1864. ] Mauter on Earthquakes. 67 
surface vertically, and soonest in this Vertical, which is the shortest 
distance between any point below and the surface, and here it only 
produces (neglecting transversals) a rapid movement up and down. 
The surface of the ground actually rises and sinks again to its pre- 
vious place, with great rapidity, and through a range that may be 
several inches or perhaps fect, dependent on how great and how near 
the blow is given below, and what is the intervening material. 
For all points around the Seismic Vertical, the wave emerges at 
slopes, called emergent angles, which become more and more nearly 
horizontal as the distance on the surface is greater. The spherical 
or quasi-spherical shell wave-form at any given distance outwards 
when cut by the earth’s surface, intersects it as a closed curve, more 
or less circular, elliptic, or oval, and the crest, so to say, of this surface- 
wave, called a coseismal line, because all bodies situated in it are 
shaken at the same instant, travels along the surface of the earth with 
a real, though not large, and with a constantly diminishing undulation, 
like a roller at sea, constantly enlarging the curvilinear area within it ; 
and as it passes “outward, objects in succession are disturbed or 
overthrown, not by the transit of the wave-form, but by the wave itself, 
that is, by the movement of the particles in motion in the wave. 
There is a certain distance outward upon the earth’s surface, all 
round the Seismic Vertical, at which it may be proved that the over- 
throwing power of the shock is a maximum, greater than anywhere, 
within or without it—within, because there the direction of normal 
movement in the wave is more nearly vertical, and hence less calcu- 
ated to upset objects standing on the ground—and without, because 
the further the shock has travelled away from the Seismic vertical, 
the more its power (to speak loosely) has decayed. This is the 
Meizoseismal circle or curve. The angle made with the Seismic vertical 
by a line drawn from any point in this curve at the surface down to 
the centre of impulse, is for the same conditions constant. 
If the impulse or blow has been accompanied by rending or frac- 
ture, or the striking or grinding together of hard or rocky masses, or 
by the rush of vapours or gases, then the wave of shock will be accom- 
panied by waves of sound. But these latter may or may not travel just 
at the same rate, or by quite the same wave-paths to the ear of a 
person upon the surface, as does that of the shock which he feels. 
Hence there may be Earthquake shocks, with or without sounds, and 
the shock may be perceived before any sound is heard, or the sounds 
may precede and herald the shock, as the awful ‘‘ bramidos” generally 
do the Earthquakes of Mexico. 
But to hearers remote from the Seismic vertical, the sounds, if any, 
will reach their ears not only through the earth, but through a longer 
or shorter intervening range of air, and hence at very different times 
and with very different amounts of repercussion and reverberation, 
although originating in one sound only, as of a single rend, or grind, 
or explosion. 
A remarkable use has been made, for the first time, of the differences 
in the character of the sounds heard nearly simultaneously, and at 
about equal distances all round the Seismic vertical, in the Report 
F2 
