[CHAPTER VII.]
HYDROSTATICS.
110. What Hydrostatics Teaches.—Hydrostatics is that branch of Natural Philosophy which treats of the pressure and equilibrium of liquids. The phenomena which it brings to view all results from the influence of the attraction of the earth upon liquids. It is for this reason that this subject calls naturally for our consideration after examining the general subject of attraction, as we have done in the previous chapters. In order to understand fully the phenomena of Hydrostatics, you must continually bear in mind the two grand characteristics of liquids. One is, that the particles move freely among each other (§ 9). The other is, that a liquid is almost entirely incompressible (§ 36).
111. Level Surface of Liquids.—It is the influence of gravitation upon liquids that gives them a level surface whenever they are not agitated by any cause. Observe how this is. A still body of water you may consider as being made up of layers of particles. Each layer will have all its particles equally attracted by the earth, and must therefore be level. If any of the particles were less attracted than their neighbors they would rise, as is the case when heat is applied, as you will see hereafter. Whenever the upper layers of the particles are disturbed by wind or any other cause, as soon as the disturbance ceases the particles will again take their places in level layers under the influence of gravitation.
112. A Comparison.—The particles of water may be compared to shot. If you have shot in a vessel, and they are heaped up in any portion of the surface, on shaking the vessel those that are highest will roll down, and the result will be a level surface. They would do this without agitation if they were as smooth as the particles of water are. If we could have a microscope strong enough to distinguish the shape of the particles of water, the surface would probably appear like the level surface of shot in a vessel. But the particles of water are so exceedingly minute that the surface of water, when entirely free from agitation, is so smooth as to constitute a perfect mirror, often feasting our eyes with another world of beauty as we look down into its quiet depths. Water was man's first mirror, and one of the most beautiful passages in the "Paradise Lost" is the description of Eve's first waking after her creation at the side of a lake, and seeing her form reflected in its smooth waters.
Fig. 66.
113. Surface of Liquids not Truly Level.—Strictly speaking, the surface of a liquid is not level, but rounding. But it is so little so that it can not be perceived unless we take into view a very large surface, as the ocean. Here it is very manifest, for whenever a ship comes into port the first thing seen from the shore is the topmost sail, the rest of the ship being concealed by the water rounded up between it and the observer. This is illustrated in Fig. 66. At a the ship is just in sight, while at b it is so near shore that the eye sees the whole of it. If the earth had no elevations of land, or if there was water enough to cover them, the water would make a perfectly globular covering for the earth, being held to it by the force of attraction. The reason for this is precisely the same as was given in § 58 for the disposition of a drop of liquid to take the globular form. As in that case, so in this, it can be demonstrated that each particle is attracted toward a common centre, and that this will produce in the freely-moving particles a uniformly rounded surface. What could thus be shown to be true if the earth were wholly covered with water, is true of the portions of water which now fill up the depressions in the earth's crust; and it can be perceived, as shown in the first part of this paragraph, in the case of any extended portion of it.
