PHYSICAL GEOGRAPHY OF THE OCEANS The cohesion of particles of water being limited, they are free to arrange themselves in obedience to the slightest impulse. This, in conjunction with its all but incompressibility, is the cause of what we may call the sensitiveness of water to disturbances ; the most common forms of which are wind waves. It being one of the characteristics of water that depression at any one point is always and necessarily accompanied by an equal elevation at another point, it follows that the impact of a moving column of air on the surface of the water causes a greater or less degree of de pression at the point of impact, and a corresponding elevation immediately beyond that point ; and thus waves, the simplest of all the disturbances to which the ocean is subject, are produced. But the disturbance in this case is temporary; the withdrawal or suspension of the disturbing element is imme diately followed by the subsidence of the elevated part of the water, and the resumption by the depressed part of its normal level. And so, although it cannot perhaps be said that every part of the ocean's surface is always uniformly level, owing to the frequent disturbances by winds, tides, &c., yet, as these are, in fact, local and temporary, the surface of the ocean is, generally speaking, level, cone sponding with the mean level of the earth as part of its circumference. The sea-level, as it is termed, then, is the
general elevation of the great mass of the ocean, and its uni formity in widely different localities is such that the distance between the surface of the water and the exact centre of the earth (allowing of course for the excess in the diameter of equa torial regions) must be equal The sea-level, being thus univer sally uniform, is adopted as the standard of all measurements of height on land or depth at sea. The earth being a sphere, the surface of the ocean is also curved, and there being nothing to obstruct the view as on land, this curvature is more apparent at sea than on land, as is strikingly shown by the disappear ance, first of the hulls, and last, of the topmasts of vessels leaving the land.
Besides this curvature of the sea as a whole, and its assumption of a generally uniform level, we have to consider other physical peculiarities of the waters of the ocean. One distinctive feature of sea-water is its saltness, a property both universal and uniform. Closely connected with its salinity are the density and colour of sea water. Other points in the physical geography of the sea, that command our attention, are its temperature, depth, and the various movements to which it is subject—waves, tides, and currents.