eighted lever, or by a spring. As soon as the steam pressure
on the face of the plug exceeds the counteracting force of the weight
or spring, the plug rises, and steam escapes until equilibrium of the
opposing forces is restored.

On stationary engines a lever safety-valve is commonly employed (Fig.
11). The blowing-off point can be varied by shifting the weight along
the arm so as to give it a greater or less leverage. On locomotive and
marine boilers, where shocks and movements have to be reckoned with,
weights are replaced by springs, set to a certain tension, and locked up
so that they cannot be tampered with.

[Illustration: FIG. 11.--A LEVER SAFETY-VALVE. V, valve; S, seating; P,
pin; L, lever; F, fulcrum; W, weight. The figures indicate the positions
at which the weight should be placed for the valve to act when the
pressure rises to that number of pounds per square inch.]

Boilers are tested by filling the boilers quite full and (1) by heating
the water, which expands slightly, but with great pressure; (2) by
forcing in additional water with a powerful pump. In either case a
rupture would not be attended by an explosion, as water is very
inelastic.

The days when an engineer could "sit on the valves"--that is, screw them
down--to obtain greater pressure, are now past, and with them a
considerable proportion of the dangers of high-pressure steam. The
Factory Act of 1895, in force throughout the British Isles, provides
that every boiler for generating steam in a factory or workshop where
the Act applies must have a proper safety-valve, steam-gauge, and
water-gauge; and that boilers and fittings must be examined by a
competent person at least once in every fourteen months. Neglect of
these provisions renders the owner of a boiler liable to heavy penalties
if an explosion occurs.

One of the most disastrous explosions on record took place at the Redcar
Iron Works, Yorkshire, in June 1895. In this case, twelve out of fifteen
boilers ranged side by side burst, through one proving too weak for its
work. The flying fragments of this boiler, striking the sides of other
boilers, exploded them, and so the damage was transmitted down the line.
Twenty men were killed and injured; while masses of metal, weighing
several tons each, were hurled 250 yards, and caused widespread damage.

The following is taken from a journal, dated December 22, 1895:
"_Providence_ (_Rhode Island_).--A recent prophecy that a boiler would
ex