THE COASTS OF SICILY. 133 



however, that this is not always the case. In the 

 steam engine the safety valves become clogged, and 

 are no longer available at the right moment ; innu- 

 merable causes, some of which are still unknown, 

 bring about the sudden evaporation of too large a 

 quantity of water. In this case the boilers burst, 

 and rending the most solid walls throw the fragments 

 far around them. Under circumstances such as 

 these, masses of fused metal weighing two tons have 

 been projected to a distance of 250 yards. Now 

 volcanoes have also their explosions, or, more cor- 

 rectly speaking, their eruptions are to a certain extent 

 one continuous explosion, and the preceding remarks 

 will show how extremely powerful must be their 

 action.* To appreciate completely the forces which 

 are put into action, it will be necessary to add to 

 the pressures that we have already calculated the 

 tumultuous liberation of vapours and gases, and the 

 frightful degree of tension to which these elastic 

 fluids must be subjected at a temperature capable of 



* The following numerical details will afford some idea of the 

 volcanic force. M. d'Aubuisson des Voisins estimates the velocity 

 of the matters ejected at the mouth of the crater to be about 400 or 

 500 yards in a second. This is very nearly the velocity of a ball 

 at the moment it leaves the cannon's mouth. On assuming that this 

 velocity is only 200 yards per second, it has been found that, during 

 certain eruptions, Vesuvius has thrown its artillery of rocks to a height 

 of more than 4,000 feet. Estimates of height are, however, almost 

 always very conjectural, and the volcanic force can be much better 

 estimated by the space which is traversed horizontally. Now the 

 volcano of Cotopaxi has ejected, to a distance of nine miles from its 

 crater, blocks often cubic yards; that is to say, masses of rock 

 weighing about 30 tons. From this we may judge of the force 

 developed in the interior of the volcano. 

 K 3 



