The Diameter Of The Whole Tube Was Nearly Equal,
And Therefore We Must Suppose That Originally It Extended To
A Much Greater Depth.
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.
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