| OCR Text |
Show 384 STRUCTURE, AND MODE OF ACTION determined, may be substituted for them; and for a comparison of determinations made at different epochs, angles of altitude so measured may even be often preferable to the complication of circumstances which more complete operations may involve. Saussure had measured Mount Vesuvius, in 1773, when the two margins of the crater, the north-western and the south-eastern, appeared to him to be of equal height. He found their height above the level of the sea 609 toises, 3,894 English feet. The eruption of 1794 occasioned a breaking down of the margin of the crater on the southern side, and a consequent inequality between the height of the two edges which the most unpractised eye does not fail to distinguish even at a considerable distance. In 1805, Leopold von Buch, GayLussac, and myself measured the height of Vesuvius three times, and found the northern margin opposite to La Somma (the Rocca del Palo) exactly as given by Saussure, but the southern margin 75 toises, or 450 French or 479 English feet, lower than he had found it in 1773. The whole elevation of the volcano on the side of Torre del Greco (the side towards which, for the last thirty years, the igneous action has, as it were, been principally directed) had at that time diminished one-eighth. The height of the cone of ashes, as compared with the whole height of the mountain, is in Vesuvius as 1 to 3; in Pichincha, as 1 to 10; and in the Peak of Teneriffe, as 1 to 22. In these three volcanic mountains, the cone of ashes is therefore, relatively speaking, highest in Vesuvius; probably because, being a low volcano, the actien has been principally by the summit. A few months ago (1822) I was enabled not only to repeat my former barometric measurements of the height of Vesuvius, but also, during the course of three visits to the summit, to make a more complete determination of all the edges of the crater. (') These determinations may not be without interest, since they include the long period of great eruptions between 1805 and 1822, and constitute perhaps the only known examination and measurement of a volcano at different epochs, in which the different parts of the examination are all truly comparable with each other. We learn from it that the margins of craters are a phenomenon of far more permanent character than had been previously inferred from passing |
