t between the interior and the exterior, will take place by
occasional little jumps at this particular centre. The fact that there
is this weak spot at which small adjustments are possible may provide,
as it were, a safety-valve for other places in the same part of
the world. Instead of a general shrinking, the materials would be
sufficiently elastic and flexible to allow the shrinking for a very
large area to be done at this particular locality. In this way we may
explain the fact that immense tracts on the earth are practically free
from earthquakes of a serious character, while in the less fortunate
regions the earthquakes are more or less perennial.

"Now, suppose an earthquake takes place in Japan, it originates a series
of vibrations through our globe. We must here distinguish between the
rocks--I might almost say the comparatively pliant rocks--which form
the earth's crust, and those which form the intensely rigid core of the
interior of our globe. The vibrations which carry the tidings of the
earthquake spread through the rocks on the surface, from the centre of
the disturbance, in gradually enlarging circles. We may liken the spread
of these vibrations to the ripples in a pool of water which diverge from
the spot where a raindrop has fallen. The vibrations transmitted by
the rocks on the surface, or on the floor of the ocean, will carry the
message all over the earth. As these rocks are flexible, at all
events by comparison with the earth's interior, the vibrations will be
correspondingly large, and will travel with vigor over land and under
sea. In due time they reach, say the Isle of Wight, where they set the
pencil of the seismometer at work. But there are different ways round
the earth from Japan to the Isle of Wight, the most direct route being
across Asia and Europe; the other route across the Pacific, America, and
the Atlantic. The vibrations will travel by both routes, and the former
is the shorter of the two."

TRANSMISSIONS OF VIBRATIONS

Some brief repetition may not here be amiss as to the products of
volcanic action, of which so much has been said in the preceding
pages, especially as many of the terms are to some extent technical in
character. The most abundant of these substances is steam or water-gas,
which, as we have seen, issues in prodigious quantities during every
eruption. But with the steam a great number of other volatile materials
frequently make their appearance. Though we have n