Propulsion by the Oar.—Parallels in the Insect World.—The “Water-boatman.”—Its Boat-like Shape.—The Oar-like Legs.—Exact mechanical Analogy between the Legs of the Insect and the Oars of the human Rower.—“Feathering” Oars in Nature and Art.—The Water-boatman and the Water-beetles.—The Feet of the Swan, Goose, and other aquatic Birds.—The Cydippe, or Beroë.—The Self-feathering Paddle-wheel.—Indirect Force.—The Wedge, Screw, and Inclined Plane.—“Sculling” a Boat.—The “Tanka” Girls of China.—Mechanical Principle of the Screw, and its Adaptation to Vessels.—Gradual Development of the Nautical Screw.—Mechanical Principle of the Tail of the Fish, the Otter, and the sinuous Body of the Eel and Lampern.—The Coracle and the Whirlwig-beetle.
Propulsion by the Oar.—Parallels in the Insect World.—The “Water-boatman.”—Its Boat-like Shape.—The Oar-like Legs.—Exact mechanical Analogy between the Legs of the Insect and the Oars of the human Rower.—“Feathering” Oars in Nature and Art.—The Water-boatman and the Water-beetles.—The Feet of the Swan, Goose, and other aquatic Birds.—The Cydippe, or Beroë.—The Self-feathering Paddle-wheel.—Indirect Force.—The Wedge, Screw, and Inclined Plane.—“Sculling” a Boat.—The “Tanka” Girls of China.—Mechanical Principle of the Screw, and its Adaptation to Vessels.—Gradual Development of the Nautical Screw.—Mechanical Principle of the Tail of the Fish, the Otter, and the sinuous Body of the Eel and Lampern.—The Coracle and the Whirlwig-beetle.
THE Boat naturally reminds us of the Boatman. In the two gnat-boats which have been described there is no propelling power used or needed, the little vessel floating about at random, and its only object being to keep afloat. But there are many cases where the propelling power is absolutely essential, and where its absence would mean death, as much as it would to a ship which was becalmed in mid ocean without any means of progress or escape. There are, for example, hundreds of creatures, belonging to every order of animals, which are absolutely dependent for their very existence on their power of propulsion, and I believe that there is not a single mode of aquatic progression employed by man which has not been previously carried out in the animal world. There are so many examples of this fact that I am obliged to select a very few typical instances in proof of the assertion.
Taking the Oar as the natural type of progression in the water, we have in the insect world numerous examples of the very same principle on which our modern boats are propelled.And it is worthy of notice, that the greater the improvement in rowing, the nearer do we approach the original insect model.
The first which we shall notice is the insect which, from its singular resemblance to a boat propelled by a pair of oars, has received the popular name ofWater-boatman. Its scientific name isNotonecta glauca, the meaning of which we shall presently see. It belongs to the order of Heteroptera, and is one of a numerous group, all bearing some resemblance to each other in form, and being almost identical in habits. Though they can fly well, and walk tolerably, they pass the greater part of their existence in the water, in which element they find their food.
Predacious to a high degree, and armed with powerful weapons of offence, it is one of the pirates of the fresh water, and may be found in almost every pond and stream, plying its deadly vocation.
Its large and powerful wings seem only to be employed in carrying it from one piece of water to another, while its first and second pairs of legs are hardly ever used at all for progression. The last pair of legs are of very great length, and furnished at their tips with a curiously constructed fringe of stiff hairs. The body is shaped in a manner that greatly resembles a boat turned upside down, the edge of the elytra forming a sort of ridge very much like the keel of the boat.
When the creature is engaged in swimming, it turns itself on its back, so as to bring the keel downwards, and to be able to cut the water with the sharp edge. From this habit it has derived the name of Notonecta, which signifies an animal which swims on its back. The first and second pairs of legs are clasped to the body, and the last pair are stretched out as shown in the illustration, not only looking like oars, but being actually used as oars.
Now, I wish especially to call the reader’s attention to the curiously exact parallel between the water-boatman and the human oarsman. As the reader may probably know, the oar is a lever of the second order,i.e.the power comes first, then the weight, and then the fulcrum. The arm of the rower furnishes the power, the boat is the weight to be moved, and the water is the fulcrum against which the lever acts.
