INTRODUCTION ix 
earliest times. It would be interesting to know whether the 
ancients possessed any accurate knowledge of the subject, as 
they did in the case of Astronomy. 
The literature of the subject is very extensive, but until 
comparatively recent years it contains much that is speculative, 
much that is inaccurate, and much that is false, as is ever the 
case with a science until it becomes quantitative. One recalls 
Lord Kelvin’s first criterion of knowledge of a phenomenon, 
viz., ““ How much of it is there?” 
If the newest literature is not entirely free from speculation 
and inaccuracy, the study now proceeds on a quantitative 
basis which enables the reader to judge for himself as to the 
value of the conclusions. 
The new Seismology as a quantitative physical science 
may be said to have started about thirty years ago, and with 
a small band of British scientists in Japan. These pioneers 
were Milne, Gray, Ewing, Perry and Knott. 
Germany and Italy may also claim pioneers about the 
same time. 
The horizontal pendulum adopted by Milne appears to 
have been independently invented in slightly different forms 
by different investigators, and it is difficult to assign priority. 
The horizontal pendulum in the forms used by Zollner and 
Rebeur-Paschwitz deserve special mention. 
The experimental discovery that an earthquake could be 
registered by a delicate pendulum at great distances from the 
focus marks the first step in the new science. It is undoubtedly 
to Milne’s vigorous personality that we owe the application of 
this fact to the study of earthquakes. On his return to this 
country in 1895 he set up his observatory at Shide in the 
Isle of Wight, and by the installation of his instruments at a 
number of stations distributed all over the earth, he inaugu- 
rated the first Seismological Service. Comparative data were 
thus obtained, and rapidly led to an increased knowledge of 
the properties of the earth. 
