[38] p. 226.—“A more complete determination of the height of all parts of the margin of the crater.”
Oltmanns, my astronomical fellow labourer, of whom, alas! science has been early deprived, re-calculated the barometric measurements of Vesuvius referred to in the preceding memoir (of the 22d and 25th of November and of the 1st of December, 1822), and has compared the results with the measurements which have been communicated to me in manuscript by Lord Minto, Visconti, Monticelli, Brioschi, and Poulett Scrope.
A. Rocca del Palo, the highest and northern margin of the Crater of Vesuvius.
| Toises. | Eng. ft. | |
|---|---|---|
| Saussure, barometric measurement computed in 1773, probably by Deluc’s formula | 609 | 3894 |
| Poli, 1794, barometric | 606 | 3875 |
| Breislak, 1794, barometric (but, like Poli, the formula employed uncertain) | 613 | 3920 |
| Gay-Lussac, Leopold von Buch, and Humboldt, 1805, barometric, computed by Laplace’s formula, as are also all the barometric results which follow | 603 | 3856 |
| Brioschi, 1810, trigonometric | 638 | 4080 |
| Visconti, 1816, trigonometric | 622 | 3977 |
| Lord Minto, 1822, barometric, often repeated | 621 | 3971 |
| Poulett Scrope, 1822, barometric, somewhat uncertain from the proportion between the diameters of the tube and cistern being unknown | 604 | 3862 |
| Monticelli and Covelli, 1822 | 624 | 3990 |
| Humboldt, 1822 | 629 | 4022 |
Most probable result 317 toises, or 2027 English feet, above the Hermitage; or 625 toises, or 3996 English feet, above the level of the sea.
B. The lowest and southern margin of the crater opposite to Bosche Tre Case.
| Toises. | Eng. ft. | |
|---|---|---|
| After the eruption of 1794 this edge became 400 (426 Eng.) feet lower than the Rocca del Palo; therefore if we estimate the latter at 625 toises (3996 English feet) | 559 | 3574 |
| Gay-Lussac, Leopold von Buch, and Humboldt, 1805, barometric | 534 | 3414 |
| Humboldt, 1822, barometric | 546 | 3491 |
C. Height of the cone of scoriæ inside the crater, which fell in on the 22d of October, 1822.
| Toises. | Eng. ft. | |
|---|---|---|
| Lord Minto, barometric | 650 | 4156 |
| Brioschi, trigonometric, according to different combinations either | 636 | 4066 |
| Or | 641 | 4098 |
Probable final result for the height of the above-mentioned cone of scoriæ 646 toises, or 4130 English feet.
D. Punta Nasone, highest summit of the Somma.
| Toises. | Eng. ft. | |
|---|---|---|
| Schuckburgh, 1794, barometric, probably computed by his own formula | 584 | 3734 |
| Humboldt, 1822, barometric, Laplace’s formula | 586 | 3747 |
E. Plain of the Atrio del Cavallo.
| Toises. | Eng. ft. | |
|---|---|---|
| Humboldt, 1822, barometric | 403 | 2577 |
F. Foot of the cone of ashes.
| Toises. | Eng. ft. | |
|---|---|---|
| Gay-Lussac, Leopold von Buch, and Humboldt, 1805, barometric | 370 | 2366 |
| Humboldt, 1822, barometric | 388 | 2481 |
G. Hermitage del Salvatore.
| Toises. | Eng. ft. | |
|---|---|---|
| Gay-Lussac, Leopold von Buch, and Humboldt, 1805, barometric | 300 | 1918 |
| Lord Minto, 1822, barometric | 307.9 | 1969 |
| Humboldt, 1822, barometric repeated | 308.7 | 1974 |
Part of my measurements have been printed in Monticelli’s Storia de’ fenomeni del Vesuvio, avvenuti negli anni 1821-1823, p. 115; but the neglected correction for the height of the mercury in the cistern has somewhat disfigured the results as there published. When it is remembered that the results given in the above table were obtained with barometers of very different constructions, at various hours of the day, with winds from very different quarters, and on the unequally heated declivity of a volcano, in a locality in which the decrease of atmospheric temperature differs greatly from that which is supposed in our barometric formulæ,—the agreement will be found to be as great as could be expected, and quite satisfactory.