I have more than once heard objections to this definition, the objectors saying that the water was a yielding substance,and therefore could not be the fulcrum. This objection, however, was easily refuted by taking a boat up a narrow creek, and rowing with the oar-blades resting on the shore, and not in the water.
Image unavailable: OAR-LEG OF WATER-BOATMAN. OAR OF BOAT. WATER-BOATMAN ROWING ITSELF. OARSMAN ROWING.
Now, the swimming legs of the water-boatman are exact analogues of the oars of a human rower. The internal muscles at the juncture of the leg with the body supply the place of the rower’s arms, the leg itself takes the office of the oar, and the body of the insect is the weight to be moved, and the water supplies the fulcrum. Even the broad blade at the end of the oar is anticipated by the fringe of bristles at the end of the leg, and its sharpened edge by the shape of the insect’s limb.
Besides these resemblances, there is another which is worthy of notice. All rowers know that one of their first lessons is to “feather” their oars,i.e.to turn the blade edgewise as soon as it leaves the water. Nothing looks more awkward than for a boatman to row without feathering. (We all must remember the eulogy on the “Jolly Young Waterman,” who “feathered his oars with skill and dexterity.”) In the first place, he must lift his oar very high out of the water, and, in the second, he will be impeded by any wind that happens to come against the blades.
The Water-boatman, however, does not lift its legs out of the water after every stroke, as a human boatman does, and therefore it has no need to feather in the same way. But there is even greater need for a feathering of some kind in the insect’s leg, on account of the greater resistance offered by water than by air, and this feathering is effected by the arrangement of the blade-bristles, which spread themselves against the water as the stroke is made, and collapse afterwards, so as to give as little resistance as possible when the stroke is completed.
InArt we have invented many similar contrivances, but I believe that there is not one in which we have not been anticipated by Nature. Putting aside the insect which has just been described, we have the whole tribe of water-beetles, in which the same principle is carried out in an almost identical manner. In the accompanying illustration, the oar, the rower, and the boat are placed above one another, and next to them are seen one of the oar-legs of the water-boatman and the insect as it appears when swimming on its back.
Then, there is the foot of the duck, goose, swan, and various other aquatic birds, in which the foot presents a broad blade as it strikes against the water, and a narrow edge as it recovers from the stroke. Some years ago, a steam yacht was built and propelled by feet made on the model of those of the swan. She was a very pretty vessel, but art could not equal nature, and at present the swan-foot propeller, however perfect in theory, has not succeeded in action. Perhaps, if some nautical engineer were to take it in hand, he would procure the desired result.
Almost exactly similar is the mode of propulsion employed by the lobster, the prawns and shrimps, their tails expanding widely into a fan-like shape as they strike against the water, and then collapsing when the stroke is withdrawn, so as to allow them to pass through the water with the least possible resistance.
The same principle is to be seen in the lively little Acaleph, for which there is unfortunately no popular name, and which we must therefore call by its scientific title ofCydippe, or Beroë, these names being almost indifferently used. When full grown, it is about as large as an acorn, and very much of the same shape. It is as transparent as if made of glass, and, when in the water, is only visible to practised eyes.
En passant, I may remark that the familiar term of “water,” when applied to diamonds, is owing to their appearance when placed in distilled water. Those which can be at once seen are called stones of the second water. Those which cannot be seen, because their refractive powers are equal to those of the water, are called “diamonds of the first water,” and are very much more valuable than the others.
As the Cydippe is, in fact, little more than sea-water,entangled in the slightest imaginable and most transparent tissue of animal fibre, it is evident that the water and the Cydippe must be of almost equal refracting power, and that therefore the acaleph must be as invisible as diamonds of the “first water.” Indeed, I have often had specimens in a glass jar which were absolutely invisible to persons to whom I wished to show them.
But an experienced eye detects the creature at once. Along its body, at equal distances, are eight narrow bands, over which the colours of the rainbow are, though very faint, perpetually rippling. This appearance is caused by the machinery which impels the body, and which seems never to cease. Each of these bands is composed of a vast number of tiny flaps, which move up and down in regular succession, so as to cause the light to play on their surfaces. And, as they move as if set on hinges, they of course offer no resistance to the water after their stroke is made.
Image unavailable: CYDIPPE AND PADDLES. PRAWN SWIMMING. FEET of DUCK. SELF-FEATHERING PADDLE-WHEEL.
