196 



THE GEOLOGIST. 



lent shower, lasting but a short period of time, and followed by a 

 gleam of sunshine which was powerful enough to produce a rapid 

 drying of the 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 Jlowing 

 of the tide. The shrinkage-cracks and the large drops of rain I have 

 heard described as instances of /o5sz7 stmshine. 



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 €hese tubes 

 is an inch and a halfjt the internal cavity is rarely circular, 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 J^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 Tlie agcrhitinating power of the electric fluid sometimes forms a mass of 2^ 

 inches diameter, with a tube above and below it ; many such interruptions have 

 been found m tlie course of a single tube, fi:om the lightning having met with 

 obstacles in its passage. 



