practice.  The eccentric hoop in marine and land engines is generally of brass; it is expedient to cast an oil cup on the eccentric hoop, and, where practicable, a pan should be placed beneath the eccentric for the reception of the oil droppings.  The notch of the eccentric rod for the reception of the pin of the valve shaft is usually steeled, to prevent inconvenient wear; for when the sides of the notch wear, the valve movement is not only disturbed, but it is very difficult to throw the eccentric rod out of gear.  It is found to be preferable, however, to fit this notch with a brass bush, for the wear is then less rapid, and it is an easy thing to replace this bush with another when it becomes worn.  The eccentric catches of the kind usually employed in marine engines, sometimes break off at the first bolt hole, and it is preferable to have a bolt in advance of the catch face, or to have a hoop encircling the shaft with the catches welded on it, the hoop itself being fixed by bolts or a key.  This hoop may either be put on before the cranks in one piece or afterwards in two pieces.

453. Q.—­Are such eccentrics used in direct acting screw engines?

A.—­No; direct acting screw engines are usually fitted with the link motion and two fixed eccentrics.

AIR PUMP AND CONDENSER.

454. Q.—­What are the details of the air pump?

A.—­The air pump bucket and valves are all of brass in modern marine engines, and the chamber of the pump is lined with copper, or made wholly of brass, whereby a single boring suffices.  When a copper lining is used, the pump is first bored out, and a bent sheet of copper is introduced, which is made accurately to fill the place, by hammering the copper on the inside.  Air pump rods of Muntz’s metal or copper are much used.  Iron rods covered with brass are generally wasted away where the bottom cone fits into the bucket eye, and if the casing be at all porous, the water will insinuate itself between the casing and the rod and eat away the iron.  If iron rods covered with brass be used, the brass casing should come some distance into the bucket eye; the cutter should be of brass, and a brass washer should cover the under side of the eye, so as to defend the end of the rod from the salt water.  Rods of Muntz’s metal are probably on the whole to be preferred.  It is a good practice to put a nut on the top of the rod, to secure it more firmly in the cross head eye, where that plan can be conveniently adopted.  The part of the rod which fits into the cross head eye should have more taper when made of copper or brass, than when made of iron; as, if the taper be small, the rod may get staved into the eye, whereby its detachment will be difficult.

455. Q.—­What species of packing is used in air pumps?