Fig. 85.
We have now to consider a very important modification of this phenomenon, namely, that whereas in the several instances illustrated by diagrams Nos. 71 to 84 inclusive, when the lower parts of the objects have entered the vanishing point, and thus disappeared to the naked eye, a telescope of considerable power will restore them to view; but in the case of a ship's hull a telescope fails to restore it, however powerful it may be. This fact is considered of such great importance, and so much is made of it as an argument for rotundity by the Newtonian philosophers, that it demands in this place special consideration. It has been already shown that the law of perspective, as commonly taught in our schools of art, is fallacious and contrary to every thing seen in nature. If an object be held up in the air, and gradually carried away from an observer who maintains his position, it is true that all its parts will converge to one and the same point--the centre, in relation to which the whole contracts and diminishes. But if the same object is placed on the ground, or on a board, as shown in diagram 74, and the lower part made distinctive in shape or colour, and similarly moved away from a fixed observer, the same predicate is false. In the first case the centre of the object is the datum to which every point of the exterior converges; but in the second
case the ground or board practically becomes the datum in and towards which every part of the object converges in succession--beginning with the lowest, or that nearest to it.
INSTANCES.--A man with light trowsers and black boots walking along a level path, will appear at a certain distance as though the boots had been removed and the trowsers brought in contact with the ground. On one occasion the author and several friends witnessed a kind of review or special drill of infantry in the open space behind the Horse Guards, at Whitehall. It was in the month of July, and the soldiers had on their summer clothing, all their "nether garments" were white, and when near to them the black well-polished boots were visible to the depth of three or four inches, standing distinctly between the white cloth of the trowsers, and the brown or yellowish gravel and sand of the parade ground. On moving a few hundred feet away, along one of the walks in St. James's Park, the three or four inches depth of black boots subtended an angle at the eye so acute that they were no longer visible, and the almost snow white trowsers of a line of men seemed to be in actual contact with the ground. Every man when turned away or whose back was towards the spectators, seemed to be footless. The effect was remarkable, and formed a very striking illustration of the true law of perspective. After observing the manœuvres for a short time. a party of soldiers were "told off" to relieve guard at St. James's and Buckingham Palaces, and on following then, down the avenue of the park we again noticed the perspective phenomenon of a line of soldiers marching apparently without feet.
A small dog running along will appear to gradually shorten by the legs, which at a distance, of less than half-a-mile will be invisible, and the body or trunk of the animal will appear to glide upon the earth.
Horses and cattle moving away from a given point upon horizontal ground, will seem to lose their hoofs, and to be walking on the bony extremities or stumps of the limbs.
Carriages similarly receding will seem to lose that portion of the rim of the wheels which touches the earth. The axles also will seem to get lower, and at the distance of one or two miles, according to the diameter of the wheels, the body of the carriage will appear to drag along in contact with the ground.
A young girl, with short garments terminating ten or twelve inches above the feet, will, on walking forward, appear to sink towards the earth, the space between which and the bottom of the frock will appear to gradually diminish, and in the distance of half-a-mile or less the limbs which were first seen for ten or twelve inches will be invisible--the bottom of the garment will seem to touch the ground. The whole body of the girl will, of course, gradually diminish as she recedes, but the depth of the limbs, or the lower part, will disappear before the shoulders and head--as illustrated in diagram 78.
These instances which are but a few selected from a great number which have been collected, will be sufficient to prove beyond the power of doubt, or the necessity for controversy, that upon a plane or horizontal surface the lowest parts of bodies receding from a given point of observation necessarily disappear before the highest.
This would be a sufficient explanation of the disappearance of a ship's hull before the rigging and mast-head; but as already stated in every one of the instances given, except that of the ship at sea, a telescope will restore to view whatever has disappeared to the naked eye. It would be the same in the case of the ship's hull were all the conditions the same. If the surface of the sea had no motion or irregularity, or if it were frozen and therefore stationary and uniform, a telescope of sufficient power to magnify at the distance, would at all times restore the hull to sight. On any frozen lake or canal, notably on the "Bedford Canal," in the county of Cambridge, in winter and on a clear day, skaters may be observed several miles away, seeming to glide along upon limbs without feet--skates and boots quite invisible to the unaided eye, but distinctly visible through a good telescope. But even on the sea, when the water is very calm, if a vessel is observed until it is just "hull down," a powerful telescope turned upon it will restore the hull to sight. From which it must be concluded that the lower part of a receding ship disappears through the influence of perspective, and not from sinking behind the summit of a convex surface. If not so it follows that the telescope either carries the line of sight through the mass of water, or over its surface and down the other side. This would indeed be "looking round a corner," a power which, nor that of penetrating a dense and extensive medium like water, has never yet been claimed for optical instruments of any kind.
