KECENT EXPERIMENTS ON FOG-SIGNALS. 311 



and fro, and which might be expected to generate cor- 

 responding 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 forma- 

 tion 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 vibra- 

 tions 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 rare- 

 factions, whereby the formation of sound-pulses is fore- 

 stalled. 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 com- 

 bustion, it may be expected that they will also vary as 

 producers of sound. This theoretic inference is com- 

 pletely verified by experiment. In a series of prelimi- 



