HYDROSTATICS. CHAPTER I. On Fluid Pressure. 1. HYDROSTATICS was originally, as the name imports, the science which treated of the Equilibrium of Fluids, or of bodies in equilibrium under the action of forces some of which are produced by the action of fluids. It is now extended so as to include many other theorems relating to the properties of fluids. 2. A fluid is a substance whose parts yield to any force impressed on it, and by yielding are easily moved among themselves. 3. This definition separates fluids from rigid bodies, in which the particles cannot be moved among each other by any force, however great, but it does not separate fluids from powders, such as flour, in which we have a collection of particles which can be moved among themselves by the application of a slight force. 4. A fluid differs from a powder in this way: the particles composing a powder do not move among themselves without friction, whereas the particles that make up a fluid move one over another without any friction. For example, if you empty a mug of flour on a table the friction between the particles will soon bring the flour to rest in more or less of a heap: whereas if you empty a mug of water the particles, moving without friction, run in all directions, and the whole body of water is spread out into a very thin sheet. S. H. 1 5. To distinguish fluids from powders we must therefore make an addition to Art. 2, and we give the following as a complete definition of a fluid. DEF. A fluid is a substance whose parts yield to any force impressed on it, and by yielding are easily moved among themselves without friction, and also act without friction on any surface with which they are in contact. This definition includes not only the bodies to which in ordinary conversation we apply the terms "fluid" and "liquid," such as water, oil, and mercury, but also such bodies as air, gas and steam. 6. Fluids may be conveniently divided into two classes, liquid and gaseous. By the term liquid we understand an incompressible and inelastic fluid. In reality all fluids with which we are acquainted are compressible, that is, a given volume of fluid can by pressure be reduced in volume. Still so great a force is required to compress to any appreciable extent such fluids as water and mercury, that we may regard them as incompressible in treating of the elements of the subject. 7. The inelastic fluids with which we are practically acquainted approach more or less to a state of perfect fluidity, but in all there is a tendency, greater or less, of adjacent particles to cohere with each other. This tendency is stronger in such fluids as oil, varnish and melted glass, than in such as water and mercury. Hence the former are called imperfect or viscous fluids. 8. The air which we breathe and gases are compressible fluids, and are endowed with a perfect elasticity, so that they can change their shape and volume by compression, and when the compression ceases they can return to their former shape and volume. 9. Vapours, as steam, are elastic fluids, but with this peculiarity: at a given temperature in a given space only a certain quantity of vapour can be contained, and if the space or the temperature be then diminished, a portion of the vapour becomes liquid, or even in some cases a solid. 10. Before proceeding further with our subject we must explain the meaning of some technical terms which we shall have to employ frequently. |