t of the
piston?

_A._--No. The valve does not move down when the piston moves down, nor does
it move down when the piston moves up; but it moves from its mid position,
to the extremity of its throw, and back again to its mid position, while
the piston makes an upward or downward movement, so that the motion is as
it were at right angles to the motion of the piston; or it is the same
motion that the piston of another engine, the crank of which is set at
right angles with that of the first engine, would acquire.

186. _Q._--Then in a steam vessel the valve of one engine may be worked
from the piston of the other?

_A._--Yes, it may; or it may be worked from its own connecting rod; and in
the case of locomotive engines, this has sometimes been done.

187. _Q._--What is meant by the lead of the valve?

_A._--The amount of opening which the valve presents for the admission of
the steam, when the piston is just beginning its stroke. It is found
expedient that the valve should have opened a little to admit steam on the
reverse side of the piston before the stroke terminates; and the amount of
this opening, which is given by turning the eccentric more or less round
upon the shaft, is what is termed the lead.

188. _Q._--And what is meant by the lap of the valve?

_A._--It is an elongation of the valve face to a certain extent over the
port, whereby the port is closed sooner than would otherwise be the case.
This extension is chiefly effected at that part of the valve where the
steam is admitted, or upon the _steam side_ of the valve, as the technical
phrase is; and the intent of the extension is to close the steam passage
before the end of the stroke, whereby the engine is made to operate to a
certain extent expansively. In some cases, however, there is also a certain
amount of lap given to the escape or eduction side, to prevent the eduction
from being performed too soon when the lead is great; but in all cases
there is far less lap on the eduction than on the steam side, very often
there is none, and sometimes less than none, so that the valve is incapable
of covering both the ports at once.

189. _Q._--What is the usual proportional length of stroke of the valve?

_A._--The common stroke of the valve in rotative engines is twice the
breadth or depth of the port, and the length of the valve face will then be
just the breadth of the port when there is lap on neither the steam nor
eduction side. Whatever lap is given,