My measurements in 1822, at the time of the Congress of Verona, when I accompanied the late King of Prussia to Naples, were made with more care and under more favourable circumstances than those of 1805. Differences of height are besides always to be preferred to absolute heights, and these show that since 1794 the difference between the heights of the edges of the crater at the Rocca del Palo and on the side towards Bosco Tre Case has continued almost the same. I found it in 1805 exactly 69 toises (441 English feet), and in 1822 almost 82 toises (524 English feet). A distinguished geologist, Mr Poulett Scrope, found 74 toises (473 English feet), although the absolute heights which he assigns to the two sides of the crater appear to be rather too small. So little variation in a period of twenty-eight years, in which there were such violent commotions in the interior of the crater, is certainly a striking phænomenon.
The height attained by cones of scoriæ rising from the floor of the crater of Vesuvius is also deserving of particular attention. In 1776 Schuckburgh found such a cone 615 toises, or 3932 English feet, above the surface of the Mediterranean: according to the measurements of Lord Minto, (a very accurate observer,) the cone of scoriæ which fell in on the 22d of October, 1822, even attained the height of 650 toises, or 4156 English feet. On both occasions, therefore, the height of the cones of scoriæ in the crater surpassed that of the highest part of the margin of the crater. When we compare together the measurements of the Rocca del Palo from 1773 to 1822, we are almost involuntarily led to entertain the bold conjecture that the north margin of the crater has been gradually upraised by subterranean forces. The accordance of the three measurements between 1773 and 1805 is almost as striking as that of those taken from 1816 to 1822. In the latter period we cannot doubt the height being from about 621 to 629 toises (3970 to 4022 English feet). Are the measurements made from thirty to forty years earlier, which gave only 606 to 609 toises (3875 to 3894 English feet), less certain? At some future day, after longer periods shall have elapsed, it will be possible to decide what is due to errors of measurement, and what to an actual rise in the margin of the crater. There cannot be in this case any accumulation of loose materials from above. If the solid trachyte-like lava beds of the Rocca del Palo really become higher, we must assume them to be upheaved from below by volcanic forces.
My learned and indefatigable friend Oltmanns has placed all the details of the above measurements before the public, accompanied by a careful critical examination of them, in the Abhandl. der königl. Akademie der Wissenschaften zu Berlin, 1822-1823, S. 3-20. May this investigation be the means of inducing geologists frequently to examine hypsometrically this low and most easily accessible (except Stromboli) of the European volcanos, so that in the course of centuries there may be obtained a frequently checked and accurate account of its periods of development!
[39] p. 235.—“Where the pressure is less.”
Compare Leopold von Buch on the Peak of Teneriffe in his Physikalische Beschreibung der canarischen Inseln, 1825, S. 213; and in the Abhandlungen der königl. Akademie zu Berlin, 1820-1821, S. 99.
[40] p. 289—“Waters of springs rising from different depths.”
Compare Arago in the Annuaire du Bureau des Longitudes pour 1835, p. 234. The increase of temperature is in our latitudes 1° of Reaumur (2°.25 of a degree of Fahrenheit) for every 113 Parisian feet (120.5 English feet), or 1° Fah. to 53.5 English feet nearly. In the Artesian boring at New Salzwerk (Oeynhausen’s Bad), not far from Minden, which is the greatest known depth below the level of the sea, the temperature of the water at 2094½ Parisian feet (2232¼ Eng.) is fully 26°.2 Reaumur, or 91° Fahr.; while the mean temperature of the air above may be taken at 7°.7 Reaumur, or 49°.2 Fahr. It is very remarkable that in the third century Saint Patricius, Bishop of Pertusa, was led by seeing the hot springs near Carthage to a very just view respecting the cause of such an increase of heat. (Acta S. Patricii, p. 555, ed. Ruinart; Kosmos, Bd. i. S. 231,—English Edition, Vol. i. p. 211.)