The spaces F' between the sides of the combustion-chambers also vary
from 5 inches to 7 inches. The width K between the back of the
combustion-chamber and the end of the boiler at the bottom in single-
ended boilers, or between the backs of the combustion-chambers at the
bottom in double-ended boilers, varies from 5 inches to 7 inches. The
width K' at the top may be the same as at the bottom, but it is usually
wider, and is sometimes as much as 12 inches.
Tubes. — The tubes used in boilers of merchant steamers are made of
wrought-iron or steel, and are usually either 3^ inches or 3 J inches in
external diameter. They are, however, sometimes as small as 2|- inches,
and occasionally as large as 4 inches in diameter. In the navy the tubes
are generally made of brass, and are then from 2^ inches to 3 inches in
diameter.
To facilitate the inserting or withdrawing of the tubes the front or
smoke-box ends are swelled —^ inch larger in diameter.
The thickness of the iron tubes, of 3 inches to 3 J inches diameter, is
usually No. 9 or No. 8 Birmingham wire gauge for steam-pressures of
80 lbs. per square inch and upwards.
The length of the tubes between the tube plates varies in practice
from 5 ft. 9 in. to 7 ft. 6 in. The average length is about 6 ft. 9 in.
The tubes are arranged in horizontal and vertical rows, as shown in
Fig. 661. The pitch is generally If times the external diameter of the
tubes (p= lf<^0*

The clear space H between the nests of tubes is usually about 11
inches. *0f a considerable number of examples which we tabulated we




/otind the j
value 12J inches.