Now let us compare these works of nature with those of art. We have already seen the parallels of the oar, and we now come to those of the paddle-wheel. When paddle-steamers were first invented, the blades were fixed and projected from the wheel, as if they had been continuations of its spokes. It was found, however, that a great waste of power, together with much inconvenience, was caused by this arrangement. Notonly was a considerable weight of water raised by each blade after it passed the middle of its stroke, but the steam power was given nearly as much to lifting and shaking the vessel as to propulsion.
A new kind of paddle-wheel was then invented, in which the blades were ingeniously jointed to the wheel, so that they presented their flat surfaces to the water while propelling, and their edges when the stroke was over. This, which is known by the name of the “Self-feathering Paddle-wheel,” was thought to be a very clever invention, and so it was; but not even the inventors were likely to have known that if they had only looked into the book of Nature, they might have found plenty of self-feathering paddle-wheels, beside the few which my limited space enables me to give.
If the reader will look at the illustration, he will see that on one side is represented the self-feathering paddle-wheel of Art, with its ingenious arrangement of rods and hinges. On the other side there comes, first, the common Prawn, shown with its tail expanded in the middle of its stroke.
Just below it is a Cydippe of its ordinary size, showing the paddle-bands, one of which is drawn at the side much magnified, so as to show the arrangement of the little paddles. As to the tentacles which trail from the body, we shall treat of them when we come to our next division of the subject of the work.
Lastly, there is a representation of the self-feathering feet of the Duck, the left foot expanded in striking the water, and the right closed so as to offer no resistance when drawn forward for another stroke. The swan’s foot shows this action even more beautifully than does that of the duck.
Wenow come to another mode of propulsion, namely, that which is not due to direct pressure of a more or less flat body against the water, but to the indirect principle of the screw, wedge, or inclined plane.
Space being valuable, I will only take two instances, namely, the well-known mode of propelling a boat by a single oar working in a groove or rowlock in the middle of the stern, and the ordinary screw of modern steamers.
Most of my readers must have seen a sailor in the act of“sculling” a boat. A tolerably deep notch is sunk in the centre of the stern, and the oar is laid in it, as shown in the central illustration, on the right-hand side. The sailor then takes the handle of the oar, and works it regularly backwards and forwards, without taking the blade out of the water. The boat at once begins to move forward, and, when the oar is urged by a strong and experienced man, can be propelled with wonderful speed. The well-known “Tanka” boat-girls of China never think of using two oars, a single oar in the stern being all-sufficient for the rapid and intricate evolutions required in their business.
Image unavailable: TAIL OF FISH. SCREW OF STEAMER. TAIL OF OTTER. “SCULLING” A BOAT. TAIL OF SEAL. ACTION OF RUDDER.
The mechanical process which is here employed is nothing more than that of the inclined plane, or rather, the wedge, the oar-blade forming the wedge, and the force being directed against the stern of the boat, and so driving it through the water.
The Rudder affords another example of a similar force, although it is used more for directing than propelling a vessel. Still, just as the scull is used not only for propelling, but for steering the boat, the rudder, when moved steadily backwards and forwards, can be used for propulsion as well as steerage. In the absence of oars, this property is most useful, as I can practically testify.
So different in appearance are the screw and the inclined plane, that very few people would realise the fact that the screw is nothing but an inclined plane wound round a cylinder, or rather, is a circular inclined plane. The ordinary corkscrew is a good example of this principle, the cylinder being but an imaginary one.
Now, if the screw be turned round, it is evident that force is applied just on the principle of the wedge, and this principle is well shown in the various screw-presses, of which the common linen-press is a familiar example, as was the original printing-press, which still survives as a toy for children.
We all know the enormous force exerted by screws when working in wood, and how, when the screw-driver is turned in the reverse direction, the instrument is forced backwards, though the operator is leaning against it with all his weight. In fact, a comparatively small screw, if working in hard wood or metal, so that the threads could not break, could lift a heavy man.
Substitute water for wood or metal, and the result would be the same in principle, though the resistance would be less. As the loss of power by friction would prevent a large vessel from being propelled by a stern oar moved like a scull, the idea was invented of applying the same kind of power by a large screw, which should project into the water from the stern of the vessel. This modification, moreover, would have the advantage of forcing the vessel forward when the screw was turned from left to right, and drawing it back when turned in the opposite direction, whereas the sculling oar would only drive it forward.
