should be such that a reasonably perfect combustion of
the fuel should be possible, and that the heating surfaces should be
capable of transmitting a large proportion of the heat produced to the
water so as to obtain a high evaporative efficiency. Further, the design
generally should be compact, not too heavy or costly, and such that the
cleaning necessary to maintain the evaporative efficiency can be easily
effected. It should also be such that the cost of upkeep will be small,
and that only an average amount of skill and attention will be required
under working conditions. It is for providing these qualities in
different degrees according to the special requirements of various
circumstances that the very different designs of the various types of
boilers have been evolved.

_Classes of Boilers._--Boilers generally may be divided into two
distinct classes, one comprising those which are generally called "tank"
boilers, containing relatively large quantities of water, and the other
those which are generally called "water-tube" boilers, in which the
water is mainly contained in numerous comparatively small tubes. There
are, however, some types of boiler which combine to some extent the
properties of both these classes. Each class has its representatives
amongst both land and marine boilers. In "tank" boilers the outer shell
is wholly or partially cylindrical, this form being one in which the
necessary strength can be obtained without the use of a large number of
stays. The boilers are generally internally fired, the furnace plates
being surrounded with water and forming the most efficient portion of
the heating surfaces. On leaving the furnace the products of combustion
are led into a chamber and thence through flues or through numerous
small tubes which serve to transmit some of the heat of combustion to
the water contained in the boiler. In "water-tube" boilers the fire is
usually placed under a collection of tubes containing water and forming
the major portion of the heating surface of the boiler. Both the fire
and the tubes are enclosed in an outer casing of brickwork or other
fire-resisting substance. In some forms of water-tube boiler the fire
is entirely surrounded by water-tubes and the casing is in no part
exposed to the direct action of the fire. In "tank" boilers generally no
difficulty is experienced in keeping all the heating surfaces in close
contact with water, but in "water-tube" boilers special provision