[king- SHAW] SOUND THROUGH EARTH AND ROCK 79 



to the operation of the oscillator could be heard in the telephone at 

 this distance.' It should be mentioned, however, that the carbon- 

 granule microphone, though sensitive, is not entirely satisfactory for 

 the reception of very faint sounds, as when the current is flowing in 

 the circuit and a very sensitive telephone receiver is employed, faint 

 scratching and hissing noises are never entirely absent and would tend 

 to make other faint sounds; these are, of course, very much emphasized 

 when an amplifier is employed in the circuit. 



(3) Experiments on the Use of the Oscillator as a Microphone: — '■ 



In its application to undersea signalling the oscillator is stated 

 to be capable of being employed as an extremely sensitive receiver. 

 This property was borne out by a few experiments carried out on 

 transmission of sound in rock, the oscillator being employed as a 

 receiving microphone. The apparatus was especially sensitive when 

 employed in conjunction with the amplifier described below; in these 

 circumstances the sensitiveness of the oscillator was remarkable and 

 conditions of hearing were very "quiet." A person speaking on a 

 calm day 10 or 20 yards from the oscillator could be heard distinctly 

 in the telephone receiver, the articulation being perfect, (the trans- 

 mitting medium was the air in this case). A number of experiments 

 were tried regarding the sensitivity of the oscillator as a detector 

 of sounds transmitted through earth and rock. With the amplifier 

 and high-resistance telephones it was found possible to hear with 

 ease blows struck with a crowbar on the solid rock as far as 150 ft. 

 and quite distinctly at about the same distance blows struck on the 

 6 ft. layer of (frozen) soil covering the bed rock in the neighbourhood 

 of the quarry. It is probable that, with better acoustic contact to 

 the bed rock and greater ratios of amplification, the range of trans- 

 mission could be very much increased. 



(4) Experiments with special Microphones suitable for the Detection 

 of very faint Vibrations : — 



For the same amount of energy propagated as waves of compres- 

 sion across unit area per second at any point of an elastic medium, the 

 amplitude of movement in rock is very much less than the 

 corresponding amplitude in water owing to the considerably 

 smaller compressibility and greater density of the solid medium. 

 In these circumstances a type of microphone suitable for the 

 detection of feeble under-water sounds is not suitable for the 



'As the noise of footsteps transmitted from distance of twenty yards through the 

 frozen ground to the water-immersed microphone in the pit could be distinctly heard 

 in the telephones, it was concluded that this method of obtaining "acoustic contact" 

 would ultimately prove to be very promising for practical purposes.' 



