The size of a reservoir is determined by the engine; it should be large enough to enable the engine to run smoothly at its maximum load for several hours consecutively. Under these conditions, the reservoir
should have a capacity of 45 to 55 gallons per horse-power for engines with "hit-and-miss" admission, and 55 to 65 gallons for engines controlled by variable admission. It is not advisable to employ reservoirs having a capacity of more than 330 to 440 gallons, the usual diameter being about 3 feet.
Fig. 62.—Incorrect arrangement of tanks and piping.
If the power of the engine be such that several reservoirs are necessary, then the reservoirs should be connected in such a manner that the top of the first communicates with the bottom of the next and so on,
the first reservoir receiving the water as it comes from the cylinder (Fig. 61).
Intercommunication of the reservoirs by means of a common top tube (a) is objectionable; and simultaneous intercommunication at top and bottom (a and b) is ineffective, so far as one of the reservoirs is concerned (Fig. 62).
Fig. 63.—Tanks connected by inclined pipes.
The reservoirs are true thermo-siphons. Consequently the water should be methodically circulated; in other words, the hottest water, flowing from the engine into the top of the first reservoir and having, for example, a temperature of 104 degrees F., is cooled off to 86 degrees F. and drops to the bottom of the reservoir, thence to be driven, at a temperature sensibly