larger plates, welded horizontal seams, drilled rivet holes, and more
perfect workmanship throughout. A modification of double-ended boiler is
that introduced by Mr. Alfred Holt. It has many decided advantages,
but is costly to make. The formation of the two ends into separate
fire-boxes leaves the bottom of the boiler free to adapt itself to the
variations of temperature to which it is exposed. The separation of the
furnaces from the combustion chamber, excepting through the opening
afforded by a connecting tube, is an advantage in the same direction,
and avoids almost entirely the racking strains due to irregular furnace
action. The weight of water carried is less, and that of the boiler
may also be made less; while the elliptical form of the two ends gives
greater steam space.

A type of boiler largely used in her Majesty's Navy, somewhat like a
locomotive boiler, is highly efficient in regard to weight and power
developed. Many examples have yielded one indicated horse-power in the
cylinders for every three square feet of heating surface, under natural
draught and with a very moderate height of funnel; and this with a
consumption of fuel not exceeding 21/2 lb. per indicated horse-power per
hour under a working pressure of 60 lb. With the aid of a steam jet in
the funnel, the heating surface per indicated horse-power has fallen
below 21/2 square feet. The large water surface afforded for escape
of steam secures almost entire freedom from priming, without the
incumbrance of steam domes; and the large combustion chamber allows of
the thorough combustion of the gases before their passage through the
tubes. The locomotive type of boiler has lately occupied the writer's
attention, with a view to its more definite introduction into marine
work. The difficulties, however, which lie in the way of applying it to
steamers going long voyages are very great. The principal difficulty
lies in the necessity of burning a large quantity of fuel in a very