The principle was right enough, but there was at first a great difficulty in carrying it out. Firstly, several turns of a large screw were used, and were found to need power inadequate to the effect. Then the screw was reduced to four separate blades, and now only two are used, as shown in the illustration, these saving friction, being equally powerful for propulsion, and running less risk of fouling by rigging blown overboard or other floating substances.
So much for Art. Now for the same principle as shown in Nature, of which I can take but a very few instances.
The first and most obvious example is that of the Fish-tail, which any one may observe by watching ordinary gold fish ina bowl. Their progression is entirely accomplished by the movement of the tail from side to side, exactly like that of the sculling oar, and moreover, like the oar, the tail acts as rudder as well as propeller.
The force with which this instrument can be used may be estimated by any one who is an angler, and knows the lightning-like rush of a hooked trout, or who has seen the wonderful spring with which a salmon shoots clear out of the water, and leaps up a fall several feet in height. This is not done, as many writers state, by bending the body into a bow-like form, and then suddenly straightening it, but by the projectile force which is gained by moving the tail backwards and forwards as a sculler moves his oar.
Perhaps some of my readers have seen the wonderful speed, ease, and grace with which an Otter propels itself through the water. As the otter feeds on fish, and can capture even the salmon itself, its powers of locomotion must be very great indeed. And these are obtained entirely by means of the tail, which is long, thick, and muscular, and can be swept from side to side with enormous force, considering the size of the animal. The legs have little or nothing to do with the act of swimming. The fore-legs are pressed closely against the body, and the hind-legs against each other. The latter act occasionally as assistants in steering, but that is all.
Then there are the various Seals, whose hind-legs, flattened and pressed together, act exactly like the tail of the fish, that of the otter, the oar of the sculler, or the screw of the steamer. Also, the eel, when swimming, uses exactly the same means, its lithe body forming a succession of inclined planes; so does the snake, and so does the pretty little lampern, which is so common in several of our rivers, and so totally absent from others.
I can only now give a short description of the woodcut which illustrates these points.
On the right hand Art is shown by the screw-blades of the modern steamer. In the middle is the ordinary mode of sculling a boat by an oar in the stern, and below it is the rudder, which, like the sculling oar, may be used either for propulsion or direction.
On the left hand we have three examples of the samemechanical powers as shown in Nature. The uppermost figure represents a fish as in the act of swimming, the dotted lines showing the movement of its tail, and the principle of the wedge. In the middle is an otter, just preparing to enter the water, and below is a seal, both of them showing the identity of mechanism between themselves and the art of man. I need not say that the mechanism of art is only a feeble copy of that of nature, but nothing more could be expected.
Whilewe are on this subject I may as well mention two more applications of the screw principle. The first is the windmill, the sails of which are constructed on exactly the same principle as the blades of the nautical screw. Only, as they are pressed by the wind, and the mill cannot move, they are forced to revolve by the pressure of the wind, just as the screw of a steamer revolves when the vessel is being towed, and the screw left at liberty.
Moreover, just as the modern screws have only two blades, so, many modern windmills have only two sails, the expense and friction being lessened, and the power not injured.
Again: some years ago there was a very fashionable toy called the aërial top. It was practically nothing but a windmill in miniature, rapidly turned by a string, after the manner of a humming-top. The edges of the sails being turned downwards, the instrument naturally screwed itself into the air to a height equivalent to the velocity of the motion.
A similar idea has been mooted with regard to the guidance of balloons, or even to aërial voyaging without the assistance of gas, but at present the weight of the needful machinery has proved to be in excess of the required lifting power.
In fine, the application of the inclined plane, wedge, or screw as a motive power, is so wide a subject that I must, with much reluctance, close it with these few and obvious examples.
Itis worth while, by the way, to remark how curiously similar are such parallels. I have already mentioned the very evident resemblance between the water-boatman, the water-beetles, and the human rower, the body of the insect being shaped very much like the form of the modern boat. I must now draw the attention of the reader to the similitude betweenthe very primitive boat known by the name of Coracle, and the common Whirlwig-beetle (Gyrinus natator), which may be found in nearly every puddle. The shape of the insect is almost identical with that of the boat, and the paddle of the coracle is an almost exact imitation of the swimming legs of the whirlwig. And, as if to make the resemblance closer, many coraclers, instead of using a single paddle with two broad ends, employ two short paddles, shaped very much like battledores.
