WE have, in previous Numbers of this Journal, advocated the use of life-belts, both on shipboard and in boats, especially in life- boats; and we have recommended the ac- quirement of the art of swimming by every one. As we think the subject very im- portant as well as interesting, we propose to devote a portion of the space in our present Number to some remarks on the general principles according to which the human body is thus made to artificially float on the water's surface, and to a brief explanation of the requirements of efficient life-belts, to- gether with a description of the one which has been adopted by the National Life-boat Institution for the use of its life-boats' crews.

Every person is aware of the property which is common to all fluids of " finding their own level," as it is vulgarly called, and that they cannot be made to stand in a heap, or be depressed into a hollow, except by the exertion of some extraneous force, which force must, therefore, be equal to the resist- ance opposed by the fluid in its endeavour to maintain its level.

It follows, then, that a solid body, placed in a fluid of greater specific gravity than itself, can only displace or push out of their level as many particles of that fluid as shall together equal its own weight, and that the excess of its bulk over the bulk of the fluid which it so displaces will remain above the surface of the fluid.

As a consequence of this law, the lighter any solid body is in proportion to the fluid in which it is immersed the higher it will float upon it. If it be of equal weight it will remain suspended in it, having no ten- dency to sink or float. If it be of greater weight it will sink, and it will then displace an amount of the fluid equal to its bulk, and having no reference to its weight.

Accordingly it is a received axiom in hydrostatics, that " all bodies which float in water displace as much of it as is equal to their weight, and all that sink as much as is equal to their bulk." Now the human body, as indeed are those of all terrestrial animals, is, when inflated by drawing in a full breath, somewhat lighter than the same bulk of water: accordingly, if a person could hold his breath for any length of time, he would for so long remain floating at its surface, although not then sufficiently so to keep the apertures of his breathing apparatus, his mouth and nostrils, above it. He is not, how- ever, able to retain his breath for more than a few moments at a time, and the action of his lungs, by successively filling and emptying his chest of air, alternately expands and con- tracts it; when the latter action takes place the bulk of his body is diminished, and it then displaces a less amount of water, and has in proportion a greater tendency to sink: it can then only be kept afloat by mechanical action, as birds support themselves, in that lighter fluid the air, by the action of their wings on it; or else by attaching to it some other bodies, which, whilst they add consi- derably to its bulk, shall but slightly in pro- portion increase its weight. By the first method this object is attained through the art of swimming; by the second, through the instrumentality of what are commonly denominated life-preservers, life-belts, or swimming-belts.

As the art of swimming is so advan- tageous, nay, so essential, to all those who have business on the waters, which, indeed, at the present day, and in this country, in- cludes most people—and as, even with a life-preserver, the knowledge of swimming gives a person a much greater mastery over himself and greater presence of mind, it may be as well that, before proceeding to more particularly define the different descriptions of life-belts, we should enter into a short ex- planation of the art of swimming.

As we have already observed, the human body differs but little in weight from its cor- responding bulk of water, and has, there- fore, but a slight tendency to sink. .This tendency may be counteracted by a simple motion of the feet alone, as in walking up a ladder or on a treadmill, which operation is ' technically called " treading water," taking ! care at the same time to keep the hands and arms under water also; but this would be a laborious operation to continue for any length of time, as the body has a greater tendency to sink when in an upright position, and the arms cannot then be conveniently brought into play. In swimming, therefore, the body is thrown forward, and so placed in a more horizontal position; by then striking back- wards and downwards with the hands and feet, it is propelled through the water, and the more rapid its motion, the lighter it will float, fox the following reason:—By the laws of motion a force or pressure applied to any plane surface exerts that force perpendicu- larly to it at whatever angle it may strike or press on it; accordingly, in the action of swimming, the advancing particles of water striking against the lower surface of the body, or, which is the same thing, the advancing body meeting the various particles of water, is pushed or pressed by them at right angles to its surface, and, therefore, in a more or less upward direction, according as the swimmer keeps his body in a more 01 less horizontal position, which pressure will also be more or less in proportion to the velocity of the progressive motion, and consequently increased pressure of the resisting particles of water. Accordingly we find, in fact, that all land animals maintain themselves at the surface of the water by propelling themselves through it, and, as is well known, even a stone or an iron shot will refuse to sink when a rapid motion has been imparted to it obliquely to the water's surface.

As the art of swimming, however, cannot be learned by any explanations, or from a theoretical knowledge alone, those who would acquire it must go into the water, and do so by practice and experiment. Most urgently, then, do we recommend every person who is not already able to swim to learn to do so, as well as every parent who has the opportunity to cause his children to be taught it, for they know not how soon they may be called on to put it in practice, either in order to save their own lives or those of others ; and even if they should not be re- quired so to exert it, their knowledge will give them greater self-command and presence of mind in any dangerous situation on the water.

Since, however, every person is not able to swim, and even those who are so may be overcome by fatigue, or may have received some injury to incapacitate them when thrown into the water, or, again, as they may be required to assist other persons who cannot swim, it will be most desirable that a boatman, when on any dangerous service, and especially a life-boatman, should be sup- plied with some extra buoyant power, which would afford him greater security, and in- crease his ability to assist others.

