have now entirely given up the construction of two-crank triple
expansion engines, because of the impossibility of equally dividing the
work between the cranks; for, although the engine when running appeared
to be perfectly balanced, the wear of the brasses of the crank having the
two cylinders was always considerably more than that of the other.
Placing the high pressure cylinder over the low pressure cylinder seemed
to give the most satisfactory results, but even these were far inferior
to those once obtained with the three cranks. We have lately constructed
some very small three-crank engines from which exceedingly good results
were obtained; the cylinders are only 111/2 inches, 17 inches, and 30
inches by 18 inches stroke, which developed 218 indicated horse power
with a consumption of 12.8 lb. of water per indicated horse power, and
this, together with some other observations, leads me to believe that the
best economical results will be obtained by running triple expansion
engines at a much higher number of revolutions than is usual, and with a
rate of expansion not less than 12 for a steam pressure not less than 140
lb. (155 absolute). The largest engines we have made of this type so far
are those of S.S. Martello, which have cylinders 31 inches, 50 inches,
and 82 inches diameter by 57 inches strokes and indicate at sea 2,400
horse power when running at 60 revolutions with steam of 150 lb.
pressure; the consumption of Yorkshire coal is 37 tons per day average
throughout a New York voyage. Had Welsh coal been used in every case, the
results would have been very much better, for, in addition to the
superior evaporative power of Welsh coal, it is slow burning and much
more easily controlled, especially on the comparatively short grates of
these modern boilers, the quick-burning Yorkshire coal causing the safety
valves to frequently blow off when working near the load pressure unless
great care is taken by the firemen.

I trust these few particulars may be of interest to the Institution, and
especially to those members of it who are particularly interested in the
commercial success of our mercantile navy. I have purposely avoided
engineering details and technicalities of any kind, giving only such
information as will tend to give British shipowners faith in that form of
engine which will undoubtedly help them to successfully tide over bad
times, and keep the bulk of the carrying trade of the world in their
hands.