RECENT EXPERIMENTS ON FOG-SIGNALS. 277 



A sound-pulse consists essentially of two parts a condensation and 

 a rarefaction. Now, air is a very mobile fluid, and, if the shock impart- 

 ed to it lack due promptness, the pulse is not produced. Consider the 

 case of a common clock-pendulum, which oscillates to and fro, and 

 which therefore 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 vac- 

 uum might be supposed to follow the bob. As a matter of fact, we 

 have nothing of this 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. 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. 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 proxim- 

 ity 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 pow- 

 der were determined. In the order of size of grain they bear the names 

 respectively of Fine-grain (F. G.), Large-grain (L. G.), Rifle Large-grain 

 (R. L. G.), and Pebble-powder (P.), (Fig. 2). The charge in each case 





WW* 



F. G. L. G. R. L. G. 



Fig. 2. 



amounted to 4^ pounds. ; four 24-pound howitzers being employed to 

 fire the respective charges. 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 pronounc- 

 ing the pebble-powder the worst sound-producer. These differences 



