In reproducing from the London the following popular article on the tides, we wish it understood that we do not invite a deluge of correspondence upon the subject. Ex. absence has warned us that there is no subject so fertile as tides. Were we to oppose no barriers to tides, they would fill our columns, overwhelm, and sink us. Tuerefore while we publish this article as a reflex of the doubts entertained by hosts of thinkers on both sides of the Atlantic, we do not on that account commit ourselves to lengthy discussions of the topic in future numbers. ” The approach of one of the highest Tides which the com- binad 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 unfavorable. For if we do not have a particularly high tide, owing to the influence of the winds being opposed to tie progress of the tidal wave, there will be the equally significant phenomenon of a singular withdrawal ofthe water at the time of low t:de. 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 tor six hours or so, they would have been well i ewarded for their journey, since at tHe time of low tide the water withdrew far within the usual limits, and stranga 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 sliItf ofa'liottom which had re nained sea.cov. eredfor 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 s\1n and moon on October 5th will be most strikingly manifested. ” Bat our object at present is less to consider the effects of the great tidal wave of October 6th, than to dwell upon some interesting effjcts and peculiarities of tidal motion. When we learn that astronomers for the most part recognize in the tid^ wave a cause which .will one day reduce the earth's rotation so effectually that instead of twenty.four hours our day will last a lunar monthwhile many astronomers believe that the same wave will at a yet more distant day bring the moon into collision with our globeit will be seen that the laws of the tides have a cosmical as well as' a local interest. They involve more important considerations 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 happened, 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, i s 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 tidt s Y” The reader may be disposed to smile at such a question. Does not every hook of geography, every popular treatise on astronomy teach us all about the tides 1 Cannot every person of average education and intelligence runrn 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 immediately under her than to the earth's ce'1ter the moon draws that water somewhat away from the earth ; and again, being nearer to the earth's center than to the wa. ter directly beyond, the moon draws the earth away from that water. Thus, underneath the moon a heap of water is raiS1Il1Il, 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.shped figure, whose longest diameter w.:;uld 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. Delaunay, a leading French astronomer, endeavored 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 travels 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. Tilis presently called up the Astronomer Royal, who gave a very clear and convincing demonstration that there would always be low 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 solusWra- tra mwrcdaw, we might readily believe that there was some flaw in his reasoning since, as every. one knows, themostemi- nent 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 is it thatthe views ofNewton 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 explp.nation actually given is preferred, on account of its greater simplicity ? That would be reasonable, i f 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, chat most Qf the consequences which astronomers 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 3he 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 90 come to form a ring of fragments around our globe. If, however, the other view is right, the moon's motion will be continnally retarded, her orbit will 'gradually widen out, and some day, presumably, we shall lose her altogether. This retarding and hastening refer to the rate at which the moon completes her revolutions round the earth. As a matter ot 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 an increased pull on her,and this pull adds more to her velocity than she lost by the check. And 'Dioe'Dioe ” Again, if the views commonly given are just, the earth's friction should cause the tidal wave to lag behind its true place. But it 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.' ” We apprehend that there is room for improvement in the cuUl'l'nt account of the tides. Many eminent men, as Whe- well, Lubbock, and Haughton, have discussed in the most elaborate and skijlful manner the laws according to which the actual tidal . wave travels along the great sea-pnths. But as jet no one has tried to reconcile the theory of Newton, which maybe called the dynamical theory of the tides, with thatcofflftlonly given in our books, which may be called the statical theory.”
This article was originally published with the title "The Tidal Wave"