CHAPTER V.

THE PRINCIPLE OF THE SPRING.--THE ELASTIC SPRING.--ACCUMULATORS.--THE SPIRAL SPRING.

Springs and their various Structure.--The Elastic Spring.--The Boy’s Catapult and its Powers.--The Pistolograph, its Principle, and Uses to which it can be put.--Leaf-rolling Caterpillars, and their Way of Work.--The Carriage Spring.--The Horse’s Hoof and its complex Structure.--Fungi and their united Power.--The Chinese Cross-bow.--The ancient Balista.--Skull of the Crocodile.--Bones of young Children.--The Spiral Spring and its many Uses.--The Toy-gun.--The Needle-gun.--Valved Brass Instruments.--Watch and Clock Springs.--The Bed Spring.--Parallels in Nature and Art.--Buffers of Railway Carriages.--Spring Solitaires.--The Bell Spring.--Spiral Springs in Vegetable Tissues.--Poison Cells of various Marine Animals.--Effects of the Spiral Springs.

ELASTIC SPRINGS.

Here we come upon a subject so large, that it is difficult to define its exact requisite limits. The principle of the elastic spring pervades all Nature, and the numerous adaptations in Art are closely, though perhaps not directly, attributable to the wide distribution of the spring in Nature.

There is, for example, the simple elasticity which enables a tree, when bowed by the wind, to spring back so soon as the pressure is removed, and which, indeed, is the power which enables a bow to propel an arrow. Then there are spiral springs innumerable, many of them so minute that they can only be seen by the aid of the microscope, and there are many springs which exhibit their elasticity by their power of extension and shortening, just as is done with the elastic fabrics which are so much in vogue at the present day, and which seem so necessary to ordinary comfort that we feel disposed to wonder how our forefathers managed without them.

We will now proceed to examine some of these springs in detail.

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THERE is one form of elastic spring which has of late years become more familiar than agreeable, namely, the toy which is learnedly called a “catapult,” though it has little in common with the ancient weapon whose name it bears.

As may be seen by reference to the illustration, it consists of one or more india-rubber straps attached to a fork-like handle, and carrying a small pouch in which is contained the missile. Although it is not remarkable for accuracy, it can throw a stone or a bullet a considerable distance, and its power can be very quickly increased by adding to the number of the straps. Thus a catapult has been made which was capable of sending a small pistol bullet through a wooden board, so that the child’s toy might really become a dangerous weapon.

Indeed, cases are known where the catapult has hurled a stone with fatal effect upon human beings. In my own neighbourhood there are many examples of glass being pierced by stones thrown from catapults just as if they had been subjected to bullets shot from firearms, the holes being quite small and round.

The power of accumulating force by increasing the number of springs was utilised by Mr. Scaife, when he invented his wonderful photographic machine which he termed the “Pistolograph,” on account of the sound which was produced when the portrait was taken.

The idea was simple enough, though the practice of it was not so easy. He wished to be able to take a photograph with an exposure of the least possible time, and thus to attain freedom and action, instead of the dull stiffness which generally characterizes photograph portraits. The mode which he adopted was by introducing a peculiarly sensitive film, which would take an impression in a mere moment, and then arranging the machine so that an exposure of more than a moment was impossible.

This was done by covering the lens with an exactly fitting door, revolving on a pivot. The axis on which the door revolved was attached to a number of india-rubber bands, exactly like those which are used for confining papers. As the power of the springs increased with their number, it naturally followed that the rapidity of the revolution was in exact ratio with the number of the bands, so that the duration of exposure to light could be measured with tolerable accuracy.

So wonderfully well did this plan succeed that photographs of eclipses were taken with perfect accuracy, a matter of great importance when time has to be considered. Horses were also taken at full gallop, so as to display their action, and the crowning achievement was the photographing of a cannon in the act of firing, and the bursting of a charged shell. So rapid is the action of the instrument, that in several cases where a cannon or mortar had been photographed, even the track of the ball or shell is visible.

It necessarily followed that when the springs caused the circular cover to revolve with such rapidity, they made it close with a sharp report, and so gave rise to the name of the machine. Moreover, as it had to be used for rapidly moving objects, it was not fixed on a pedestal, but was held in the hands, while aim was taken at the object, just as with a pistol. When the observer thought that he had his aim correct, he touched a trigger, round spun the cover, and the photograph was taken.

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ON the right hand of the illustration is seen the Catapult, made with several springs, and on the left is shown an example of the Accumulator as formed by Nature.

