"I would the more earnestly press upon you the trial of this method, because, though I think I shall satisfy you that your body is lighter than water, and that you might float for a long time with your mouth free for breathing, if you would put yourself into a proper posture, and would be still and forbear struggling, yet till you have obtained this experimental confidence in the water, I cannot depend upon your having the necessary presence of mind to recollect the posture and the directions I gave you relating to it; the surprise may put all out of your mind. "Though the legs, arms, and head of a human body, being solid parts, are specifically somewhat heavier than fresh water, yet the trunk, particularly the upper part, from its hollowness, is so much lighter than water, as that the whole of the body, taken altogether, is too light to sink wholly under water, but that some parts will remain above until the lungs become filled with water, which happens from drawing water to them instead of air, when a person in the fright attempts breathing while the mouth and nostrils are under water. "The legs and arms are specifically lighter than salt water, and will be supported by it, so that a human body cannot sink in salt water, though the lungs were filled as above, but for the greater specific gravity of the head. Therefore, a person throwing himself on his back in salt water, 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 should perceive any tendency to it. "In fresh water, if a man throw himself on his back near the surface, he cannot continue in that situation but by proper action of his hands in the water; if he have no such action, the legs and lower part of the body will gradually sink till he comes into an upright position, in which he will continue suspended, the hollow of his breast keeping the head uppermost. "But if in this erect position, the head be kept upright above the shoulders, as when we stand on the ground, the immersion will, by the weight of that part of the head that is out of the water, reach above the mouth and nostrils, perhaps a little above the eyes, so that a man cannot long remain suspended in the water with his head in that position. "The body continuing suspended, as before, and upright, if the head be leaned quite back, so that the face look upward, all the back part of the head being under water, and its weight consequently being in a great measure supported by it, the face will remain above water quite free for breathing, will rise an inch higher at every inspiration, and sink as much at every expiration, but never so low that the water may come over the mouth.