The second marine engine to which I wish to call your attention is one
that has been made with a view to great economy. The principles
followed in its construction are among those suggested by the
President (Sir W.G. Armstrong) in his address. He (you will remember)
pointed out that the direction in which economy in the steam engine
was to be looked for was that of increasing the initial pressure;
although at the same time he said that there were drawbacks in the
shape of greater loss, by radiation, and by the higher temperature at
which the products of combustion will escape. We must admit the fact
of the latter source of loss, when using very high steam, it being
inevitable that temperature of the products of combustion escaping
from a boiler under these conditions must be higher than those which
need be allowed to escape when lower steam is employed; although I
regret to say that in practice in marine boilers working at
comparatively low pressures the products are ordinarily suffered to
pass into the funnel at above the temperature of melted lead. But with
respect to the loss by radiation in the particular engine I am about
to mention--that of Perkins--there is not as much loss as that which
prevails in the ordinary marine boilers, because the Perkins boiler is
completely inclosed, with the result that while there is within the
case a boiler containing steam of 400 lb. on the square inch, and the
fire to generate that steam, the hand may be applied to the casting
itself, which contains the whole of the boiler, without receiving any
unpleasant sensation of warmth. By Mr. Perkins's arrangement, using
steam of 400 lb. in the boiler, it was found, as the result of very
severe trials, conducted by Mr. Rich, of Messrs. Easton and Anderson's
firm, and myself--trials which lasted for twelve hours--that the total
consumption of fuel, including that for getting up steam from cold
water, was just under 1.8, actually 1.79 lb. per gross indicated
horse-power per hour. That gross indicated horse-power was obtained in
a manner which it is desirable should always be employed in steamboat
trials. It was not got by using as a divisor the horse-power of the
most favorable diagram obtained during the day; but it was got from
diagrams taken during the regular work; then, every half-hour, when
the pressure began to die down, from coal being no longer put upon the
fire, diagrams taken every quarter of an hour, and then toward the
last, every five minutes; and the total number of foot pounds were
calculated from these diagrams, and were used to obtain the gross
indicated horse-power.
Further, so far as could be ascertained by the process of commencing a
trial with a known fire, and closing that trial at the end of six
hours, with the fire as nearly as possible in the same condition, the
consumption was 1.66 lb. of coal per gross indicated horse-power per
hour. So that, without taking into account the coal consumed in
raising steam from cold water, the engine worked for 1-2/3 lb. of coal
per horse per hour. I think it well to give these details, because
undoubtedly it is an extremely economical result.