Where the interval between the passage of successive wave-crests was noted, we can tell the actual breadth of the wave. Thus, at the Samoan Isles, in 1868, the crests succeeded each other at intervals of sixteen minutes, corresponding to eight minutes between crest and trough. But we have seen, that if the waves were one mile in breadth, the corresponding interval would be only four seconds, or only one 120th part of eight minutes: it follows, then, that the breadth of the great wave, where it reached the Samoan Isles in 1868, was about 120 miles. Now a wave extending right athwart the Pacific Ocean, and having a cross breadth of more than 120 miles, would be discernible as a marked feature of the disc of our earth, seen under the conditions described above, either from Mercury or Venus. It is true that the slope of the wave’s advancing and following surfaces would be but slight, yet the difference of the illumination under the sun’s rays would be recognizable. Then, also, it is to be remembered that there was not merely a single wave, but a succession of many waves. These travelled also with enormous velocity; and though at the distance of even the nearest planet, the apparent motion of the great wave, swift though it was in reality, would be so far reduced that it would have to be estimated rather than actually seen, yet there would be no difficulty in thus perceiving it with the mind’s eye. The rate of motion indeed would almost be exactly the same as that of the equatorial part of the surface of Mars, in consequence of the planet’s rotation; and this (as is well known to telescopists), though not discernible directly, produces, even in a few minutes, changes which a good eye can clearly recognize. We can scarcely doubt then that if our earth were so situated at any time when one of the great waves generated by Peruvian earthquakes in traversing the Pacific, that the hemisphere containing this ocean were turned fully illuminated towards Venus (favourably placed for observing her), the disturbance of the Pacific could be observed and measured by telescopists on that planet. Unfortunately there is little chance that terrestrial observers will ever be able to watch the progress of great waves athwart the oceans of Mars, and still less that any disturbance of the frame of Venus should become discernible to us by its effects. We can scarce even be assured that there are lands and seas on Venus, so far as direct observation is concerned, so unfavourably is she always placed for observation; and though we see Mars under much more favourable conditions, his seas are too small and would seem to be too shallow (compared with our own) for great waves to traverse them such as could be discerned from the earth.