26 
PEOF. MELDOLA—THE EAETHQUAKE 
kind occurred; this (i sulphureous vapour ” must have arisen out of 
his inner consciousness. As far as my inquiries have gone, I do not 
think any connection can he shown between any such natural 
phenomena and earthquakes, and in this instance at all events there 
is nothing to show that the disturbance was accompanied by any 
remarkable atmospheric movements. 
Japan (which is very favourably situated for the purpose) is the 
only country in the world where the science of seismology is being 
systematically and practically carried out, with the aid of very 
efficient instruments, by which it is possible to make very accurate 
observations. It had always been supposed hitherto, on the 
assumption of Mallet, that an earthquake was simply a wave of 
elastic compression passing through the materials of the earth’s 
crust, and causing the particles at the surface to oscillate to and fro 
in a closed curve, the amplitude and period depending upon the 
intensity of the originating impulse. Nearly all the calculations 
formerly made with respect to the maximum velocity of displace¬ 
ment as deduced from the projection of bodies were based on the 
supposition that the movement at the surface was of the nature of a 
sudden jerk; but these have been pointed out by Professors Ewing 
and Milne to be devoid of value. The old view of earthquake- 
motion might hold good in the case of an instantaneous impulse 
originating at a point in a homogeneous solid of uniform elasticity 
in all directions, but such conditions are never realised in 
actual earthquakes. In the first place, the disturbance never 
originates at a mere point, but always along an axis of greater or 
less extent, such as a fissure or fault. The vibrations thus set up, 
in travelling to the surface, undergo so many changes of direction 
by reflection and refraction in passing through the very hetero¬ 
geneous materials of which the earth’s crust is composed, that the 
motion of a particle at the surface is, in all cases where the move¬ 
ment has been traced by seismographs, of an extraordinary degree 
of complexity. Professor Ewing, who has had the opportunity of 
recording a large number of earthquakes in the Plain of Yedo, 
in Japan, summarises his results as follows — 
“ Automatic records given by seismographs confirm what has 
been said, on theoretical grounds, as to the complexity which 
earthquake-motions may be expected to present. They show 
that, as observed at a station on the surface of the earth, an 
earthquake consists of a very large number of successive vibrations 
—in some cases as many as three hundred have been distinctly 
registered. These are irregular both in period and amplitude, and 
the amplitude does not exceed a few millimetres, even when the 
earthquake is of sufficient severity to throw down chimneys and 
crack walls; while in many instances the greatest motion is no 
more than a fraction of a millimetre. The periods of the principal 
motions are usually from half a second to a second, but, as has been 
already said, the early part of the disturbance often contains 
vibrations of much greater frequency. The earthquake generally 
begins and always ends very gradually, and it is a noteworthy 
