64 O. C. LESTER. IR: 
After completion of this profile, a hole was dug at the shot point 
and the top of the water table was determined to be at a depth of 
slightly less than 8 feet. On the assumption that below the water table 
all air in a free state is displaced by water, whose density and elasticity 
are of the same order of magnitude as the earth under consideration, 
and hence has no material effect on the velocity of transmission of 
sound through such substance, both the shot point and recorders 
were lowered to the top of the water table. At this depth the shorter 
or “‘weathered” portion of the profile was re-shot with the result 
shown in curve (B), Figure 2. 
TIME IN SECONDS , 

DISTANCE IN FEET 
Fic. 2 
’ 
On examination, this curve indicates an “‘unweatherec”’ velocity 
(5,200 feet per second), which passes through the origin with only that 
curvature which is to be expected with a slight increase of velocity 
with depth. At the same recording distances from the shot point, the 
“weathered” portion of the time-distance curve has been eliminated 
with the elimination of the “aerated layer.’’ The obvious conclusion 
is that where the water table is sufficiently shallow the thickness of 
the aerated layer is determined by its position. However, if the water 
table be at considerable depth, the ‘“‘weathered’’ thickness is deter- 
mined by the depth to which any appreciable amount of air penetrates. 
This explains, in a measure, the sometimes extreme variation of 
“weathered” thickness with different surface materials. It would also 
predict a change of weathered thickness with wet and dry seasons. 
Though these experiments have not been carried out to such an 
extent as to make the conclusions indisputable, it may be said that 
all the experimental data obtained favor the theoretical predictions 
resulting from the assumption of an “‘aerated”’ layer. 
394 
