_Answer._--In horizontal engines there are generally two gun-metal
screw-plugs on the top of the cylinder, one over each end and in front
of the piston; when a diagram is to be taken, these plugs are taken out
and other screws put in their places, to which a copper pipe is
attached; the screw plugs are 1 inch in diameter, also the copper pipes;
and exactly mid-way on the copper pipe is a small cylinder which moves
on a pivot, by means of a string with a turn round it. One end of the
string is fixed by a clip on the connecting rod, the other end anywhere
to keep the string tight, so that by the movement of the steam entering
the cylinder at either end, and the connecting rod working backwards and
forwards, the small cylinder is made to turn frontways and backways; and
within the small cylinder is another cylinder very much smaller; it has
a tiny piston within it, and as the steam presses on the little piston
at every stroke of the engine, a pencil from the outer cylinder is fixed
in a slot and marks the movements of the little piston on a roll of
prepared paper, slid over the inner cylinder for that purpose, the
pencil being kept up to the paper by means of a small steel spring. This
diagram on the paper cylinder, not only is used for determining the
power of the engine, but for detecting any irregularity in the
slide-valve movements. Every hour during the trial the finished diagram
is torn off the roll and a fresh one started, and when time is up the
engine is stopped and the diagrams compared. Then commence the
calculations, which are gone through somewhat in this manner: the common
multiplier is found by multiplying the area of the piston in inches by
the speed of the piston in feet per minute and the product divided by
32,000 (Watt's horse-power), then the effective mean pressure found on
the diagram is multiplied by the common multiplier, and the quotient
will be the _indicated_ horse-power of the engine.
46. _Question._--How is the consumption of coals apportioned to the
horse-power of the engine at the finish of the trial?
_Answer._--The consumption of coals in pounds is divided by the product
of the indicated horse-power and the time in hours. The quotient is the
quantity consumed per horse-power per hour.
47. _Question._--Would the quality of the coals used in the trial be of
the same quality as will be used in the ordinary working of the boiler
after the trial is ended?
_Answer._--No; the coals which are used in the trial are generally the
best Welsh, not shovelled up indiscriminately, but carefully
hand-picked, weighed and wheeled into the stoke-hold; the engine during
the trial is lavishly supplied with oils and tallow, with great
regularity. After the trial, and the horse-power is indicated, the
boiler resumes her ordinary work; the stoker is ever after expected to
create sufficient steam with very inferior coals to develop the same
amount of power in the engine as was done in the trial. I think that is
very unfair to the stoker.
* * * * *
"Let the finish give you pleasure" was the last headline in my last
school copybook in the long, long ago; and it has given me as much
pleasure to begin this catechism as to finish it; it has given me
pleasure to offer to brother stokers my very long experience in stoking,
and kindred vocations, such as hydraulics, steam-pipe joint making,
water-pipe joint making, engine driving, etc., in the hope that in the
perusal of this catechism they may find something to their advantage.
And with my best wishes for their future success, remain their true
friend.