Puslapio vaizdai

nodes, they are not only total, but last for some length of time.

When the sun is eclipsed, the total darkness is confined to one particular spot of the earth, as the moon's

[ocr errors]
[ocr errors]


shadow is not large enough to cover the earth. The lunar eclipses, on the contrary, are visible from every

[blocks in formation]

part of the earth, where the moon is above the horizon.


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 highwater, at the spot 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 hav 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. order to render the explanation more simple, let us supthe 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 at

traction of the moon at в 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 direction as the sun. Both then act in conjunction 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 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 attraction, 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.


The science of the mechanical properties of fluids is called Hydrostatics. A fluid is a substance which yields to the slightest pressure.

Fluids are divided into two classes, distinguished by the names of liquids, and elastic fluids or gases, which latter comprehends the air of the atmosphere, and all the various kinds of air with which chemistry makes us acquainted. We shall confine our attention at present to the mechanical properties of liquids or non-elastic fluids.

Water, and liquids in general, are little susceptible of being compressed, or squeezed into a smaller space than that which they naturally occupy. This is supposed to be owing to the extreme minuteness of their particles, which, rather than submit to compression, force their way through the pores of the substance which confines them, as was shown by a celebrated


experiment, made at Florence many years ago. hollow globe of gold was filled with water, and on its being submitted to great pressure, the water was seen to exude through the pores of the gold, which it covered with a fine dew. But more recent experiments, in which water has been confined in strong iron tubes, prove that it is susceptible of compression.

Liquids are porous, like solid bodies, but the pores are too minute to be discovered by the most powerful microscope. The existence of pores in liquids can be ascertained by dissolving solid bodies in them. If we melt some salt in a glass full of water, the water will not overflow, and the reason probably is, that the particles of salt will lodge themselves in the pores of the liquid, so that the salt and water together will not occupy more space than the water did alone. If we attempt to melt more salt than can find room within these pores, the remainder will subside to the bottom, and, occupying the space which the water filled before, oblige the latter to overflow. A certain proportion of spirit of wine may also be poured into water without adding to the bulk, as the spirit will introduce itself into the pores of the water.

Fluids show the effects of gravitation in a more perfect manner than solid bodies; the strong cohesive attraction of the particles of the latter in some measure counteracting the effect of gravity. In a table, for instance, the strong cohesion of the particles of wood enables four slender legs to support a considerable weight. Were the cohesion so far destroyed as to convert the wood into a fluid, no support could be afforded by the legs; for the particles no longer cohering together, each would press separately and independently, and would be brought to a level with the surface of the earth.

This deficiency of cohesion is the reason why fluids can never be formed into figures or maintained in heaps ; for though it is true the wind raises water into waves, they are immediately afterwards destroyed by gravity. Thus liquids always find their level. The definition of the equilibrium of a fluid is, that every part of the sur

« AnkstesnisTęsti »