The reader may probably be acquainted with the Leaf-rolling Caterpillars, of which there are so many. I had often inspected these curled leaves, and, on comparing them with the size of the caterpillars, had noticed that the muscular strength of the insect was quite inadequate to the work which was done. That much of it was owing to the “bowsing” system, which has already been described when treating of the Toggle-joint, was very probable, but that some other force must be employed was evident.

On unrolling a leaf, the hidden force was at once explained, and showed itself to be a system of accumulators exactly like those of the pistolograph or the catapult. The caterpillar spins successive belts of silken threads, and affixes them to the leaf, as shown in the illustration. These threads are nearly as elastic as the india-rubber bands of the catapult, and accordingly draw the leaf together. Another set of belts is added above the former, and, as they harden and contract in the air, they roll the leaf still further. The first row is then shortened and tightened, and a third and fourth row are added in the same fashion. So elastic are these belts, that if the leaf be carefully handled it can be almost wholly unrolled, and will spring back again as soon as the force is removed.

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ANOTHER form of accumulated force may be seen in the ordinary Carriage Spring, one of which is shown in the illustration. It is made of a number of strips of elastic steel lying upon each other, and suffered to play upon each other by means of slots and rivets. The weight being placed in the centre, it is evident that this very ingenious spring is really an elastic girder, yielding to sudden pressure, and recovering itself when that pressure is removed.

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INGENIOUS as is this spring, it has many parallels in Nature, one of which is here given.

It is popularly thought the hoof of the horse is a solid mass of horn destined to protect the feet against hard and rough ground. Such certainly seems to be the opinion of farriers, who, in shoeing horses, act exactly as if the horn of the hoof were structureless; whereas it is a marvel of complicated mechanism. On looking at the exterior of a horse’s hoof, it will be seen to be marked with a vast number of very fine, but easily visible longitudinal lines, looking as if they were scratches from a very fine needle. If the hoof be removed from the foot, and examined upon the interior, it will be seen that each of the apparent scratches signifies the edge of a very thin plate of horn, not so thick as the paper on which this book is printed. The hoof, in fact, is built up of multitudinous plates of horn, set side by side, and each acting as a separate spring. It is this beautiful structure which allows the horse to tread without a jar being sent through its whole system by every step which it takes.

A similar structure is to be found in all hoofed quadrupeds, and is especially noticeable in the case of the Elephant. All those who have watched the walk of an Elephant, no matter what its size may be, must have been struck with the curious noiselessness of its movements. Its weight may be measured by tons, and yet the enormous animal steps as noiselessly as a cat. On examining one of the hoofs, after it is removed from the foot, the cause of this marvellously silent tread is perfectly evident. The whole of the hoof is composed of nearly parallel horny plates, and by their united action they produce the required result.

Each plate in itself is very feeble, but, when united as they are at the ends, they afford mutual support to each other. Similarly the separate feathers in a couch would be crushed by a comparatively slight weight, but when a number are confined together they support each other, and form the soft, yielding couch with which we are so familiar. Horsehair, when used as the stuffing for a couch or chair, acts in the same way, and so do the fine filaments of wool when used under the name of “flock.”

Another good example of the power of accumulated force, although it has no direct relation to the spring, is the well-known fact that fungi, which are separately so fragile, are capable of lifting and retaining in the air stones so large that two men could hardly carry them. Were the stones laid down upon the fungi, the latter would be crushed, but, as they grow beneath the stones, they accumulate their powers, and slowly, but certainly, raise the weight from the ground.

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THIS very principle of accumulated force has long been used in weapons of war, and I possess several examples of such weapons. One of them is a Chinese repeating Cross-bow, which was taken at the capture of the Peiho Fort, and was really a formidable wall-instrument, carrying a reserve of arrows, and delivering them with great rapidity. In point of fact, it consists of three bows, placed upon each other, and playing upon each other just as do the portions of a carriage spring. Such strength is thus obtained, that the bow cannot be drawn by hand, but is worked with a lever, as shown in the illustration. The whole machinery of the weapon, including the self-notching and self-supplying system, is very interesting, but is outside our present object. The very powerful bow of the ancient Balista was made on the same principle, and was strong enough to throw large stones and wooden beams.

I also have bows in my collection which are strengthened on the same principle, though not exactly in the same manner. There are several Indian, Chinese, and Japanese bows which are curved almost like the