To supply this deficiency, various descrip- tions of life-belts, &c., have of late years been designed; but as all are not equally efficient to supply it, and especially as all are not alike adapted for the use of a person who must undergo great physical exertion with his belts on, such as rowing in a boat against a head sea, we will offer an opinion on the qualities which we conceive a boat- man's life-preserver (and it is such which we particularly have in view) should possess.

1st. It should contain as much buoyant power as would support one man with his head and shoulders above the water, although he might be disabled, by injury or otherwise, from swimming, or as would enable a man, if a swimmer, to support with ease another person besides himself.

2nd. It should not be liable to lose its buoyant property by any accident to which it might be exposed, such as by a heavy blow, or by absorption of water.

3rd. It should be of such a pliant, elastic, or soft nature, as to conform to the shape of the body, and not to prevent the free use of the arms and upper part of the person, or to confine the chest so as to impede the action of the lungs, on which the capability of enduring prolonged exertion or fatigue much depends.

4th. It should be of such a form as would offer as little resistance as possible to the water when propelled through it, and not thereby to retard its wearer's progression by swimming.

And here again, as man with advantage so constantly does, we may profitably betake ourselves to the great laboratory of Nature for our type and model; and although we may find ourselves but poor copyists after all, we may yet derive from that source some useful practical ideas, as well as, in a moral point of view, a profitable lesson on our own imperfect capacities and restricted powers.

We have stated above that the bodies of man and of the land animals are of nearly the same specific gravity as water; the same may also be said of birds, although, from their bones being hollow instead of being solid or filled with marrow, they are some- what lighter than the former, yet we find that the large class of birds of the aquatic kinds, whose functions are chiefly performed on the surface of the water, such as the swan, goose, duck, and sea-gull, float lightly upon it, with a large portion of their bodies buoyed up above it; and here we have at once a beautiful model before us.

These birds are covered with an exceed- ingly light but thick covering of down and feathers, which, from being, whilst the ani- mals are alive and in health, of an unctuous nature, prevents the absorption of water by capillary attraction, and, from the peculiarity of its disposition over their bodies, forms a surface quite impervious to water.

The bulk of one of these birds is thus greatly increased, whilst its weight is but slightly so, and thus it acquires its extra buoyancy. The shape of its body, with its broader and flatter breast than those of land birds, is also calculated to enable it readily to maintain its equilibrium, and to propel itself lightly and with considerable velocity over the surface of the water. We may also remark, in passing, that a further evidence of design and beauty of arrangement is shown by the position in which the head and neck of these tribes are placed, as compared with those of land animals. In the latter it is continued, when in its natural position, in the same line, or nearly so, as the spinal cord and the back, which in man raises his head into an elevated and upright position, and in the greater portion of the lower ani- mals maintains in a horizontal one, enabling them the more readily to procure their food, which is the chief concern they have to attend to. In the swimming-birds, however, whose bodies are required to be horizontally on the water, that they may the better float and swim, but yet whose heads need to be raised above it, the neck, when erect, is nearly at a right angle to the back, and is of that flexible nature which enables the animal to turn its head about in every direction, and either to immerse it in the water in search of food, or keep it raised above it at pleasure, To return, however, to our more imperfect machinery to effect the same ends. Although we cannot equal the contrivances of Nature, yet we may, so far as we can, imitate them; and in doing so in this instance, our endea- vour will be to attach a sufficient amount of buoyant power around the body to raise the head and shoulders perpendicularly above the water's surface, whilst this shall be effected without confining the limbs or im- peding the free action of the lungs and the muscles of the arms and chest. On these two qualities will the efficiency of a life-belt chiefly depend, yet neither has been suffi- ciently considered by the inventors and makers of them. It seems, indeed, to have been generally supposed that the least pos- sible amount of buoyancy which would suffice to raise the mouth above the surface of the water was all that was required, and, ac- cordingly, many of the belts which are sold in the shops have only buoyant power equal to 6, 8, or 10 Ibs.: a little consideration will, however, make it apparent that the largest amount of buoyancy which can be obtained without seriously incommoding the wearer, and depriving him of the free use of his limbs, is no more than is requisite.

In the 1st place, then, it must be remem- bered, that the same amount of buoyancy which would suffice in smooth water to raise the mouth above the surface, would be quite insufficient to do so in even a moderately rough sea, 2ndly, That boatmen, and especially fishermen, have often thick boots and other heavy clothing on them which will increase their tendency to sink.

Srdly, That when thrown out of a boat, other persons unprovided with life-belts may cling to them for support.

Lastly. That if from either of the above- named causes a person should find himself more deeply immersed than he had antici- pated, his presence of mind would be more likely to desert him than if he felt himself buoyed up with his head well above the I water, without any exertion on his own part, i These considerations are all of importance, snd none more so than the last named, for '•.

unquestionably more persons have been drowned, including even the best of swim- mers, by the loss of their presence of mind than from any other cause, We believe that a life-belt, for an adult person of average size, ought to have, at the least, buoyant power equal to 20 Ibs., and as much more as can conveniently be obtained.

