The arms should be folded across, below the breast, or compressed against the hips, and the legs employed as in front swimming, except as to time and extent. They should perform their action in half the usual time, or two strokes should be taken in the time of one; because, acting perpendicularly, each stroke would otherwise raise the swimmer too much, and he would sink too low between the strokes, were they not quickly to follow each other. They should also work in about two-thirds of the usual space, preserving the upper or stronger, and omitting the lower or weaker, part of the stroke. There is, however, another mode of treading water, in which the thighs are separated, and the legs slightly bent, or curved together, as in a half-sitting posture. Here the legs are used alternately, so that, while one remains more contracted, the other, less so, describes a circle. By this method, the swimmer does not seem to hop in the water, but remains nearly at the same height. Pl XXIII. f. 3 represents both these methods, and shows their peculiar adaptation to relieve drowning persons. BACK SWIMMING. In swimming on the back, the action of the thoracic member is weaker, because the swimmer can support himself on the water without their assistance. The muscular contractions take place principally in the muscles of the abdominal members, and in those of the anterior part of the neck. Though little calculated for progression, it is the easiest of all methods, because, much of the head being immersed, little effort is required for support. For this purpose, the swimmer must lie down gently upon the water; the body extended; the head kept in a line with it, so that the back and much of the upper part of the head may be immersed; the head and breast must remain perfectly unagitated by the action of the legs; the hand laid on the thighs (Plate XXIV. fig. 1), and the legs employed as in front swimming, care being taken that the knees do not rise out of the water.--(Plate XXIV. fig. 2.) The arms may, however, be used in various ways in swimming on the back. In the method called winging, the arms are extended till in a line with each other; they must then be struck down to the thighs, with the palms turned in that direction, and the thumbs inclining downward to increase the buoyancy, (Plate XXIV. fig. 3); the palms must then be moved edgewise, and the arms elevated as before (Plate XXIV. fig. 4); and so on, repeating the same actions. The legs should throughout make one stroke as the arms are struck down, and another as they are elevated. The other mode, called finning, differs from this only in the stroke of the arms being shorter, and made in the same time as that of the legs.