Volcanoes. 169 



From these observations it appears that the phenomena 

 which precede volcanic eruptions of the same class, universally- 

 present a remarkable similarity of character, though they 

 greatly vary in power. The energy chiefly depends upon the 

 force required to open a passage for the liquefied mass, and 

 the attendant gaseous fluids. Hence it is that the phenomena 

 which result from the activity of a subaqueous volcano are 

 probably very different from those which attend an aerial 

 eruption. Particular attention, therefore, must always be 

 paid to the situation of the vent, and the density of the fluid 

 resisting the volcanic energies. On this account it is neces- 

 sary that we should in this and the following section take a 

 separate view of these classes. 



If the vent of a habitual volcano, whether aerial or sub- 

 marine, be much obstructed by the accumulation of a large 

 quantity of the lava resulting from a previous eruption, greater 

 power will be required to force the heated matter through the 

 crust of the earth, than if no such obstruction were presented. 

 And if it be necessary to form an entirely new vent, a still 

 greater force must brought into action, and that in proportion 

 to the solidity, position, and weight of the superposed mass. 

 There is not, we believe, a single instance within the range 

 of historical records, in which a new vent has been formed 

 in a country never before the seat of volcanic action. It is not 

 uncommon to hear of the eruption of volcanoes that have been 

 supposed extinct, nor is it improbable that entirely new vents 

 are sometimes formed in the neighbourhood of those situations 

 where the same power has before exerted its energies. In 

 both these cases the great obstruction presented to the volcanic 

 agent by superposed rocks has been removed by former 

 eruptions, and therefore but little increase of power is required 

 to force a passage for the melted materials. We might, 

 perhaps, deduce from this fact, either that the volcanic energy 

 has decreased, or that there is such a connection in the sub- 

 terranean abyss, that the weakest portion of the earth's crust 

 is broken, which is a volcanic vent ; but we forbear to theo- 

 rise. 



Having premised these general observations, we may now 

 enquire into the character of those phenomena which precede 

 volcanic eruption. The most invariable of these is earth- 

 quake. The same night that Lima was destroyed by earth- 

 quake, four new volcanic vents were formed in the Andes. In 

 the year 447, the earth was convulsed for six months, almost 

 without intermission, from the Black to the Red Sea ; and oji 

 the 20th of May, 520, Antioch was destroyed, and 250,000 of 

 the inhabitants buried in its ruins. Soon after the earthquake at 



