LOCOMOTIVE ENGINE. 
469 
fective pressure on the piston must have been only 10 lbs. on the square inch, or 
but one fifth of that of the steam in the boiler. This shews the great loss of 
power in working the engine so quickly; the loss at 14 miles an hour having been 
very little. This loss of power is lessened by making the wheels larger, the velo 
city of the piston being by that means diminished; and they are consequently made 
as large as is practicable. 
The power of a locomotive engine is limited only by the evaporating power of the 
boiler, or the number of cylinders’ full of steam that can be supplied in a given time, by 
which the velocity of the piston is determined; while in other steam engines the size 
of the cylinders is the limit to the power, as a sufficient quantity of steam to supply 
them can be readily obtained by increasing the size or number of the boilers, which 
cannot be done in a locomotive. This engine is capable of evaporating 77 cubic feet 
of water per hour, or eight gallons in a minute ; and the large amount of this power 
causes its great superiority to the old locomotives, which could evaporate only about 
16 cubic feet per hour. 
The consumption of fuel per mile for every ton of the gross load is about a quarter 
of a lb., and that of the water is rather less than a quarter of a gallon; the con 
sumption increasing to nearly one half, according as the engine is less fully loaded, 
being proportionally greater with a light load; the consumption of water, when 
working with a full load, is a cubic foot per hour for each horse power, which is 
also the usual proportion in stationary engines although they condense the steam. 
About 8 lbs. of fuel is required to evaporate a cubic foot of water, being nearly 
the same as in stationary engines; but in the old locomotives as much as 18 lbs. 
was required, in consequence of their having so small a heating surface, which was 
only about two and a half square feet for each foot of water evaporated per hour; 
the proportion in the present one being five and a half square feet, and in stationary 
engines as much as eight. 
The great perfection of the present locomotives, and their superiority to the old 
ones, is caused not so much by the application of new inventions to them, as by the 
combination of many former ones, and the uniting together several plans which, sepa 
rately, would be but of small value. Their great power and velocity, for example, 
could not have been obtained without the rapid means of generating steam afforded 
by the use of the tubes; and the tubes would have been useless, without the 
powerful draught produced by the blast, which increases in intensity with the 
velocity, and with the necessity for its increased action.