In Fig. 333 is shown an illustration from which the principle governing the artesian well, may be studied. Wherever this well is to be found, it will also be found that the strata composing the earth's surface is of two kinds, the one permeable to water, such as sand, gravel, etc., the other impermeable to water, such as clay. Let it be supposed, as shown in Fig. 333, that in a natural basin of greater or less extent, there are two impermeable layers, A B and C D, inclosing between them a permeable layer, E F. The rain water falling on the part of this permeable layer which comes to the surface, will filter down through it, and following the natural fall of the ground will collect in the hollow of the basin, from which it cannot escape, owing to the impermeable nature of the layers above and below it. The result is, that the space between the impermeable layers fills with water throughout its course.

If now a hole is bored from the surface, through the upper impermeable layer and into the water-bearing stratum, the water escaping, will endeavor to reach the level from which it has descended, the result being that it will gush out to a height depending on the difference between the levels at which the boring is made and the point at which this stratum comes to the surface. The water which feeds an artesian well often comes from a distance of sixty or seventy miles, and these wells are sometimes capable of delivering several hundreds of gallons of water per minute. In the action of the artesian well, it is not atmospheric pressure that forces the water up, but the pressure of the column of water between the well and the point where the water-bearing stratum comes to the surface.

Fig. 333.   The Artesian Well.

Fig. 333. - The Artesian Well.

Capillarity is a characteristic of liquids, which as seen in a preceding chapter, is often met with in connection with traps. While the phenomena of capillary action are numerous, there is only one feature of the subject which will directly interest the plumber. This feature is the fact that when a body in which very minute tubes or pores exist is immersed in water, the liquid will rise through these minute tubes to heights much above the level of the water itself. In order that this may happen, the substance immersed must be capable of being moistened by the water. The action is not due to any pressure or to action of the atmosphere, but to a sort of attractive force between the substance immersed and the liquid. If, for instance, a fine glass tube is thus immersed, the liquid will be seen to rise in it to a considerable height above the level of the water. The application of this principle to plumbing, appears in the destruction of trap seals by capillary action.

Fig. 334.   Destruction of Trap Seal by Capillary Action.

Fig. 334. - Destruction of Trap Seal by Capillary Action.

In Fig. 334 is shown a trap into which a collection of lint, etc., dips. This material reaching from the trap seal into the outlet presents a danger to the trap seal much greater than would at first appear. By capillary action, the water in the trap is drawn up through this collection of lint, threads, etc., and deposited in the outlet a drop at a time. In the course of a comparatively short time the entire seal of the trap may be drawn out in this manner.

The friction of liquids is a matter which applies directly to supply work. In flowing over a surface, against the surface of a pipe, for instance, a liquid meets with more or less resistance, and in addition, there is a certain amount of friction between the molecules or particles of the water itself. This is called fluid friction. Fluid friction does not depend in any way upon the pressure of the liquid against the surface with which it comes in contact. It increases with the roughness of the surface and with the area of the surface, and within certain limits it increases with the velocity of the liquid.

Friction has greater effect in the case of small pipes than in large pipes. The reason for this is that in small pipes a larger proportion of the liquid comes in contact with the surface of the pipe than in large sizes. In long lines of pipe the friction may be of such an amount as to entirely cut off the flow of water.

Bends and other fittings add greatly to the friction in pipes. It is for this reason that changes in direction of pipes are often made by bending the pipe rather than by using bends or elbows.

The effects of heat present a subject which is also of very great interest to the plumber. The theory of heat now generally accepted, is that it is a form of motion. According to this theory, all bodies are made up of infinitely small particles or molecules, which are in constant motion. When heat is applied these vibrations increase, and expansion of the body results. If heat is continued, the expansion at last becomes so great that the particles will not hold together, the body loses its form and passes into the liquid state. If now heat is still further applied, by virtue of the continued expansion of its particles, the liquid mass will become transformed into a gas or vapor. When heat is withdrawn, the reverse action of contraction takes place.

Expansion and contraction of different metals varies considerably. In the calking of lead joints on cast-iron pipe this is to be seen. A metal or composition of metals has long been sought which in cooling will not contract so much as lead will, for as any workman well knows, when it cools, calking lead will draw away from both surfaces with which it is in contact. Type metal is about the only composition which answers this requirement, as its expansion and contraction are very slight. This metal, however, is not only too expensive for this purpose, but is too hard for calking.