84 MODERN SEISMOLOGY 
there, produced no perceptible effect on the Eskdalemuir seis- 
mographs. 
It is now the custom for observatories to exchange bulletins, 
and for many years Milne has published (“British Assoc. 
Reports ”) annual tables of data from all sources. An annual 
table is now also issued under the auspices of the International 
Association of Seismology (Strassburg). The importance of 
such bulletins and tables can hardly be overrated. 
They enable one to confirm or correct inferences and greatly 
extend our knowledge of the number of earthquakes which 
occur at all points of the earth. Milne estimates that the 
annual output from all sources is nearly 60,000 earthquakes. 
It has long been noted that the seismograms obtained at a 
given station showan extraordinary similarity for separate earth- 
quakes that occur in the same region of the Earth. In some 
cases the seismograms might almost be superposed. This is 
a matter deserving careful investigation as it points not perhaps 
so much to a difference of the properties of the interior of Earth 
in different directions, as to a characteristic origin of the earth- 
quakes occurring in one and the same region. 
While there is a general agreement that an earthquake is 
caused by a rupture of the rocks within the earth’s crust, we 
have no very definite knowledge as to the primary cause of the 
rupture. It is not unnatural to look for such a cause in the 
tidal stresses of solar and lunar origin. In particular we might 
look for a preponderance of the number of earthquakes at the 
times of syzygy of Sun, Earth,and Moon. Such investigations 
have been made but do not appear to result in clear evidence 
of such an association (Milne, “ Earthquakes,” p. 250). Another 
way of dealing with the occurrence of earthquakes, and which 
is well known in connexion with the analysis of meteorological 
and magnetic data, is to express the observations by a Fourier 
series in terms of the time, either solar or lunar. Such investi- 
gations have been made by Knott (“ Proc. R.S.,” 1897) and by 
Davison (“ Phil. Trans. A.,” Vol. 184, 1893). These have 
been critically examined by Schuster (‘‘ Proc. R.S.,” 1897). 
A question arises as to what should be included in the data 
submitted to analysis. It is known that a large earthquake is 
