16 
Proceedings of Eoyal Society of Edinburgh. [sess. 
this, and every wave is registered — following its predecessor with 
more or less rhythmic precision, the intensity of the disturbance 
getting less and less, till at about 34 minutes past the hour the 
disturbed trace runs into normal curve once more. 
However, one singular feature in the trace, which is clearly 
visible in the latter part of the disturbed curve, must also be 
explained, viz., the running of the vibration lines into each other 
by a zigzag kind of trace. The most plausible explanation appears 
to be the following. 
Assuming the normal curve to be the zero for the time, ordinary 
vibrations caused by a momentary disturbance would take place 
above and below this zero with equal amplitudes. But from the 
nature of the appearance of the zigzag curve, it seems as if the 
advancing seismic wave of condensation caused a decrement in 
the declination, and compelled vibrations about a displaced zero 
higher up the curve ; while during the receding wave, as it passed 
away, the reverse and opposite effect followed, and the declination 
increasing, the vibrations were caused about a lowered zero, each 
phenomenon, of course, having its maximum and minimum effect. 
Fourteen such waves in all can be detected on the trace, the 
second or third of which — more possibly the second — was the most 
intense, and each complete wave was followed by another after an 
average interval of about two minutes,— the maximum effect of the 
advancing and the retreating parts of the wave being recorded at 
the average interval of about one minute. 
Shock. 
Maximum Effect 
showing Decreased 
Declination. 
Maximum Effect 
showing Increased 
Declination. 
Disturbance. 
1 
16 h. 6' 
16 h. 7' 
Commenced at 
16 h. 5' 
2 
8-5 
9*5 
3 
10-5 
11-5 
4 
12-5 
13-5 
5 
15 
16 
6 
17 
18 
7 
19-5 
20-5 
8 
21-5 
22-5 
9 
24 
25 
10 
26 
27 
11 
28 
29 
12 
30 
31 
13 
3D5 
32-5 
Ended at 
14 
33-5 
16 h. 34' 
