190

Examples of Loss of Weight in Water.

of water in the scale of a weighing beam, and putting a fish into the water, found that scale preponderating just as much as if the fish had been weighed dry in the scale-pan. This experiment may be readily performed by balancing a leech, and afterwards transferring the leech to a balanced beaker of water. The weight will be found to be the same whether the leech is in the scale-pan or in the balanced beaker, and whether the animal is moving in the water or adhering to the side of the glass.

In the sense now explained, water is said to have no weight in water. The least force will raise a bucket of water from the bottom of a well to the surface; but if the bucket be then lifted at all farther, its weight is felt, just in proportion to the part of it raised above the surface.

"Floating."

322. "A body lighter than its bulk of water will float in water, submerged to an extent proportioned to its specific gravity or relative weight compared with water."

The reason of this is clear. A block of wood, that it may

remain floating out of the water, as represented in fig. 85, must have, in its whole volume, only the weight of the water which the immersed part of it displaces, because the upward pressure which supports its weight is just equal to this weight of water. If it be lighter than this, it will rise farther; if heavier, it will sink farther,

until the exact balance be produced.

Hence any floating body which weighs a pound, displaces just a pound of water, whatever be the size of the body, and whether it be cork, or wood, or glass, or iron. This is experimentally shown by putting such bodies in succession to float in a vessel full of water. The water displaced by each will run over the sides of the vessel, and will always weigh just a pound.

Hence an empty basin, whether of glass, or porcelain, or metal, weighing a pound, will sink in water only as far as a wooden one of the same external dimensions and of the same weight; and the weight of the basin may be in the substance of which it is formed, or in anything else put into it as a load.

Hence an iron boat floats just as high out of the water as a wooden one of similar form and size, provided the iron be propor

Floating Stability.—Centre of Buoyancy.

191

tionally thinner than the wood, and the vessel not heavier on the whole. An empty metallic pot or kettle may be seen floating with a great part of it above the surface of the water. Prejudice for a long time deterred men from building iron boats or ships, though possessed of many advantages over wood.

Hence a ship carrying a thousand tons of cargo will just draw as much water, or float to the same depth, whether the cargo be of cotton or of lead; and the exact weight of any ship and cargo may be determined by finding how much water the floating ship displaces, which can be known by the depth to which she sinks in the water. In canal boats, which are generally of a simple form, this affords a ready means of ascertaining the quantity of their load.

323. A floating body sinks to the same depth whether the mass of iiquid surrounding it be great or small. This is seen when a bowl is placed first in a pond, and then in a second bowl only so much larger that a few spoonfuls of water suffice to fill up the interval between them. One ounce of water may thus float a thing weighing several pounds—another form of the hydrostatic paradox. Even if a large ship were received into a dock, or case, so nearly fitting its form that there were only half an inch of interval between it and the walls of the dock, it would float as completely, when the few hogsheads of water required to fill this interval up to its usual watermark were poured in, as if it were on the high sea. In canal locks, when the boats are made nearly to fit the space where they have to rise and fall, the cost of water to work the lock is much diminished.

"Stability of floating; centre of buoyancy."

324. Similar reasoning to that which proves that the whole weight of a body acts as if it were lodged in the point called its centre of gravity, proves also that the whole buoyancy of a body, or rather the upward push of the fluid in which a body is immersed, acts as if lodged in the point which was the centre of gravity of the fluid displaced. This point consequently is called the centre of buoyancy.

A floating body, therefore, to be stable in its position, must either have its centre of gravity exactly below the centre of buoyancy,—in which case it resembles a pendulum hanging at rest, or it must have a very broad basis or bearing on the water, so that any inclination must cause the centre of gravity to ascend,-in which latter case it resembles a cradle or rocking-horse.

192

The Art of Swimming.

Hence arises, in the stowing of a ship's cargo, the necessity of putting the heavy merchandize low down, and often of putting iron ballast under all the ordinary merchandize. Hence also, comes the danger of having cargo or ballast which is liable to shift its place. A ship loaded entirely with loose stones is sometimes lost by a high wave making it incline for a moment so much that the load shifts to one side, which will then be kept down. For a similar reason, a cargo of salt, sugar, or saltpetre has a peculiar danger attached to it, for if the ship leak, or admit water, part of the cargo may be dissolved and be then pumped out with the bilge water, leaving the ship with altered trim.

Bladders used by beginners in swimming are dangerous, unless secured so as not to shift towards the lower part of the body.

"Swimming."

325. The human body, in an ordinary healthy state with the chest full of air, is lighter than water, and will float with nearly half the head above water; and in order to breathe, it is only necessary to keep the body at rest and the face uppermost.

