PROPAGATION OF SEISMIC WAVES 157 
operate when tilted at any angle between the vertical and the hori- 
zontal. The tilting produced some change in the constants of the 
instrument, but this change was of no importance in the investigation. 
The idea of the experiment was to tilt the geophone toward the shot 
until its axis became perpendicular to the direction of the first mo- 
tion produced by the shot. Successive tests were made to determine 
the angle of tilt at which the direction of first motion on the seismo- 
gram showed a reversal. (This angle could easily be determined to 
one or two degrees in the range of distances studied.) Figure 4 shows 
Observed © 
Computed — 
Distance (ft) 

40 nT SOmcOm 10s 
Emergence Angles 
Fic. 5—Relation of emergence angle to distance traversed. 
the set of observations taken at 5 feet. In Figure 5 emergence angle 
Oo is plotted against the distance traversed by the wave along the 
surface. The curve drawn represents the theoretical values of the 
angle as determined from Eq. (3) using the values of a and d obtained 
from the time-distance curves. The observed values, indicated by the 
points, agree with the calculated values within the limits of experi- 
mental error. It was not considered necessary to correct for the differ- 
ence between true and apparent emergence angles.’ 
TRAVEL-TIME BETWEEN ANY TWO POINTS ON A PATH 
From Eq. (4) the relation 
dy/dv = (v2 — 6?)!/227a, (5) 
E. Wiechert, Gott. Nachr. (1907), No. 1. 
817 
