THE TIDAL WAVE.

THE approach of one of the highest Tides which the combined attraction of the sun and moon can possibly raise, has made many of us look up our acquaintance with the laws of Tidal Motion. Every one has satisfied himself why the coming spring tide will be higher than usual. We know that the moon will be near the equinoctial when new, and also near her perigee ; and that the combination of these circumstances at a season of the year when the tidal wave raised by the sun is unusually high, must necessarily result in causing a very remarkable tide, even though the winds should be unfavourable. For if we do not have a particularly high tide, owing to the influence of the winds being opposed to the progress of the tidal wave, there will be the equally significant phenomenon of a singular withdrawal of the water at the time of low tide. A few years ago, when a very high tide was expected on the shores of France, the winds drove back the sea, and many who had come from far inland to witness the great influx of water returned disappointed. But had they waited for six hours or so, they would have been well rewarded for their journey, since at the time of low tide the water with- drew far within the usual limits, and strange sights were revealed to the wondering fishermen who lived along that shore. Wrecks of forgotten ships were to be seen half-buried in the ooze and slime of a bottom which had remained sea-covered for centuries. Old anchors were disclosed to view, with the broken cables attached to them, on which the lives of many gallant men had once depended, so that every parted strand seemed the record of a lost life. And crawling things and stranded fish showed how far the great sea had retreated within its ordinary bounds. We may, therefore, expect that results well worth noting will under any circumstances accompany the tidal action of October 6th, on which day the effects of the conjunction of the sun and moon on October 5th will be most strikingly manifested.

But our object at present is less to consider the effects of the great tidal wave of October 6th, than to dwell upon some inte- resting effects and peculiarities of tidal motion. When we learn that astronomers for the most part recognize in the tidal wave a cause which will one day reduce the earth's rotation so effec- tually that instead of twenty-four hours our day will last a lunar month,—while many astronomers believe that the same wave will at a yet more distant day bring the moon into collision with our globe,—it will be seen that the laws of the tides have a cosmical as well as a local interest. They involve more important consi- derations than whether the water in the Thames will rise a foot or two higher than usual at Vauxhall Bridge on any particular day. And though many thousands of years must elapse before either of the events looked forward to by astronomers shall have hap- pened, yet we cannot but look with deep interest into the long vista of the coming centuries. To the astronomer, at any rate,

the study of what will be, or of what has been, is as interesting even as the study of what is.

But at the very threshold of the inquiry we are met by the question, "Do any of us know the law of the tides ?" The reader may be disposed to smile at such a question. Does not. every book of geography, every popular treatise on astronomy, teach us all about the tides? Cannot every person of average education and intelligence run through the simple explanation of the tidal wave ?

Certainly it is so. Most of us suppose we know, in a general way (and that is all that we at present want), how the moon or sun draws a tidal wave after it. The explanation which nine

hundred and ninety-nine (at least) out of every thousand would give runs much on this wise:—Being nearer to the water imme-

diately under her than to the earth's centre, the moon draws that.

water somewhat away from the earth ; and again, being nearer to. the earth's centre than to the water directly beyond, the moon.

draws the earth away from that water. Thus, underneath the moon a heap of water is raised, and at the directly opposite point. a heap of water is left (so to speak). So that were it not for the effects of friction, the water would assume a sort of egg-shapedfigure, whose longest diameter would point directly towards the moon.

And not only is this the explanation which is invariably given. in popular treatises, but scientific men of the utmost eminence.

have adopted it, as correctly exhibiting the general facts of the

case. Recently, for example, when Mr. Adams had published his proof that the moon's motion is gradually becoming accelerated

in a way which the lunar theory cannot account for, M. Delau- nay, a leading French astronomer, endeavoured to prove that in, reality it is the earth's rotation which is diminishing, instead of the moon's motion which is increasing. He thought the tidal wave, continually checked by the earth's friction as it travela against the direction of her rotation, would act as a sort of "break," since its friction must, in turn, check the earth. And in discussing this matter he took, as his fundamental axioms, the law of tidal motion commonly given in our books of geography

and astronomy. This presentlycalled up the Astronomer Royal, who. gave a very clear and convincing demonstration that there would always be /ow water under the moon, if there were no friction.

But this is not all, nor is it even the most remarkable part of the case. Eminent as the Astronomer Royal deservedly is, and.

especially skilful as we know him to be in questions such as the. one we are considering, yet if he were solus contra mundum, we. might readily believe that there was some flaw in his reasoning,. since, as every one knows, the most eminent mathematicians have. sometimes misconceived the bearings of a perplexing problem.

But, as Mr. Airy himself pointed out, Newton and Laplace were both with him !

How it is that the views of Newton and Laplace, admittedly the very highest authorities which could be quoted, have found no place in our treatises of astronomy ? Their views have never been disproved. In fact, as we have seen, one of the most eminent of our mathematicians, in re-examining the question, has come to precisely the same conclusion. Can it be that the explanation. actually given is preferred, on account of its greater simplicity ?

That would be reasonable, if the two explanations were accordant,. but they happen unfortunately to be wholly opposed to each other, and therefore one of them must be false. Those who teach us our geography and astronomy ought to look to this.

The worst of it is, that most of the consequences which astro- nomers ascribe to the action of the tidal wave depend on the.

choice we make between the rival theories. If the ordinary view is right, the moon's motion is continually being hastened by the attraction of the bulging tidal wave, and this hastening will bring the moon into a smaller and smaller orbit until at last she will be brought into contact with the earth, unless, as Professor Alexander Herschel suggests, she should crumble under the increased effects of the earth's action, and so come to form a ring of fragments around our globe. If, however, the other view is right, the moon's motion will be continually retarded,* her orbit will gradually widen out, and some day, presumably, we shall lose her altogether.

Again, if the views commonly given are just, the earth's friction. should cause the tidal wave to lag behind its true place. But if Newton, Laplace, and Airy are right, then, to use the words of the last-named astronomer, "the effect of friction will be to accelerate the time of each individual tide."

*The retarding and hastening mentioned in the above paragraph refer to the rate at which the moon completes her revolutions round the earth. As a matter of fact, paradoxical as it sounds, it is a continual process of retarding which eventually hastens the moon's motion. Every check on the moon's motion gives the earth sa. Increased pull on her, and this pull adds more to her velocity than she lost by the check. And vice versa. We apprehend that there is room for improvement in the current account of the tides. Many eminent men, as Whewell, Lubbock, and Haughton, have discussed in the most elaborate and skilfnl manner the laws according to which the actual tidal wave travels along.the great sea-paths. But as yet no one has tried to recon- cile the theory of Newton, which may be called the dynamical theory of the tides, with that commonly given in our books, which may be called the statical theory.