In salt water, therefore, a person throwing himself on his back, and extending his arms, may easily lie so as to keep his mouth and nostrils free for breathing; and, by a small motion of the hand, may prevent turning, if he perceive any tendency to it. In fresh water, a man cannot long continue in that situation, except by the action of his hands; and if no such action be employed, the legs and lower part of the body will gradually sink into an upright position, the hollow of the breast keeping the head uppermost. If, however, in this position, the head be kept upright above the shoulders, as in standing on the ground, the immersion, owing to the weight of the part of the head out of the water, will reach above the mouth and nostrils, perhaps a little above the eyes. On the contrary, in the same position, if the head be leaned back, so that the face is turned upwards, the back part of the head has its weight supported by the water, and the face will rise an inch higher at every inspiration, and will sink as much at every expiration, but never so low that the water can come over the mouth. 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.