (6). Kreil’s ingenious instrument is not devoid of some serious objections. 
It partakes of those common to all pendula; and these will be further 
perplexed when the annular dish #77 is filled with mercury, which 
will form a second (fluid) attached pendulum with a time of oscilla- 
tion of its own, and differing largely from that of the pendulum which 
suspends it. Very little value, however, can be attached to the indi- 
cations to be afforded by the very small amount of mercury that can 
be caused to spill out, owing to the very small are of oscillation that 
the whole instrumeifit can be afforded to make by construction. The 
most serious objection, however, lies in the method of flexible sus- 
pension adopted for the whole pendulous part of the instrument, viz., 
by two short thin plates or ribbons of tempered steel, whose respective 
vertical planes are at right angles to each other, the object being to 
allow of oscillation in any direction, but prevent rotation upon the 
vertical axis. Whenever a somewhat energetic disturbance shall 
be given to a pendulum so suspended—so as to cause oscillation in 
a vertical plane, diagonal to the crossing planes of the two suspend- 
ing ribbons, ¢orsion of each of these arises, and violent twisting 
movements (by jerks) of the pendulum itself result, producing sudden, 
jerking, rotatory oscillations of the bob (the cylinder containing the 
clockwork, &e.) round the axis of the pendulum. These must of 
course interfere with and derange any true results as indicated by 
the tracing-pencil, which must also record all such accidental 
moments, and probably derange the rate of the clock. 
There does not appear, however, to be any insuperable difficulty in 
devising another mode of suspension for the instrument, that might 
at least remove this defect. 
Such are some of the main objections to the seismometric instruments 
themselves, hitherto proposed. It remains to consider the difficulties intro- 
duced by the nature of the movements we require to observe and record 
with them, as they actually take place in nature. What we want to find 
is the true direction of emergence of the normal earth-wave, with its dimen- 
sions and velocity, at a given point upon the earth’s surface. This, were the 
earth a perfectly homogeneous elastic solid, though much easier, would still 
be attended with grave difficulties ; one of these, which must ever remain 
instrumentally insuperable, consists in the fact that the emergent wave on 
leaving the free outlying stratum of the earth’s surface, differs both in dimen- 
sions and in velocity from the same wave in the previous parts of its deep 
transit. Future and more perfect knowledge of the laws of imperfectly elastic 
bodies in wave-transmission will, it may be expected, enable us to calculate 
the latter from the observed final part of the transit. 
Far, however, from being homogeneous, every portion of our earth’s crust 
that we are acquainted with consists of various “ couches,” or masses of 
materials, differing in elasticity, density, and degree of discontinuity, in the 
character, directions, and openness or closeness of the discontinuant fissures, 
