The lighter a substance is that is immersed in water, the more there will be of it above the surface. Take two blocks of wood of different weights though of the same size. Suppose the heaviest one, A, Fig. 89, is one third lighter than the same bulk of water. One third of it will be above the surface. If the other, B, is half the weight of water, half of it will be above the surface. We should say, then, that the specific gravity of the wood in the first block is two thirds of that of water, and the specific gravity of the wood in the second is one half that of water.
138. Illustrations.—There are many interesting facts that illustrate the principles which I have developed. A stone is lifted much more easily in water than in air, because of the support afforded by the upward pressure of the water. A boy will often wonder why he can lift a very heavy stone to the surface, but can get it no farther. When a bucket of water is drawn up a well much less exertion is required to raise it through the water than through the air after it emerges from the water. While it is in the water you raise only the bucket itself, the water in it having no weight, being sustained by the water around it. But when it comes to the air you have the weight of the water added to that of the bucket. When a person lies in a bath for some time, on raising his arm from the water it seems to be very heavy. The reason is, that it has had for so long a time the support of the water that when it is lifted into the air the want of this support is sensibly felt, just as we perceive the difference between raising a bucket of water through water and raising it through air. It is said that Archimedes took in the full idea of the principles of specific gravity as his limbs felt the liquid support of a bath, and so overjoyed was he with the discovery, that he ran home crying out all the way, "Εὕρηκα! εὕρηκα!"—I have found it! I have found it! It was a rational joy, for he had found a principle of immense value to science and to the world.
139. Boats and Life-Boats.—A boat of iron will float with as much of it out of water as one of wood of the same size, provided that the iron be made so thin that the boat is not heavier than the wooden one. For what is it that floats? Not the iron or wood, but a wooden or iron boat filled with air. If it were filled with water instead of air it would sink, the specific gravity of the materials of which it is built being on the whole of greater specific gravity than water. Life-boats have in their structure either a large quantity of cork or air-tight vessels of tin or copper, and in this way they are made so light that they will float even when filled with water.
As the weight of a body can be estimated from the quantity of water which it displaces, we can very readily estimate the weight of the load of a canal-boat, as its form is so simple and regular. In order to do this we must first know how far the boat sinks in the water when empty, or, in other words, how much water it displaces.
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."
When a person is drowned the body sinks because in the struggle much of the air in the lungs is lost, just as the fish sinks when his air-bladder is contracted. It is, however, so little heavier than water after this is done, that it very readily rises when any gas is produced in it by putrefaction. It is a common popular notion that firing cannon over the water will raise the drowned. But it can produce no effect, unless perhaps the agitation caused by the concussion may hasten a very little the rising of a body which from commencing putrefaction is about to rise.
In wading a river the feet press upon the bottom with only a force equal to the weight of half the person's head, this being the difference between the weight of the body and the weight of the same bulk of water. Now this pressure is not sufficient to give a sure footing against even a moderate current. Many persons have been drowned from ignorance of this fact. A man carrying a load may often ford a river safely where without a load to press him down, and thus give him a sure footing, he would be carried down the stream. So a man may walk in deep water upon broken glass with impunity.
Fig. 90.