es?

_A._--Yes, in marine boilers this is a constant source of danger, which is
only to be met by attention on the part of the engineer. If the water in
the boiler be suffered to become too salt, an incrustation of salt will
take place on the furnaces, which may cause them to become red hot, and
they may then be collapsed even by their own weight aided by a moderate
pressure of steam. The expedients which should be adopted for preventing
such an accumulation of salt from taking place within the boiler as will be
injurious to it, properly fall under the head of the management of steam
boilers, and will be explained in a subsequent chapter.

CHAPTER VI.

PROPORTIONS OF ENGINES.

* * * * *

STEAM PASSAGES.

312. _Q._--What size of orifice is commonly allowed for the escape of the
steam through the safety valve in low pressure engines?

_A._--About 0.8 of a circular inch per horse power, or a circular inch per
1-1/4 horse power. The following rule, however, will give the dimensions
suitable for all kinds of engines, whether high or low pressure:--multiply
the square of the diameter of the cylinder in inches by the speed of the
piston in feet per minute, and divide the product by 375 times the pressure
on the boiler per square inch; the quotient is the proper area of the
safety valve in square inches. This rule of course supposes that the
evaporating surface has been properly proportioned to the engine power.

313. _Q._--Is this rule applicable to locomotives?

_A._--It is applicable to high pressure engines of every kind. The
dimensions of safety valves, however, in practice are very variable, being
in some cases greater, and in some cases less, than what the rule gives,
the consideration being apparently as often what proportions will best
prevent the valve from sticking in its seat, as what proportions will
enable the steam to escape freely. In Bury's locomotives, the safety valve
was generally 2-1/2 inches diameter for all sizes of boiler, and the valve
was kept down by a lever formed in the proportion of 5 to 1, fitted at one
end with a Salter's balance. As the area of the valve was 5 square inches,
the number of pounds shown on the spring balance denoted the number of
pounds pressure on each square inch of the boiler.

314. _Q._--Is there only one safety valve in a locomotive boiler?

_A._--There are always two.

315. _Q._--And are they always pressed down by a spring b