Image unavailable: WHIRLWIG BEETLE AND PADDLES. CORACLE AND PADDLE.
General Sketch of the Subject.—The Mast of Wood and Iron.—Analogy between the Iron Mast and the Porcupine Quill.—The Iron Yard and its Shape prefigured by the same Quill.—Beams of the Steam-engine.—Principle of the Hollow Tube in place of the Solid Bar.—Quills and Bones of Birds.—Wheat Straws and Bamboos.—Structure of the Boat.—The Coracle, the Esquimaux Boat, and the Bark Canoe.—Framework of the Ship and Skeleton of the Fish.—Compartments of Iron Ship and Skull of Elephant.—The Rush, the Cane, and the Sugar-cane.—“Stellate” Tissue and its Varieties.
General Sketch of the Subject.—The Mast of Wood and Iron.—Analogy between the Iron Mast and the Porcupine Quill.—The Iron Yard and its Shape prefigured by the same Quill.—Beams of the Steam-engine.—Principle of the Hollow Tube in place of the Solid Bar.—Quills and Bones of Birds.—Wheat Straws and Bamboos.—Structure of the Boat.—The Coracle, the Esquimaux Boat, and the Bark Canoe.—Framework of the Ship and Skeleton of the Fish.—Compartments of Iron Ship and Skull of Elephant.—The Rush, the Cane, and the Sugar-cane.—“Stellate” Tissue and its Varieties.
HAVING now treated of the raft, the boat, the ship, and their various modes of propulsion and guidance, we come to the subsidiary appliances to navigation, if they may be so called in lack of a better name.
First in importance is necessarily the mast; and the yards, which support the sails, are naturally the next in order. Then there come the various improvements in the building of vessels; namely, the substitution of planks fastened on a skeleton of beams for a mere hollowed log, and the subsequent invention of iron vessels with their numerous compartments, giving enormous strength and size, with very great comparative lightness.
Then we come to the various developments of the ropes or cables, by which a vessel is kept in its place when within reach of ground, whether on shore or at the water-bed. Next come the different forms of anchors which fasten a vessel to the bed of the ocean, of grapnels by which she can be made fast to the shore, or of “drags,” which at a pinch can perform either office, and can besides be utilised in searching for and hauling up objects that are lying at the bottom of the sea.
Next we come to the boat-hook, which is so useful either as a temporary anchor, or as a pole by which a boat can be propelled by pushing it against the shore or the bed of the water; and then to the “punt-pole,” which is only used for the latter purpose.
Lastly, we come to the life-belt and life-raft, which are now occupying, and rightly, so much of the public attention. These subjects will be treated in their order in the present chapter, and I hope to be able to show the reader that in all these points nature has anticipated art.
I presume that most, if not all, of my readers are aware of the rapidly extending use of iron in ship-building, not only in the standing rigging, but in the material of the vessel. First there came iron “knees,”i.e.the angular pieces of wood which strengthen the junctions of the timbers. Formerly these were made of oak-branches, and, as it was not easy to find a bough which was naturally bent at such an angle as was required for a “knee,” such branches were exceedingly valuable. Iron, however, was then employed, and with the best results. It was lighter than the wooden knee, was stronger, could be bent at any angle, and took up much less space.
By degrees iron was used more and more, until vessels were wholly made of that material. Then the masts, and even the yards, were made of iron, and, strange as it may appear, were found to be lighter as well as stronger than those made of wood. Of course, the masts and yards were hollow, and it was found by the engineers that in order to combine lightness with great strength, the best plan was to run longitudinal ridges along the inside of the tube.
A section of one of these masts is given at Fig. B, and taken from the drawings of one of our largest engineering firms. The reader will see that the mast is composed of rather slight material, and that it is strengthened by four deep though thin ribs, which run throughout its length.
When I first saw this mast I was at once struck with the remarkable resemblance between it and the quill of the Porcupine. These quills, as all anglers know, are very light, and of extraordinary strength when compared with their weight. Indeed, they are so light that they are invaluable as penholders to those who are obliged to make much use of their pen. Ihave used nothing else for a very long time, and the drawing of the Porcupine quill which is here given at Fig. A was made from a small piece cut from the top of the penholder which I have used for some fifteen years, and with which all my largest and most important works were written, including the large “Natural History,” “Homes without Hands,” “Man and Beast,” &c., &c. A portion of the same quill is also shown of its real size.
