Various scientists have calculated, from the weight of the water displaced, that the bulk of the adult body is equal to about two and a quarter to two and a half cubic feet. The weight of this quantity displaced by total immersion is about i56 pounds. There are, of course, certain circumstances which cause the specific gravity of the body to vary. If the quantity of fat is proportionally large the specific gravity will be dimi nished, and a fat person will float more easily than a thin, muscular swimmer. A large proportion of bone tends at once to increase the specific gravity of a person, and it is for this reason that thin men find it so difficult to float. Yet it does not always follow that a man heavily weighted with bone cannot float as well, or as easily acquire the art, as a fat man. If the chest is large and capacious, and the respiratory organs are healthy, a man can, with slight practice, easily counteract the weight of the skeleton.
Women are of less specific gravity than men, the skeleton is smaller, and there is a greater proportion of fat ; hence they can learn to float much more easily. With children the bones are much lighter, the quantity of fatty matter is usually abundant, and they can therefore float more easily, if properly taught, than adults.
A person with a large and capacious chest floats better than one whose chest is small and contracted. The body of a floating person rises slightly out of the water during inspiration, and correspondingly sinks during expiration. If the lungs are emptied while the face is under water, and cannot again be replenished, the specific gravity becomes greater and the body sinks. Under ordinary conditions, a. human body in a healthy state, with the chest full of air, is quite buoyant ; and anyone possessed of the knowledge necessary to ensure proper management of the lungs, and imbued with confidence, will float with about half the head out of the water ; but to do this it is essential that the head be inclined well back and every other part of the body kept below the surface. If the breathing be unnatural and irregular, the body, owing to the bones and all other portions except fatty matter being of greater specific gravity than will sink during every expiration. It is most important that. every teacher should know and fully recognise the main principles which govern all attempts to float or propel the body through the,water.