arises from the weight of the fluid, when the fluid is not contained in a closed vessel. The former pressures, or those which we have spoken of as arising from some extraneous or impressed forces, are the same at every part of the vessel; the latter pressures, or those which arise from the weight of the fluid, are greatest at the lowest parts, and their magnitude depends altogether on the position of the part.

71. Form of Surface. - The preceding articles have treated of the properties of fluids not considered subject to gravity, and contained in a closed vessel, so that the form which any portions of their surfaces assume will depend on the shape of the containing vessel; but we shall now consider fluids as subject to gravity, and free; that is, their surfaces being at liberty to assume their natural forms. Under these circumstances, let the surfaces of a fluid contained in any number of vessels which communicate and stand near each other be observed; it will be seen that all the surfaces are in the same horizontal line: for if a line touch any two of the surfaces, that line produced will touch all of them, and will appear parallel to the horizon, so that all the surfaces will be in the same horizontal plane. If the surface of a fluid contained in a large tub be observed, every part of it will appear to be in the same horizontal plane.

This, which is thus derived from observation, may be shewn to be the necessary consequence of the action of gravity on the particles of a fluid mass. We know, from experience, that gravity produces the same effects on every particle of matter, situated at the same distance from the centre of the earth. Now all points near each other on the earth's surface, and at the same distance from the earth's centre, may be considered as situated in the same horizontal plane. Hence, when the surface is horizontal, every particle will be equally acted on by gravity, and there will be equilibrium; but if the surface be not horizontal, some particles will be more acted on than others;

unequal pressures will consequently be transmitted in every direction, and the fluid will not be at rest.

A small extent therefore of the surface of still water is horizontal; and the same is true for all other fluids subject to gravity, so that a small portion of the surface of the atmosphere which envelopes us is also horizontal; since this also is subject to the action of gravity.

The preceding reasoning may be considered as sufficient when gravity is the only force to which the fluid is subject. But we may have many other forces acting on a fluid; the form of its free surface will depend upon these forces, and be subject to the following invariable condition; "That the surface must at every point be perpendicular to the resultant of the forces which act at that point.' This theoretical condition is evidently verified by the preceding remarks, for gravity being the only force, and its direction being parallel for all near points, any small extent of surface must be a plane.

72. Surfaces perpendicular to resultant of forces.-The application of the principle, that the surface of a still fluid is perpendicular to the direction of the force which acts upon it, shews that a large extent of water must be nearly spherical, while at the same time a small portion is apparently plane. The same principle must be applied in many other remarkable phenomena. When the particles are acted on by any other force than gravity, the surface will not then be perpendicular to gravity only, but to the resultant of the forces which act upon it. Thus, the centrifugal force arising from the rotation of the earth, when combined with gravity, is compounded into a third force, and it is the resultant of these to which gravity is perpendicular. Hence it is that the surface of the sea, and also of the earth, is more curved at the equator than at the poles. Near great mountains, whose attraction is sufficient to cause a deviation in the plumb-line (Art. 63), the surface of still water will not have its regular form, but it will adapt itself so as to be perpendicular to the resultant

of the forces which act upon it; and similarly, when the moon passes over any place, its attractive force compounded with gravity gives a resultant whose direction is different from the direction of gravity; and it is this attempt of the surface constantly to adapt itself to the proper form which gives rise to the periodic motions of the tides.

But there are many other phenomena referable to the same principle; it is known perfectly well that water in any vessel is elevated at the edges where it is in contact with the vessel, and that mercury is depressed; and when the fluid is contained in small tubes, the effect is so great that the whole surface becomes curved-being concave in water, and convex in mercury. Now gravity is not the only force which acts here; there are two other forces, the mutual attractions of cohesion which subsist between the particles of the fluid, and of adhesion betwixt them and the containing vessel. It is to the resultant of these three forces that the fluid surface is to be perpendicular; and the degree of curvature depends on the ratio of the intensity of the two molecular forces.

73. Level Surface. In many cases the term level is the same as horizontal, and this may be taken as its true meaning, when only a small extent of surface is referred to; but it has a very different meaning when we speak of a great extent of surface, as the sea, a large lake, or extended plain. The horizontal plane or surface is in these cases a tangent plane to the level surface. We may define

a level as that surface into which water forms itself when subject to the laws of gravity; and we must see what sort of surface observation and theory will decide this to be.

The earth's figure may be considered as spherical; for the slight deviation from this form may be disregarded in the present inquiry. Now, the inequalities which exist on the surface of the earth do not exist at all on the surface of a calm sea; but this surface, if continued uninterruptedly, would present a surface uniformly curved, and very nearly spherical. Such, then, is the form, accord

ing to observation. If, now, we consider the action of gravity, it will be evident that a surface of no other form can be at rest. If the surface have not all its points at the same distance from the earth's centre, the particles will be unequally acted on, and there can only be equilibrium when they have all settled down into a spherical form. This surface, then, of equilibrium, or spherical surface, as we may at present consider it, is a level surface, and if the whole globe were fluid, as once was probably the case, or entirely covered with water, there would be but one level. But as the world now exists, there are many surfaces, and consequently many levels; these are situated at different distances from the earth's centre, and one level is said to be above or below another, according as it is at a greater or a less distance from the centre of the earth.

The vast ocean of water which surrounds our globe, furnishes a level to which all other levels are referred. The level of the sea is the standard by which we compare the elevations of the mountains and continents of different parts of the earth. Thus, some large tracts of land, as one in the interior of Africa, are below the level of the sea, and the question of the permanency of the level of the ocean, that is, whether any change is taking place in the distance of this surface from the centre of the earth, is one of the most interesting questions of physical geology.

The level of different seas has been made the subject of the researches of several philosophers. It appears that the water of the ocean is constantly flowing into the Mediterranean through the straits of Gibraltar; whether or not this is compensated for by a constant under current is not known. If this under current exists, it is caused by the different densities of the upper and lower strata; the water of the Mediterranean being more dense than that of the ocean, flows out underneath it. If this under current does not exist, we must suppose that the Mediterranean

loses by evaporation more water than it receives from the Nile, the Rhone, the Danube, and the numerous smaller streams which are emptied into it; and that the water of the ocean flowing in to compensate this loss, keeps the level at the height requisite for equilibrium. With respect to other seas, we may remark, that it seems clearly made out, that the level of the Red Sea is considerably above the level of the Mediterranean.* The level of the Caspian Sea is said to be several hundred feet below the North Sea;† as this, however, does not communicate directly with the ocean, there is no level to be preserved. The interesting question is, how came this depression of level? There is distinct evidence, from the nature of the soil, its chemical constitution, and the presence of salt water shells, that salt water once existed at a vast distance from its present shores.

74. Fluids rise to their Level.—All the points on the same level being at the same distance from the earth's centre, and consequently subject to the same action of, gravity, it will follow from the principles already laid down, that a fluid cannot be at rest unless every part of its surface is on the same level. This is illustrated by the well-known fact, that a fluid will always sink or rise to the same level; when this is attained, the fluid is at rest.

The supply of towns with water furnishes a grand illustration of this law. The ancient method of supplying towns by means of aqueducts is now almost entirely superseded by long pipes, laid under ground and above ground, and turned in any direction. A reservoir is selected in some situation more elevated than the places to which the water is to be supplied. Pipes are conveyed from it in every direction, and the water may be laid on,' that is,

* The French engineers state the average difference of level to be about 30 feet.

+ The difference in level is stated to be 100 metres, about 326 feet.