VOLCANOES. 



259 



iEtna. The colossal summits of the Andes, Cotopaxi and Tungu- 

 rahua, scarcely have an eruption once in a century. The Peak of 

 TeoerifFe seemed to be extinguished for ninety two years, when it 

 made its last eruption by a lateral opening in 1798. In this interval, 

 Vesuvius had sixteen eruptions." The greatest eruptions of lava 

 from Mtm and Vesuvius are always from the sides of these moun- 

 tains ; but these lateral eruptions finish by an ejection of ashes and 

 flames from the crater at the summit of the mountain. In the Peak 

 of TenerifFe, an eruption of lava from the summit has not taken 

 place for ages; and in the recent great eruption of 1798, the crater 

 remained inactive, nor did its bottom fall in. 



The observation of M. Humboldt, that lofty volcanoes have the 

 longest periods of repose, will not be found universally correct. 

 The small volcano of Volcano, one of the Lipari islands, was in a 

 dormant state for thirteen hundred years, while the volcano of Popo- 

 catapetl, fourteen leagues from Mexico, which is nearly eighteen 

 thousand feet above the level of the sea, seems to be in a state of 

 constant activity. It was ascended by Lieutenant William Glennie, 

 in 1827. The volcano rises from a country that is 8216 feet above 

 the sea ; its sides are thickly wooded with pine forests to the height 

 of nearly 13,000 feet: beyond this altitude vegetation ceased en- 

 tirely. The ground consisted of loose black sand of considerable 

 depth, in which numerous fragments of pumice and basalt were dis- 

 persed ; above this, were several projecting ridges of loose fragments 

 of basalt, arranged one above another. At the summit, the mercury 

 subsided to 15.63 inches. The crater appeared to extend one mile 

 in diameter ; the interior walls consisted of masses of rock, arranged 

 perpendicularly, and marked by numerous vertical channels, filled, 

 in many places, with black sand. Four horizontal circles of rock, 

 differently coloured, were also noticed within the crater. From the 

 edges of the latter, as well as from its perpendicular walls, several 

 small columns of vapour arise, smelling strongly of sulphur. The 

 noise was incessant, resembling that heard near the sea shore during 

 a storm. At intervals of two or three minutes, the sound increased, 

 followed by an irruption of stones : the larger fell again into the cra- 

 ter, the smaller were projected into the ravine through which the 

 party had ascended. 



The volcano of Popocatapetl is, perhaps, the loftiest active volca- 

 no that has been ascended, and yet, according to Humboldt, it some- 

 times pours out currents of lava from the summit. 



Those who are acquainted with hydrostatics, and know the im- 

 mense power that would be required to raise even a column of water 

 from the level of the sea to the top of Popocatapetl, jEtna, or Ten- 

 eriflfe, will not be surprised that, in lofty volcanic mountains, the lava 

 forces itself out of the sides, and rarely rises to the top of the crater. 

 It has been calculated, that the force required to raise a column of 

 lava to the height of the summit of TenerifFe, (twelve thousand five 



