he water level, and abate the risk of water being carried
over into the engine with the steam. With all these conditions the boiler
must be as light and compact as possible, and must be so contrived as to be
capable of being cleaned and repaired with facility.

253. _Q._--Supposing, then, that you had to proportion a boiler, which
should be capable of supplying steam sufficient to propel a steam vessel or
railway train at a given speed, or to perform any other given work, how
would you proceed?

_A._--I would first ascertain the resistance which had to be overcome, and
the velocity with which it was necessary to overcome it. I should then be
in a position to know what pressure and volume of steam were required to
overcome the resistance at the prescribed rate of motion; and, finally, I
should allow a sufficient heating and fire grate surface in the boiler
according to the kind of boiler it was, to furnish the requisite quantity
of steam, or, in other words, to evaporate the requisite quantity of water.

254. _Q._--will you state the amount of heating surface and grate surface
necessary to evaporate a given quantity of water?

_A._--The number of square feet of heating or flue surface, required to
evaporate a cubic foot of water per hour, is about 70 square feet in
Cornish boilers, 8 to 11 square feet in land and marine boilers, and 5 or 6
square feet in locomotive boilers. The number of square feet of heating
surface per square foot of fire grate, is from 13 to 15 square feet in
wagon boilers; about 40 square feet in Cornish boilers; and from 50 to 90
square feet in locomotive boilers. About 80 square feet in locomotives is a
very good proportion.

255. _Q._--What is the heating surface of boilers per horse power?

_A._--About 9 square feet of flue and furnace surface per horse power is
the usual proportion in wagon boilers, reckoning the total surface as
effective surface, if the boilers be of a considerable size; but in the
case of small boilers the proportion is larger. The total heating surface
of a two horse power wagon boiler is, according to Boulton and Watt's
proportions, 30 square feet, or 15 ft. per horse power; whereas, in the
case of a 45 horse power boiler the total heating surface is 438 square
feet, or 9.6 ft. per horse power. In marine boilers nearly the same
proportions obtain. The original boilers of the Great Western steamer, by
Messrs. Maudslay, were proportioned with about 10 square feet of flue