

60 L. D. LEET AND W. M. EWING 
second. A vacuum-tube driven tuning fork controlled the interruption of a 
beam of light, thereby producing timing lines, which extended the full width 
of the record, at intervals of 0.01 sec. 
The electrical circuit which was used to detonate the dynamite from the 
recording location also served for the registration on the record of the in- 
stant of the explosion. A modified polarized relay in this circuit was actuated 
both when the firing circuit was closed and when it was broken by the ex- 
plosion. A mirror attached to the moving vane of the relay and in the path 
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of the beam from light-source to seismograph effected the actual registration. 
In Fig. 2, the upward jog in all seismograph lines marks the deflection caused 
by closing the firing circuit, and the downward jog a few hundredths of a 
second later, the instant of the explosion. 
IV. Frrine Positions 
At Quincy and Rockport, several abandoned water-filled quarries were 
used as firing locations. The charges were placed against the quarry walls and 
204 