If the reader will cut a Porcupine quill at right angles, make a thin section of it, and place it under the microscope, or even under an ordinary pocket lens, he will see that the exterior is composed of a very thin layer of horny matter, and the interior filled with a vast number of tiny cells, which are formed much on the same plan as the pith of elder and other plants. The analogies of the pith will be treated in another page.
Image unavailable: PORTION OF PORCUPINE QUILL. COMPLETE QUILL. BAMBOO. SECTION OF PORCUPINE QUILL MAGNIFIED. SECTION OF ENGINE BEAM. IRON YARD AND YARD-ARM. ENGINE BEAM. SECTION OF IRON MAST.
But were the quill merely a hollow tube filled with pith, it would be too weak to resist the strain to which it is often liable. Consequently it is strengthened by a number of internal ribs, composed of the same horny material as the outer coat, and arranged in exactly the same way as those of the mast.
There are yet other points in the structure of the Porcupine quill which might be imitated with advantage in the mast. In the first place, the internal ribs are much more numerousthan those of the mast, but they are very much thinner, and taper away from the base, where the greatest strain exists, to the end, where they come to the finest imaginable edge. This modification of structure enables the outer shell of the quill to be exceedingly thin and light, and, moreover, gives to the whole quill an elasticity which is quite wonderful, considering its weight and strength.
Then, in the iron mast the exterior is quite smooth, whereas in the Porcupine quill it is regularly indented, exactly on the principle of the corrugated iron, which combines great strength with great lightness. And I cannot but think that our iron masts might be made both lighter and stronger if the shell were thinner, the internal ribs made like those of the Porcupine quill, and the shell corrugated instead of being quite smooth. The internal cells of the quill are, of course, not needed in the mast, as they are intended for nutrition, and not for strength.
Beingon this subject, we may take the shape of the Porcupine quill, and compare it with that of the ship’s yard. It will be seen that the two are so exactly similar in form that the outline of one would answer perfectly well for the other. The only perceptible difference is, that in the ship’s yard both ends are alike, whereas in the Porcupine quill the end which is inserted in the skin is rounded and slightly bent, while the other end is sharply pointed.
The principal point to be noticed in the form of both quill and yard is, that they become thicker in the centre, that being the spot on which the greatest strain comes, and which, in consequence, needs to be stronger than any other part. While holding and balancing the pole which Blondin uses to preserve his balance when walking on the high rope, I was struck with the fact that the pole, which is heavily weighted at each end, had to be strengthened in the middle, exactly on the principle of the Porcupine quill and the ship’s yard. It could not, of course, be thickened, as the hands could not grasp it, but it had to be furnished with additional strengthening. And the necessity of such strengthening is evident from the fact that on one occasion the pole did break in the middle, so that any one of less nerve and presence of mind must have been killed.
Bearing in mind, then, that in a rod or pole the centre is the part which most requires to be strengthened, we can see, in cases too numerous to mention, how art has followed, though perhaps unconsciously, in the footsteps of nature. Take, for example, the beam of a steam-engine, such as is given in the sketch, and for which the great engine at Chatham acted as model. The reader will observe that in this case the beam is gradually thickened towards the centre, the ends, where the strain is slightest, being comparatively small.
Another point also must be noticed. Equal strength could have been obtained had the beam been solid, but at the expense of weight, and consequent waste of power. Lightness is therefore combined with strength by making the beam consist of a comparatively slight centre, but having four bold ridges, as shown in the section given in the accompanying illustration. This plan, as the reader will see, is exactly the same as that which is adopted in the iron mast and porcupine quill, except that the ridges are external instead of internal. The same mode of construction is employed in ordinary cranes, the principal beam of which is almost identical in form with that of the engine, both being thickest in the centre, and both strengthened with external ridges.
There are also other analogies between the hollow mast and natural objects. Keeping still to the animal world, we find the quill feathers of the flying birds to supply examples of the combination of great strength with great lightness and very little expenditure of material. Their wing bones, too, are hollow, communicating with the lungs, and are consequently light as well as strong.
