In order to be successful in his work, the plumber of to-day must have at his command not only a knowledge of the practical requirements of his trade, but also a knowledge of the theory that underlies it. Every action that takes place in the drainage or supply system depends in some way upon some natural law, as also the action of various devices employed on the plumbing system.

It will be evident then that a knowledge of certain of these actions, and the laws governing them, is necessary to the plumber, in order that he may be able to understand and solve the various problems that are constantly arising in his work. Possibly to a greater extent than in almost any other trade the efficiency of the plumber consists largely in his ability to avoid trouble and to successfully get out of trouble that he already finds himself in.

Of the various phenomena to be considered, that of atmospheric pressure is perhaps of first importance. The atmosphere may be considered as a very light fluid, of certain depth, surrounding the earth. Although of small density, the weight of the atmosphere, which is computed to be about 45 miles in depth at the sea level, is very appreciable. Experiment has demonstrated that at sea level, the pressure of the atmosphere amounts to about 14.7 pounds per square inch. In ordinary computations, atmospheric pressure is considered as 15 pounds per square inch. Now in a square foot of surface, there are 144 square inches. Therefore, the pressure of the atmosphere on each square foot amounts to about 2160 pounds, or over a ton. This will serve to indicate what an immense pressure the atmosphere is capable of exerting.

That this pressure does not result in crushing everything that it comes in contact with, is due to the fact that the same pressure is exerted from all sides, thus equalizing the effect. Thus in the case of the human body, atmospheric pressure is exerted from without and from within, the two forces balancing each other and producing no results. If, however, a vacuum is formed on one side of an object, and atmospheric pressure is exerted on the other side, the obvious result is the crushing in of the object.

The action of atmospheric pressure, combined with the formation of a vacuum or partial vacuum, is to be met in a variety of ways in the plumbing system, some of the more common instances being the following:

Whenever siphonage takes place, whether it is in connection with traps, or' in connection with the range boiler, or the action of water-closets and various other fixtures, the action is due to the creation of a vacuum on one side, and the pressure of the atmosphere on the other. This also underlies the action of pumps. In the siphonage of a trap, a vacuum must necessarily have first been formed on the sewer side of the trap seal. When this condition has been fulfilled, there is nothing to oppose the atmospheric pressure exerted on the house side of the seal, and the result is that the contents of the trap is forced out by the pressure of the atmosphere. It will be understood, by the way, that a vacuum is a portion of space from which the air has been exhausted.

The siphonage of boilers occurs in the same way as the siphonage of traps. In the case of the boiler, however, the vacuum is generally formed by the passage out of the pipe supplying the boiler of the water held in it, while the vacuum on the drainage system is often produced by the passage past the entrance of the trap of a body of waste, through the pipe into which the trap connects. In the operation of the pump, the action is as follows: As the piston of the pump is raised, it causes the formation of a vacuum in the suction pipe, and the action of atmospheric pressure on the surface of the water in the well forces the water up through the suction pipe. If the vacuum were perfect, and there was not the friction of the pipe to be considered, the water would rise in the suction pipe nearly 34 feet above the level of the water in the well. A more or less imperfect vacuum and the friction of pipe and fittings results in cutting down this distance considerably, depending on the conditions that exist in each case.

The same principle underlies the action of the vacuum valve as well as other devices used on the plumbing system. Under ordinary conditions, the vacuum valve remains closed, owing to the pressure of the water against the valve. When a vacuum occurs on the inner side of the valve, however, atmospheric pressure opens the valve, admitting air, and thus preventing the siphonage of the boiler. Various other examples might be mentioned, if necessary, of the effect of a vacuum and atmospheric pressure in connection with the plumbing system.

Atmospheric pressure is exerted not only downward, but in all directions. Hydrostatics is another subject which holds much interest for the plumber. Hydrostatics treats of the conditions of liquids in equilibrium, and of the pressures which they exert. One of the principles demonstrated in the study of this subject is that liquids are almost incompressible. This principle is made use of in the hydraulic ram. On the other hand, air is extremely compressible. The air chamber is often used on pumping and sup-Ply systems, the compressibility of air allowing the air chamber to take up any shock which may occur on the piping. When the air has become exhausted from the air chamber, however, and it ha3 become filled with water, the latter being incompressible, the results of shocks through the pipes again become noticeable.

Another very important principle is the following: Pressure exerted anywhere upon a mass of liquid is transmitted undiminished in all directions, and acts with the same force on all equal surfaces, and in a direction at right angles to those surfaces. This principle may be demonstrated by means of the vessel shown in section in Fig. 332. In this vessel, the several openings are fitted with pistons of equal area. If pressure is exerted on the surface of the water in the vessel by means of a piston working through another opening, the six pistons shown will be pressed out in the direction of the arrows, a distance equal to that through which the additional piston is forced in. In any body of water, the pressure at any level is proportional to the depth, and the pressure is the same at all points on that level. The pressure of a column of water one square inch in area, and one foot high, is equivalent to .434 lbs. Therefore, to find the pressure of a column of water, multiply its head in feet, by .434. The pressure which the upper layers of a liquid exert on the lower layers, causes the latter to exert an equal reaction in an upward direction. This it will be seen, is a necessary consequence of the principle of transmission of pressure in all directions. This upward pressure is called the buoyancy of the liquid. The pressure exerted by a liquid, due to its weight, on any portion of the liquid, or on the sides of the vessel in which it is contained, depends on the depth and density of the liquid, but depends in no way upon the shape of the vessel or upon the quantity of the liquid. Another principle, which is well understood, is the fact that water will always strive to reach its own level. The fact, however, that this principle underlies the action of the artesian well is probably not so well understood.

Fig. 332. - Direction of Pressure.