For ships at sea during an earthquake feel such a blow from it (though it does them no harm) that the sailors often rush upon deck fancying that they have struck upon a rock; and the force which could give a ship, floating in water, such a blow as that, would be strong enough to hurl thousands of tons of water up the beach, and on to the land.
But there is another way of accounting for this great sea wave, which I fancy comes true sometimes.
Suppose you put an empty india-rubber ball into water, and then blow into it through a pipe.

Of course, you know, as the ball filled, the upper side of it would rise out of the water.

Now, suppose there were a party of little ants moving about upon that ball, and fancying it a great island, or perhaps the whole world--what would they think of the ball's filling and growing bigger?
If they could see the sides of the basin or tub in which the ball was, and were sure that they did not move, then they would soon judge by them that they themselves were moving, and that the ball was rising out of the water.

But if the ants were so short-sighted that they could not see the sides of the basin, they would be apt to make a mistake, because they would then be like men on an island out of sight of any other land.

Then it would be impossible further to tell whether they were moving up, or whether the water was moving down; whether their ball was rising out of the water, or the water was sinking away from the ball.