Passing to the vegetable world, we find a familiar example of this structure in the common Wheat Straw. The ripe ear is so heavy, when compared with the amount of material which can be spared to carry it, that if the stalk were solid it would give way under the mere weight of the ear. Moreover, the full-grown corn has to endure much additional weight when wetted with rain, and to resist much additional force when bowed by the wind, so that a slight and solid stalk would be quite inadequate to the task of supporting the ear.
The material of the stalk is therefore utilised in a different manner, being formed into a hollow cylinder, the exterior ofwhich is coated with a very thin shell of flint, or “silex” as it is scientifically termed. The result of this structure is that the stem possesses strength, lightness, and elasticity, so as to be equal to the burden which is laid upon it.
Then there is the common Bamboo, which is little more than a magnified straw, being constructed in much the same manner, and possessing almost the same constituents of vegetable matter and silex.
Perhaps the most extraordinary of the tubal system is to be found in the remarkable plant of Guiana called by the natives Ourah, and scientifically known by the name ofArundinaria Schomburgkii. Like the bamboo, it grows in clusters, and has a feathery top, which waves about in the breeze. But, instead of decreasing gradually in size from the base upwards, the Ourah, although it runs to some fifty feet in height, is nowhere more than half an inch in diameter. The first joint is about sixteen feet in length, and uniform in diameter throughout.
It is scarcely thicker than ordinary pasteboard, and yet so strong and elastic is it, that it can sustain with ease the weight and strain of its feathery top as it blows about in the breeze. The natives of certain parts of Guiana use this reed as a blow-gun, and I have a specimen, presented to me by the late Mr. Waterton, which is eleven feet in length.
So the reader will see that when engineers found that hollow iron beams were not only lighter, but stronger than solid beams, they were simply copying the hollow beams formed by Nature thousands of years ago.
Anothergreat improvement in ship-building now comes before us.
We have already seen that the earliest boats were merely hollowed logs, just as Robinson Crusoe is represented to have made. But these had many disadvantages. They were always too heavy. They were liable to split, on account of flaws in the wood, and if a large vessel were needed, it was difficult to find a tree sufficiently large, or to get it down to the water when finished.
So the next idea was to build a skeleton, so to speak, of light wooden beams, and to surround it with an outer clothing, orskin, if it may be so termed. As far as I know, the two original types of this structure are the Coracle of the ancient Briton, and the birch-bark Canoe of the North American Indian, and it is not a little remarkable that both exist to the present day, with scarcely any modification.
The Coracle has been already represented on page22. It is, perhaps, or was in its original form, the simplest boat in existence, next to the “dug-out.” In the times of the very ancient Britons, who were content with blue paint by way of dress, and lived by hunting and fishing, the Coracle was a basin-shaped basket of wicker-work, rather longer than wide, and covered with the skin of a wild ox. This was sufficiently light to be carried by one man, and sufficiently buoyant to bear him down rapids, if he were a skilful paddler, and, of course, formed a considerable step in civilisation.
The modern Coracle is identical in form, and almost in material. The frame is still oval and basin-shaped, and made of wicker, but the outer covering is not the same. An ox-hide is an expensive article in these days, and, especially when wetted, is very heavy. So the modern Coracle builder covers the wicker skin with a piece of tarpaulin, which is much cheaper than the ox-hide, much lighter, is equally water-tight, and has the great advantage of not absorbing moisture, so that it is as light after use as before.
The Esquimaux make a boat on very similar principles. It is simply hideous in form, resembling a huge washing tub in shape, but, as it is only intended for the inferior beings called women, this does not signify.
Best, most perfect, and most graceful of all such boats is the Birch-bark Canoe of the North American Indians, whose shape has evidently been borrowed from that of a fish. I have seen many of these canoes, and have now before me several models which are exactly like the originals, except in point of size. Instead of being mere elongated bowls, like the coracle, they are long and slender, swelling out considerably in the middle, and coming to an almost knife-like edge at each end. Both stem and stern are alike, so that the canoe can be paddled in either direction, and, as one of the paddlers always acts as steersman, no rudder is needed.
The mode of construction is perfectly simple. The labour isdivided between the sexes: the women cut large sheets of bark from the birch-trees, scrape and smooth them, and then sew them together, so as to form the outer skin, or “cloak” as it is called, of the canoe. Meanwhile the men are making the skeleton of strips of white cedar-wood, and binding them into shape with thongs made of the inner bark of the same tree, just like the “bass” of our gardeners. The “cloak” is then gradually worked over the skeleton, sewn into its place, and the canoe is finished. A figure of this canoe, as completed, is given in the same illustration as that which represents various forms of boat, page 7.
