76 Mr. Mater on the Dynamics of Earthquakes. 
direction, from vertically upwards, to horizontally, in any azimuth, through the 
surface and crust of the earth, from any centre of impulse, or from more than 
one, and which may be attended with tidal and sound waves dependent upon 
the former, and upon circumstances of position as to sea and land. 
Our knowledge is not at present sufficiently advanced as to the laws of large 
waves of elasticity passing through solids, to be able to do more than rudely to 
predict the many strange alterations of the original wave which will be produced 
by particular local circumstances, such as by its passing from low to high land, 
from hard to soft rock, or vice versd, round great axes of hard rock, and round 
great bodies of inland waters, or through masses of softer rock, reposing on much 
harder, and suddenly reduced as to depth, in which case a single shock will pro- 
bably be divided into two or three in quick succession, and varying in direc- 
tion. 
I return, therefore, now to the great sea wave, which, after having been elevated 
at the same origin and instant of time with the earth wave, we left pursuing its 
course to land, over the deep ocean. While over the profound depth of the 
ocean, its course, though greatly slower than that of the earth wave, will still be 
exceedingly rapid ; and, like all deep sea tidal waves, it will have an equal slope 
before and behind; its amplitude will be very great, and its slope so gentle, that 
it might even pass under a ship without being noticed. It is here, but a long, 
low swell, of enormous volume, and with an unbroken surface; so it continues 
until it reaches the “‘edge of soundings,” and here a new phenomenon is mani- 
fested. It is capable of proof theoretically, and the fact is daily observable in 
tidal estuaries or rivers, that when the tidal wave (such a wave as we are here 
considering) leaves the open sea, its front and rear slopes being equal in length 
and similar in form, and advances into the shallow water of an estuary or near the 
shore, its front slope becomes short and steep, and its rear slope long and gentle ; 
advancing still further, the rear slope becomes more and more flattened about the 
middle of its length, then becomes depressed at this point, and as the wave con- 
tinues to advance this depression deepens, and at last the wave becomes broken 
into two or more smaller waves (see Plate IL., figs. 5, 3, and 7) ; and when the 
depth of water below its mean surface becomes less than the altitude of these 
waves, they will lose their equilibrium, become broken, and topple over, falling 
in breakers” upon the shore. It is unnecessary here to enter more fully 
