PROPER MOVEMENTS OR CURRENTS OF THE OCEAN However violent the wind, the " waves " it produces scarcely ever exceed forty feet from crest to trough, and are therefore essentially surface-disturbances. The "tides," on the contrary, are the result of an attraction, exerted chiefly by the moon, and affecting the whole mass of the water. Wind-waves are also virtually local and temporary, depending on a constantly fluctuating cause, while the tides are periodical, regularly flow ing and ebbing twice a day. Waves and tides are thus two distinct movements of the waters of the ocean—the former all but constantly varying, the latter recurring regularly and uni formly ; but the nature of the motion in each case is exactly the same—both wind-waves and tidal-waves are mere undula tions, or up-and-down movements of the water, without any progressive motion. As we have already explained in Art. 109, the form of the wave alone moves, and not the water. Occa sionally, indeed, a purely undulating movement of the water is converted into a " wave of translation," as along shallow coasts, or in narrow inlets. A wind-wave may be propagated for hun dreds, and a tidal-wave for thousands, of miles, yet the nature of their motion is such that a floating substance would move vertically only, simply rising and falling with the undulation, but retaining the same actual position. But leaves, fruits, and branches of tropical plants are often found scattered along the shores of remote northern seas, and must therefore have been "drifted" from the equatorial regions towards the poles.
This fact alone would suffice to prove that there must be a progressive motion of the water itself otherwise there could be no actual transference of substances from one place to another. That there is a movement of the sea other than waves and tides has been practically proved in many ways. Sailors often enclose a piece of paper, with the date and position written on it, in a bottle, which is then thrown overboard. Numbers of bottles thus consigned to the sea have been found hundreds and thousands of miles distant from the place where they were thrown over. Some are soon cast ashore ; others move freely
about for many years. Maury says of one bottle thrown over off Cape Horn being picked up on the coast of Ireland more than twenty years after. Of two others, thrown overboard in the South Atlantic, off the coast of South Africa, one was found at Trinidad, in the West Indies, and the other was picked up on the coast of Guernsey, in the English Channel. Most probably both journeyed together towards the West Indies, where one was cast ashore, while the other pursued its course eastwards across the Atlantic and up the English Channel to Guernsey. In this way alone a pretty accurate notion of the general drift of the sea in different localities could be gained, but, the com mencement and end of the journey of each bottle being known, its course could only be imagined. And as a thorough know ledge of the "rivers of the ocean" is most important to the navigator, these currents and drifts have been minutely traced and investigated ; and instead' of being, as formerly, regarded with doubt and fear, their position, direction, and velocity are now so well known, that a voyage may be materially shortened by eluding unfavourable drifts, and taking full advantage of the " flow " in the direction of the wished-for port. The various drifts and currents that thus retard or increase the speed of vessels, constitute what is termed the horizontal cir culation of the waters of the ocean. As we shall again see, there is also a vertical circulation, or a gradual interchange of the whole mass of the ocean from the surface to the lowest depths. The study of the latter is certainly most interesting, but that of the former is not only theoretically interesting, but is also of vast practical importance ; its axioms being daily and hourly applied in "shaping the courses" of thousands of vessels speeding across the boundless "world of waters."