68 Original Articles. | Jan. 
addressed to the Royal Society of the examination made on the facts 
of the Neapolitan Earthquake of 1857, by employing them to deter- 
mine approximately from their varying character the form of the focal 
surface or cavity, or of the subterranean locus of the centre’ of effort, 
—and the method will no doubt hereafter, when more largely and 
completely applied, yield very important results. Space forbids us, 
however, here to do more than mention it, and refer to the Report in 
question. 
These, then, are the waves produced by a single impulse, and con- 
stituting an Earthquake whose origin is inland. But should the orzgin 
be under the sea, then at the point passed through by the Seismic ver- 
tical and around it, the sea-bottom is, as on land, suddenly upheaved, 
and again dropped down; or it may be, as by submarine volcano, 
actually broken up altogether, and steam, lava, and floods of lapilh, 
and so forth, may be then belched forth under water. In either case 
there is forced up a volume of water upon the sea’s surface just 
above, or several of these in succession, and as each mass falls again 
it assumes the horizontal form of a circular liquid wave of trans- 
lation—and these are propagated outwards over the surface of the sea, 
like the circles or ring-shaped waves on a pond, when a pebble is 
dropped into it. The altitude and breadth of these waves depend 
mainly upon the magnitude of the disturbance of the bottom, and on 
the depth of water above it; the rate of their propagation outwards 
has nothing to do directly with elasticity, it is dependent simply 
upon the square root of the depth of the water traversed by the wave 
on its surface. Ifthe ocean continued everywhere of the same depth, 
and the original impulse came from a single point, or circular disc, 
then the horizontal plan of the crest of any one of these waves would 
always remain a circle; but the depth varies—and as that part of 
the expanded circle which is over a deeper part moves on much faster 
than portions moving over shallow water, or approaching shores—so 
the circles soon get distorted into various other closed curves, and the 
original radial direction of translation outwards gets changed to any 
extent —so that a wave might, without any reflection, even double back 
upon its original line of progress. 
When the long flat swell of such waves, as they are originated on 
the deep sea, approaches the shores and reaches shoal water, their 
fronts become steeper and steeper, and they finally roll in upon the 
shore, as the great sea waves of South American and other Earthquakes, 
so much dreaded wherever they have been once experienced. ‘They 
are often so large that they only topple over as breakers after they 
have rolled in unbroken masses far inland. 
Such was the wave that swept, in one unexpected deluge, thousands 
of people off the Quay at Messina, and which in some South American 
Earthquakes have inundated devoted cities like Valparaiso and Callao, 
with a frowning crest 80 feet in height. Not that the wave while 
it was far out at sea possessed anything like this altitude,—but just as 
the Atlantic tide wave,—when constricted in the Bay of Fundy, or 
in our own Bristol Channel reaches 70 or 40 feet; so does the Harth- 
quake sea-wave rise and get steep in the narrow and shallow waters. 
