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.