eferred
to. This feebleness is due to the prompt 'reciprocating flow' of the
air between the incipient condensations and rarefactions, whereby the
formation of sound-pulses is forestalled. Stokes, however, has taught
us that this flow may be intercepted by placing the edge of a card in
close proximity to one of the corners of the fork. An immediate
augmentation of the sound of the fork is the consequence.

The more rapid the shock imparted to the air, the greater is the
fractional part of the energy of the shock converted into wave motion.
And as different kinds of gunpowder vary considerably in their
rapidity of combustion, it may be expected that they will also vary as
producers of sound. This theoretic inference is completely verified
by experiment. In a series of preliminary trials conducted at
Woolwich on the 4th of June, 1875, the sound-producing powers of four
different kinds of powder were determined. In the order of the size
of their grains they bear the names respectively of Fine-grain
(F.G.), Large-grain (L.G.), Rifle Large-grain (R.L.G.), and
Pebble-powder (P.) (See annexed figures.) The charge in each case
amounted to 4.5 lbs. four 24-lb. howitzers being employed to fire the
respective charges.

FIG. 9.

There were eleven observers, all of whom, without a single
dissentient, pronounced the sound of the fine-grain powder loudest of
all. In the opinion of seven of the eleven the large-grain powder
came next; seven also of the eleven placed the rifle large-grain third
on the list; while they were again unanimous in pronouncing the
pebble-powder the worst sound-producer. These differences are
entirely due to differences in the rapidity of combustion. All who
have witnessed the performance of the 80-ton gun must have been
surprised at the mildness of its thunder. To avoid the strain
resulting from quick combustion, the powder employed is composed of
lumps far larger than those of the pebble-powder above referred to. In
the long tube of the gun these lumps of solid matter gradually resolve
themselves into gas, which on issuing from muzzle imparts a kind of
push to the air, instead of the sharp shock necessary to form the
condensation of an intensely sonorous wave.

These are some of the physical reasons why guncotton might be regarded
as a promising fog-signal. Firing it as we have been taught to do by
Mr. Abel, its explosion is more rapid than that of gunpowder. In its
case the air particles,