140. =Specific Gravity of Animals.=--Birds have a much less specific gravity than animals that walk, in order that they may mount up easily in the air. Their light feathers increase greatly their bulk, as you may see whenever a bird is stripped of them. Besides this, the bones are hollow and communicate with the lungs. Birds that swim, as ducks, swans, etc., have so small a specific gravity--that is, are so large in proportion to their weight--that but a small part of the body is under water, and the motion of their feet is not required at all to sustain them, but only, like the action of oars, to propel them along. Insects are of small specific gravity, those that fly the most swiftly being the lightest. Fishes are very nearly of the same specific gravity with water, and hence require but little muscular effort to move about in their element. They are assisted much in rising and falling by a contrivance by which they can instantaneously alter their specific gravity. They have an air-bladder, which they can dilate or contract at pleasure. When dilated, the bulk of the fish is increased and his specific gravity lessened, and he rises easily and at once. By compressing it he as readily sinks. 141. =Specific Gravity of the Human Body.=--The human body, when the chest is filled with air, is so much lighter than water that it will float with about half the head above the surface. A knowledge of this fact, with proper presence of mind, might ordinarily save persons from drowning; for if the body be put in the proper position, the feet downward and the head thrown backward, the nose and mouth will be out of the water. So little is required in the way of support to keep the whole head out of water, that persons who can not swim are often saved from drowning by catching hold of very small pieces of wood. An oar would support half a dozen men, if they would be satisfied with keeping only the head out of water; but if each one struggle to get his whole body upon the oar, they may all be lost.[1] A life-preserver is a great aid in preservation from drowning, for it diminishes the specific gravity of the body. It is commonly an air-tight bag fastened round the upper part of the body, which can be filled by blowing into it through a pipe which has a valve in it. "On the great rivers of China," says Dr. Arnot, "where thousands of people find it more convenient to live in covered boats upon the water than in houses on the shore, the younger male children have a hollow ball of some light material attached constantly to their necks, so that in their frequent falls overboard they are not in danger."