76 MODERN SEISMOLOGY 
It is perhaps needless to remark that the zero of a pendu- 
lum is continually changing. Such changes may be merely 
instrumental or due to local temperature change. As such they 
are of little scientific interest, and are rather a serious nuisance, 
and every care should be taken to remove such sources of 
change. Careful examination shows, however, that part of 
the change of zero is regular and of considerable scientific 
importance. The most marked effect in point of magnitude is 
that which occurs on the seismograms of the Milne pendulum, 
e.g. at Ryde, Isle of Wight, which show in a manner visible 
to the eye a regular sinusoidal movement of the zero agreeing 
precisely with the rise and fall of the Channel tides. There 
seems little doubt that the rock strata bend under the influence 
of this periodic alteration of load in the Channel basin. Dr. 
Milne has kindly sent me the specimen record, Plate 7B. 
Such visible effects are not howevershown at inland stations, 
and it is only by careful analysis of results extending over long 
intervals that the existence of periodic movement in the pen- 
dulum zero can be detected. The effects, although small, 
derive importance from their association with earth tides and 
the theory of the physical properties of the Earth. 
The acceleration of gravity g at any point of the Earth’s 
surface is not exactly constant either in magnitude or direction, 
but on account of the attraction of the Sun or Moon it under- 
goes small changes. The potential of these additional forces 
at any point is expressed by a solid spherical harmonic of 
order 2 and may be written 
te 
W, =3mg 3 
wherein 7z is the mass of the Sun or Moon, 
cis the distance of the Sun (or Moon) from the 
Earth’s centre, 
(cos?@’ — 1) 
g the normal acceleration of gravity, 
a the mean radius of the Earth, 
vy the geocentric radius to the point, and 
@ the geocentric zenith distance of the Sun or Moon. 
The solar effect is about half that of the Moon. 
If now # is any direction on the Earth’s surface perpendi- 
