These tidal waves are also experienced in the Ganges, the Amazon, and at Bordeaux, as well as in China and elsewhere.
Fig. 700.—Tidal Attraction.
It may well be imagined that the tides also affect the land, and the theory of these ocean movements is a very interesting study. We have already referred to it under Astronomy, for the Sun’s and Moon’s attraction is the main cause of the phenomenon, which is so familiar and yet so strange. But the consideration of the tides must be again entered upon here ere we proceed to view the effects of the sea upon the land, and how the physical geographical features alter.
Isaac Newton rightly attributed the cause of the tides to the attraction of the moon and sun. Spring tides occur when both luminaries are above the meridian, and the neap, or low tides, happen when the sun and moon are farthest apart. The highest tides are perceived after a new or full moon; the lowest, after she has passed the first or third quarter. In January the spring tide is highest of all, because the earth is nearest to the sun then, and his force of attraction, added to that of the moon, causes a very high tide. With the assistance of the accompanying diagrams we shall be able to make the tidal phenomena clear.
Fig. 701.—Tidal Attraction.
Suppose the moon to be at M, the point J (the sea) will be nearest to the moon and will be attracted, while the earth will exercise a retarding power to a certain extent. This attraction of the water from its usual level causes a kind of vacuum, into which the surrounding water flows and causes a high tide at H. At the opposite side the earth, not the water, is most attracted, and then the water rushes in to a certain extent to fill the vacancy left by the earth’s movement towards the moon. Another high tide is therefore caused at L, but not so high as the tide upon the opposite side, as the Moon is so much nearer the latter. The tide, then, is only the natural movement of the sea water to fill up the space the earth and other portions of the watery mass have vacated in obedience to lunar and solar attraction, which is, to a certain extent, counterbalanced by the attraction and resistance of the earth.
The neap tides are caused by the opposing forces of attraction of the sun and moon. The sun, as it were, pulls one way, the moon the other. The latter (being nearer) having twice the power of the former, causes a tide indeed, but it is a low one. The spring tide occurs when sun and moon together attract the water.