Page:Popular Science Monthly Volume 16.djvu/198

 would flow to the westward. For two reasons, however, it would be confined to equatorial regions: first, the centrifugal force there is greatest; and, second, the meridians converge as we near the poles.

This second reason will appear evident if we suppose a body of water of five degrees area and any depth to set out from the equator toward either pole. At each remove it would find the linear dimensions of a degree smaller. The depth remaining constant, its volume would be too great for an area of five degrees square in latitude 30°, still more so for one in latitude 60°, and so on. This constant crowding in extra-tropical zones would therefore constitute an opposing force sufficient to confine the flow of water to a zone where its volume would undergo little or no compression—that is, in the vicinity of the equator.

Arriving, then, at the eastern shores of the continents to the westward of those from which it started—at the North and South American shores, for instance, having started from Europe and Africa—and being banked up by constantly arriving volumes of water, it would be forced to the northward and to the southward along the coast-line of each continent; it would then flow to the eastward in high latitudes until reaching the western shores of the continents from which it started, where, owing to the divergence of the meridians toward the equator and the greater centrifugal force at that parallel, it would flow from the north and from the south along the shore-lines of the continents until reaching the intertropical zone, where it would again start westward on its circuit.

Imagine this system of circulation once set up, and nothing is more natural than that it should continue while the earth revolves; indeed, a glance at any current chart of the world will suffice to show the force of this reasoning.

It will now be seen how important a part the thermometer and hydrometer play in the discovery of oceanic currents: by the former a difference of temperature, and by the latter a difference of density, is quickly detected; and, if a decided difference of either kind is found, a permanent current may be fairly inferred. A consideration of the winds, whether an accidental gale, the constant trades, or the seasonal monsoon, may lead us to deduce intelligently whether a set that may have been experienced for days is a temporary surface-flow or a permanent current. So, also, keeping in view the range of the barometer for a few days—the locality and amount of its rise or fall—may assist in deciding whether a certain set be due to its extreme range or not. A consideration of the rotation of the earth is of assistance only in determining the general direction of the great ocean-currents.

A few other entries of the log-book require a passing notice. The direction of the wind is indicated by a vane in connection with a compass, and its force is estimated according to the following scale: