340 Analysis of Scientific Books and Memoirs. 



of repression, and produce elevations of the superficial strata, earthquakes, 

 &c. The author thus distinguishes hetween the general or primary, and 

 the local or secondary expansive forces, each having their peculiar force : 

 the first residing in the general subterranean bed of heated rock ; the se- 

 cond in minor and less deeply seated foci. The laws thus determined 

 hold good, whatever the scale of magnitude of the phenomena they give 

 rise to, whether the elevation of a few square yards of rock, or of a whole 

 continent ; a quiescent interval of a few hours, or of centuries. 



Every habitual volcano acts, therefore, as a safety-valve to the globe, the 

 caloric which emanates from its anterior passing off by means of this 

 vent into outer space. But these eruptions are necessarily accompanied by 

 circumstances tending to impede their continuance, and they are thus, in 

 the generality of cases, rendered intermittent. Where the opposing forces 

 of expansion and repression are in equilibrio, the volcano is in the first of 

 the phases noticed above. Where they oscillate frequently about an equi- 

 librium, in the second. Where the oscillations are on a large scale, in 

 the third. The first case must necessarily be very rare. In the instance of 

 Stromboli, our author attributes the permanence of its eruption solely to 

 the peculiar form of the crater ; the aperture of the volcano having a high 

 and sloping ridge only on one side; on the other a precipitous slope down 

 to the sea, which is there unfathomable. Owing to this remarkable figure, 

 less than one half of the scoriae projected from the aperture at each explo- 

 sion fall again into it, and, consequently, there can be no accumulation of 

 fragments on the surface of the lava within the vent, which always re- 

 mains level, or nearly so, with the mouth of this aperture, without being- 

 discharged otherwise than in fragments tossed up by the bubbles of va- 

 pour which escape from it. The volcano of Bourbon again, a similar ex- 

 ample of almost continual eruption, is shown by the author to owe this 

 character to another peculiarity of form. This volcanic mountain is a 

 complete obtuse cone, and there exists at the apex an almost permanent 

 source of a very fluid and glassy lava, which slowly boils over the lips of 

 the circular orifice, and flows rapidly on all sides down the steep slopes 

 of the cone. Thus, in this, as in the former instance, the force of repres- 

 sion remains fixed, and that of expansion being always slightly in excess, 

 the eruption is permanent. Where, however, these forces are so nearly in 

 equilibrio, a very slight addition to that of repression may stop the erup- 

 tion for a certain time, and Mr P. S. supposes that even changes in the 

 density of the atmosphere will occasionally produce this effect ; and that 

 a permanently active volcano, whose phenomena are, according to him, 

 occasioned by the ebullition of water in the focal lava, from constant and 

 uniform additions of caloric to it, will be as sensible as the barometer to 

 variations in the pressure of the atmosphere on the surface of the sup- 

 ported column of lava ; the ebullition ceasing for a few minutes or hours 

 as the density of the atmosphere increases, and increasing in energy as it 

 is diminished. Observation 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 



