Water motors for general power purposes, of light nature but requiring comparatively high speed, are made on the rotary plan, a jet impinging on the blades. Others, often used for oscillating fans, operating air-bellows for church organs, etc., have reciprocal motion, the water being handled in a cylinder much as steam is in a reciprocating steam engine. Air-compressors for light duty, operated by water; are made on the reciprocal plan, and also in a way to fill and dump alternately a pair of pivoted buckets, the water pressure expelling the air into an accumulator by filling the bucket with water until it becomes overbalanced, when it falls and trips a waste-valve in the bottom, and at the same time cuts off the supply to one bucket and turns it into the other.

Knowledge of these and kindred devices for producing motion by water-pressure, is not considered a part of the plumber's curriculum; but it is to his interest to learn about them when he can do so without interfering with studies that should take precedence by reason of more immediate importance.

When a pressure tank - the so-called pneumatic plan - is used, the supply piping for plumbing fixtures is essentially the same as for street pressure; but when the supply is by gravity, from a tank, new problems present themselves. The type of tank used may in some cases be decided by reasons other than adaptability or simple preference. If of iron, the tank must have a safe-pan to intercept condensation, unless it is insulated from the air, which is difficult and expensive except when the lightness of the metal requires casing for support.

Any shape with flat bottom provides for retaining much sediment that would otherwise flow down with the water. Closed cylindrical tanks, those with merely a pipe-opening to the air, have not even this redeeming feature. Open, rectangular, lead-lined tanks, with loose cover, serve best. An overflow two sizes larger than the supply to tank (never less than 2 inches' diameter) should always be put in near the top. Roof water is sometimes led directly into an attic tank, to avoid pumping. The tank is then divided so that one portion will act as a sort of filter, the water, after subsidence, finding its way into the distributing portion through a screened opening in the partition, some inches above the bottom. This plan requires a large tank, with extraordinary support.

The water is never so well filtered as it may be, if the regular yard cistern with intermediate filter is used; and, all things considered, this is not a plan advisable to follow. The house supply should be taken from a little above the bottom, and well screened to prevent accidental choking. The valve-controlling distribution may be an ordinary stop-cock with an air-pipe carried from immediately below it to above the overflow level, terminating in a position to discharge into the tank, so that air can enter to drain the line; Or it may be the regular cistern valve, so arranged, or - which is far better - a hollow stopper valve, with pipe stem extending to above the overflow level, having a chain attached to the stem, and terminating at a convenient point downstairs so that the supply can be stopped at will without going up to the tank. The hollow stem will admit air to the service when the water is off, and there will be no danger of accidental breakage or freezing, as is the case when the necessary relief pipe is carried up outside the tank wall. A standing bath waste fitting can be adapted to admirable service in this capacity; a strainer fitted in the collar of the waste inlet takes the place of the usual screen-hood, and the inlet is just far enough above the bottom to avoid trouble from sediment. The tell-tale pipe should be taken from the bottom of the overflow pipe near the tank, and should discharge where it can easily be observed while pumping is in progress - over the kitchen sink, if the pump is beside the sink. If the closets are to be flushed by valves instead of individual tanks, a separate supply with cut-off should be put in for them.

Pumping into the bottom of the tank, and taking the cold-service branches from the pipe thus used to fill it, should never be practiced ' The little difference in head against which the pump must work when pumping over the top, is too small to be considered when compared with the disadvantages of the combined service and pump delivery,' even though one line of small pipe is thereby saved. Failure of the single line prohibits service to the fixtures, and pumping into the tank, too; moreover, water that has been pumped is likely to find its way back to the cistern through leaky pump-valves, and there is more trouble in draining both the house pipes and the pump. In placing stop and watse cocks in tank installations, care is necessary to set the right end up, as the water is usually feeding down, instead of up as when direct pressure prevails. Thus, when a cock is set properly, air sometimes enters the waste-hole to cause the line to drain out at some other point - just the reverse of what happens in direct work. However, by bringing the main cold service to the kitchen, and feeding back with the various lines from a manifold, instead of branching out with the cold water on the downward course, the stop-cock work will be about the same as on direct work, after the manifold is reached.

The supplying of hot water to the fixtures should be as direct as possible in all jobs where circulation is desired. Dipping the supply from the top of the reservoir to below the sink level, in order to secure a handy location for the stop-cocks, and ease in taking care of the drain-water, is most certain to interfere with circulation, and not infrequently makes it a matter of impossibility.

The hot-service connection of a tank installation should continue up to and over the tank, as should the main lines of cold service, if convenient, when feeding upward. Also, as there is no street main to give relief, it is good practice to carry a line from the hot-service opening in top of reservoir directly to the tank, and over it, without stop-cocks or branches, so that there will be no ordinary means of closing it. This line will make it impossible to shut off all means of egress for steam and vapor, and may prevent serious accidents otherwise possible.