112 WEATHERBY, BORN, AND HARDING 
TABLE I 
First Section Last Section 
o-100 Feet 300-400 Feet 
Incompressibility K 3-85X10% dynes/cm?. 5-17X10% dynes/cm?. 
Rigidity n 1.21X104% dynes/cm?. I.56X10" dynes/cm?. 
Compressibility £8 2.60X 10-8 dynes/cm?. 1.93 X10—2 dynes/cm?. 
Poisson’s ratio o 0.357 0.363 
Young’s modulus E 3.29X10% dynes/cm?. 3-91X10% dynes/cm?. 
Considering the fact that these values are representative of less 
penetration than the average values obtained by Leet and Ewing,® 
there is a remarkable agreement between their data and those pre- 
sented here. 
Although their average velocity values are somewhat higher, they 
are undoubtedly influenced by the high velocity portion of the time- 
distance curve since their profile extended to 4,000 feet. 
SOURCES OF ERROR 
In order to obtain an idea of the accuracy of the readings in this 
work, we may first consider the errors from a theoretical standpoint 
and then compare this with the observed variations. 
Since the micrometer microscope makes possible the reading of 
distances on the records to 0.02 millimeter, and since the time interval 
measured (0.01 second) covered, on an average, 1 centimeter, a read- 
ing of a given point on the record could be made to 0.00002 second. 
In each determination of time, it was necessary to measure the two 
timing lines a and 6 and the instant of arrival of energy c (Fig. 2). 
The maximum total error of reading would be obtained when the 
errors in all three readings were additive in the same direction and 
would therefore be .co006 second. However, it was not found possible 
to pick the instant of arrival ¢ to this degree of precision. From an 
analysis of a large number of records it was concluded that the total 
error in picking the instant of arrival and in measuring the three neces- 
sary intervals on any one record did not exceed 0.00015 second. Since 
the total time corresponding with a distance of 100 feet is approxi- 
mately 0.0063 second, the maximum error to be expected in reading 
any one trace would be approximately 2.5 per cent. In determining 
the average time for a given span a sufficiently large number of rec- 
ords were used so that the average value obtained is thought to be 
correct to within at least 2 per cent. 
The distances along the profile used were laid out with a steel tape 
with a probable error negligible compared with the possible error 
made in measuring the time. 
5 L. Don Leet and W. Maurice Ewing, “Velocity of Elastic Waves in Granite,” 
Physics (March, 1932), p. 160. 
634 
