\ 
24.4, Dr. C. Davison—Founders of Seismology— 
velocity of earth-waves, and the advantage of founding earthquake 
observatories in various parts of the world. There is perhaps little 
‘that is really novel in the whole memoir, little, if anything, that 
the present-day student of seismology would have occasion to consult, 
and his account of Michell’s work is strangely inaccurate and 
incomplete. Its chief merit is that it does form an attempt to 
explain the more important phenomena of earthquakes by the 
light of one guiding principle, and thus, as he says, to bring them 
within the range of exact science. As W. Hopkins wrote in the 
following year, he treated the subject “ in a more determinate manner 
and in more detail than any preceding writer ”’. 
The principle referred to is that an earthquake is “the transit 
of a wave of elastic compression in any direction, from vertically 
upwards to horizontally, in any azimuth, through the surface and 
crust of the earth, from any centre of impulse, or from more than 
one, and which may be attended with tidal and sound waves 
dependent upon the former, and upon circumstances of position 
as to sea and land’! At the same time, he realized that the waves 
of vibration may “ become complicated by movements of permanent 
elevation or depression in the land . . . the effects of which it may 
often be difficult or impossible subsequently to separate.” 
Mallet gives a very full description of the sea-waves which 
accompany earthquakes. Although he does not follow Michell 
in separating the visible waves from the waves of vibration on land, 
he distinguishes two kinds of sea-waves—the forced sea-wave, as 
he calls it, and the great sea-wave. His explanations of the origin 
of both are partially incorrect. The forced sea-wave, which 
accompanies the earth-waves and is “ carried upon its back, as it 
were’, is merely the wave of compressional vibrations in water. 
If the great sea-wave, which sweeps in some time later, were initially 
raised by the earth-wave, there would be a great sea-wave with 
every strong submarine earthquake. He does, however, notice 
that the sea-wave may become “‘ complicated by movements of 
permanent elevation or depression in the land ”’, and to this extent 
his explanation is no doubt correct. 
Great stress is laid on the importance of determining the velocity 
of the earth-wave, not so much for the interest of the question in 
itself, as on account of its geological applications. Mallet was 
aware that the velocity in any rock must depend on the elastic 
constants of the rock. If we were to measure these constants for 
all the different rocks, the knowledge of the velocity in a given 
submarine earthquake, he suggested, would enable us to predict 
1 Many earlier writers had regarded earthquakes as due to the passage of 
waves of vibration, e.g. J. Michell in 1760, Thomas Young in 1807 (Lectures 
on Natural Philosophy, vol.i, p. 717), and D. Milne in 1841 (Hdin. New Phil. 
Journ., vol. xxxi, pp. 262, 275-7). In his report on the theories of elevation 
and earthquakes (Rep. Brit. Assoc., 1847, pp. 33-92), W. Hopkins, treating 
the theory as obvious or well known, gives an account of wave-motion in 
solids and liquids that may still be read with advantage. 
