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. [Illustration: Fig. 90.] 142. =How to Ascertain the Specific Gravity of Solids.=--It results from the upward pressure of water that a body weighs less in water than in air. Take a piece of gold or any other substance, _a_, Fig. 90 (p. 107), and weigh it suspended as you see from one of the scales. Introduce the gold now into a cup of water, and you will find that a part of the weight must be taken from the opposite scale to preserve the balance. The weight which you take from the scale will be the weight of a quantity of water equal in bulk to the piece of gold; for the immersed body is supported with a force equal to the weight of the water it displaces (§ 137). By comparing, therefore, its weight in water with its weight in air we determine its specific gravity. Thus if a lump of gold weigh nineteen ounces, and on being weighed in water weighs eighteen, it will prove that gold is nineteen times as heavy as water. And if a lump of copper weigh nine ounces in air and eight in water, it is nine times as heavy as water. Calling water, therefore, 1, the specific gravity of gold is 19, and of copper 9. It is obvious that a body of the same specific gravity with water would weigh nothing when immersed in water, for it would be supported with an upward pressure precisely equal to its own weight, just as the same bulk of water is. A pound of water, therefore, will weigh nothing in water. The experiment can easily be tried. Weigh a glass bottle, suspended on one arm of the scale-beam, and then put a pound of water in it. On immersing it in water it will be balanced if you take out the pound weight in the opposite scale.