[king-shaw] sound through EARTH AND ROCK 83 



of the induction-coil employed, may be increased advantageously 

 until the resistance of the secondary is of this order of magnitude. 



In the experiments already described, an induction coil, capable 

 of giving a six-inch spark with the usual hammer make-and-break 

 arrangement, was employed. The magnification obtained with a 

 high-grade 3,000 ohms telephone receiver was quite large, and this 

 was arrangement employed throughout these experiments. 



By the use of several "amplifiers" of this type employed in 

 cascade much higher magnification may be obtained: in the case of 

 contact microphones a limit is set by slight spurious noises which 

 are never entirely absent and are magnified in proportion to external 

 sounds. 



The study of current amplifiers constitutes an important field of 

 research in connection with problems of telephony and telegraphy. 

 In this connection it is interesting to note that by the use of three 

 "Audion" amplifiers in cascade De Forest claims to have obtained a 

 magnification of 125 times. More recently the experiments of 

 Langmuir^ of the General Electric Co. have led to the development of 

 amplifiers on the principle of thermionic currents for which a current 

 magnification of several hundred fold is claimed. It is worth noticing 

 that the recent achievements in long-distance ratio-telephony have 

 been accomplished by the use of amplifiers of the type just 

 mentioned. - 



Summary and Conclusions. 



Section 1. A search through the scientific literature of the subject 

 was made with a view to finding out to what extent sound was capable 

 of being propagated mid received through earth and rock. 



Modern seismological observations indicate that solid bed rock 

 provides a good medium for the transmission of elastic waves of 

 disturbance (microseisms) through long distances: the evidence thus 

 brought forward would lead one to expect that sound-waves could be 

 transmitted and received through this medium. 



Section 2. The use of the Fessenden Submarine Telegraph 

 Oscillator provides an extremely powerful means of setting up 

 vibrations in earth and rock. 



Section 3. An account is given of a few experiments carried out 

 at Macdonald College, Ste. Anne de Bellevue, with a Fessenden 



^Langmuir, I., "The Pure Electron Discharge and its AppHcation in Radio- 

 telegraphy and Telephony," General Electric Revieiv, 18, pp. 327-339, May, 1915; 

 Electrician, Vol. 75, May 21, 1915, pp. 240-245; Institutiie of Radio Engineers, Proc. 

 Vol. 3, Sept., 1915, pp. 261-286. 



^Electrical World, Vol. 66, Oct. 9, 1915, pp. 788-791. 



