ELASTIC WAVES IN AMHERST SANDSTONE 15 
character of the rock. In other words, the ‘‘moisture cycle’ appears 
to be reversible. This is evidenced by the fact that points 1 and 2 fall 
so nearly on the curve determined by the remaining points. This fact 
must be taken into consideration in seeking for an explanation of the 
observed phenomena. 
Figure 5 is a graph derived from the observed data. In this figure 
the calculated velocities in feet per second are plotted against the 
percentage of water added to the dry bar, the weight of the latter 
being taken as the base. This curve shows most clearly the very small 
BAR VELOCITY - FEET/ SEC. 


° | 2) 3 4 5 6 7 
WATER ADDED IN PER CENT BY WEIGHT 
Fic. 5.—Variation of bar velocity of elastic waves in Amherst sandstone 
as water content is changed. 
amount of moisture necessary to cause a great decrease in velocity. 
The addition of slightly more than 2 per cent of water to the dry bar 
decreases its velocity from 7,640 feet per second to 4,600 feet per 
second, which is a reduction of 40 per cent. The addition of 4 per cent 
more water, however, decreases the velocity to 4,415 feet per second, 
which is a further reduction of only 2 per cent. 
In order to formulate an explanation for the phenomena described, 
it is first necessary to determine what property of the bar is affected 
by the moisture. The bar velocity is a function of the value of Young’s 
modulus and the density of the bar, being given by the relationship 
v= /E/p 
where v is the bar velocity 
E is Young’s modulus 
- and p is the density of the bar. 
A decrease of velocity may therefore be caused by an increase in 
density, by a decrease in the value of Young’s modulus, or by both. 
The addition of water does, of course, increase the density, but this 
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