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 flap valve exterior to the pump.  If delivery valve seats be put in the mouth of the air pump at all, the best mode of fixing them appears to be that adopted by Messrs. Maudslay.  The top of the pump barrel is made quite fair across, and upon this flat surface a plate containing the delivery valve is set, there being a small ledge all round to keep it steady.  Between the bottom of the stuffing box of the pump cover and the eye of the valve seat a short pipe extends encircling the pump rod, its lower end checked into the eye of the valve seat, and its upper end widening out to form the bottom of the stuffing box of the pump cover.  Upon the top of this pipe some screws press, which are accessible from the top of the stuffing box gland, and the packing also aids in keeping down the pipe, the function of which is to retain the valve seat in its place.  When the pump bucket has to be examined the valve seat may be slung with the cover, so as to come up with the same purchase.  For the bucket valves of such pumps Messrs. Maudslay employ two or more concentric ring valves with a small lift.  These valves have given a good deal of trouble in some cases, in consequence of the frequent fracture of the bolts which guide and confine the rings; but this is only a fault of detail which is easily remedied, and the principle appears to be superior to that of any of the other metallic air pump valves at present in common use.

[Illustration:  Fig. 41.  TRUNK AIR PUMP.  Scale 3/4 inch to 1 foot.]

457. Q.—­Are not air pump valves now very generally made of india rubber?