haps seem so, that the mighty sea-wave which, on May 10, rushed in upon the shores of the group of Sandwich Islands, would have been discernible from Venus, supposing an observer there had been watching the earth with a telescope as powerful as the best yet made on this earth. The wave was caused, as we know, by a tremendous subterranean disturbance in Peru a few hours earlier. Here, at least, was the centre of subterranean action, for a land wave also travelled from that region along the Pacific coast of Mexico, and was felt at the Sandwich Isles, where the Kilanea volcano was set in motion almost at the same time that the sea-wave came in. But there can be no doubt whatever that, as in the case of the great Peruvian earthquake of August, 1868, the sea-wave had its origin not in the local subterranean disturbances, but in the great upheaval by which Iquique and other places were destroyed. We shall, no doubt, hear before long, as in that case, of the arrival of the great wave at the Samoa Isles, at the Japanese Archipelago, on the shores of New Zealand, Australia, and so forth. Now, the great circular wave which spread on May 10 last from the Peruvian shore as a centre athwart the entire Pacific was probably not felt by a single ship in the open sea, any more than the still vaster wave of the 13th and 14th of August, 1868, and for the same reason. With a height of some fifteen feet (or thirty feet vertical difference between crest and hollow), the wave had yet so gentle a slope that, though it rushed at the rate of three or four hundred miles an hour across the Pacific, the rise and fall of a ship upon its surface would be altogether imperceptible. The great sea-wave, as Mallet long since pointed out, consists, in the deep ocean, of "a long, low swell of enormous volume, having an equal slope before and behind, and that so gentle that it might pass under a ship without being noticed." And we are told, in fact, by a modern writer, that during the rush of the great sea-wave across the Pacific on August 13-14, 1868, though where the wave reached island shores it seemed as though the land were first sinking bodily into the ocean and then rising bodily out of it, "there was not one among the hundreds of vessels which were sailing upon the Pacific when it was traversed by the sea-wave in which any unusual motion was perceived."
How, then, it may be asked, can we suppose that a wave which was not perceived by those actually sailing upon the ocean traversed by it, could have been visible with suitable telescopic power from a distant planet? The very circumstance which rendered the rise and fall of ships upon the sea-waves of 1868 and of last May imperceptible, assures us that the progress of the wave would so have been visible. Besides its enormous range in length, for when it struck the Sandwich Isles its crest must have formed the arc of a great curve, having for radius the distance of sixty-three hundred miles, separating that group from Peru, the wave had great breadth, otherwise, its height being about thirty feet, the rapid advance of the wave would have caused a rapid rise and fall, instead of a slow motion only discernible along shore-lines. Probably the distance from valley to valley, on either side of the mighty crest of the wave, was not less than two hundred miles in the open sea. So far as mere dimensions, then, are concerned, the great wave would certainly have been visible from a planet placed as Venus is, when most favorably situated for observing the earth. To show this, it is only necessary to point out that Venus is then much nearer to us than Mars ever is, that the entire diameter of Mars is but about forty-five hundred miles, while the radius of the great wave, when it reached the Sandwich Isles, was fully six thousand miles, and that its probable breadth of two hundred miles very far exceeds the breadth of many of the well-known markings upon the planet Mars.
But it may be asked how the wave would become discernible at all, viewed, as it were, from above. How should an observer in Venus know that the highest part of the wave was thirty feet or so nearer to him than the hollow of the valleys on either side of it? The way in which the wave would become visible corresponds in some degree to the way in which those strange radiations which extend from several of the lunar craters are visible, though they have very little elevation, cast no perceptible shadows, and are many of them undiscernible when other lunar features are clearly seen, and become discernible only when those other features are scarcely visible at all. Under the sun's rays, the two opposite faces of the advancing waves would be differently illuminated. One face, a hundred miles broad, be it remembered, would catch the light more fully than the ocean as yet undisturbed, while the other would catch the light less fully. Thus the mighty arc of the wave would appear as a double arc, one-half of its breadth being bright, the other (relatively) dark. We do not say