For all these reasons, though the impetus given by the fall of the body into water occasions its sinking to a depth proportioned to the force of the descent, its natural buoyancy soon impels it again to the surface, where, after a few oscillations up and down, it settles with the head free.
Unfortunately, ignorant people stretch the arms out to grasp at anything or nothing, and thereby keep the head under; for the arms and head, together exceeding in weight one-tenth of the body, cannot remain above the surface at the same time. The buoyancy of the trunk, then and then only, occasions the head and shoulders to sink, the ridge of the bent back becoming the portion exposed; and, in this attitude, water is swallowed, by which the specific gravity is increased, and the body settles to the bottom. It is, therefore, most important to the safety of the inexperienced to be firmly convinced that the body naturally floats.
To satisfy the beginner of the truth of this, Dr. Franklin advises him to choose a place where clear water deepens gradually, to walk into it till it is up to his breast, to turn his face to the shore, and to throw an egg into the water between him and it—so deep that he cannot fetch it up but by diving. To encourage him to take it up, he must reflect that his progress will be from deep to shallow water, and that at any time he may, by bringing his legs under him, and standing on the bottom, raise his head far above the water. He must then plunge under it, having his eyes open, before as well as after going under; throw himself towards the egg, and endeavour, by the action of his hands and feet against the water, to get forward till within reach of it. In this attempt, he will find that the water brings him up against his inclination, that it is not so easy to sink as he imagined, and that he cannot, but by force, get down to the egg. Thus he feels the power of water to support him, and learns to confide in that power; while his endeavours to overcome it, and reach the egg, teach him the manner of acting on the water with his feet and hands, as he afterwards must in swimming, in order to support his head higher above the water, or to go forward through it.
If, then, any person, however unacquainted with swimming, will hold himself perfectly still and upright, as if standing with his head somewhat thrown back so as to rest on the surface, his face will remain above the water, and he will enjoy full freedom of breathing. To do this most effectually, the head must be so far thrown back that the chin is higher than the forehead, the breast inflated, the back quite hollow, and the hands and arms kept under water. If these directions be carefully observed, the face will float above the water, and the body will settle in a diagonal direction. ([Plate XXIII.] fig. 1.)
In this case, the only difficulty is to preserve the balance of the body. This is secured, as described by Bernardi, by extending the arms laterally under the surface of the water, with the legs separated, the one to the front and the other behind: thus presenting resistance to any tendency of the body to incline to either side, forward or backward. This posture may be preserved any length of time. ([Plate XXIII.] fig. 2.)
The Abbé Paul Moccia, who lived in Naples in 1760, perceived, at the age of fifty, that he could never entirely cover himself in the water. He weighed three hundred pounds (Italian weight), but being very fat, he lost at least thirty pounds in the water. Robertson had just made his experiments on the specific weight of man; and everybody was then occupied with the Abbé, who could walk in the water with nearly half his body out of it.
Plate XXIII
Swimming—Buoyancy in Water.