is very little and very narrow, and although this is
attempted to be overcome by exceedingly wide ports, sixteen inches in
width in many cases in locomotive work, this great width adds largely
to the unbalanced area of the valve. The exhausting functions of the
valve are materially changed at the short cut off, and when much lap
is added to overcome this defect, there usually takes place a choking
of the exhaust port. You might inquire, why not make the port wider,
but this would increase the minimum amount of load on the valve, and
this must not be overlooked. Then the cut off is a fixed one, and we
can govern only by throttling the pressure we have raised in the
boiler or by using a cut off governor and the consequent wastes of an
enormous clearance space. You will observe, therefore, that the plain
slide valve engine gives the most general satisfaction at about
two-thirds cut off and a very low economic result. The best of such
engines will require forty-five to fifty pounds of steam per horse
power per hour, and to generate this, assuming an evaporation of nine
pounds of water to a pound of coal, would require between five and six
pounds of coal per horse power per hour. And the only feature that the
valve has specially to commend it is its extreme simplicity and the
very little mechanism required to operate it.

Yet this is of considerable importance, and in consideration of some
special features at its latest cut off, the attempt has been many
times made to take advantage of these features. For instance, at 90 deg.
advance, the valve opens very rapidly indeed and fully satisfies our
requirements of a perfect valve. This is one good point, and in this
position also the exhaust and compression can be regulated very
closely and as desired without much lap, and as the opening of the
exhaust port comes with the eccentric at its most rapid movement the
release is very quick and as we would have it. This is only possible
at the most uneconomic position of the valve as regards cut off.

The aim of many engineers has been to take advantage of these matters
by using the valve with 90 deg. angular advance of eccentric ahead of
crank, for the admission, release, and compression of the steam, and
provide another means of cutting off, besides the one already referred
to, viz., cutting off the supply of steam to the chest, and overcome
the objection in this one of large clearance spaces. This is done by
means of riding cut off