248 Dr. C. Davison—Founders of Sersmology— 
least or no disturbance are the central areas of great oceanic or terr- 
oceanic basins and the greater islands existing in shallow seas. 
The modern and more accurate method of mapping by epicentres, 
rather than by disturbed areas, has led to greater detail in our 
knowledge, but the main laws of seismic distribution’ are those which 
Mallet has so clearly established. 
Methods of Investigation—Though they are not explicitly stated, 
there can be little doubt, I think, that the methods of investigation 
which Mallet afterwards developed, were partly present in his 
mind when he wrote his early papers. Even in the first edition 
(1849) of the Admiralty Manual of Scientific Inquiry, to which, 
on Darwin’s recommendation, he was invited to contribute, there is 
no direct account of them in the section on the observation of 
earthquake phenomena. They are described for the first time, and 
very fully described, in his report on the Neapolitan earthquake, 
and it is some indication of the importance which he attached to 
them that the main title which he gave to these two large volumes 
s “‘ The First Principles of Observational Seismology ”’ 
The fundamental object of Mallet’s inquiry was to ascertain the 
surface-position and depth of the seismic focus. The methods 
which he proposed for this purpose are well known. By observations 
on the direction of fissures in large and uniform buildings, of the fall 
of columns, and of the projection of detached masses of masonry, 
he sought to determine the azimuth or horizontal direction of the 
shock, and such directions at two or more places would suffice to 
determine the position of the point vertically above the seismic 
focus, which we now know as the epicentre. Once this point is 
determined, a single observation of the angle of emergence, as given 
by the inclination of fractures in walls, would lead to the depth of, 
at any rate, one point within the focus. 
Various writers before Mallet had suggested the use of observations 
on the direction of the shock. The earliest case known to me, which 
I have already quoted (p. 105), is that of J. Michell, in 1760. D. Milne 
used the method in 1841 to determine the epicentre of the Comrie earth- 
quakes, which were at that time frequent, the directions at different 
places being obtained chiefly from the records of inverted pendulums. 
He also made the valuable suggestion that “if instruments could 
be invented which at different places would indicate, not merely the 
relative intensity of the shocks, but the direction in which they acted 
on bodies, means would be obtained of determining the point in 
the earth’s interior from which the shocks originated ’’'—a method 
which, forty years later, occurred to J. Milne, and was used by him, 
and afterwards by Omori and Hirata, to determine the depth of 
the seismic foci of some Japanese earthquakes. Lastly, W. Hopkins, 
in giving (1847) the same method for locating the epicentre, adds 
the important remark that the direction observed should be that at 
1 Rep. Brit. Assoc., 1841, p. 48; 1842, pp. 96-7; 18438, p. 121; Hdin. 
New Phil. Journ., vol. xxxi, 1841, pp. 276-7. 
