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?

_A._--Metallic packing has in some instances been employed in air pump
buckets, but its success has not been such as to lead to its further
adoption. The packing commonly employed is hemp. A deep solid block of
metal, however, without any packing, is often employed with a satisfactory
result; but this block should have circular grooves cut round its edge to
hold water. Where ordinary packing is employed, the bucket should always be
made with a junk ring, whereby the packing may be easily screwed down at
any time with facility. In slow moving engines the bucket valve is
generally of the spindle or pot-lid kind, but butterfly valves are
sometimes used. The foot and delivery valves are for the most part of the
flap or hanging kind. These valves all make a considerable noise in
working, and are objectionable in many ways. Valves on Belidor's
construction, which is in effect that of a throttle valve hung off the
centre, were some years ago proposed for the delivery and foot valves; and
it appears probable that their operation would be more satisfactory than
that of the valves usually employed.

456. _Q._--Where is the delivery valve usually situated?

_A._--Some delivery valve seats are bolted into the mouth of the air pump,
whereby access to the pump bucket is rendered difficult: but more commonly
the delivery valve is a fl