362 Analysis of Scientific Booh. 



cular that we feel the difficulty of giving a view in analysis of 

 these details, for want of the same aid, so here particularly we 

 must refer our readers to the original work. 



At page 58, he makes some observations of the power of at- 

 mospheric pressure separately considered, in modifying the vol- 

 canic action on the principle of a restraining force, and concludes 

 that the effect is sufficiently marked to render the lava in the 

 crater subject to an influence varying with the barometric pres- 

 sure. The action will be analogous to that which affects the 

 boiling point of water under similar circumstances ; and, in proof 

 of this effect, Stromboli is used as a barometer by the natives. 

 The author even considers that this pressure exerted upon the 

 volcanic aperture, may act so powerfully, as to affect the interior 

 of the earth, so as to regulate the appearance or otherwise of 

 earthquakes ; it being supposed, in this case, that a similar pres- 

 sure is acting on those vents " contemporaneously over a vast 

 extent of the earth's surface." 



Thus he also considers that hurricanes may be the cause of 

 volcanic eruptions and earthquakes, instead of being the effects, 

 as has been supposed ; their action consisting in diminishing, as 

 is well known, the pressure on the barometer, and thus in re- 

 moving a portion of the restraining force. He admits, however, 

 that the volcanic eruptions re-act upon the atmosphere, producing 

 various meteoric phenomena, such as hurricanes themselves, as 

 well as the lightnings and rains formerly mentioned. 



We proceed now to the third chapter, which is entitled " Dis- 

 position of Volcanic Products." 



The obvious result of eruption, in the manner formerly de- 

 scribed, is to produce a volcanic cone, containing at the apex, 

 which is truncated, an inverted hollow cone, which is the crater. 

 The form of the cone is modified by various causes, such as the 

 sizes of the fragments, their cohesion, <^c, while that of the 

 crater will also depend on such causes, and further on the form 

 of the fissure, which is the vent. And thus when the fissure is 

 prolonged, the crater may be elliptical ; or there may be more 

 than one volcanic cone ; or, lastly, a ridge may be the result. 

 Thus, also, an original irregularity of the ground may modify the 

 forms, as is also found to happen from violent winds occurring 

 during the period of the eruption. Lastly, the emission of lava 

 from particular points is a frequent cause of irregularity in the 

 forms of volcanic mountains. Very obviously this happens when 

 the lava in ebullition fills the crater, and then breaking down by- 

 its weight the feeblest side, flows down the surface of the hill. 

 The cone, thus broken at the summit, sometimes remains in that 

 state ; while at others, eruptions of scoria following the flow of 

 the lava, replace it, and even sometimes so as to conceal the ori- 

 ginal orifice. 



