is illustrated. In each of these figures, S represents the sun, M the moon, and E the solid portion of the earth. The dotted line inclosing the earth is the solar tide-wave,

and upon this in the line of the three bodies, is heaped the lunar tide-wave, the boundary of which is the outer curved line.

693. In Fig. 82, is exhibited the phenomenon of the neap tides. The moon is in quadrature, 90° from the sun, and the two bodies evidently counteract each other's influence in producing their respective tides. The solar tide wave, as in the preceding figure is represented by the dotted oval line, and the lunar tide wave by the unbroken curved line.

694. Since a difficulty is sometimes experienced in understanding how a spring tide is produced when the sun and moon are on opposite sides of the globe, we will explain this point a little more particularly. In Fig. 80, where the sun and moon are on the same side of the earth, it is the time of new moon, and a spring tide

Describe these phenomena in full, and explain from Figures 80, 81, and 827

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occurs. From the reasoning employed in Arts. 681-2, it will be perceived that the waters at O are heaped up by

the excess of attraction exerted by the sun and moon, to draw the waters from the centre of the earth, thus rendering them lighter than usual. Around O' there is a deficiency of solar and lunar attraction, and the waters in this region are drawn down less than usual toward the centre of the earth, they are consequently more bulky than common and accordingly swell outward.

695. In Fig. 81, the sun and moon are on opposite sides of the earth. The moon is at her full, and a spring tide now also occurs. At O the sun produces a high tide by his excess of attraction and the moon here causes a high tide by her deficiency of attraction, since that which is the nearest hemisphere to the sun is the farthest from the moon. At Of the moon's excess of attraction gives rise to the lunar high tide, while the sun's deficiency of attraction causes here likewise solar high tide; for in this case the hemisphere which is nearest to the moon is the most remote from the sun.

696. TIME OF THE TIDE. In Art. 679, we have said that there exists a marked correspondence between the motions of the tide, and those of the moon. If the waters moved with perfect freedom, the lunar tide wave would

Show why a spring tide occurs at full moon?

be highest at any place when the moon was upon the merilian of the place; and the solar tide wave highest when the sun was upon its meridian. But the waters do not at once obey the action of the sun and moon, on account of their inertia; and they are also retarded in their motion by the friction produced in their passage over the bed of the sea and the sides and bottoms of channels. It thus happens that the high tide does not occur at any place until the moon has passed its meridian several hours.' The interval between high tide and the moon's meridian passage is however not constant, but varies in different places.

697. PRIMING OR LAGGING OF THE TIDE. The actual high tide at any part is produced by the union or superposition of the solar and lunar tide waves. Now on account of the changing relative positions of the sun and moon, these waves do not so unite as to make the high tides recur at any port at the expiration of exactly equal intervals of time. The tide days therefore, are not of the uniform length of 24h. 50m. 28sec., but vary somewhat in duration, and this variation is quite marked about the time of the new and full moon.

698. EFFECT OF DECLINATION ON THE HEIGHT OF THE TIDE. The highest point of the tide wave, tends to place itself directly beneath the body which raises it, so as to be exactly in the line joining the centre of this body and the earth. If therefore the sun and moon were always found in the plane of the equator, the tides would be highest in the equatorial regions, while a constant low tide would exist at the poles. But these luminaries are not thus situated, since, owing to the obliquity of the ecliptic, they have an apparent motion north and south of the equator; the sun departing from the

1. At Dunkirk, for instance, high water occurs half a day after the moon passes its meridian, at St. Malo's six hours, and at the Cape of Good Hope, one and a half hours.

Why does not the high tide occur at any place when the moon is exactly on its meridian? How long a period sometimes elapses after the moon has passed the meridian before the high tide happens? Is the interval between the time of high tide at any place, and the moon's meridian passage, invariably the same? Explain what is meant by the priming and lagging of the tide? Are the tide days of uniform length? When is the variation greatest? Why do the declinations of the sun and moon influence the tides?

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equator about 23 degrees, while the moon attains a declination of 29° on one side, and about 17° on the other.

699. These changes in the position of the sun or moon accordingly affect the height of the tide at any particular place. When the moon, for example, has her greatest northern declination, the daily high tides will be highest in all those places in the northern hemisphere where the moon is above the horizon, and lowest where she is below the horizon. In the southern hemisphere the phenomena are reversed; the daily high tides being highest at all those places where the moon is beneath the horizon, and lowest in all the regions where she is above the hori zon. A glance at Fig. 83, proves these statements.

700. ACTUAL HEIGHTS OF THE TIDE. The theoretical height of the tide does not correspond to the real height. This difference is owing to local causes, such as the union of two tides or the rushing of the tide wave into a narrow channel. In the latter case the advance of the tide is often very rapid, and the water rises to a great elevation. Thus within the British Channel, the sea is so compressed that the tide rises 50 feet at St. Malo's, on the coast of France. In the Bay of Fundy, the tide swells to the height of 60 or 70 feet. Here, according to Prof.

State why these changes in the position of the sun or moon affect the height of the tide at any particular place? Why does not the theoretical height of the tide at any place correspond with the actual height?

Whewell, the tide wave of the South Atlantic, meets the tide wave of the Northern Ocean, and their union raises the surface of the sea to the height just mentioned. On the vast Pacific, where the great tide wave moves without obstruction, the rise of the water is only about two feet on the shores of some of the South Sea Islands.

701. DERIVATIVE TIDES. The tides perceptible in rivers, and in seas communicating with the ocean, are termed derivative tides; inasmuch as they are not produced by the immediate action of the sun and moon, but are portions of the great oceanic tide waves, which flow in from the open sea.

702. The derivative tides ascend the large rivers of the globe to a great distance from their mouths; but their upward progress is so much retarded by their friction against the banks, and the various impediments they encounter, that several tides in some instances, are found at the same time along the same river. Thus, at the Straits of Pauxis, in the Amazon, five hundred miles from its mouth, the tide is distinctly perceptible; and so much is it retarded in ascending this mighty stream, that at the time of the equinoxes, for three successive days, five tide waves, rising to the height of from 12 to 15 feet, follow each other daily up the river.

703. NO TIDES EXCEPT ON THE OCEAN, AND ON SEAS CONNECTED WITH IT. Inland seas and lakes have no perceptible tides. None have ever been observed in the Caspian sea, or in any of the great North American Lakes. This is owing to the fact that the attractive forces exerted by the moon upon the waters of a lake are so nearly the same in every part, that no sensible difference can exist; and as the tides are caused by the differences that occur in the amount of attraction, it follows that where there is no difference there is no tide. These remarks apply with greater force to the attraction of the sun. It is only in the ocean that the expanse of water is sufficiently great to cause such an inequality of action, both in the lunar and solar attraction, as to produce tides. Of

State the cases cited? Explain derivative tides? State what is said respecting derivative tides ascending long rivers? Give the facts in regard to the Amazon? Have tides been noticed in lakes and inland seas? Why do they not occur in such waters?