nd much slanted on the other, whereby a greater strength
is secured, without creating any disposition to split the nut. In side
lever engines it is a judicious practice to add a nut to the top of the
piston rod, in addition to the cutter for securing the piston rod to the
cross head. In a good example of an engine thus provided, the piston rod is
7 in. in diameter, and the screw 5 in.; the part of the rod which fits into
the cross head eye is 1 ft. 5-1/2 in. long, and tapers from 6-1/2 in. to
6-13/16 in. diameter. This proportion of taper is a good one; if the taper
be less, or if a portion of the piston rod within the cross head eye be
left untapered, as is sometimes the case, it is very difficult to detach
the parts from one another.

449. _Q._--Which is the most beneficial construction of slide valve?

_A._--The best construction of slide valve appears to be that adopted by
Messrs. Penn for their larger engines, and which consists of a three ported
valve, to the back of which a ring is applied of an area equal to that of
exhaustion port, and which, by bearing steam tight against the back of the
casing, so that a vacuum may be maintained within the ring, puts the valve
in equilibrium, so that it may be moved with an inconsiderable exercise of
force. The back of the valve casing is put on like a door, and its internal
surface is made very true by scraping. There is a hole through the valve so
as to conduct away any steam which may enter within the ring by leakage,
and the ring is kept tight against the back of the casing by means of a
ring situated beneath the bearing ring, provided with four lugs, through
which bolts pass tapped into bosses on the back of the valve; and, by
unscrewing these bolts,--which may be done by means of a box key which
passes through holes in the casing closed with screwed plugs,--the lower
ring is raised upwards, carrying the bearing ring before it. The rings must
obviously be fitted over a boss upon the back of the valve; and between the
rings, which are of brass, a gasket ring is interposed to compensate by its
compressibility for any irregularity of pressure, and each of the bolts is
provided with a ratchet collar to prevent it from turning back, so that the
engineer, in tightening these bolts, will have no difficulty in tightening
them equally, if he counts the number of clicks made by the ratchet. Where
this species of valve is used, it is indispensable that large escape valves
be applied