The main cold supply from the tank is controlled by stop-cock No. 9. Just below the cock a small pipe is branched in, and carried up and curved over the top of the tank, to admit air when it is desired to drain the pipe. D acts as a drain to the hot-water pipes of both reservoirs. The sediment pipes of the reservoirs are also connected to it.

To aid the reader in tracing the different pipes easily, all the hot-water pipes are represented by heavy black lines, and the cold-water pipes by double parallel lines. C1 is the cold supply to tank-pressure reservoir; C2, tank supply, cold water, to fixtures on the second and third floors; C3 cold supply to street pressure reservoir; and C4, cold supply from street pressure to fixtures on the first floor. FF are hot and cold faucets at the kitchen sink, the hot being on the left side. H is the main hot supply of the tank system;H', hot supply from the tank reservoir to the fixtures on the second and third floors; H1, main hot supply from the street-pressure reservoir; and H3, hot supply from the street reservoir to the fixtures on the first floor.

It will be noticed that each of the supplies has been carried up as high as the top of the tank, and curved at the end so that they will discharge into it, which, in the case of hot supplies fig. 98. Double-Reservoir Installation for Heating Combined.

Direct and Tank Supply. Two Independent Reservoirs, might occur from steaming. The extension of the fixture supplies to the top of the tank with ends left open, insures that they will drain themselves when the water is shut off; and also cushion the pressure when the faucets are turned off quickly, the same as air-chambers do on direct-pressure systems.

Each Connected to a Water-Back.

The hot and cold supplies from the street reservoir might be left off at the point where the branches are made on the first floor, without causing any material difference in the working. In that case, however, there would be no vapor relief for the reservoir through the hot-water pipe; and when the cocks were shut off, none of the pipe would drain unless the faucets were opened upstairs. As it is, the main line will drain whether the faucets are open or not; and there is also the advantage of the air-cushion and relief as well.

Of the cocks which are located over the kitchen sink, only those on the pipes leading to fixtures on the upper floors - are stop and waste cocks. The others are plain stops which prevent any chance of causing them to waste continually by some error in using them. Stops and waste would be of little value on the lines above the sink which lead direct to the reservoirs, because it is not particularly desirable to drain-any of the pipe between the cocks and the reservoirs while the cocks mentioned are shut off.

The branches aal, bbl, and ccl, are of 5/8-inch pipe, and supply fixtures on the first floor from the street pressure, and on the second and third floors from the tank pressure. W and W represent Water-backs, both of which are in the same firebox of the range. One of them is connected to the tank reservoir by means of circulating pipes 12 and 14, while the other isconnected to the street reservoir by pipes 11 and 13. The sediment pipes of the reservoirs are controlled by cocks 4 and 5. Both of the sediment pipes discharge into the general drainpipe D. The overflow pipe of the tank is indicated by X Y and Y are vacuum valves situated over the kitchen sink. They communicate with the reservoirs through branches from the main hot supplies.

By referring to the engraving, the reader will see that there is no way to cut off communication between the reservoirs and the vacuum valves. With the valves placed at the sink as shown, the weight of the water in the 'vertical pipe above the valves must be overcome before air will enter the reservoirs. If desired, the valves may be placed in the heads of the reservoirs, and a pipe carried over to the sink to take care of the drippings. In this style of double-boiler work, vacuum valves are not so important as they are in systems having one reservoir within the other, because the reservoirs here described work under about the same conditions as those in ordinary single-reservoir jobs. The tell-tale pipe discharges over the kitchen sink, and is indicated by Z. Cock 6 is to drain the hot-water pipe from the street reservoir, and cock 7 drains the hot pipe from the tank reservoir.

Cock 8 is placed in a connection where, when turned on, it allows the tank pressure to by-pass check valve No. 3. By this means, both systems may be worked under high-pressure duty when the street pressure is off. In a case where the street pressure is constant for the fixtures on the first floor, but does not reach the second, cock 8 will seldom have to be used, and it should then be of a type having a removable handle.

One point gained by bringing the pipes down and up, as shown by the loops over the sink, is, that every stop can be reached from the floor without the aid of a ladder. The fixtures on the upper floors can be shut off without interfering with the supply to kitchen sink or other fixtures that may happen to be on the lower floors.

The sizes of the pipes shown in this installation, which have not already been given, are as follows: B and C, 3/4-inch; C1, C2, C3, C4, 5/8-inch; D, 3/4-inch; FF 5/8-inch; H, H1 H1 H3, 5/8-inch; X, l1/2-inch; YY, 1/2-inch; Z, 3/8-inch; 11 and 12, 1-inch; 13 and 14, 3/4-inch. Cocks 4 and 5 are 3/4-inch; 6, 7, and 8, 5/8-inch; and 10, 1 1/4-inch.

Plumbers habitually having this type of work to contend with - New Yorkers, for instance - become ultra-skilful in meeting the difficulties presented by variable pressure. The range of variation may cover the second floor of one building and the third of another, according to elevation. The fixtures on the floor with intermittent street supply can be placed wholly on the tank system, only at the expense of pumping all the water used in them. To take advantage of the street pressure reaching those fixtures at certain hours, four cocks are arranged so that one handle will turn all of them at once - two closing the tank hot and cold supply from the fixtures on that floor, and two admitting instead the street-pressure hot and cold.

There are many interesting features in piping water for municipal service, but it is not in the province of this work to consider them.