NOTES
EXPLANATORY OP
THE TIDAL CHARTS OF THE WORLD
AND OF
BY
CONTENTS.
INTRODUCTION,
I— TIDAL CHART OF THE WORLD,
II— TIDAL CHART OF THE BRITISH SEAS,
Page
1
2
ib.
INTRODUCTION.
the earth presented a uniform globe, with a belt of sea of very great and uniform depth encircling it round the equator, we should have a rise of tide, or high-water, following the sun every day from east"West after mid-day, and another succeeding it after mid-night, when the sun had passed the meridian of our antipodes. This phenomenon Sir Isaac Newton deduced from the laws of the gravi-^tion to each other of the mass of the sun, the mass of the earth, and the mass of the water. In like manner, each transit of the moon over the meridian would bring after it also a tide or high-water; and° Win g to the smaller distance of the moon, its effect would be greater in raising the water than the effect of the sun, in a proportion nearly six-fold. In this state of matters we might expect four tides a-day. These,however, would so modify each other as to produce only two great tides compounded of the four. When the transit of the sun and moon took place together, their joint effect would raise a tide measured bythe sum of the effects; that is, the lunar tide would be increased one-sixth by the solar tide. But, on the other hand, when the transit of the moon was about six hours later or earlier than the sun’s, the low-w ater of the one would diminish the effect of the high-water of the other, and so the lunar tide would be diminished by one-sixth. Thus the mean tidal elevation of high-water would vary in nearly the pro-portion of 5 to 7. At new and full moon the tides would range higher (forming spring-tides), and at the first and third quarters the tides (forming neap-tides) would range lower. Where there were twenty-° ne ^ eet of rise of high-water above low-water at springs, there would only be fifteen feet at neaps. Had the earth been formed as we have supposed, with a great equatorial canal, or central belt of ocean, thePhenomena might have taken place as we have stated, with great regularity, and we should not have required to form a tidal map or chart, for the crest of high-water would have been nearly (notexac tly) a meridional line following the sun and moon at certain very simple intervals. This simple case is, however, very different from the fact, the heights of successive tides and their intervals vary much111 ' va y we have described, but the direction of motion and the epoch or date of high-water are exceedingly various; so different, indeed, that no adequate notion of tidal phenomena can be formed, withouts °me su °h tidal charts as accompany this paper.
Looking at the Chart of the World, we observe that there is no great equatorial sea extending east and west round the globe, in such a belt as has been imagined. On the contrary, the great continentsCr °ss the equator nearly at right angles; the Atlantic running nearly north and south, between Europe, Africa , and the Americas. The chief region of water appears in reality to surround the south pole, andwe recollect how peculiar the nature of the Pacific is, how much it is intercepted with coral reefs and islands, and insular continents, we shall readily perceive how far it is removed from the condition of anTutorial canal of uniform, unimpeded, great depth; presenting, on the contrary, a coral barrier nearly impervious to tidal action.
The source of the tides is therefore to be sought in the great reservoir of ocean round the southern pole. This polar reservoir is agitated on opposite sides by the moon in its alternate lower and uppertransits, and by the sun in a less degree. Here the great central agitation seems to commence, and hence on all sides it seems to flow northwards. From the south pole this great agitation flowslnt0 the Indian Ocean, and proceeding northward, the great tide-wave strikes with violence on the shores of Hindostan, and finally breaks in the mouths of the Ganges , where it expends its force on the shoresln Ae form of the well-known and terrific bore of the Hoogly. The Atlantic , in like manner, receives from the southern reservoir its great wave of tide, which passes northward with impetuosity, and expends1 s forces on the shores of Britain and North America, where again it becomes the enormous stream-tide of the Bristol Channel, and the destroying surge of the Bay of Fundi , so well known to all mariners. From*he south, in like manner, the Pacific should receive its great tide, were it not barricaded out by innumerable submarine steppes, and its thousand coral reefs, and its myriads of happy islands, to whose calmS6as no propagation of this great horizontally-acting wave can gain access. It is by depth and uninterrupted bottom only that a great wave like the tide can force entrance,—it is only the small waves
FaiSe d by a local tempest, that travel over the surface_an action like the tide, extending uniformly to all depths of ocean, cannot be propagated on a superficial coating alone. The tides are built out of the
Pacific by submarine works, and enter it alone and with difficulty by the eastern side of America , where, diffused and rapidly diminishing, the tide extends a certain way through the more open parts of theSea ’ c °ntinually diminishing in intensity. In the North Pacific we have neither the bores of a Hoogly, nor the terrific tides of a Bay of Fundi . It is thus that we shall best understand the tides of thesen ° r thern seas, in tracing them back to their genesis by the joint actions of the sun and moon on the waters of the Southern Polar Ocean. There we must conceive that the elevation and horizontal motion are firstc °mmunicated to a vast mass of deep water, which, imparting this impulse to all the surrounding mass, sets in motion the waters of all the southern mouths of our seas, and propagates to our shores, long after^eir original force has been impaired, the effects of the derangement of the equilibrium of that central reservoir from which alone our tides are derived. It is this course of transmission of tides from south ton ° n h b y means of a great tidal wave, which is exhibited in the annexed Chart of the World. The chart exhibited now enables us to follow the history of a tide.
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