THE PRESIDENTIAL ADDRESS. ll 
North Sea, and by the deformation due to the alternating load which they 
impose on the bottom. In Chicago, near the centre of the American 
- continent, these influences were absent. 
The question may be asked, What is the precise degree of rigidity which 
is indicated by these observations, or by others which have been referred 
to ? Various answers have been given, based on observations of the tides, 
of the lunar deflection of the vertical, and of the period of the earth’s 
Eulerian mutation, on which I have not touched. The estimates have varied 
greatly, but they are all high, some of them extremely high. That they 
should differ among themselves is not surprising. The material is certainly 
not uniform, either in its elastic properties or the conditions to which it is 
subject, so that we can only speak of the rigidity of the earth as a whole 
in some conventional sense. Larmor and Love have shown that all the 
information that can be gathered, whether from the tides or from the 
Eulerian mutation, can be condensed into two numerical constants. This 
leaves a large degree of indeterminateness as to the actual distribution of 
elasticity within the earth. It is at all events certain that in regard to 
tidal forces the great bulk of the material must be highly rigid. 
In leaving this topic, it may be recalled that it was in this same connec- 
tion that Kelvin was led to initiate the method of Harmonic Analysis as 
applied to the tides, as well as to accomplish much brilliant mathematical 
work, whose importance is by no means limited to the present subject. 
The whole theory of the tides and cognate cosmical questions afterwards 
became the special province of George Darwin, but after his death, work 
on the tides was almost at a standstill, until it was resumed by Professor 
Proudman and his associate Dr. Doodson in the recently established Tidal 
Institute at Liverpool. They have already arrived at results of great 
theoretic as well as practical interest, some of which I understand are to be 
brought before the Association at this meeting. 
Within the last twenty years or so light has come on the elastic properties 
of the earth from a new and unexpected quarter, viz. from a study of the 
propagation of earthquake shocks. It is pleasant to recall that this has 
been largely due to efforts especially fostered, so far as its means allowed, 
by this Association. To John Milne, more than to anyone else, is due the 
inception of a system of widely scattered seismological stations. The 
instruments which he devised have been improved upon by others, notably 
by Galitzin, but it is mainly to his initiative that we are indebted for 
such insight as has been gained into the elastic character of the materials 
of the earth, down, at least, to a depth of half the radius. It may be 
