DISTRIBUTION OF COMPRESSED AIR.

Tubing.-Tubes are usually of cast or wrought iron, the former being preferable for large diameters, and the latter for those below 3.9 to 5.7 inches. Wrought-iron has the advantage of lightness and flexibility. Tubes of riveted sheet-iron are sometimes used for transmitting blast to furnace. Copper tubing is employed where flexible joints and sections of peculiar shape are needed. Lead tube is of little value, and rubber tubes are used for flexible connections. The latter are usually- lined with wire-spiral, and covered with canvas.

Diameter of Tubing.-The following table shows the losses of pressure in millimetres of mercury which occur in conduits 1,000 metres in length, and of diameters increasing from .1 to .35 metre, velocity of air from 1 to 6 metres: The results of practice at the tunnels of Mont Cenis and St. Gothard, Saarbruck mines, and elsewhere in Europe, show the following diameters of pipes to be advisable: For principal conduits, east-iron, from 5.8 to 9.7 inches; for secondary conduits (generally drawn tubing), 2.9 to 5.8 inches; for extreme branches (always drawn tubing), 1.9 to 2.9 inches; for flexible connections (rubber), 0.9 to 1.9 inch.

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Reservoirs.-Under ordinary conditions, the capacity of the reservoir should represent 10 or 15 times the consumption of air in cubic feet per minute when the air is used variably, as in rock-drills; in cases of regular employment, 4 or 5 times the consumption per minute may be taken as the rule. In a large number of instances, reservoirs ranging from 706 to 2,824 cubic feet have been found of ample size.

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