274 FRAGMENTS OF SCIENCE 



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 pen- 

 etration, 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- 



