The difference in the height of waves, as observed at places lying close to each other, is probably due to the configuration of the coast, the interference of outlying islands, reefs, &c.—causes which would produce similar effects in the height of tide. As a wave approaches shallow water it gradually increases in height, its front slope becomes steep, and its rear slope gentle, until finally it topples over and breaks. This increasing in height of waves is no doubt connected with the destruction of Talcahuano and Callao, which are situated at the head of shallow bays. Valparaiso, which is on the edge of deep water, has never been overwhelmed.[78] Another case tending to produce anomalies in the character of waves would be their reflection and mutual interference, the reflections due to the configuration of the ocean bed and coast lines. The complete phenomena which may accompany a violent submarine disturbance are as follows:— By the initial impulse of explosion or lifting of the ground, a ‘great sea wave’ is generated, which travels shorewards with a velocity dependent upon its size and the depth of the ocean. At the same instant, a ‘sound wave’ may be produced in the air, which travels at a quicker rate than the ‘great sea wave.’ A third wave which is produced, is an ‘earth wave,’ which will reach the shore with a velocity dependent on the intensity of the impulse and the elasticity of the rocks through which it is propagated. This latter, which travels the fastest, may carry on its back a small ‘forced sea wave.’ On reaching the shore and passing inland, this ‘earth wave’ will cause a slight recession of the water as the ‘forced sea wave’ slips from its back. As these ‘forced sea waves’ travel they will give blows to ships beneath which they may pass, being transmitted from the bottom of the ocean to the bottom of the ships like sound waves in water. At the time of small earthquakes, produced, for example, by the explosion of small quantities of water entering volcanic fissures, or by the sudden condensation of steam from such a fissure entering the ocean, aqueous sound waves are produced, which cause the rattling and vibrating jars so often noticed on board ships.