When the chest is empty, as in the act of expiration, its tendency is to sink. The specific gravity of the body depends on the specific gravities of its constituent solids. About 72 per cent. of the weight of the human body consists entirely of water; its specific gravity, therefore, depends on the remaining 28 per cent. of dry solids. The fat is the lightest part of the body, its specific gravity being o'92, while bone is the heaviest. This has a specific gravity of 2:01. Hence, the floating and sinking of the human body will depend greatly on the relative proportion of these two constituents; the other parts of the body, such as muscle and other soft structures, having nearly the specific gravity of water.

If the facts connected with the buoyancy of the body were gencrally and practically understood, it would lead to the saving of more lives, in cases of shipwreck and other accidents, than all the lifepreservers yet contrived.

The reasons that in water-accidents so many people are drowned who might easily be saved are chiefly the following:

Ist. They believe that the body is heavier than water, and therefore, that unless continued exertion be made, they must sink. Hence, instead of lying quietly and a little on the back, with the face only

Avoidable Causes of Drowning.

193

out of the water, they generally assume the position of a swimmer, in which the face is downwards, and the whole head has to be kept out of the water to allow of breathing. To do this requires practice; and if a person cannot swim, the first attempt at floating in this position will prove a disastrous failure.

2nd. The body raised for a moment by any exertion above the floating level, sinks as far below that when the exertion ceases; and the plunge terrifies the unpractised and renders them easier victims to their fate.

3rd. They make a wasteful exertion of strength to prevent water entering the ears, not thinking that it can fill only the outer car, as far as the drum, and that this is of no consequence.

4th. They generally attempt in their struggle to keep their hands free above the surface, forgetting that any part of the body held out of the water, in addition to the face which must be out, requires an additional effort to support it. The tendency of the body to sink diminishes just in proportion to the quantity immersed ; because all those parts which are out of water, not being supported by the water, become so much additional absolute weight to the portion immersed. This is indeed one of the most frequent causes of death by drowning.

5th. If the accident occurs at sea, they cannot, like the practised swimmer, choose the proper interval for breathing, which is when the crest of a wave has passed over and the head is for an instant above water.

6th. The chest should be kept as full of air as possible, which. without other effort, will cause nearly the whole head to remain above water. If the chest be once emptied, while the face is under water and the person cannot inhale again, the body remains specifically heavier than water, and will sink.

So little is required to keep a swimmer's head above water, that an oar will suffice as a support to several people, provided no one attempts to keep more than his head out of the water; but one or two, wishing to have as much of the security as possible, may submerge all the others.

for pre

The most common contrivances, called life-preservers, venting drowning, are strings of corks put under the chest or neck, or air-tight bags applied round the upper part of the body, and filled when required, by those who wear them blowing into them through valved pipes attached.

326. The most recent and complete of these inventions is the

life-saving dress of Captain Boyton. It not only sustains the body beyond possibility of sinking, but it is at the same time so adjusted as to enable the wearer to make his way through the water and perform a variety of movements. The dress is made of vulcanized rubber, and is rendered buoyant by means of five airchambers, which can be inflated or emptied through small tubes fitted with stop-valves. An upper chamber forms a complete air pillow for the head, and has the effect of always keeping the face uppermost and raised out of the water. A second chamber covers the entire breast, a third the back, and two others the legs. The whole of these are so adjusted to the body by a belt, as to be quite water-tight. The only part of the dress from which there is a possibility of getting wet is the face, but this is remedied by fully inflating the air-pillow at the back of the head. This draws the only aperture tightly round the chin, cheeks, and forehead. The feeling is said to be that of reclining at full length on an air mattress. It is not a swimming, but a floating-dress. It would require a weight of 300 pounds to submerge the body. Protected by this dress, the Captain on one occasion threw himself overboard in a rough sea off the rocky coast of Ireland, and after passing seven hours in the water he landed safely on the shore. A more surprising feat, however, was an attempt to cross the British Channel at night. On this occasion the Captain, properly invested in his life-saving costume, passed nearly fifteen hours in the sea-water, during which period it is calculated that he paddled himself or was drifted by the tide, a distance of forty or fifty miles. In May, 1875, he succeeded in crossing the Channel from the coast of France, and landed at Dover after twenty-two hours' immersion in the sea.

327. On the great rivers of China, where thousands of people find it more convenient to dwell in covered boats than in houses upon the shore, the younger children have a hollow ball of some light material constantly attached to their necks, so that in their frequent falls overboard they are not in danger.

Life-boats have a large quantity of cork mixed in their structure, or of air-tight vessels of thin copper or tin plate; so that, even when the boats are filled with water, a considerable part floats above the general surface.

Swimming is much easier to quadrupeds than to man, because the natural motion of their legs is that which best supports them in swimming. Man is at first the most helpless of creatures in water, though by his superior intelligence he can become so expert in this