500 REPORT—1883. 
radial fissure extending outwards from the chimney of the volcano, so that we see 
the position and plane of this fissure shows it to be no exception to the rule. The 
centre of violence on the surface occupies the same relation to the axis of the 
volcano as does Monte Rotaro, Montagnone, and Cremate, the three most normally 
placed centres of eruption, so that an outburst at this point would have just the 
position from which we should expect it to occur. 
In the case of an active volcano, the radial fissure extends outwards from the whole 
length of the chimney, as pointed out by Mallet ; but when we have to deal with a 
case similar to Ischia, the canal has no doubt been plugged by a mass of trachyte 
that for at least some thousands of years has been able to gradually cool from the 
surface downwards. Any escape, therefore, of unconsolidated igneous matter 
would probably occur at the point of least resistance, and so would be into 
the fragmentary material, in preference to following the plugged chimney. The 
fissure would, therefore, have a tendency to branch out, forming an angle with 
the main axis and extending itself by spasmodic ruptures, followed by immediate 
injection of igneous matter, and would not send the maximum impulse in an 
upward direction perpendicular to the horizon, but inclined, just as we see denoted 
by the eccentricity of the different isoseismal ellipsoids. 
There is yet another point to be cleared up. Fontana is not included in the 
isoseismals, yet it suffered rather severely. Very few of the walls of the houses 
were damaged, but hardly a roof, of the arch masonry type especially, escaped being 
fissured and cracked in such a way that the fractures usually assumed a circular 
form in the centre of the vault, with others radially extending outwards. The 
wooden beams over the doors and windows were bent down or broken by a piece 
of masonry of a A shape, included in fractures extending from each upper corner 
of the opening in the wall. There was also no evidence of lateral movement by 
injuries. The people remarked a strong subsultory shock, followed by a slight 
undulatory one. 
There is, therefore, plenty of evidence to show that we have here a series of 
injuries depending upon a wave isstiing from the earth vertically or nearly so. 
How can we explain this ? 
We have in one earthquake two seismic verticals, one surrounded by a district 
in which the damage diminishes more or less gradually as we recede from the 
mesoseismal area; in the other we have a small mesoseismal (?) area surrounded by 
districts quite uninjured. 
The following explanation has appeared to the author to clear up the difficulty, 
and therefore perhaps he may be permitted to give it. 
As already stated, Fontana occupies the centre of the great crater of Epomeo, 
and therefore lies immediately over the old chimney, which in all probability is 
filled by an old plug of consolidated trachyte which must descend to the igneous 
reservoir. Any increase of tension in the general mass of igneous matter that 
might determine the further rupture of a collateral fissure would result in the 
conduction of any changes of pressure or vibrations along the column of highly 
elastic trachyte, whilst the same earth-waves would be annulled or absorbed by the 
inelastic tufas surrounding it, so that the blow would be struck perpendicularly to 
the surface and in a small area, with well-defined borders. The undulatory 
sensations after the principal local shock were these that were derived from the 
great centre of impulse beneath Casamenella. 
The earthquake occurred at an epoch of general seismic activity throughout 
Europe, although its own vibrations were only communicated to short distances. 
It was felt at Vivara, Procida, Bacoli, and Misenum, and at Ventotene and Ponza 
very slightly. At Naples and Vesuvius, even the most delicate seismographs were 
undisturbed. This is comprehensible when we think over the geology of the 
district, and remember its composition of tufas of different but always low elas- 
ticity inclined at every imaginable angle, so that the earth-wave would be refracted 
and reflected every few yards, besides being absorbed by the inelastic medium. 
But, moreover, it is possible to show that the earth-wave in passing from the 
focus to the instruments in a straight line would have to be transferred from the 
earth to the sea, thence to the air, thence again to the earth, or directly to the 
