_a._ Air and vacuum chambers in accordance with the sizes given in the following table must be provided with all pumps. If the air chamber is cast iron, the pump manufacturers must warrant that it has been subjected to a hydraulic test of 400 lbs. per square inch before it is connected to pump. It is to be thoroughly painted inside and out to diminish its porosity. SIZE OF VACUUM AND AIR CHAMBERS. ====================+===================+================ | Vacuum Chamber is | Air Chamber is | to contain:— | to contain:— --------------------+-------------------+---------------- 500-gallon pump. | 13 gallons. | 17 gallons. 750 „ „ | 18 „ | 25 „ 1,000 „ „ | 24 „ | 30 „ 1,500 „ „ | 30 „ | 40 „ --------------------+-------------------+---------------- The air chamber, combined with connections for discharge pipe, relief valve, and hose valves, should be carefully designed to make the whole weight as small as possible. Keeping this weight down makes the pump run steadier and brings less strain on the flanges at high speeds. An air chamber of hammered copper and warranted tested under a hydraulic pressure not less than 300 lbs. per square inch is a little better than cast iron as it holds air better, and being lighter it wrenches and strains the pump less when running fast and shaking, but because it costs from $25 to $50 more than cast iron, it is not often adopted. _b._ The vacuum chamber must be attached to the pump in the most direct way practicable, but provision must be made for attaching it in such manner as not to prevent readily taking off the cylinder heads. _c._ Every vacuum chamber should be provided on one side near the top with a 1/4-inch pipe hole plugged. This to be used for attaching a vacuum gauge if desired. 41. PRESSURE GAUGE. _a._ A pressure gauge of the Lane double tube spring pattern with 5-inch case, must be provided with the pump, and connected near to inboard side of air chamber, as shown in Fig. 12, by a 1/4-inch cock, with lever handle. The dial of this gauge should be scaled to indicate pressures up to 240 lbs. and be marked “WATER.” This kind of gauge is used on locomotives and is the best for withstanding the vibration which causes fire-pump gauges to be often unreliable. Moreover, this double spring form is safer against freezing. 42. HOSE VALVES. _a._ Hose valves must be attached to the pump (and included in its price) as follows:— For the 2 stream or 500-gal. pump, 2 hose valves. For the 3 stream or 750-gal. pump, 3 hose valves. For the 4 stream or 1,000-gal. pump, 4 hose valves. For the 6 stream or 1,500-gal. pump, 6 hose valves.