12 Proceedings of Royal Society of Edinburgh. [sess. 
wave shown by declination and vertical force magnetographs later 
by about one minute. 
With regard to the disturbed trace of the declination magneto- 
graph, on the other hand, it is difficult to conceive how condensa- 
tion or distortional waves could affect the suspended magnet 
under an exhausted receiver, so as to set up large vibrations in it 
simulating magnetic action. The magnet was disturbed mechani- 
cally, no doubt, by the seismic waves, and these disturbances were 
typical of mechanical action. The three motions (besides the 
vibratory motion) observed in the declinometer at the time of 
disturbance showed (1) motion of the whole magnet parallel to 
itself (east and west) ; (2) motion of the magnet as a whole north 
and south ; and (3) the slight bobbing motion of the ends of the 
magnet up and down. But neither of these motions would show 
increase of the scale reading, nor would vibratory motion, if set up 
by mechanical causes, permanently show increase or decrease of 
declination. It is true that violent motion of any one of the types 
referred to may so disturb the magnet as to bring one end or the 
other of the magnet in slight contact with the damper, which may 
result in vibrations ; but the intensity of the mechanical disturb- 
ance was feeble, and the motion observed at the time of the dis- 
turbance showed no evidence of any great, much less violent 
movements. Finally, the peculiar character of the trace of this 
instrument, as will be seen. later on, leaves no reasonable doubt 
that the disturbance was due to magnetic action which must 
have accompanied the seismic disturbance, possibly as effect of 
a cause. 
The case of the horizontal force magnetograph is, however, 
different. From the peculiar nature of the bifilar suspension, it 
becomes obvious that the tension of the wires being a principal 
factor in the general formula for the condition of equilibrium, it 
must be affected by any sudden displacement of the points of 
suspension. A distortional wave, for instance, suddenly lifting 
the points of suspension, would result in increasing the tension, and 
any sudden depression would for a moment reduce the tension. 
A lateral movement, also, is likely to temporarily alter the con- 
ditions of equilibrium, and this magnetograph therefore appears to 
be disturbed by the combined action of both the mechanical and 
