104 Mr. Mauuet on the Dynamics of Earthquakes. 
Fig. 5. The diagrams N, O, and P, are sections, showing the change of form at succes- 
sive periods, a, b,c, &c., which a tidal or great sea wave undergoes when advancing into an 
estuary or bay, or into shallow water. The front and rear faces are similar, and equally 
sloped, while the wave is over deep water. As this shallows, the forward slope becomes 
steep and impending, the rearward slope flattened, then hollow; and finally the wave 
becomes divided into two or more smaller waves, as at f and g. N refers to a small tide 
or low wave; O and P to large tides or great sea waves of the largest magnitude. 
Fig. 7 illustrates the phenomena presented by the great sea wave, on coming in shore 
upon a gently sloping beach, which has uniformly shallowed from the deep sea. The 
wave here comes in solitary and unbroken, but with a steep and impending front. When 
advanced to G,a great recession of water from the beach occurs. Advancing still further, 
as in H, the shallowness of the water divides the wave, and it falls in a succession of 
breakers upon the land. 
Comparing Figs. 3 and 7, it is obvious that the most violent and destructive sweep on 
land, of the great sea wave, will occur in circumstances, where the depth of water, close 
in to the land, is just sufficient to carry in the wave as one solitary unbroken mass, with a 
steep and impending front. If the water be profoundly deep close to land, and the latter 
<‘jron bound,” the wave comes in with both slopes alike, and does little or no damage. 
Puate IIT. 
Fig. 2. A section showing the general relations of the earthquake phenomena in their 
successive occurrence. A submarine eruption has just taken place. The earth wave and 
forced sea wave have just arrived at land, marked by a tower falling upon the shore. 
The shock has passed the ships at sea, but the great sea wave and its minor successors 
have as yet not reached them, on their way towards the land. 
Fig. 4 is intended to explain the apparent recession of the sea, at the moment that the 
forced sea wave arrives at the shore with the earth wave or shock. In the section L, the 
undulation of surface of the earth wave or shock is shown (much exaggerated) passing 
along the bed of the sea, which is here shallow, and carrying along upon its crest, and 
with its own velocity, the small aqueous undulation, or heap of pushed up water, denomi- 
nated ‘* the forced sea wave.” In the diagram M, the undulation of the earth wave has 
just reached the shore, has elevated the beach, at the ordinary tide-mark by a height 
equal to its own, and, therefore, has apparently depressed the surface of the water at this 
point by the same amount. The earth wave here leaves the small forced sea wave behind 
it, and the latter breaks and falls in small breakers, or ripples, upon the beach. 
