just lift the safety-valve up to that which is sufficient, if
exerted during an extremely small space of time, to tear open the
boiler itself.

26. This reasoning ought, however, to be admitted with
caution; and perhaps some inducement to examine it carefully may
be presented by tracing it to extreme cases. It would seem, but
this is not a necessary consequence, that a gun might be made so
long, that it would burst although no obstacle filled up its
muzzle. It should also follow that if, after the gun is charged,
the air were extracted from the barrel, though the muzzle be then
left closed, the gun ought not to burst. It would also seem to
follow from the principle of the explanation, that a body might
be projected in air, or other elastic resisting medium, with such
force that, after advancing a very short space it should return
in the same direction in which it was projected.

NOTES:

1. See Poisson's remarks, Ecole Polytec. Cahier, xxi, p. 191.



Chapter 3

Regulating Power

27. Uniformity and steadiness in the rate at which machinery
works, are essential both for its effect and its duration. The
first illustration which presents itself is that beautiful