MANUAL FOR NAVAL CADETS.

CHAPTER I.

WATER AND AIR.

THE Theory of naval architecture is founded on those branches of the science of watery fluids which are comprehended in the term Hydrodynamics. Those are-Hydrostatics, which treat of their pressure when at rest; Hydraulics, which treat of them in motion; and Pneumatics, which treat of the pressure and motion of air and other elastic fluids.

The Questions then arise, what is water? and what is air? Every substance which comes under the cognizance of our senses is called matter, and is made up of atoms, each of which, however minute, has length, breadth, and thickness, and occupies a space into which another cannot enter until the first has been displaced. If, for example, we immerse a solid in a vessel full of water, a portion will overflow equal in bulk to that of the body which is submerged. This property of matter is called its Impenetrability.

In some bodies the atoms are closer together than in others, and, although of similar volume, would have greater gravity. This is the Density of matter.* The quantity of space which

*The earth is a mass of matter, about 5 times heavier than an equal volume of water; or, what is the same, the mean density of the earth is 53. The total weight of the earth is more than 6,000,000,000 billions of tons.

B