SEISMIC EFFECTS 75 
into shallower water would maintain their period but diminish 
in wave length and speed, while the amplitude of movement 
at a depth equal to the wave length would be 1/500 of the sur- 
faceamplitude. This would seem to provide adequate margin 
for explaining an observed earth amplitude of I micron= 
‘OOI mm. even at a considerable distance from the area of 
origin. 
The case of waves set up by wind in an ocean of moderate 
depth, such as the North Sea (average depth about 100 m.) is 
more complex, but is soluble on the lines indicated by Lamb 
(“ Hydrodynamics”) and seems to merit investigation with a 
view to explaining the microseismic movement observed in 
Western Europe. 
Dr. Klotz of Ottawa, who has studied the effects observed 
there by means of a Bosch Seismograph, is of opinion that the 
largest effects are associated with cyclonic areas in the North 
Atlantic, and he suggests that the microseismic movement 
may appear in West Europe before the cyclone arrives. If - 
this should prove to be the case it would be a most valuable 
addition to meteorological knowledge. 
In Chapter I. we have observed that a pendulum, whether 
of simple or of horizontal type, indicates by its relative 
motion not only horizontal acceleration applied to the pier, 
but also tilting. It also indicates accelerating effect applied 
to the mass ina horizontal direction. If these effects are applied 
very slowly, the inertia and frictional terms in the equation of 
motion have no influence and the pendulum simply shows a 
gradual change of its zero position. The equation is now of 
the form 
0=(@ — gry) or 18 = (% — gy) 
wherein @ and ¢ are measured to the right and is measured 
in the anti-clockwise direction. 
The changes of zero are shown by all mechanical pendu- 
lums, but it must be remembered that here the electro- 
magnetic method of registration is of no avail, since the zero 
position of the galvanometer is not dependent on the zero 
position of the pendulum itself. 
