Footnote 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.
Footnote 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.
Footnote 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]