196 
THE GEOLOGIST. 
lent shower, lasting but a shoi-t period of time, and followed by a 
gleam of sunshine which was powerful enough to produce a rapid 
drying of tlie sand, and the formation of shrinkage-cracks, which, 
together with the depressions formed by the rain-drops, the latter 
often so strikingly marked as to indicate the very direction of the 
shower, became covered by a thin layer of sand on the next flowing 
of the tide. The shrinkage-cracks and the large drops of rain I have 
heard described as instances ot fossil sunshine. 
Fulgurites were first discovered by the shepherd Herman in 1711, 
whose specimens are still preserved in the Museum at Dresden ; 
Hentzen next found them, in 1805, and he was the first to recognise 
their true nature. They are generally compressed in form, mostly 
hollow, and taper in their descent into the sand vertically. Some are 
distinctly furcated, and in many specimens lateral branches, also tu- 
bular and from two to three inches long, and not exceeding a quarter 
of an inch at the point of junction, proceed from various parts of the 
parent tube. These small branches gradually bend downwards, and 
assume a more or less conical form, terminating in abrupt and closed 
points. If the lightning has encountered the resistance of pebbles, or 
has passed through wet sand, the tube becomes not only contorted 
and twisted in its course, but is also much flattened and compressed. 
Mr. Irton found a tube at Drigg, which was hollow for eight or nine 
inches, then became completely solid without any central perforation, 
while lower down it again assumed the rugged and tubular condition.* 
The extreme diameter (or bore (?) of the lightning) of these tubes 
is an inch and a half;t the internal cavity is rarely cii'cular, being 
either triangular, quadrangular, pentangular, oval, or some irregular 
figure ; their length, as stated before, is from a few feet to upwards 
of forty feet; the thickness of their walls has reached -Y^th of an inch, 
and their largest circumference from two to five inches, as I have 
determined by actual measurement. 
Nearly all the tubes met with have numerous longitudinal furrows 
and indentations on their external surface, and have not inaptly been 
* Trans. Geol. Soc. vols. ii. and v. 
t The agghitinating power of the electric fluid sometimes fomis a mass of 2i 
inches diameter, with a tube above and below it ; many such interruptions have 
been found in the course of a single tube, from the lightning having met with 
obstacles in its passage. 