Having decided on the amount of buoy- ancy, the questions next arise as to the fittest material to be employed, and the best shape to be adopted. The only two descriptions of material worth notice are the rival ones of cork and of Macintosh cloth inflated with air. Horsehair and dried rushes have each been employed, but they are untrustworthy The advantages of inflated belts are— their greater buoyancy compared with their bulk, and their greater portability, which latter quality makes them especially conve.- nient for travellers, since when emptied of air, they fold up and stow in a small com- pass, whereas an efficient cork belt must be of considerable size, and cannot therefore be stowed away in the portmanteau or carpet bag. The inflated belt will therefore, no doubt, take precedence of any others in the hands of tourists and travellers, to whom portability is an object of importance.

The disadvantages of inflated belts are— their liability to get punctured or torn, or to decay from being put away damp after use, or to have their inflating metal valves out of order from corrosion or otherwise; from any of which causes their whole effi- ciency would be destroyed. Again, in the hands of life-boatmen, amidst the hurry and anxiety of launching to the aid of a wrecked crew, they are liable to be only partially inflated, or the valves to be only half screwed up, and so to adroit the escape of the en- closed air.

An inflated belt, having four compart- ments separately inflated, has been invented by the Inspector of Life-Poats to the Na- tional Life-Boat Institution, which has, in 3 great measure, got over the danger arising from risk of puncture, or injury to the in- flating valves, as without being more incom- modious than the ordinary belts, it has buoy- ancy equal to 30 Ibs,; and if even two of its compartments should be disabled, the re- maining two would be sufficient to float its wearer. It is, however, necessarily more expensive than those made with one com- partment and one valve only.

The advantages of cork as a material for . life-belts are its durability, and its non- liability to injury from puncture, fracture, or damp, so that it will bear the rough usage of ordinary boat work. By being divided into many narrow pieces, it can also be made more flexible and yielding to the body of the wearer than an inflated belt.

The only disadvantage of a cork belt is, that from its greater weight it requires to be of larger size than an inflated belt, and as it must always retain its full size, it cannot be stowed away in a small compass when not ia use. For a life-boatman's belt, however, portability is not of much consequence, and the advantages of cork are so great in other respects, that that material has been selected by the National Life-Boat Institution for the belts of its life-boats' crews.

These belts are, however, of a new con- struction, designed by the Inspector to the Institution, Commander J. R. WARD, R.N.

They have much greater buoyancy than any kind of cork belt previously introduced, and have other important peculiarities.

1. Their buoyant power is from 20 to 24 Ibs.

2. The cork is uncovered, so that its quality can always be discerned, and it is divided into many narrow pieces, each of which is separately sewn on to a strong linen or duck belt, covering the body from the arm-pits to below the hips.

3. These pieces of cork are distributed in two rows, one above, and the other below the waist, the belt being secured closely about the body by strings passed round the waist, between the two rows of cork, and being further secured by other strings crossed over the shoulders, as men's trouser braces are worn. This division of the cork into two rows is one of the most important distinc- tions of these belts, as a sufficient quantity of cork to afford double the buoyancy of the ordinary cork belts can be thus attached, and in a manner which is much less incon- venient to the wearer than the lesser quan- tity in one row, which, not being secured round the waist, but round the chest, inter- feres with the free action of the lungs, and the muscles of the shoulders and arms.

By this arrangement, in these belts, the trunk of the body is enveloped in cork, so attached as to be perfectly flexible, and to allow of all the ordinary movements of the body without inconvenience, being a nearer approach, in principle, to the buoyant covering of the swimming birds than any other belt, and forming also, like theirs, a' great protection to the body against injury from a blow, and a preservative of its heat in cold weather.

About 800 of these belts have been sup- plied by the National Life-boat Institution to the crews of its life-boats and those of others during the last three years, and they have given universal satisfaction to those who have used them. A few have also been supplied to some of the vessels chartered by the Emigration Commissioners.

We consider that it would be a great boon to the merchant seamen of our country if every merchant vessel were compelled by Act of Parliament to have on board, and stowed on the upper deck, as many of these belts as formed the number of her crew, so that in the event of their having to desert their vessel from wreck, or leakage, or fire, and take to their boats, each man might be supplied with an apparatus which, as it would make it impossible for him to sink, would, doubtless, be often the means of saving his life.

As the inventor of these belts has declined making any profit by them, they are made very cheaply; and when their durable character is considered, the expense of pro- viding them for his ship's crews would not be worth consideration by any shipowner, whilst the satisfaction to him would be great, if they were ever instrumental in saving the lives of those employed by him; and if they should never have occasion to be used, he would be amply'recompensed by the addi- tional regard and respect which his seamen would entertain towards him, on perceiving this instance of his humanity and kind feeling towards them.

These belts, although not patented, have been hitherto manufactured only by Mr, JOSEPH BIRT, junior, of No. 5, Wellclose- square, London Docks, under the general superintendence of the inventor.