Upon the sea the law of perspective is modified because the leading condition, that of stability in the surface or
datum line, is changed. When the surface is calm the/ hull of a vessel can be seen for a much greater distance than when it is rough and stormy. This can easily be verified by observations upon fixed objects at known distances, such as light-ships, light-houses, sea walls, head-lands, or the light-coloured masonry of batteries, such as are built on the coast in many parts of the world.
In May, 1864, the author, with several gentlemen who bad attended his lectures at Gosport, made a number of observations on the "Nab" light-ship, from the landing stairs of the Victoria Pier, at Portsmouth. From an elevation of thirty-two inches above the water, when it was very calm, the greater part of the hull of the light vessel was, through a good telescope, plainly visible. But on other occasions, when the water was much disturbed, no portion of the hull could be seen from the same elevation, and with the same or even a more powerful telescope. At other times, when the water was more or less calm, only a small portion of the hull, and sometimes the upper part of the bulwarks only, could be seen. These observations not only prove that the distance at which objects at sea can be seen by a powerful telescope depends greatly on the state of the water, but they furnish a strong argument against rotundity. The "Nab" light-ship is eight statute miles from the Victoria pier, and allowing thirty-two inches for the altitude of the observers, and ten feet for the height of the bulwarks above the water line, we find that even if the water were perfectly smooth and stationary, the top of the hull should at all times be fourteen feet below the horizon. Many observations similar to the above have
been made on the north-west light-ship, in Liverpool Bay and on light-vessels in various parts of the sea round; Great Britain and Ireland.
It is a well known fact that the light of Eddystone lighthouse is often plainly visible from the beach in Plymouth Sound, and sometimes, when the sea is very calm, persons sitting in ordinary rowing boats can see the light distinctly from that part of the Sound which will allow the line of sight to pass between "Drake's Island" and the. western end of the Breakwater. The distance is fourteen statute miles. In the tables published by the Admiralty, and also by calculation according to the supposed rotundity of the earth, the light is stated to be visible thirteen nautical or over fifteen statute miles, yet often at the same distance, and in rough weather, not only is the light not visible but in the day time the top of the vane which surmounts the lantern, and which is nearly twenty feet higher than the centre of the reflectors or the focus of the light, is out of sight.
A remarkable instance of this is given in the Western Daily Mercury, of October 25th, 1864. After lectures by the author at the Plymouth Athenæum and the Devonport Mechanics' Institute, a committee was formed for the purpose of making experiments on this subject, and on the general question of the earth's form. A report and the names of the committee were published in the Journal above referred to; from which the following extract is made.
"OBSERVATION 6TH.--On the beach, at five feet from the water level, the Eddystone was entirely out of sight."
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.
At page 60 it is shown that the surface of the sea appears to rise up to the level or altitude of the eye; and that at a certain distance, less or greater, according to the elevation of the observer, the line of sight and the surface of the water appear to converge to a "vanishing point," which is in reality "the horizon." If this horizon were formed by the apparent junction of two perfectly stationary parallel lines, it could, as before stated, be penetrated by a telescope of sufficient power to magnify at the distance, however great, to which any vessel had sailed. But because the surface of the sea is not stationary, the line of sight must pass over the horizon, or vanishing point, at an angle at the eye of the observer depending on the amount of "swell" in the water. This will be rendered clear by the following diagram, fig. 85.
Let C, D, represent the horizontal surface of the water. By the law of perspective operating without interference from any local cause, the surface will appear to ascend to the point B, which is the horizon, or vanishing point to the observer at A; but because the water undulates, the line A, B, of necessity becomes A, H, S, and the angular direction of this line becomes
less or greater if the "swell" at H increases or diminishes. Hence when a ship has reached the point H, the horizon; the line of sight begins to cut the rigging higher and higher towards the mast-head, as the vessel more and more recedes. In such a position a telescope will enlarge and render more visible all that part of the rigging which is above the line A, H, S, but cannot possibly restore that part including the hull, which is below it. The waves at the point H, whatever their real magnitude may be, are magnified and rendered more obstructive by the very instrument (the telescope), which is employed to make the objects beyond more plainly visible; and thus the phenomenon is often very strikingly observed, that while a powerful telescope will render the sails and rigging of a ship beyond the horizon H, so distinct that the different kinds of rope can be readily distinguished, not the slightest portion of the hull, large and solid as it is, can be seen. The "crested waters" form a barrier to the horizontal line of sight as substantial as would the summit of an intervening rock. And because the watery barrier is magnified and practically increased by the telescope, the paradoxical condition arises, that the greater the power of the instrument the less can be seen with it.
Thus have we ascertained by a simple Zetetic process, regardless of all theories, and irrespective of consequences, that the disappearance of the hull of an outward bound vessel is the natural result of the law of perspective operating on a plane surface, but modified by the mobility of the water; and has logically no actual connection with the doctrine of the earth's rotundity. All that can be said for it is, that such a phenomenon would exist if the earth were a globe; but it cannot be employed as a proof that the assumption of rotundity is correct.
219:1 "Lighthouses of the World," p. 36.