Although these pumps are not expected to be designed for a regular working water-pressure of 240 or 320 lbs., it is expected that bolts, shells, rods, etc., will be figured to stand this comparatively quiet, temporary, high pressure, exclusive of further allowance for initial strain due setting up of bolts, with a factor of safety of at least four. This high test pressure is analogous to the custom of proving all common cast-iron water pipes to 300 lbs. and all common lap-welded steam pipes to 500 lbs. per square inch, and common water-works gate valves to 400 lbs., even though these are to be regularly used at much less pressure. We are assured that castings no heavier than at present used by the best makers will stand this test, _if properly shaped and liberally bolted_. _g._ For requirements for stuffing boxes, see Art. 22. 28. WATER PLUNGERS AND BUSHINGS. _a._ The “inside plunger and bushing” is preferred for all situations where the water is free from grit or mud. _b._ Water-plungers must be of solid brass or bronze, and the bushing in which they slide must also be of brass or bronze. The composition of the plunger and its bushing should be of very hard, though dissimilar alloys, to ensure good wearing qualities. For material and size of piston rods and lock for nuts, see Art. 20. With poor alignment or bad workmanship or lack of skill in mixing the alloys, brass plungers are liable to score and give trouble; but with proper selection of alloys and true cylinders accurately aligned, they can be made to run all right wherever iron ones can. It is quite a fine point to get these wearing surfaces just right; and _this is wherein the experience, skill and shop practice of one maker is likely to be much superior to that of another working under the same specification_. _c._ The length of machined cylindrical bearing within the partition must be not less then 2 inches. The plunger bushing must have a faced seat transverse to its axis against partition, forming a water-tight ground joint not less than one-half inch wide. Any rubber gasket or other compressible packing for making this joint water-tight is not acceptable. _d._ The construction of bushing and hole in partition must be such that a cylindrical shell for use with a packed piston can be interchangeably inserted in its place and secured by the same bolts. This can readily be arranged and enables a packed piston to be inserted in place of a plunger subsequent to the installation of the pump with a minimum of expense, should this become desirable from change of conditions at any future time. _e._ Small transverse grooves cut within the sliding surface of the plunger bushing, with a view to lessen the leakage, are not acceptable. Although a slight advantage in this respect for clean water, they are a disadvantage on the whole, as dirt catches in them in the ordinary situation and cuts the plungers.