1326 EUGENE McDERMOTT 
The question of identifying reflections naturally arises. The most 
valuable aid in this identification is the use of several recording instru- 
ments, the movements of which are photographed simultaneously on the 
same strip of paper. These recorders are arranged as shown in Figure 9. 
The output of each of these recorders is amplified electrically by the de- 
sired amount, thus being made to actuate a galvanometer. The motions 
of the several galvanometers are photographed on the same strip of paper. 
This strip also has timing lines photographed on it so that the time of 
arrival of each reflection may be determined. Obviously, in Figure 9 
the lengths of the four reflection paths from the shot point to the four 
recorders are not very different. If the recorders are fairly close to the 
shot point, the time differences between these paths may not be more 
than a few thousandths of a second and in general are not more than 
SHOT FPE COR DER S 

\ POINTS OF 
NX} Ue REFLECTION 
t\ 
| 
VA 
Fic. 9.—Multiple recorders. 
44 
