Meafure of EleBricity.—'Defcrlption of Gibraltar. -i\:<^ 



quantity of furfacc is greatly increafed; in which cafe the velocity of the eleflric fluid 

 may be fuppofeJ infufficient for tlie whole charge to exift in the condu£iing wire at one amt 

 the fame time; or its impetus may be Icfs ; or, laftly, there may be a confiderablc waftc 

 from the conducing power of the air through tire very thin ftratum of air through which 

 the explollon at laft paliles. This laft fuppofiiion is far from being merely coujeftural. 

 For by fome experiments on the charge of a plate of air in eonneftion with the gold leaf 

 eleftrometer, of which I may hereafter give an account, I find that even m very low in- 

 tenfities the elecStric matter will pafs in a conftant ftream through intervals of about one 

 inch ; a-conclufion ■»i'hich nright likewifc hi deduced from die very fmall duration of the 

 ufual charge in the prime condu£lor. 



^Yith regard to the curious fa£l of the fpontaneous explofion of a clean jar ; vhlch, when 

 damped by breathing into it, was found much more capable of retaining its charge, there ap- 

 pears to be fome difficulty in the theory. I have mentioned my fuppofition that Mr. Brook's 

 experiment of foiling the jar with The hand, triglit have fucceeded from the pcrfpirable mat- 

 ter having operated as a varnifli to the expuVfion -of moifture from the atr. Mr. Cuthbert- 

 -fon's experiment overthrows this notion, i am now inclined to think that it depends upon 

 undulation. The bare furfacc of clean giafs may become charged in fucceflive zones of the 

 contrary e5e6lriciLies, as Prieftley anfi others have fliewrn. When thefe zones have acquir- 

 ed the requifite interifity, they may explode into each other, and produce an undulation in 

 the whole charge, which may greatly favour its flight through the interval from coating to 

 coating. But when irhe furface is covered with diftinft infulated particles of moillure, 

 •the efcape from particle to particle mull be by fmallev leaps ; the tranquillity of the charge 

 will be fcarcdy at all difturbed ; and the fpontaneous explofion wiJl not take place until the 

 intenfity has become fo great, as to carry it through a'fpcce equal to thefum of the inter- 

 >vals between any one rcw of particles which form a line from coating to coating. 



V. 



AJhtrt Mineralogical I)efcnption of the Mountain of Gibraltar. By Majof IMRIE. 



[Concluded from Page 187, Vol. II.] 



A' 



-T no great diflance from where thefe cryftals are found,'upon the fame fliope of the moun- 

 tain, but rather nearer to the level of the fea, a ftratum of argillaceous matter has been laid 

 open, divided into many thin beds, the broadeft of which does not exceed a foot in thicknefs. 

 Its general colour is of a whitifli grey, with a fmall mixture of yellow ; and it is divided 

 tranfverfely by ftraight fepta or cracks, both fides of which arc covered with dendritical 

 figures of a yellowifti brown colour, beautifully reppefenting the obje£ls-of landfcape. At 

 the weftcm bafe of the mountain, on a level with the fea, by which it is wafted, a very ex- 

 tenfive ftratum occurs of the fame nature as the "laft defcribed, bearing from north to fouth, 

 parallel with and dipping towards the mountain nearly at an angle of 40 degrees. 



In fome parts of the weftern Hope of the mountain, towards the fouth, are found nefts of 

 a dark red ftiivery clay, in which are imbedded flints of a dirty fap green colour : of thofe ho 

 regular ftratutn is to be perceived ; many of them are unfhapely maffes j but the^y in genc- 



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