VELOCITY OF ELASTIC WAVES IN GRANITE 59 
The distance travelled by the waves and the time required constituted the 
observed data. Distances were scaled from the maps reproduced in Fig. 1. 
The time plotted against the distance yielded a time-distance graph which 
was a straight line through the origin. The velocity, the reciprocal slope of 
this line, was computed by a least square solution. 
BIRCH RIOGE CAMP 
' 
SHOT & RECORDING 
LOCATIONS 
OF THE 
WESTERLY AREA 
SHOT & RECORDING 
LOCATIONS 
OF THE 
QUINCY AREA 
S 
oe 
> 
< 
s 
r 
2 
~ = 
300 FT 
—— 

Fig. 1. 
Ill. INSTRUMENTS 
The seismograph used at Quincy consisted of three components, two 
horizontal and one vertical, mounted on a single base. The instrument was 
always oriented so that one of the horizontal components recordéd vibrations 
on a line from shot to recorder, while the other, at right angles to this, 
recorded vibrations transverse to that line. These are designated respectively 
as the longitudinal and transverse components. The longitudinal component 
had a free period of 0.77 sec., the transverse, of 0.62 sec., and the vertical, 
of 0.55 sec. Each was critically damped by a magnetic device of conventional 
design. The static magnification, effected by mechanical and optical means, 
was about one thousand. 
At Westerly and Rockport, a vertical component seismograph was used. 
It had a free period of approximately two tenths of a second and was criti- 
cally damped by a piston working in a cup of oil. The static magnification, 
also mechanical and optical, was about twelve thousand. 
Both of the instruments recorded photographically on a strip of bromide 
paper one inch wide, moving at a speed of about twenty centimeters per 
203 
