250 Dr. C. Davison—Founders of Sersmology. 
pass within a circle, 24 geographical miles in radius, with its centre 
close to the village of Caggiano. Of twenty-six separate wave-paths, 
twenty-three start from the vertical line through this centre at 
depths of less than 74 geographical miles. The mean depth of the 
focus he estimated at 52 geographical miles, and the probable 
vertical dimension of the focal cavity at about 3 geographical miles. 
Some of the azimuths, however, pass outside the Caggiano circle. 
From their various points of intersection Mallet concluded that the 
focal cavity was a curved fissure 9 geographical miles in horizontal 
length. The mean velocity of the earth-waves he found to be 788 feet 
per second. The amplitude and maximum velocity of the vibrations 
naturally varied greatly. At Polla, which is close to the epicentre, 
he estimated the amplitude at 23 inches, and the maximum velocity 
at 13 feet per second.! 
Conclusion—To sum up Mallet’s contributions to seismology is 
not an easy task. One of the most important is also the 
most intangible—his influence on the different points of view from 
which earthquakes were regarded, say in 1845 and after the lapse 
of twenty years. Fifteen years after his death and nearly forty 
years after the Neapolitan earthquake, I was struck by the fact that 
of more than two thousand observers of the Hereford earthquake 
of 1896, one in every five gave unasked the direction of the shock. 
In the large towns the proportion rose to one in every three. 
“ It is given to no man,” said Mallet, “ so to interpret nature that 
his enunciation of her secrets shall remain for ever unmodified 
by the labours of his successors.” Mallet’s work was unfortunately 
no exception to this rule. Some of it may now be obsolete, but much 
ofitremains. Several of the terms in daily use—seismology, seismic 
focus, angle of emergence, isoseismal line, and meizoseimsal area—are 
due to him. The position of the epicentre may still be determined 
by observations on the direction of the shock, especially by the mean 
of a large number of observations. As regards the depth of 
the focus, the results derived from such observations are much 
less certain. All that can be said is that they perhaps indicate the 
order of magnitude of the true depth. His experiments on the 
velocity of earth-waves are interesting, though their bearing on 
the actual problem is somewhat remote. But, by his perception of 
the nature of earthquake-motion, by the construction of isoseismal 
lines, by the compilation of his great catalogue, by his statement 
of some of the laws which govern the distribution of earthquakes 
in time and space, and, above all, by his investigation—the first 
rational investigation—of a great earthquake, Mallet has placed 
the science of seismology under a debt, which those who have 
followed in his steps would be the last to underestimate. 
* A more detailed account of Mallet’s investigation is given in my Study of 
British Harthquakes (Contemporary Science Series), pp. 7-44. 
