The Tides. The tides are produced by the attraction of the moon. The cohesion of fluids being much less than that of solid bodies, they more easily yield to the power of gravity, in consequence of which, the waters immediately below the moon are drawn up in a protuberance, producing a full tide, or what is commonly called high-water, at the loot where it happens. According to this theory, you would imagine we should have full tide only once in twenty-four hours - that is, every time that we were below the moon - while we find that we have two tides in the course of twenty-four hours, and that it is high-water with us and with our antipodes at the same time.
This opposite tide is rather more difficult to explain than that which is drawn up beneath the moon. In order to render the explanation more simple, let us suppose the earth to be everywhere covered by the ocean.
M is the moon, ABCD the earth. Now, the waters on the surface of the earth about A being more strongly attracted than in any other part, will be elevated, the attraction of the moon at B and C, being less; but still it will be greater there than at D, which is the part most distant from the moon. The body of the earth will, therefore, be drawn away from the waters at D, leaving a protuberance similar to that at A; so that the tide A is produced by the waters receding from the earth, and the tide D by the earth receding from the waters.
The influence of the sun on the tides is less than that of the moon; for observe that the tides rise in consequence of the moon attracting one part of the waters more forcibly than another part; it is this inequality of attraction which produces full and ebb tides. Now, the distance of the sun is so great, that the whole globe of the earth is comparatively but as a point, and the difference of its attraction for that part of the waters most under its influence, and that part least subject to it, is but trifling; and no part of the waters will be much elevated above or much depressed below their general surface by its action. The sun has, however, a considerable effect on the tides, and increases or diminishes them as it acts in conjunction with, or in opposition to, the moon.
The moon is a month in going round the earth; twice during that time, therefore, at full and at change, she is in the same direc-tion as the sun. Both then act conjointly on the earth, and produce very great tides, called spring-tides as represented at A and B; but when the moon is at the intermediate parts of her orbit, the sun, instead of affording assistance, weakens her power by acting in opposition to it; and smaller tides are produced, called neap-tides.
Since attraction is mutual between the moon and the earth, we produce tides in the moon ; and these are more considerable, in proportion as our planet is larger. Neither the moon nor the earth in reality assume an oval form, for the land which intersects the water destroys the regularity of the effect. The orbit of the moon being nearly parallel to that of the earth, she is never vertical but to the inhabitants of the torrid zone ; in that climate, therefore, the tides are greatest, and they diminish as you recede from it and approach the poles; but in no part of the globe is the moon immediately above the spot where it is high tide. All matter, by its inertia, offers some resistance to a change of state ; the waters, therefore, do not readily yield to the attraction of the moon, and the effect of her influence is not complete until some time after she has passed the meridian.
The earth revolves on its axis in about twenty-four hours; if the moon were stationary, therefore, the same part of our globe would, every twenty-four hours, return beneath the moon ; but as during our daily revolution the moon advances in her orbit, the earth must make more than a complete rotation in order to bring the same meridian opposite the moon: we are about three-quarters of an hour in overtaking her. The tides, therefore, are retarded, for the same reason that the moon rises later, by three-quarters of an hour every day. This, however, is only the average amount of the retardation. The time of the highest tide is modified by the sun's attrac-tion, and is between those of the tides which would be produced by the separate action of the two luminaries. The action of the sun, therefore, makes the interval different on different days, but leaves the average amount unaffected.