Читать книгу Nature's Teachings - John George Wood - Страница 5

The Cable and its Variations.—Material of Cables.—Hempen and Iron Cables, and Elasticity of the latter.—Natural Cables.—The “Byssus” of the Pinna and the common Mussel.—The Water-snail and its Cable.—A similar Cable produced by the common White Slug.—The Principle of Elasticity.—Elastic Cable of the Garden Spider.—Tendrilous Cables of the Pea and the Bryony.—The Vallisneria, and its Development through the Elastic Cable.—Proposed Submarine Telegraph Cable.—The Anchor, Grapnel, and their Varieties.—Natural Anchors.—Spicule of Synapta.—The Grapnel, natural and artificial.—Ice-anchor and Walrus Tusks.—The Mushroom Kedge.—The Flesh-hook.—Eagle-claw.—The Grapple-plant of South Africa.—The Drag.

AMONG the most important accessories to a ship are the Cable, by which she can be anchored to the bed of the sea, and the ropes called “warps,” by which she can be fastened to the land.

Perhaps my readers may not know the old riddle—“How many ropes are there on board a man-of-war?” The non-nautical individual cannot answer, but the initiated replies that there are only three, namely, the man-rope, the tiller-rope, and the rope’s-end, all the others being “tacks,” “sheets,” “haulyards,” “stays,” “braces,” &c.

Formerly cables were always made of hemp, enormously thick, and most carefully twisted by hand. Now, even in small vessels, the hempen cable has been superseded by the iron chain, and this for several reasons.

In the first place, it is much smaller in bulk, and therefore does not occupy so much room. In the next place, it is even lighter than the hempen cable of corresponding strength; and, in the third, its specific gravity—i.e. its weight when compared with an equal bulk of water—is so great, that when submerged, it falls into a sort of arch-like form, and so attains an elasticity which takes off much of the strain on the anchor, and protects it from dragging.

We will now look to Nature for Cables.

The natural cable which will first suggest itself is evidently that of the Pinna Shell (Pinna pectinata), which fixes its shell to some rock or stone with a number of silk-like threads, spun by itself, and protruding from the base, just as a vessel on a lee shore throws out a number of cables. The threads which compose the “byssus,” as it is called, are only a few inches in length, and apparently slight. They are, however, really strong, and by acting in unison enable the shell, though sometimes two feet in length, to be held firmly to the rock. I may here mention that they have been occasionally woven into gloves, and other articles of apparel, to which their natural soft grey-brown hue gives a very pleasing appearance.

A still more familiar instance of a natural marine cable is given by the common Mussel, which can be found in thousands on almost every solid substance which affords it a hold. Even copper-bottomed ships are often covered with Mussels, all clinging by their natural cables, and it is thought that the cases which sometimes occur of being poisoned by eating Mussels, or “musselled,” as the malady is called by the seafaring population, are due to the fact that the Mussels have anchored themselves to copper, and have in consequence imbibed the verdigris.

Passing from salt to fresh water, we come to a natural cable which is very common, and yet, on account of its practical invisibility, is almost unknown, except by naturalists. I refer to the curious cable which is constructed by the common Water-snail (Limnæa stagnalis), which has already been mentioned in its capacity of a boat.

This creature has a way of attaching itself to some fixed object, such as a water-lily leaf, by means of a gelatinous thread, which it can elongate at pleasure, and by means of which it can retain its position in a stream, or in still water can sink itself to the bottom, and ascend to the same spot. This cable seems to be made of the same glairy secretion as that which surrounds the egg-masses which are found so plentifully on leaves and stones in our fresh waters, and, like that substance, is all but invisible in the water, so that an inexperienced eye would not be able to see it, even if it were pointed out.

Slight, gelatinous, and almost invisible in the water as is this thread, its strength is very much greater than might be supposed. Not only can a mollusc be safely moored in the water by such a cable, but it can be actually suspended in the air, as may be seen from a letter in Hardwicke’s Science Gossip for 1875, p. 190:—

“Last summer (September 29) I met with the following unusual fact. In a green-house, from a vine-leaf which was within a few inches of the glass … a slug was hanging by a thread, which was more than four feet in length, not unlike a spider-web, but evidently much stronger.

“The slug was descending by means of this thread, and, as the glutinous matter from the under part of the body was drawn out by the weight of the creature, it was consolidated into a compact thread by the slug twisting itself in the direction of the hands of a clock, the power of twisting being given by the head, and the part of the body nearest the head being turned in the direction of the twist. There was no tendency to turn in the contrary direction. Evidently the thread became hard as soon as it was drawn away from the body.

“By wetting the sides of slips of glass, I secured two specimens of the thread. In one of these, part was stretched, and part quite loose, the latter appearing flat when seen through a microscope. The thread, which was highly elastic, was increased about three inches in a minute. The slug was white, and about an inch and a half in length.”

