The use of the straddle fitting is shown in Fig. 265, from which it will be seen that a fitting of this style not only adds greatly to the appearance of the work, but saves labor and the use of a number of fittings.

FIG. 266.-Wash-tray Bibbs.

Regarding the running of the hot- and cold-water pipes side by side, when so run horizontally the hot water should ordinarily be run above the cold water, as the heat radiated from the hot water

FIG. 267.-Use of Wash-tray Fittings.

will affect the cold water to a greater degree if the latter is run above. The male and female union elbows and tees shown in Fig. 264 represent another valuable type of fitting. It will be clear that fittings of this type are of great value where the work

is close, and there is not sufficient room in which to use a tee or elbow and a union.

In Fig. 266 are shown several different styles of wash-tray bibbs. In the use of the ordinary faucets on wash trays, with the handle upright, there is often difficulty, owing to the projection. of the bibb handles so high up as to prevent the use of covers on the trays.

The wash-tray fittings to be seen in Fig. 267 are fittings which allow neat work to be installed in connection with wash trays. By means of them, the wash-tray bibbs may be kept on a straight line, as shown in Fig. 267. In the use of ordinary tees and elbows, the bibbs are on two horizontal lines, the hot-water bibbs being on a line above the cold-water bibbs. The wash-tray elbows and tees are each made in two styles, one to be used on the upper line, throwing the bibb opening down, and the other to be used on the lower line, to throw the bibb opening up.

CHAPTER XXIII

RANGE BOILERS-THEORY OF CIRCULATION

THE subject of cold-water supply, which has already been considered, is a comparatively simple matter. On the other hand, the subject of hot-water supply involves numerous difficulties which do not enter into the former branch of work. Indeed, the proper installation of the hot-water supply system is often a most perplexing problem, one demanding a knowledge of the theory which underlies the action of hot water, and the application also of knowledge which comes by experience. In this branch of plumbing construction, more than in any other, practical experience counts for a great deal. It is the employment of a knowledge of the theory of the subject in combination with the practical knowledge that comes by experience, that produces the perfect system of hot-water supply.

Many a system of hot-water supply produces results which, to the person unacquainted with the subject, would seem to be all that is required of it, whereas, if the same system had been properly constructed, it would have been able to do better work, and a greater amount of it. A great deal of the supply work that is installed, is like all other construction work-it will answer the purpose, while at the same time it is not capable of performing its work as it should be performed. As in the case of the drainage and vent systems, the perfect operation of the hot-water supply system depends very largely on the observance of the small points which, to the uninitiated, would often appear to be of small consequence. By the observance of these small points, one workman will install a system which will produce excellent results, while another workman, careless in observing these same points, will construct a like system from which only poor or fair results are to be obtained.

The principle of circulation underlies all hot-water supply

work, as well as all heating systems, whether hot water, steam, or hot air, and a thorough understanding of this principle is necessary to an understanding of either subject. For the purpose of this explanation, it is necessary to understand something of what is known as the molecular theory. According to this theory, all bodies, regardless of their composition, are made up of molecules or particles, these particles being so minute, and of such numbers, that they cannot be estimated. These particles are in constant motion or vibration, the path which each molecule traverses being so small as to be immeasurable. When heat is applied to some bodies, metals, for instance, these particles are set in more violent vibration, and the result, as is well known, is the expansion of the body. If heat continues to be applied of sufficient intensity, the vibrations of the molecules become so rapid and so violent that they refuse to hold together, and separate from the main body, that is, the body melts. Thus it will be seen that by means of the molecular theory, the action of expansion and contraction, the melting of metals and many other phenomena may be explained. Indeed, it is by means of this theory that innumerable actions and operations may be explained.

The molecular theory may be applied much more extensively than the above, but the simple statement as given, will be sufficient for the present purpose. As a means of illustration, suppose that a block of ice be subjected to heat. In its original form, the ice is a solid, compact mass. As the heat is applied, however, according to the above explanation, the particles or molecules of the ice begin to vibrate more rapidly, finally refusing to hold together longer, and the ice melts and forms water. If heat continues to be applied to the water its temperature rises and its molecules expand. The vibration of these molecules finally reaches such a stage that it is stronger than atmospheric pressure, and in their expanded state, being lighter than the air, they rise into the air in the form of steam. Thus, by the application of heat, the original block of ice has been changed to water, which is a fluid, and the water in turn has been changed to steam, which is a vapor. Each of these actions is an example of the action of circulation.

Now, when this same principle is applied to the range boiler, as shown in Fig. 268, the action is the following: