A MIGHTY SEA-WAVE. 195 



were traversed by the great sea-wave, there was not one in 

 which any unusual motion was perceived." The inference 

 is clear, that the slope of the advancing and following faces 

 of the great wave was very much less than in the case above 

 imagined ; in other words, that the breadth of the wave 

 greatly exceeded one mile amounting, in fact, to many 

 miles. 



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 suc- 

 teeded each other at intervals of sixteen minutes, correspond- 

 ing to eight minutes between crest and trough. But we have 

 seen, that if the waves were one mile in breadth, the corre- 

 sponding interval would be only four seconds, or only one 

 izoth 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 Mer- 

 cury 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 appa- 

 rent 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 

 planef 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. 



