These Dimensions Are However Small,
Compared To Those Of The Tubes From Drigg, One Of Which
Was Traced To A Depth Of Not Less Than Thirty Feet.
The internal surface is completely vitrified, glossy, and
smooth.
A small fragment examined under the microscope
appeared, from the number of minute entangled air or perhaps
steam bubbles, like an assay fused before the blowpipe.
The sand is entirely, or in greater part, siliceous; but some
points are of a black colour, and from their glossy surface
possess a metallic lustre. The thickness of the wall of the
tube varies from a thirtieth to a twentieth of an inch, and
occasionally even equals a tenth. On the outside the grains
of sand are rounded, and have a slightly glazed appearance:
I could not distinguish any signs of crystallization. In a
similar manner to that described in the Geological Transactions,
the tubes are generally compressed, and have deep
longitudinal furrows, so as closely to resemble a shrivelled
vegetable stalk, or the bark of the elm or cork tree. Their
circumference is about two inches, but in some fragments,
which are cylindrical and without any furrows, it is as much
as four inches. The compression from the surrounding loose
sand, acting while the tube was still softened from the
effects of the intense heat, has evidently caused the creases
or furrows. Judging from the uncompressed fragments, the
measure or bore of the lightning (if such a term may be used)
must have been about one inch and a quarter. At Paris, M.
Hachette and M. Beudant [11] succeeded in making tubes, in
most respects similar to these fulgurites, by passing very
strong shocks of galvanism through finely-powdered glass:
when salt was added, so as to increase its fusibility, the tubes
were larger in every dimension, They failed both with
powdered felspar and quartz. One tube, formed with
pounded glass, was very nearly an inch long, namely .982,
and had an internal diameter of .019 of an inch. When we
hear that the strongest battery in Paris was used, and that
its power on a substance of such easy fusibility as glass was
to form tubes so diminutive, we must feel greatly astonished
at the force of a shock of lightning, which, striking the sand
in several places, has formed cylinders, in one instance of at
least thirty feet long, and having an internal bore, where not
compressed, of full an inch and a half; and this in a material
so extraordinarily refractory as quartz!
The tubes, as I have already remarked, enter the sand
nearly in a vertical direction. One, however, which was less
regular than the others, deviated from a right line, at the
most considerable bend, to the amount of thirty-three degrees.
From this same tube, two small branches, about a
foot apart, were sent off; one pointed downwards, and the
other upwards. This latter case is remarkable, as the electric
fluid must have turned back at the acute angle of 26 degs.,
to the line of its main course. Besides the four tubes which
I found vertical, and traced beneath the surface, there were
several other groups of fragments, the original sites of which
without doubt were near. All occurred in a level area of
shifting sand, sixty yards by twenty, situated among some
high sand-hillocks, and at the distance of about half a mile
from a chain of hills four or five hundred feet in height. The
most remarkable circumstance, as it appears to me, in this
case as well as in that of Drigg, and in one described by
M. Ribbentrop in Germany, is the number of tubes found
within such limited spaces. At Drigg, within an area of
fifteen yards, three were observed, and the same number
occurred in Germany. In the case which I have described,
certainly more than four existed within the space of the
sixty by twenty yards. As it does not appear probable that
the tubes are produced by successive distinct shocks, we must
believe that the lightning, shortly before entering the ground,
divides itself into separate branches.
The neighbourhood of the Rio Plata seems peculiarly subject
to electric phenomena. In the year 1793, [12] one of the
most destructive thunderstorms perhaps on record happened
at Buenos Ayres: thirty-seven places within the city were
struck by lightning, and nineteen people killed. From facts
stated in several books of travels, I am inclined to suspect
that thunderstorms are very common near the mouths of
great rivers. Is it not possible that the mixture of large
bodies of fresh and salt water may disturb the electrical
equilibrium? Even during our occasional visits to this part
of South America, we heard of a ship, two churches, and a
house having been struck. Both the church and the house
I saw shortly afterwards: the house belonged to Mr. Hood,
the consul-general at Monte Video. Some of the effects were
curious: the paper, for nearly a foot on each side of the line
where the bell-wires had run, was blackened. The metal had
been fused, and although the room was about fifteen feet
high, the globules, dropping on the chairs and furniture, had
drilled in them a chain of minute holes. A part of the wall
was shattered, as if by gunpowder, and the fragments had
been blown off with force sufficient to dent the wall on the
opposite side of the room. The frame of a looking-glass was
blackened, and the gilding must have been volatilized, for a
smelling-bottle, which stood on the chimney-piece, was coated
with bright metallic particles, which adhered as firmly as
if they had been enamelled.
[1] Hearne's Journey, p. 383.
[2] Maclaren, art. "America," Encyclop. Brittann.
[3] Azara says, "Je crois que la quantite annuelle des pluies
est, dans toutes ces contrees, plus considerable qu'en Espagne."
- Vol. i. p. 36.
[4] In South America I collected altogether twenty-seven
species of mice, and thirteen more are known from the works
of Azara and other authors.
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