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wUm-M* il paffe*. Some of tlie adfcititious clouds appear 

 bte while fringfs at the Ikirtsof the thunder-cloud, or under 

 Okc body of il, but they keep continuaUy growing darktr 

 and darker, as they approach to unite with it. 



When the thunder-cloud is grown to a great file, its lower 

 (urface it often ragged, particular parts being detached to- 

 wards the earth, but Hill conneded with the reft. Some- 

 Onet the lower furface fwells into various large protube- 

 rances, bending uniformly toward the earth. And fome- 

 timet one wliole fide of the cloud will have an inchnation to 

 Uk firth, and the extremity of it will nearly touch the 

 earth. When the eye is under the thunder-cloud, after it is 

 OTown larger, and well formed, it is feen to fink lower, and 

 to 4arken prodigioufly ; at the fame time that a number of 

 rmall .idfcititious clouds (the origin of which can never be 

 perceived) are feen in a rapid motion, driving about in very 

 inicertiiin directions under it. While tliefe clouds are agitated 

 with the moll rapid motions, the rain generally falls in the 

 greatell plenty, and if the agitation be exceedingly great, it 

 commonly hails. 



While the thunder-cloud is fwelliiig, and extending its 

 branches over a large tra<ft of country, the lightning is feen 

 to dart from one part of it to another, and often to illu- 

 minate its whole mafs. When the cloud has acquired a fuf- 

 ficient extent, the hghtning ftrikes between the cloud, and 

 the earth, in two oppolite places, the path of the hghtning 

 lying through the whole body of the cloud and its branches. 

 The longer this lightning continues, the rarer does the cloud 

 grow, and the lefs dark is its appearance ; till, at length, it 

 breaks in different places, and fhews a clear ilvy. When the 

 thunder-cloiid is thus difperfed, thofe parts which occupy 

 the upper regions of the atmofphere are equally fpread, and 

 very thin ; and thofe that are underneath arc black, but thin 

 too ; and they vanilh gradually, without being driven away 

 by any wind. 



Thefe thimder-clouds were fometimes in a pofitive as well 

 as negative ftate of eleftricity. The eleftricity continued 

 longer of the fame kind, in proportion as the thunder-cloud 

 was fimple and uniform in its direftion ; but when the light- 

 ning changed its place, there commonly happened a change 

 in the electricity of the apparatus over which the clouds 

 pafTed. It would change fuddenly after a very violent flafii 

 of hghtning, but the change would be gradual when the 

 lightning was moderate, and the progrefs of the thunder- 

 cloud flow. Beccar. Lettere dell' EUetricifmo, p. 107 ; or 

 Prieftley's Hift. Eleftr. vol. i. p. 397, &:c. See LlGHT- 



.NING. 



TuUKDER-Z/'ou/t, in EUSrkity, is an inftrument invented 

 by Dr. James Lind of Edinburgh, for illuftrating the man- 

 ner by which buildings receive damage from hghtning, and 

 to evince tlie. utility of metallic conduftors in preferving 

 them from it. 



A [PlaU XV. EUc!ndty,J!g.2.) is aboard about three 

 quarters of an inch thick, and lliaped hke the gable-end of 

 a houfe. This board is fixed perpendicularly upon the bot- 

 tom board B, upon which the perpendicular glafs pillar 

 C D 15 alfo fixed in a hole about eight inches diftant from 

 the bafis of the board A. A fquare hole 1 L M K, about 

 a quarter of an inch deep, and nearly an inch wide, is made 

 in the board A, and is filled with a fquare piece of wood, 

 nearly of the fame dimenfions. It is nearly of the fame di- 

 menfions, becaufe it muft go fo eafily into the hole, that it 

 may drop off, by the kaft (baking of the inftrument. A 

 wire, L K, is faftened diagonally to this fquare piece of wood. 