Now we come to the elastic system of the Chain Cable, and find it anticipated in Nature in various ways.

One curious example was that of a Spider, which found its wheel-like net in danger from a tempestuous wind. The Spider descended to the ground, a depth of about seven feet, and, instead of attaching its thread to a stone or plant, fastened it to a piece of loose stick, hauled it up a few feet clear of the ground, and then went back to its web. The piece of stick thus left suspended acted in a most admirable manner, giving strength and support, and at the same time yielding partly to the wind.

By accident the thread became broken, and the stick, which was about as thick as an ordinary pencil, and not quite three inches in length, fell to the ground. The Spider immediately descended, attached another thread, and hauled it up as before. In a day or two, when the tempestuous weather had ceased, the Spider voluntarily cut the thread, and allowed the then useless stick to drop.

A curious example of the elastic cable is seen in the egg-case of the Dog-fish, which is given on page 35. The egg-case is formed like that of the common skate, and has a projection from each of its angles. But the projections, instead of being mere flattened horns, are lengthened into long elastic strings, tapering towards the ends, and twisted spirally, like the tendrils of a grape-vine.

These tendril-like appendages twist themselves round seaweeds and other objects, and, on account of their spiral form, can hardly ever be torn from their attachments. Sometimes after a storm the egg is thrown on the shore, still clinging to the seaweed, but to find an egg detached is very rarely done.

I have already mentioned the tendrils of the vine, and their great strength. The reader may remember the corresponding cases of the Pea and the Bryony, the latter being a most remarkable example of the strength gained by the spiral form. It clambers about hedges, is exposed to the fiercest winds, has large and broad leaves, and yet such a thing as a Bryony being blown off a hedge is scarcely, if ever, seen. I never saw an example myself, though I have had long experience in hedges.

Another excellent example of this principle is found in the Vallisneria plant, which of late years has become tolerably familiar to us through the means of fresh-water aquaria, though it is not indigenous to this country.

In this plant the elastic power of the spiral cable is beautifully developed. It is an aquatic plant, mostly found in running waters, and has a most singular mode of development. It is diœcious—i.e. the male, or stamen-bearing, and the female, or pistil-bearing flowers, grow upon separate plants.

It has to deposit its seeds in the bed of the stream, and yet it is necessary that both sets of flowers should be exposed to the air and sun before they become able to perform their several duties. Add to this the fact that the male flower is quite as small in proportion to the female as is the case with the lac and scale insects, and the problem of their reaching each other becomes apparently intricate, though it is solved in a beautifully simple manner.

Fertilisation cannot be conducted by means of insects, as is the case with so many diœcious terrestrial plants, and it is absolutely necessary that actual contact should take place between them. This difficult process is effected as follows:—

The female flowers are attached to a very long spiral and closely coiled footstalk, and, when they are sufficiently developed, the footstalk elongates itself until the flower rests on the surface of the water, where it is safely anchored by its spiral cable, the coils yielding to the wavelets, and keeping the flower in its place.

Meanwhile the tiny male flowers are being developed at the bottom of the river, and are attached to very short footstalks. When they are quite ripe they disengage themselves from their footstalks, and rise to the surface of the river. Being carried along by the stream, they are sure to come in contact with the anchored female flowers. This having been done, and the seeds beginning to be developed, the spiral footstalk again coils itself tightly, and brings the seeds close to the bed of the stream, where they can take root.

There are other numerous examples, of which any reader, even slightly skilled in botany, need not be reminded, most of them being, in one form or another, modifications of the leaf or the petal, which, after all, are much the same thing. The vine and passion-flower are, however, partial exceptions.

I may here mention that soon after the failure of the first Atlantic telegraph cable, an invention was patented of a very much lighter cable, enclosed in a tube of india-rubber, and being coiled spirally at certain distances, so that the coils might give the elasticity which constitutes strength. The cable was never made, its manufacture proving to be too costly; but the idea of lightness and elasticity, having been evidently taken from the spiral tendrils of the bryony, was certainly a good one, and I should have wished to see it tried on a smaller scale than the Atlantic requires.

As a natural consequence, after the cable comes the Anchor, which in almost every form has been anticipated by Nature, whether it be called by the name of anchor, kedge, drag, or grapnel.

On the accompanying illustrations are shown a number of corresponding forms of the Anchor, together with a few others, which, although they may not necessarily be used in the water, are nevertheless constructed on the same principle—i.e. for the purpose of grappling.

One of the most startling parallels may be seen on the right hand of the illustration, the figure having been drawn from an old Roman coin. On the other side of the same illustration may be seen an anchor so exactly similar in form, that the outline of the one would almost answer for that of the other. This object is a much-magnified representation of a spicule which is found on the skin of the Synapta, one of the so-called Sea-slugs, which are so extensively sold under the name of Bêche de Mer. It forms one of the curious group called the Holothuridæ.

