198 FRAGMENTS OF SCIENCE. 
A sound-wave consists essentially of two parts a con- 
densation, 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 pene- 
tration, 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 referred 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 combus- 
tion, it may be expected that they will also vary as pro- 
ducers 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- 
