REFLEXION OF ELASTIC WAVES 137 
with the reflected longitudinal wave and so will amplify that wave to 
a much greater degree than waves of lower frequency. The longi- 
tudinal wave propagated through surface beds or refracted from deeper 
beds is found to be very quickly dissipated, and when recording re- 
flected events from appreciable depths is not a serious factor. The 
sound wave in air can be eliminated toa great extent by burying the 
seismographs at shallow depths. Micro-seismic unrest and audible 
frequency disturbances such as telephone wire hum and rustling of 
trees is usually of sufficiently high frequency to permit of its being by- 
passed to earth through condensers in the amplifier or of such low 
frequency as not to affect the unit. 

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M7 
Fic. 2——Elements of reflexion seismograph unit. 
INSTRUMENTS 
Instrumental design has followed widely separated paths, but 
there is evidence of convergence of these paths as the objective sought 
is more clearly visualised. 
Electrical detectors, usually referred to as seismometers, are uni- 
versally used, some employing moving coils in electromagnetic or 
permanent magnetic fields, some the carbon button microphone, and 
others the piezo-electric type of detector. Amplification of the weak 
electrical image can be obtained by audio-frequency vacuum-tube 
amplifier circuits employing from one to many stages. Recording of the 
image is through either oscillographs or string galvanometers. In 
average commercial work timing of the interval between the explosion 
and the arrival of an event at a seismometer must be accurate within 
limits of 0.1%. This accuracy is readily obtainable by means of an 
electrically maintained tuning-fork which may be arranged either to 
drive a synchronous motor or a shutter which periodically interrupts 
the light beam of the galvanometer. Experience has shown that the 
greatest energy is carried by the component of the reflected longitu- 
dinal wave having a frequency ranging from 30 to 60 cycles per 
second, and it is standard practice to peak the frequency amplification 
7197 