Each of these anchors is affixed to a sort of open-worked shield, as shown above, and on the left hand; and it is a curious fact that in the various species of Synapta the anchor is rather different in form, and the shield very different in pattern. They are lovely objects, and I recommend any of my readers who possess a microscope to procure one. They need a power of at least 150 diameters to show their full beauties.

An ordinary Grapnel is here shown, and in the corresponding position on the opposite side is an almost exactly similar object, except that it is double, having the grapnel at both ends of the stem. This is a spicule of a species of sponge, and is one of the vast numbers of which the sponge principally consists.

Next to the sponge-spicule is a still more perfect example of a natural Grapnel. This is the head of an internal parasite called Echinococcus, which holds itself in its position by means of the circle of hooks with which the head is surrounded. These hooks are easily detached, and have a curious resemblance to the claw of the lion or tiger.

On the left-hand side is a representation of a parasitic crustacean animal called Lernentoma, which adheres to various fishes, and is mostly found upon the sprat, clinging to the gills by means of its grapnel-shaped head.

On the right hand of the accompanying illustration is an ice-anchor, copied from one of those which were taken out in the Arctic expedition of 1875. Opposite is the skull of the Walrus, the tusks of which are said to be used for exactly the same purpose. Below are ice-hooks, also used for the same expedition.

The next illustration exhibits a butcher’s hook and a common porter’s hook, by which he lifts sacks on his back; and opposite them are some sponge-spicules, the similarity of which in form is so remarkable that the former might have been copied from the latter.

Our next sketch shows a remarkable example of similitude in form. There are certain small anchors called Kedges, which are very useful for mooring a boat where no great power of resistance has to be overcome, and a large anchor would be cumbersome. One of these is called, from its shape, the “Mushroom Kedge,” and is very useful, as, however it may be dropped, some part of the edge is sure to take the ground. This Kedge is shown on the right hand of the illustration, and the Mushroom, from which its shape was borrowed, is seen on the left.

We now come to some more examples of the principle of the Grapnel, some of which are applied to nautical, and others to terrestrial objects.

The right-hand upper figure represents the “Flesh-hook,” used for taking boiled meat out of the caldron, so familiar to us by the reference to it in Exodus xxvii. 3, and the still better-known allusion to its office in 1 Samuel ii. 13, 14. In the former passage, even the material, brass, which was really what we now call bronze, is mentioned, and it is a curious fact that all the specimens in the British Museum, from one of which the drawing was taken, are made of bronze. I need hardly state that the hollow handle is meant to receive a wooden staff.

On comparing this figure with that of the Eagle’s foot on the opposite side, the reader cannot but be struck with the exact resemblance between the two. Indeed, there is very little doubt that the flesh-hook was intentionally copied from the foot of some bird of prey. Perhaps the Osprey would have furnished even a better example than the Eagle, the claws being sharper and more boldly curved, so as to hold their slippery prey the better.

On the left hand of the next illustration is a figure of the seed-vessel of the Grapple-plant of Southern Africa, drawn from a specimen in my collection. The seed-vessel is several inches in length, and the traveller who is caught by a single hook had better wait for assistance than try to release himself. The stems of the plant are so slender, and the armed seed-vessels so numerous, that in attempting to rescue one portion of the dress, another portion becomes entangled, and the traveller gets hopelessly captured. Besides the hooks of the seed-vessels, the branches themselves are armed with long thorns, set in pairs. The scientific name of this plant is Uncinaria procumbens, the former word signifying “a hook,” and the latter “trailing.” It is also known by the popular name of Hook-plant.

In the late Kafir wars the natives made great use of this and other plants with similar properties, their own naked, dark, and oiled bodies slipping through them easily and unseen, while the scarlet coats of the soldiers were quickly entangled, and made them an easy mark for the Kafir’s spear. In this way many more of our soldiers were killed by the spears than by the bullets of their enemies.

Opposite to the Grapple-plant is shown the common Drag, which is utilised for so many purposes. Generally it is employed for recovering objects that have sunk to the bottom of the water, and its use by the officers of the Humane Society is perfectly well known, the Drag being sometimes affixed to the end of a long pole, like the flesh-hook already described, and sometimes tied to a rope.

It can also be used as an anchor, after the manner of a kedge, and has been often employed in naval engagements for the purpose of drawing two ships together, and preventing the escape of the vessel which is being worsted. My relative, the late Admiral Sir J. Harvey, K.B., used drags in this manner, and secured two French ships, one on either side, namely, L’Achille and Le Vengeur. The first was sunk, and the second captured.