70 Mr. Mautet on the Dynamics of Earthquakes. 
its direct course, and, passing horizontally through deep, elastic strata, may reach 
the surface vertically at distant points, and produce those shocks upwards, from 
directly below, sometimes spoken of in earthquake narratives. 
Again, if the seat of disturbance be at a great depth, the earth wave must 
reach the surface in its immediate neighbourhood in vertical and diagonal lines of 
undulation, and produce similar effects as to shock as those last spoken of. This 
is the case with shaken countries close to volcanoes, where the seat of impulse is 
often close underneath.* 
The earth wave which reaches the shore (from an origin beneath the sea) is 
a real undulation of the surface: it is a wave, whose magnitude depends upon 
the elasticity of the crust of the earth which has received the original impulse, and 
upon the nature and force of that impulse; whose vertical height appears to vary 
from a fraction of an inch to several feet; generally, its vertical height seems to 
be from two to three feet in great earthquakes, and its length variable, according 
to the depth and elasticity of the strata which it affects. 
Now, at the moment of its origin, the earth wave is vertically below the great 
sea wave, which is raised by the initial impulse upon the surface of the ocean ; 
both move landwards together, but the earth wave rapidly outstrips the sea wave, 
because, while the velocity of the former depends upon a function of the elasti- 
city of the crust of the earth, the velocity of the sea wave, depends upon a func- 
tion of the depth of the sea (see Plate II. Figs. 2, 3, p.). Leaving, therefore, 
for the present, the sea wave in its progress towards the land, we will follow the 
earth wave in its silent and rapid transit beneath the deep ocean, and trace what 
takes place when it gets into soundings, and at last reaches the devoted shore. 
While passing under the deep water of the ocean it gives no trace of its pro- 
gress at the surface, in all probability ; but, as it arrives in soundings, and gets into 
water more and more shallow, the undulation of the bottom, the crest of the long, 
flat-shaped earth wave, brings along with it—carries upon its back, as it were,—a 
corresponding aqueous undulation, slight, long, and flat, upon the surface of the 
water. This, which, adopting Airy’s nomenclature, might be called the “‘ forced sea 
wave” of earthquakes, has no proper motion of its own; it is simply a long, low 
ridge of water, pushed up at the surface by the partial elevation of the bottom 
* See Professor Ferrara’s Account of the Earthquake at Catania in 1833, Silliman’s Journal, 
vol. ix, p. 216, 
