At any time when the sea is calm and the weather clear, the light of the Eddystone may be seen from an elevation of five feet above the water level; and according to the Admiralty directions, it "maybe seen thirteen nautical (or fifteen statute), miles," 1 or one mile further away than the position of the observers on the above-named occasion; yet, on that occasion, and at a distance of only fourteen statute miles, notwithstanding that it was a very fine autumn day, and a clear background existed, not only was the lantern, which is 80 feet high, not visible, but the top of the vane, which is 100 feet above the foundation, was, as stated in the report "entirely out of sight."
There was, however, a considerable "swell" in the sea beyond the breakwater. That vessels, lighthouses, light-ships, buoys, signals, and other known and fixed objects are sometimes more distinctly seen than at other times, and are often, from the same common elevation, entirely out of sight when the sea is rough, cannot be denied or doubted by any one of experience in nautical matters.
The conclusion which such observations necessitate and force upon us is, that the law of perspective, which is everywhere visible on land, is modified when observed in connection with objects on or near the sea. But how modified? If the water were frozen and at perfect rest, any object on its surface would be seen again and again as often as it disappeared and as far as telescopic or magnifying power could be brought to bear upon it. But because this is not the case--because the water is always more or less in motion, not only of progression but of fluctuation and undulation, the "swells" and waves into which the surface is broken, operate to prevent the line of sight from passing absolutely parallel to the horizontal water line.If it is argued that "there are times when the surface of the sea is perfectly calm, and that at such times at least, if the earth is a plane, the telescope ought to restore the hull of a ship, irrespective of distance, providing its power is great enough to magnify it," the reply is that practical experiments have proved that during what is called a "
dead calm," the undulations or waves in the water amount to
more than 20 inches, as will be seen from the following extracts:--
ON THE DIMENSIONS OF OCEAN WAVES."This interesting subject was very fully entered into at a recent meeting of the Academy of Sciences, by
Admiral Coupvent de Bois:
"It is not easy to ascertain the height of the waves of the ocean; nevertheless, the method adopted for the purpose is capable of affording sufficiently exact results. The point in the shrouds corresponding with a tangent to the tops of the highest waves is ascertained by gradually ascending them, and making observations until it is reached. That point being determined, the known dimensions of the ship give the height of the waves above the line of flotation, which corresponds with the horizon of the sea, in the trough of the wave. In this way the following results were obtained:--
"The lengths of the waves have also been measured, and it has been found that, for example, waves of 27 feet in height, are about 1640 feet in length."
It is well known that even on lakes of small dimensions and also on canals, when high winds prevail for some time in the same direction, the ordinary ripple is converted into comparatively large waves. On the "Bedford Canal," during the windy season, the water is raised into undulations so high, that through
a powerful telescope at an elevation of 8 inches, a boat two or three miles away will be invisible; but at other times, through the same telescope the same kind of boat may be seen at a distance of six or eight miles.During very fine weather when the water has been calm for some days and become as it were settled down, persons are often able to see with the naked eye from Dover the coast of France, and a steamer has been traced all the way across the channel. At other times when the winds are very high, and a heavy swell prevails, the coast is invisible, and the steamers cannot be traced the whole distance from the same altitude, even with a good telescope.Instances could be greatly multiplied, but already more evidence has been given than the subject really requires, to prove that when a telescope does not restore the hull of a distant vessel it is owing to a purely special and local cause...