_The action of the vacuum chamber is practically the reverse of that of the air chamber._ The object of the vacuum chamber is to facilitate changing continuous into intermittent motion. The moving column of water compresses the air in the vacuum chamber at the ends of the stroke of the piston, and when the piston starts the air expands (thus creating a partial vacuum above the water) and aids the piston in setting the column of water in motion again. Thus the flow of water into the suction chamber of the pump is much more uniform during each stroke of the piston than without the vacuum chamber, and consequently the pump can be run at higher speeds without increasing the loss due to slippage and without “slamming” of the valves. Vacuum chambers should be slightly larger than the suction pipe and of considerable length rather than of large diameter and short. The size of the neck is substantially the same as in the air chamber. [Illustration: FIG. 204.] PIPING A PUMP. Fig. 204 on the opposite page represents the pipe connections, etc., of a pump with the delivery opening on the opposite side. D represents the _foot valve and strainer_ placed on the lower end of the suction, which should be not less than a foot from the bottom of the well; the distance named provides for the gradual filling of the well. C is the _suction pipe_ proper, screwed into _the elbow_, E, which changes its direction into the suction chamber, which contains _the strainer_, A. This strainer can be removed for cleaning by lifting _the bonnet_ secured by stud bolts on top. In connecting large pumps it is customary to attach a vacuum chamber, F, which in the absence of any regular pattern, may be made of a piece of pipe of the same diameter as the suction and screwed into a ^T^, instead of the elbow, E, with a regulation screwed cap on top as shown in the dotted lines. A _priming pipe_ is shown by the letter J, often used to fill the pump on starting. The _discharge pipe_ connection is shown at G with the _air chamber attached_. This figure is introduced for the purpose of showing an approved method of piping a pump. It may be observed that the flange joints in this design are so arranged that they may be disconnected without unscrewing any part of the suction pipe; this feature is almost essential in view of needed repairs. The foregoing description of the parts of a pump relate to the water end solely; there remain the more complex and widely differing parts of the steam-end which constitute the distinguishing characteristics of the pumps built by the different makers. There remain also the particular parts belonging to the large pumping engines, air-pumps, etc. These will be described under their respective chapters with much added and essential matter. Particular details as to the conditions of service under which it is proposed to operate pumps are to be found on the next page. CONDITIONS OF SERVICE REQUIRED OF A PUMP.