Aërostatics.
WE will begin this chapter with the only two modes at present known by which man can ascend from the earth or descend to it with safety, namely, the Balloon and the Parachute, the latter being generally attached to the former, and detachable at pleasure.
The Balloon is, in fact, as its name imports, a large, hollow, air-tight ball, filled with some substance lighter than ordinary air. The original Balloons by Montgolfier were filled with heated air exactly like our toy fire-balloons. Just as the supply of hot air is kept up in them by a sponge dipped in lighted spirits of wine, so in Montgolfier’s balloons the same object was attained by straw which was kept continually burning in a grate.
There were, however, two disadvantages about this plan. The first was the great danger of fire, which on one occasion did ignite a balloon when at a great height. The second was the perpetual labour required in keeping the fire alight. Straw burns very rapidly, and so the aëronaut had no opportunity of making those meteorologic observations in which consist almost the entire value of the balloon.
Then it was thought that hydrogen gas, being about fourteen times lighter than ordinary air, would answer the purpose, and such has proved to be the case. Formerly the gas was made at great expense from sulphuric acid and zinc, but it is now found that the common coal-gas is quite as efficient, very much cheaper, and fills the balloon much more rapidly.
The same principle, though not the same form, is found in Nature.
There are certain tiny spiders called Gossamers, which have a curious power of floating in the air. They have been seen on the tops of lofty spires, and they are sometimes so numerous that the air is full of their floating webs, and the ground is white with those that have descended.
Their mode of ascent is this. They climb to the top of some elevated object, if it be only a grass-blade. They then pour out a tuft of long, slender threads, which shortly begin to tend upwards. As soon as the Spider feels the pull, it crawls upon the web, and sails away into the air. The duration and height of the ascent depend much on the wind and character of the atmosphere.
The web ascends because it is for the time lighter than the atmosphere. But, as it gradually becomes laden with the moisture that more or less fills the air, it becomes heavier than the atmosphere, and gently sinks to the ground.
What may be the object of these aërial voyages no one knows. They may be for the purpose of capturing minute insects, or they may be for mere amusement. But in either case they are highly instructive, as showing the principle on which the balloon was framed.
The little Gossamer Spider is shown on the left hand of the illustration, clinging to its floating web. I believe that the Gossamer is not a single species of Spider, but that there are many species which deserve the name, being able to float in the air when they are small, but losing that capacity as they increase in size and weight.
Now we come to another branch of the same subject, namely, the safe descent from a great height by means of the Parachute.
On the right hand of the illustration is the ordinary Parachute as it appears when open and closed, in either case having somewhat the appearance of a large umbrella. It is hung to the balloon in its closed state, and when detached it falls rapidly for a yard or two with startling rapidity. The pressure of the air thus forces the ribs open, and gives sufficient assistance to the atmosphere to insure a gentle fall.
On one memorable occasion, when the late Albert Smith was in the car of a balloon upwards of a mile from the ground, the balloon burst. Fortunately it burst so completely, that the silk was driven into the closely meshed netting, and formed an extemporised parachute, which took the voyagers to the earth with safety, except some rather severe bruises.
Children often amuse themselves with miniature parachutes. They take a square piece of thin paper, tie threads to the four corners, and then bring the ends together, a cork taking the place of the car. They then launch it from a high window, and should there be a favourable breeze, it is wonderful how far it will be carried before it comes to the ground.
Once, when a boy of eleven, and consequently thoughtless, I set a chimney on fire by one of these Parachutes. I wished to see whether it would go up the chimney, and come out at the top. Unfortunately it was caught by a flame as it was launched, flew up in full blaze, and, as the chimney needed sweeping, the result was inevitable.
In the centre of the illustrations, and at the top, are two examples of a well-known natural Parachute called the Dandelion seed. The resemblance to the real Parachute is wonderful, the actual seed occupying the place of the car, and fulfilling the same office, i.e. keeping the seed upright until it reaches the ground.
When the tuft is closed, as is the case before the pretty ball of seeds bursts from the green envelope in which they had been confined during the process of development, its form bears the same startling resemblance to the Parachute.
Passing from the vegetable world, there will be seen three examples of Natural Parachutes. Several others will be mentioned, but we have no space for description or figure. It will be seen, however, that the one principle which characterizes them all is the exposure to the air of a flattened and large surface, in proportion to the size of the object.
Before beginning the description, however, I must mention that nearly all animal parachutes can to a certain extent guide their course, while neither the balloon, the gossamer, the parachute, nor the various winged seeds have the least power of guidance, but must follow every current of air in which they may happen to float.
The upper figure represents a Flying Squirrel.
There are many species of Flying Squirrel, but they all agree in one point. The skin of their sides is modified into a very thin fold, which extends as far as the feet.
It is very elastic, so that when it is not in use it falls into folds or wrinkles, and is hardly perceptible. But should the Squirrel wish to pass from one tree to another, without coming to the ground, it spreads its legs as widely as possible, so as to stretch the membrane into a wide, flat surface. It then boldly springs into the air, and sweeps upon its mark with a sort of skimming movement. Except that it does not revolve, it passes through the air much after the fashion of an oyster-shell when thrown horizontally.
Many mammalia are constructed after a similar fashion, such as the Colugo, or Flying Monkey, the Flying Mice, and the Flying Phalangists, or “Opossums,” as they are popularly called.
In the centre is the Flying Dragon, or small lizard, which very probably gave rise to the fabled Dragons in which our ancestors so devoutly believed. Indeed, on looking back at the old illustrated works on Natural History, there can be but little doubt on the subject.
In this creature, the ribs, instead of the legs, carry the flat and elastic membranes. When simply crawling on the branches, after the manner of tree-lizards, the ribs lie flat against the sides, and the membranes collapse, so that the shape of the body is little different from that of any crawling lizard.
But the ribs are movable at will, and, when the creature wishes to pass from one tree to another, it extends the ribs, stretches the membranes, and launches itself into the air, exactly as has been narrated of the Flying Squirrel.
The lowest figure represents a most extraordinary animal, called the Flying Frog. Only one specimen is believed to be known, and that was discovered in Borneo by Mr. Wallace.
Here we have an analogy with the bats of the present day and the pterodactyles of the past, namely, the elongation of the toes, and the stretching of a web between them. In the two latter animals, however, only the toes of the two fore-legs are elongated, whereas, with the Flying Frog, the elongation is found in both pairs of limbs. The ends of the toes are furnished with adhesive pads, like those of the tree-frogs, to which it is probably related.
By means of the four membranes, the creature is able to sweep through the air for some distance, and, indeed, this power was the reason why it was caught. It was seen to skim from one tree to another, and was immediately secured. Had it remained sticking on the tree, it would probably have escaped observation.