ea of water
level, or an insufficient width between the flues for the free ascent of
the steam and the descent of the superincumbent water, the evil may be
abated by the addition of circulating pipes in some part of the boiler,
which will allow the water to descend freely to the place from whence the
steam rises, the width of the water spaces being virtually increased by
restricting their function to the transmission of a current of steam and
water to the surface. It is desirable to arrange the heating surface in
such a way that the feed water entering the boiler at its lowest point is
heated gradually as it ascends, until toward the superior part of the flues
it is raised gradually into steam; but in all cases there will be currents
in the boiler for which it is proper to provide. The steam pipe proceeding
to the engine should obviously be attached to the highest point of the
steam chest, in boilers of every construction.

297. _Q._--Having now stated the proportions proper to be adopted for
evaporating any given quantity of water in steam boilers, will you proceed
to show how you would proportion a boiler to do a given amount of work? say
a locomotive boiler which will propel a train of 100 tons weight at a speed
of 50 miles an hour.

_A._--According to experiments on the resistance of railway trains at
various rates of speed, made by Mr. Gooch, of the Great Western Railway, it
appears that a train weighing, with locomotive, tender, and carriages,
about 100 tons, experiences, at a speed of 50 miles an hour, a resistance
of about 3,000 lbs., or about 30 lbs. per ton; which resistance includes
the resistance of the engine as well as that of the train. This, therefore,
is the force which must be imparted at the circumference of the driving
wheels, except that small part intercepted by the engine itself, and the
force exerted by the pistons must be greater than that at the circumference
of the driving wheel, in the proportion of their slower motion, or in the
proportion of the circumference of the driving wheel to the length of a
double stroke of the engine. If the diameter of the driving wheel be 5-1/2
feet, its circumference will be 17.278 feet, and if the length of the
stroke be 18 inches, the length of a double stroke will be 3 feet. The
pressure on the pistons must therefore be greater than the traction at the
circumference of the driving wheel, in the proportion of 17.278 to 3, or,
in other words, the mean pressure