ylinders together 790, total, 1,188; and
temperature of hot well 100 deg. Fahr. Then with feed heating the same
engine might work as follows: The feed might be heated to 220 deg. Fahr.,
and the percentage of steam from the first receiver required to heat it
would be 12.2 per cent.; the indicated horse power in the high pressure
cylinder would be as before 398, and in the intermediate and low
pressure cylinders it would be 12.2 per cent, less than before, or 694,
and the total would be 1,092, or 92 per cent. of the power developed
without feed heating. Meanwhile the heat to be added to each pound of
the feed water at 220 deg. Fahr. for converting it into steam would be 1,005
units against 1,125 units with feed at 100 deg. Fahr., equivalent to an
expenditure of only 89.4 per cent. of the heat required without feed
heating. Hence the expenditure of heat in relation to power would be
89.4 + 92.0 = 97.2 per cent., equivalent to a heat economy of 2.8 per
cent. If the steam for heating can be taken from the low pressure
receiver, the economy is about doubled. Other feed heaters, more or less
upon the same principle, have been introduced. Also others which heat
the feed in a series of pipes within the boiler, so that it is
introduced into the water in the boiler practically at boiling
temperature; this is economical, however, only in the sense that wear
and tear of the boiler is saved; in principle the plan does not involve
economy of fuel.

_Auxiliary Supply of Fresh Water_.--Intimately associated with the feed
is the means adopted for making up the losses of fresh water due to
leakage of steam from safety valves, glands, joints, etc., and of water
discharged from the air pumps. A few years ago this loss was regularly
made up from the sea, with the result that the water in the boilers was
gradually increased in density; whence followed deposit on the internal
surfaces, and consequent loss of efficiency, and danger of accident
through overheating the plates. With the higher pressures now adopted,
the danger arising from overheating is much more serious, and the
necessity is absolute of maintaining the heating surfaces free from
deposit. This can be done only by filling the boiler with fresh water in
the first instance, and maintaining it in that condition. To do this two
methods are adopted, either separately or in conjunction. Either a
reserve supply of fresh water is carried in tanks or the supplementary
feed is distilled fro