This valve seat should rest in an inverted position, and can be so fitted up as to be readily removed. The valve stems can be of the removable type screwing into the seat, but must be made long enough to receive a check nut on the opposite side of seat. This will effectually lock the stem in place. _h._ Care must be taken to arrange the water passages through and about these priming checks, so as to avoid all air pockets and so as to reduce to a minimum the possibility of the valves becoming choked up by refuse. _i._ The valve seats, stems and all parts must be of composition and of strong rugged design, so fitted up that there is the least chance for the rubber valves to stick, and with all parts securely put together the valves must be readily accessible. _j._ The valve springs must have only sufficient strength to keep the valves on their seats, so that they will freely open even with the low head of priming water often existing. [Illustration: FIG. 17.] _k._ There must be provided, and attached to the top of each plunger chamber, a brass check valve and air cock with umbrella top, as shown in Fig. 17. This cock and valve must have a clear passageway through them—the full equivalent of a 1/2-inch bore. The check valve is to permit the outflow of air, but to prevent the influx when the plunger is sucking. Cocks with lever handles are used, as these show clearly whether they are open or shut. _l._ There must also be provided a 2-inch brass gate valve for the general control of the water to the four-check valves. The hand wheel of this valve must be marked as per Fig. 15. The lettering must be very clear, open, and distinct, not liable to be obscured by grease, and of a permanent character. It is essential for a properly working pump that the main 2-inch priming-valve should be closed as soon as the pump is primed. Otherwise, water will be drawn from the priming tank, lessening the lifting power of the pump through the main suction, and if this is continued the priming tank will often be exhausted and air drawn into the pump, interfering with its proper action. It is for this reason that the marking on the priming valve is required. For all average situations, either method of priming permits of getting the pump under way in a very few minutes, but, for cases where the suction pipe is over 300 or 400 feet in length, or sometimes where the lift is over 18 feet, or where there is a combination of long length and lift within these limits, so much time is consumed in exhausting the air from the suction pipe that it becomes desirable to supplement this method.