Volcanos. 119 



occasioned by the ebullition of water in the focal lava, from con- 

 stant and uniform additions of caloric to it, will bd as sensible as 

 the barometer to variations in the pressure of the atmosphere on 

 the surface of the supported column of lava ; the ebullition ceas- 

 ing for a few minutes or hours as the density of the atmosphere 

 increases, and increasing in energy as it is diminished. Obser- 

 vation confirms this opinion. Stromboli is made use of as . a 

 weather-glass, and securely relied on by the fishermen of the 

 Lipari isles; other volcanos likewise have been Observed to 

 augment their activity in tempestuous weather, and, in general, 

 to be most violent in the stormy season of the year. Earth- 

 quakes also have been often observed to coincide in time with 

 hurricanes or violent storms; and the author notices the proba- 

 bility that a diminution of the pressure of the atmosphere, taking 

 place simultaneously on a large extent of the surface, beneath 

 which the ever-active force of expansion is continually pressing 

 upwards, and often restrained by only the slightest degree of 

 superiority in the combined forces of repression, may occasionally 

 give the predominance to the former force, and determine one 

 of those partial elevations of the crust of the globe to which he 

 attributes the phenomena of earthquakes. 



ll lt is remarked, that an individual volcano may occasionally 

 pass from one of the phases, distinguished above, into another; 

 or may even exist in two phases at once, having a double system 

 of operations, corresponding to two different foci, seated one 

 below the other ; the latter perhaps at a considerable elevation in 

 the chimney or main vent of the volcano, and giving rise to 

 minor and frequent eruptions ; the former at a much greater 

 depth, and productive of rare and violent paroxysmal eruptions. 

 It is obvious that the last system must be in activity wherever 

 the supply of caloric is in a faster ratio than its drain through 

 the activity of the upper focus. The paroxysmal eruptions 

 leave usually a prodigiously wide and deep crater, which is sub- 

 sequently filled up by degrees by the eruptions of the minor and 

 upper focus. Such alternations of minor and paroxysmal erup- 

 tions appear to have produced the colossal crateral cavities of 

 volcanic countries, most of which have one or more recent cones 

 rising from within their circuit, such as Vesuvius within the 

 crater of Somma : the Peak of Teneriffe, and the cone of Cha- 

 horra, from the circus described by Von Buch ; that of Bourbon 

 from the successive circuses described by St. Vincent ; those of 

 Volcano, Astroni, the lake of Roneiglione, &c. &c." 



The views of the author respecting the mode in which the 

 crystals of volcanic substances are supposed to be brought into 

 a state of fluidity, more or less perfect, are somewhat novel. If 