The last improvement is that which was caused by the necessity for large vessels, when planks or iron plates were fastened over the skeleton. But, in all these cases, the vessel is built on the principle of the thorax of a vertebrate animal, that of the whale or a fish being an admirable example. It only needs to take the skeleton of a whale, turn it on its back, and the ribs will be seen to form an almost exact reproduction of those of any ship being built in the nearest dockyard.
Image unavailable: RIBS OF FISH. RIBS OF SHIP.
I have now before me the spine and ribs of a herring. The fish was over-boiled, and the flesh fell off the bones as it was being lifted out of the dish, leaving most of the ribs in their places. When held with the spine downwards, and viewed from one end, the resemblance to the framework of a ship is absolutely startling, the ribs representing the beams, and the spine taking the place of the keel. I have also before me a sketch representing a section of a Fijian canoe, and it is remarkable that even the very curve of the ribs of the herring is reproduced in those of the canoe.
Whether the Fijians derived this peculiar and beautiful curve from the ribs of a fish I cannot say, but think it very likely.
A stillgreater improvement in ship-building now comes before us, and this also has been anticipated both in the animal and vegetable kingdoms. There are so many examples of this anticipation that I can only give one or two.
The improvement to which I refer is that which is now almost universally employed in the construction of iron ships, namely, the making the outer shell double instead of single, and dividing it into a number of separate compartments. Putting aside the advantage that if the vessel were stove, only one compartment would fill, we have the fact that the ship is at the same time enormously strengthened and very light in proportion to her bulk.
Image unavailable: SECTION OF ELEPHANT SKULL. TRANSVERSE SECTION OF IRON SHIP. STELLATE TISSUES. LONGITUDINAL SECTION OF IRON SHIP.
Perhaps the best, and certainly the most obvious, example of this principle in the animal world is to be found in the skull of the Elephant. The enormous tusks, with their powerful leverage, the massive teeth, and the large and weighty proboscis,require a corresponding supply of muscles, and consequently a large surface of bone for the attachments of these muscles. Now, were the skull solid in proportion to its requisite size, its weight would be too much for the neck to endure, however short and sturdy it might be. The mode of attaining expanse of surface, together with lightness of structure, is singularly beautiful.
Perhaps some of my readers may not be aware that the bone of the skull consists of an outer and inner plate, with a variable arrangement of cells between them. In many animals, such, for example, as man, where the jaws are comparatively feeble, and the teeth small and light, the size of the skull is practically that of the brain, to which it affords a covering. The same structure may be observed in the skull of the common sparrow, where, as in man, the two bony plates are set almost in contact.
But in the elephant these external and internal plates are set widely apart, and the space between them is filled with bony cells, much resembling those of a honeycomb. They are, in fact, just the same cells as those which exist in the skull of man and sparrow, but they are very much enlarged, and in consequence give a large surface, accompanied with united strength and lightness.
There are many other examples in the animal kingdom, but our limited space will not allow them to be even mentioned.
Asto the vegetable examples of this principle, they are so multitudinous that only a very slight description can be given of them.
I suppose that most boys have seen a “cane” (whether they have felt it or not is not to the purpose), and some boys have made sham cigars from pieces of cane. In either case they must have noticed that the cane is not solid, but is pierced with a vast number of holes, passing longitudinally through it, and is, in fact, a collection of little tubes connected and bound together by a common envelope.
The Sugar-cane, if cut across, is seen also to consist of multitudinous cells, which, however, are not hollow, but filled with the sweet liquid from which sugar is obtained by boiling. Then there are many of our common English plants, like theordinary rush or reed, which are very slight in diameter in comparison with their length, and in which the cells are still further strengthened and lightened by the projection of their sides into a number of points which meet each other, and leave interstices between them. This modification of the cellular system is called “Stellate” (or star-like) Tissue, and two examples of it are given in the illustration, one being taken from the common rush, and the other from the seed-coat of the privet. A very good specimen of stellate tissue may be obtained by cutting a thin section of the white inner peel of the orange.
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.
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 comparedwith 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.
Wewill now look to Nature for Cables.