marine engines, whether paddle or screw, if moving at a slow rate
of speed, the eccentric is generally loose upon the shaft, for the purpose
of backing, and is furnished with a back balance and catches, so that it
may stand either in the position for going ahead, or in that for going
astern. The body of the eccentric is of cast iron, and it is put on the
shaft in two pieces. The halves are put together with rebated joints to
keep them from separating laterally, and they are prevented from sliding
out by round steel pins, each ground into both halves; square keys would
probably be preferable to round pins in this arrangement, as the pins tend
to wedge the jaws of the eccentric asunder. In some cases the halves of the
eccentric are bolted together by means of flanges, which is, perhaps, the
preferable 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