Diving and Divers.
It has already been mentioned that the flexible tubes used by modern divers are constructed on the model of several structures belonging to the animal and vegetable kingdoms.
We will now see how they are utilised.
In the earlier stages of the diver’s art the Diving-bell afforded the only means of gaining access to the bed of the sea, even in comparatively shallow waters. The mode in which this result was obtained was simple enough, and though it carried with it the germs of still greater improvements, was but limited and uncertain in its action.
The reader is probably aware that if a vessel be filled with air, no liquid can obtain admittance until a corresponding amount of air be set free. Suppose, for example, that an empty tumbler be inserted over a basin of very clean water, and pressed downwards, it will be found that scarcely any water will enter it, the air having taken up all the available space, and only allowing as much space as may be accounted for by its faculty of compression.
It is evident, therefore, that if an enlarged tumbler could be lowered to the bed of the sea, a man might be enclosed within it, and for a time be able to support life by means of the air contained within the “bell,” as this enlarged tumbler was popularly called.
It is equally evident that within a short time the air within the bell must be exhausted, and that, unless a fresh supply could be introduced, the diver within the bell would be as effectively drowned as if there were no bell at all.
The accompanying illustration is a kind of chart, so to speak, of the mode in which air was formerly supplied to the bell.
On the right hand is seen a section of the Diving-bell itself, together with the seat on which the divers can rest. There is also an escape-valve at the top of the bell, by which the vitiated air can pass away; but, as it is not essential to the subject in hand, and is rather complicated in structure, it has been omitted.
Immediately on the left of the bell is a cask, to which several heavy weights are attached. This cask contained compressed air, and, after it was lowered by the side of the bell, the end of the flexible tube was taken into the bell, the tap turned, and the compressed air rushed into the bell, taking the place of that which had been exhausted by respiration, and was allowed to pass through the escape-valve. I may mention that the divers unexpectedly discovered that, when they were breathing compressed air, they could dispense with respiration for a wonderfully long time, the amount of oxygen taken in at a single breath being enough to renovate the blood more than could be done by several ordinary inspirations.
On the left hand of the illustration is seen a sketch of the nest of the now familiar Water-spider (Argyronetra aquatica), taken from some specimens in my possession.
The Water-spider is really a remarkable being. Itself a denizen of air, breathing our earthly atmosphere just as we do, and as capable of being drowned as ourselves, it nevertheless passes nearly the whole of its existence under water, and in that strange locality lays its eggs and rears its young. How this wonderful feat is performed we shall now see.
When the female Water-spider wishes to deposit her eggs, she looks out for a suitable locality, and, being a good diver, tests the various aquatic herbage until she has found a favourable spot, and then sets to work on her remarkable nest, which I believe is quite original in zoology.
After stretching a few stout threads by way of a scaffolding, she attaches to the plant a small silken cell, shaped very much like an acorn, but not so large. Ascending to the surface of the water, she contrives to clasp a bubble of air between her last pair of legs, and, laden with this airy treasure, dives below.
As soon as she has reached the entrance to the cell, which is always below, she loosens her hold of the air-bubble. It at once rises into the cell, and expels a proportionate amount of water. Not many of these journeys are required before the nest is filled with air, and then the diminutive architect spends the greater part of its time in holding on to the mouth of the little diving-bell, and supporting life by means of the air within it.
This nest, as the reader will see, is an exact representation of the various diving schemes in which air-bells are the chief portions of the machinery, although the air is conducted into them after a different fashion.
We now come to another mode of diving, in which the bell is practically superseded by the flexible tube, which allows to the diver far more range than can be obtained by the bell. In this case the diver wears a peculiar dress, the chief part of which is a helmet so constructed that air can be introduced to it from above the surface of the water, and, after respiration, can escape by means of a valve.
Air is pumped into the tube by assistants above water, and, as the tube is long and elastic, the diver can move about with considerable freedom. As is the case with the diving-bell, the diver’s tube is strengthened by an internal spiral wire, so that it is always open, however it may be bent or twisted.
The right-hand figure of the illustration represents the diver examining part of a sunken vessel. The tube through which he breathes is seen passing to the surface of the water, and so is the line by which he gives his signals to his comrades above. In his hand he holds a lamp which can burn for a limited time, being connected by a smaller but similarly constructed tube to a vessel of compressed air.
On the left hand of the same illustration are shown the curious Rat-tail Maggots, as they are popularly called. They are the larvæ of the common Drone-fly (Eristalis tenax), which is so common towards the end of summer, and looks so curiously like a bee.
These creatures pass their larval life buried in the mud and below the surface of the water, and yet are obliged to breathe atmospheric air. This they do by means of the long appendages which have gained for them the name of Rat-tails. These “tails” are very elastic, and are capable of elongation and contraction to a wonderful extent.
When the creature is undisturbed, it lies buried in the mud with its head downwards, and its tail extended so that it reaches the surface of the water. Within this tail are two air-tubes, which are connected with the principal tracheæ, which have already been mentioned. They are wonderfully elastic, and, when the tail is extended to its utmost limit, are nearly straight. When, however, the tail is contracted, the tubes become self-coiled by their own elasticity, and shrink into the base of the tail.
As the tail is very transparent, it is easy to see how these movements are conducted. The larvæ, which may be found in almost any stagnant water, should be placed in a tall and narrow glass. Some mud should be placed at the bottom of the glass, which should then be filled with water to the depth of three inches or so.
When the mud has quite subsided, and the water become clear, the long slender tails of the larvæ will be seen so elongated that their tips reach just above the surface of the water. A magnifying-glass will easily show the two tubes within the tail.
Let the glass be but slightly tapped, and all the tail is withdrawn in a moment, so as to be out of reach of external danger. The magnifying-glass will then show the two tubes lying contracted in the base of the tail, and taking astonishingly little space, considering the amount of elongation which they can sustain. And, on examining the various bends and curves of the tubes, the value and power of the spiral spring will at once be seen. True, they are very small, but in Nature all things go by comparison, and our whole earth itself is as a grain of sand upon the seashore among the grandeurs of the visible universe.