 Another wire, I H, of the fame thicknefs, having a brafs ball, 

 H, fcrewed on its pointed extremity, is, faftened upon the 



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board A : fo alfo is the wire M N, which is (haped in 3 

 ring at O. From the upper extremity of the glafs pillar 

 C D, a crooked wire proCLids, having a fpring focket F, 

 through which a double knobbed wire Hips perpendicularly, 

 the lower knob, G, of which falls juft above the knob H. 

 The glafs pillar D C muft not be made very faft into the 

 bottom board ; but it muft be fixed fo that it may be pretty 

 eafily moved round its own axis, by whicli means the brafs 

 ball G may be brought nearer or farther from the ball H, 

 without touching the part E F G. Now when the fquare 

 piece of wood L M I K (which may reprefent the fhulter 

 of a window or the like ) is fixed into the hole fo that the 

 wire L K ftands in the dotted reprefentation I M, then the 

 metallic communication from H to O is complete, and the 

 inftrument reprefents a houfe furniftied with a proper me- 

 talhc conduftor ; but if the fquare piece of wood L M I K 

 is fixed fo that the wire L K ftands in the diretlion L K, as 

 reprefented in the figure, then the metallic conduftor H O, 

 from the top of the houfe to its bottom, is interrupted at 

 I M, in which cafe the houfe is not properly fecured. 



Fix the piece of wood L M I K, fo that its wire may be 

 as reprefented in the figure, in which cafe the metallic con- 

 ductor H O is difcontinued. Let the ball G be fixpd at ' 

 about half an inch perpendicular diftance from the ball H, 

 then, by turning the glafs pillar D C, remove the formei- 

 ball from the latter ; by a wire or chain conneft the wire 

 E F with the wire Q of the jar F, and let another wire or 

 chain, faftened to the hook O, touch the outfide coating of 

 the jar. Conneft the wire Q with the prime conduftor, and 

 charge the jar ; then, by turning the glafs pillar D C, let 

 the ball G come gradually near the ball H, and when they 

 are arrived fufficiently near one another, you will obferve, 

 that the jar explodes, and the piece of wood L M I K is 

 pufhed out of the hole to a confiderable diftance from the 

 thunder-houfe. Now the ball G, in this experiment, repre- 

 fents an eleftritied cloud, which, when it is arrived fuf- 

 ficiently neai- the top of the houfe A, the eleftricity ftrikes 

 it, and as tliis houfe is not fecured with a proper conduftor, 

 the explofion breaks part of it, i. e- knocks off the piece of 

 wood I M. 



Repeat the experiment with only this variation, viz. that 

 this piece of wood I M is fituated fo that the wire L Iv may 

 ftand in the fituation I M ; in which cafe the conduftor 

 H O is not difcontinued ; and you will obferve that the ex- 

 plofion wiU have no effeft upon the piece of wood L M, 

 this remaining in the hole unmoved ; which fiiews the ufe- 

 fulnefs of the metallic conduftor. 



Farther, unfcrew the brafs ball H from the wire H I, fo 

 that this may remain pointed, and with this difference only 

 in the apparatus repeat both of the above experiments, and 

 you will find that the piece of wood I M is in neither cafe 

 moved from its place, nor any explofion will be heard, which 

 not only demonftrates the preference of conduftors with 

 pointed terminations to thofe with blunted ones, but alfo 

 ftiews that a houfe furniftied with ftiarp terminations, al- 

 though not furniftied with a regular conduftor, is alfo fuf- 

 ficiently guarded againft the effefts of lightning. 



Mr. Hcnly, having connefted a jar containing 509 fquare 

 inches of coated furface with his prime conduftor, obferved 

 that if it was fo charged as to raife the index of his eleftro- 

 meter to 60°, by bringing the ball on the wire of the thun- 

 der-houfe, to the diftance of half an inch from that connefted 

 with the prime conduftor, the jar would be difcharged, and 

 the piece in the thunder-houfe thrown out to a confiderable 

 diftance. Ufing a pointed wire for a conduftor to the 

 thunder-houfe, iiiftead of the knob, the charge being the 



fame 



