[kixg-shaw] sound through EARTH AND ROCK 77 



required to operate the dynamotor by means of a saltwater trough 

 arranged potentiometer fashion. The oscillator was set up on a 

 slab of solid bed rock with the oscillator plate face downwards, resting 

 on three rubber blocks I" thick mounted on wooden blocks 3" high. 



Contact with the solid bed rock was effected by means of a small 

 screw-jack consisting of a threaded |" rod of steel which could be 

 screwed up into a solid nut so as to give a rigid connection between the 

 centre of the oscillator-plate and the bed rock; the screw-jack was 

 adjusted so that about one-half of the weight of the oscillator (total 

 weight = 1300 lbs.) was taken up by the central screw-jack. It is 

 evident that this connection could only be considered as a makeshift 

 for a few preliminary experiments; more elaborate connections by 

 means of heavy steel tie-rods connected to the copper vibrator of the 

 oscillator and concreted into the bed rock to a depth of 4 ft. or 5 ft. 

 would have necessitated a considerable outlay of time and expense 

 beyond resources available at the time. xA.t a distance of about 100 

 ft. from the oscillator, in a deep part of the quarry, was a pond of water 

 about 4 ft. to 5 ft. deep. It has been hoped to try experiments on 

 transmission by immersing the oscillator in this pool and testing for 

 reception of signals by means of a sensitive submarine microphone 

 immersed in water at the bottom of pits dug down through the earth 

 to the bed rock, following in this matter a suggestion made to one of 

 us (L.V.K.) by Prof. Fessenden. Before experiments had reached this 

 stage, however, the pond referred to had frozen over and this, .together 

 with the difficulty of working in cold weather, made it impossible to 

 continue the experiments further. 



The method of immersion in water will probably prove to be the 

 most satisfactory, both for the emission of sound waves and their 

 reception. In the writers' opinion, the best conditions for sending 

 would be secured by excavating a circular well about 12 ft. deep and of 

 somewhat greater diameter than the oscillator, into the bed rock, the 

 oscillator being suspended free from contact with the sides, about 8 

 ft. below the surface of the level of the water accumulated in the pit. 

 As the compressibility of water is only about is that of solid rock, 

 fairly good "accoustic contact" between the oscillator and the rock- 

 medium should be obtained in this way through the water. 



Receiving Microphones. 



(1) Experiments were first tried by employing the marine micro- 

 phone loaned to one of us (L.V.K.) by the Submarine Signal Co. 

 and employed in tests on acoustic soundings. This microphone was 

 of the usual carbon-granule type mounted in a water-tight metal 

 casing, one face of which consisted of a flexible copper diaphragm 



