_Answer._--The safety-valve, but it is not always a safety-valve, when
it is weighted to twice the amount the boiler is certified to be worked
at safely. As an instance: Amongst the many engines employed at the
Midland Extension Works, St. Pancras, was a light steam crane for
hoisting earth from the deep excavations, there were in use small wooden
skips, and the pressure of steam was forty-five lb.; but after a time
there arrived large iron skips that the crane could not lift, even when
empty; there were about twenty men depending on the crane for their work
and the navvy-ganger was anxious for "something to be done," and the
crane man hinted about weighting the safety-valve, and no sooner said
than almost done; the safety spring balance was screwed down, and a
railway chair suspended from it by strong copper wire, and the steam
allowed to rise until it reached ninety lb. on the inch, and the big
iron skips were hoisted with their load of heavy ballast as easily as
the wooden ones had been. The boiler _happened_ to stand it.