764 
PROCEEDINGS OE SECTION H. 
been uniform as regards the whole earth, and that the force of earth- 
quakes may for a cycle of years have been invariably confined, as it is 
now, to large but determinate spaces. Gradually, however, this force 
shifts its position, so that other regions for ages at rest become 
in their turn the grand theatre of action.” But though violent 
earthquakes are confined to limited regions, complete quiescence is 
nowhere to be found. Professor Milne finds the soil of Japan in a 
constant state of vibration. It is the same in Italy. So much is this 
the case there that many of the towus now have building regulations 
providing for their security. Even the soil of Britain is never 
absolutely at rest. So this subject should have interest for mankind 
everywhere. 
I do not propose to read you an elaborate essay on earthquake 
phenomena, but will confine myself to such as may, in my opinion, 
have led to our modern so-called earthquake-proof construction. 
Earthquake phenomena have been divided into three kinds — 
namely, the explosive, the horizontally progressive, and the vorticose. 
The first is described by Humboldt, and has a violent motion 
directly upward, like the explosion of a mine, by which the crust of 
the earth is broken up, and bodies on the surface are thrown into 
the air. The best example of this is the great earthquake of Eiobamba, 
which occurred in 1797. In this case the nature of the shock was 
precisely as if a fearful explosion had taken place immediately beneath 
the fated town. The earth was not only broken up and fissured in 
many places, but, what was most characteristic, bodies lying on the 
surface, among which were bodies of men, were actually thrown 
upward 200 feet in the air, and were afterwards found on the top of a 
hill on the other side of the river. 
The velocity due to this height of projection has been estimated 
at 80 feet per second, and Mallet thought that it was probably the 
greatest velocity recorded, or perhaps at present possible, on our earth ; 
and is, as he lias calculated, 5*33 times greater than the velocity of the 
Neapolitan earthquake of 1857, regarding which his great work is 
written. We may well despair of constructing anything capable of 
resisting such a shock. 
Le Conte is of opinion that the horizontally progressive earth- 
quake must be regarded as tlie true type, and describes it in this 
way : — “ A concussion occurs deep beneath the surface — a succes- 
sion of spherical earth-waves emerges first immediately over the 
focus. From this point a series of rapidly enlarging spheres or 
shells radiate. If the elasticity of the earth and the velocity of the 
waves are equal in all directions, the surface waves or shells will 
spread in concentric circles ; but if the elasticity of the earth and the 
velocity of the waves are greater in one direction than auother, their 
form will he elliptical. It is easy to suppose that the shock will be 
most severe immediately over the point where the concussion occurs, 
or, as it has been termed, the seismic vertical. The waves here will 
emerge at steep angles, with a vibratory up-and-down motion 
dangerous to the stability of buildings. As the waves recede from this 
point, the angle of emergence will become gradually less until the 
tremors die away. Unless it may be over the focus, the danger to 
buildings in this case will be much less than in the explosive.” 
