EXPLANATORY ADDITIONS.
[108]. P. 363.—“A more complete determination of the margins of the Crater of Mount Vesuvius.”
My astronomical fellow-labourer, Oltmanns, who was unhappily too early lost to science, has re-calculated the barometric measurements I made on Mount Vesuvius (from the 22nd to the 25th of November, and on the 1st of December, 1822), and compared the results with those yielded by the measurements given to me in manuscript by Lord Minto, Visconti, Monticelli, Brioschi, and Poulett Scrope.
| A. Rocca del Palo, the highest northern margin of the Crater of Vesuvius, was estimated by— | |
|---|---|
| Feet. | |
| Saussure, in 1773, barometrically, probably according to Deluc’s formula | 3894 |
| Poli (1794), barometrically | 3875 |
| Breislak (1794), barometrically, although, as in the case of Poli, it is uncertain what formula was used | 3920 |
| Gay-Lussac, Leopold von Buch, and Humboldt (1805), barometrically, according to the formula of Laplace, as in all the following barometric results | 3856 |
| Brioschi (1810), trigonometrically | 4079 |
| Visconti (1816), trigonometrically | 3977 |
| Lord Minto (1822), barometrically, and frequently repeated | 3971 |
| Poulett Scrope (1822). This calculation is somewhat uncertain, owing to the unknown relation of the diameters of the tubes to those of the cistern | 3862 |
| Monticelli and Covelli (1822) | 3990 |
| Humboldt (1822) | 4022 |
| The most probable final result is 2026 feet above the hermitage, or 3996 feet above the level of the sea. | |
| B. The lowest south-eastern margin of the Crater, opposite Bosche Tre Case. | |
| After the eruption of 1794, this margin was 426 feet lower than the Rocca del Palo, consequently, if the latter be estimated at 3996 feet, it would be | 3570 |
| Gay-Lussac, Leopold von Buch, and Humboldt (1805), barometrically | 3414 |
| Humboldt (1822), barometrically | 3491 |
| C. The elevation of the cone of scoriæ that fell into the Crater on the 22nd October, 1822. | |
| Lord Minto, barometrically | 4156 |
| Brioschi, trigonometrically, according to different combinations— | |
| Either | 4067 |
| Or | 4099 |
| The most probable final result for the height of the cone of scoriæ that fell in during the year 1822, is 4131 feet. | |
| D. Punta Nasone, the highest summit of the Somma. | |
| Shuckburgh (1794), barometrically, probably according to his own formula | 3734 |
| Humboldt (1822), barometrically, according to the formula of Laplace | 3747 |
| E. Plain of the Atrio del Cavallo. | |
| Humboldt (1822), barometrically | 2577 |
| F. Base of the cone of ashes. | |
| Gay-Lussac, Leopold von Buch, and Humboldt (1805), barometrically | 2366 |
| Humboldt (1822), barometrically | 2482 |
| G. Hermitage of Salvatore. | |
| Gay-Lussac, Leopold von Buch, and Humboldt (1805), barometrically | 1918 |
| Lord Minto (1822), barometrically | 1969 |
| Humboldt (1822), again barometrically | 1974 |
Some of my measurements have appeared in Monticelli’s Storia de’ fenomeni del Vesuvio, avvenuti negli anni 1821–1823, p. 115, but owing to the correction of the height of the mercury in the cistern having been omitted, the numbers are not given with perfect exactness. When it is remembered that the results contained in the above table were obtained with barometers of very different construction, at different hours of the day, during the prevalence of various winds, and on the unequally heated declivity of a volcano, in a locality where the decrease of the atmospheric temperature differs very considerably from that assumed in our barometrical formulæ, the amount of correspondence between the various results will appear sufficiently satisfactory.
My measurements of 1822, at the time of the Congress of Verona, when I accompanied the late King to Naples, were conducted with more care and under more favourable circumstances than those of 1805. Differences of elevations are moreover always preferable to absolute elevations. These differences show, that since 1794, the relative condition of the margins of the Rocca del Palo and of that towards Bosche Tre Case had remained almost the same. I found, in 1805, for the height, 441, and in 1822, nearly 524 feet. A distinguished geologist, Mr. Poulett Scrope, obtained 473 feet, although his absolute heights for these two margins of the crater appear somewhat too low. So inconsiderable a variation in a period of twenty-eight years, and during violent disturbances in the interior of the mountain, is undoubtedly a remarkable phenomenon.
The height to which the cones of scoriæ rise from the bottom of the crater at Vesuvius also deserves special attention. Shuckburgh found in 1776 a cone of this nature to be 3932 feet above the level of the Mediterranean; and, according to Lord Minto—a remarkably exact observer—the cone of scoriæ which fell in on the 22nd of October, 1822, was even 4156 feet high. On both occasions therefore the cone of scoriæ in the crater exceeded the highest point of the margin of the crater. On comparing the measurements of Rocca del Palo from 1773 to 1822, one is almost involuntarily led to hazard the bold conjecture that the northern margin of the crater has been gradually upheaved by subterranean forces. The correspondence of the three measurements made between 1773 and 1805 is almost as striking as in those between 1816 and 1822. No doubt can be entertained as to the height being from 3970 to 4021 feet during the latter period. Ought less confidence to be attached to the measurements made thirty or forty years previously, and which only gave from 3875 to 3894 feet? After a longer lapse of time the question may be decided, as to how much is attributable to errors of measurement, and how much to the upheaval of the margin of the crater. There is here no accumulation of loose masses from above; if therefore the solid trachytic lava strata of the Rocca del Palo actually rise, we must assume that they are upheaved from below by volcanic forces.
My learned and indefatigable friend, Oltmanns, has published the details of all these measurements with critical remarks.[[RH]] Would that this work might incite geognosists to enter upon a series of hypsometric observations, by which, in the course of time, Vesuvius, which is, excepting Stromboli, the most accessible of all European volcanos, may be thoroughly understood in all periods of its development.
[109]. p. 371—“At elevations 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, aus den J. 1820–21, s. 99.
[110]. p. 373—“Springs which rise from different depths.”
Compare Arago in the Annuaire du Bureau des Longitudes pour 1835, p. 234. The increase of the temperature is in our latitudes 1° Fahr. for nearly every 54 feet. In the Artesian boring at the New Salt-works (Oeynhausen’s Bath) near Minden, which is the greatest known depth that has been reached below the surface of the sea, the temperature of the water at 2231 feet, is fully 91° Fahrenheit, whilst the mean upper temperature of the air may be assumed at 49°·3 Fahr. It is very remarkable that, even in the third century, Saint Patricius, bishop of Pertusa, should have been led, from the thermal springs near Carthage, to form a very correct view of such an increase of heat.[[RI]]