ion and violently explosive
energy are demonstrated, seemed to single it out as a substance
eminently calculated to fulfil the conditions necessary to the
production of an intense wave of sound. What those conditions are we
shall now more particularly enquire, calling to our aid a brief but
very remarkable paper, published by Professor Stokes in the
'Philosophical Magazine' for 1868.

The explosive force of gunpowder is known to depend on the sudden
conversion of a solid body into an intensely heated gas. Now the work
which the artillerist requires the expanding gas to perform is the
displacement of the projectile, besides which it has to displace the
air in front of the projectile, which is backed by the whole pressure
of the atmosphere. Such, however, is not the work that we want our
gunpowder to perform. We wish to transmute its energy not into the
mere mechanical translation of either shot or air, but into vibratory
motion. We want _pulses_ to be formed which shall propagate themselves
to vast distances through the atmosphere, and this requires a certain
choice and management of the explosive material.

A sound-wave consists essentially of two parts--a condensation and a
rarefaction. Now air is a very mobile fluid, and if the shock
imparted to it lack due promptness, the wave is not produced. Consider
the case of a common clock pendulum, which oscillates to and fro, and
which might be expected to generate corresponding pulses in the air.
When, for example, the bob moves to the right, the air to the right of
it might be supposed to be condensed, while a partial vacuum might be
supposed to follow the bob. As a matter of fact, we have nothing of
the kind. The air particles in front of the bob retreat so rapidly,
and those behind it close so rapidly in, that no sound-pulse is
formed. The mobility of hydrogen, moreover, being far greater than
that of air, a prompter action is essential to the formation of
sonorous waves in hydrogen than in air. It is to this rapid power of
readjustment, this refusal, so to speak, to allow its atoms to be
crowded together or to be drawn apart, that Professor Stokes, with
admirable penetration, refers the damping power, first described by
Sir John Leslie, of hydrogen upon sound.

A tuning-fork which executes 256 complete vibrations in a second, if
struck gently on a pad and held in free air, emits a scarcely audible
note. It behaves to some extent like the pendulum bob just r