1152 



METEOROLOGY. 



[I.ICIITNIXG, no. 



made up of watery vesicles, are electrical 

 ictoni ; and being surrounded by the atmosphere 

 on all lido*, are necessarily insulated. ' Fur the sake of 

 illustration, it will now suffice to assume that neither 

 .-presented is electrically excited at 

 this period of the description, and li.-n.v tint the marks 

 + and are for the present misplaced. Let it now be 

 named that the cloud A become! positively el. .-inn. -.i. 

 that is to say, charged with positive <'. wiug 



to some natural cause unnecessary here to explain ; ami 

 li -t the results of this condition be traced out Firstly, 

 tliere is nut in all nature, and there rnn,:ut If, such a 

 condition as that of independent electric excitation; in 

 t words, there cannot be one body positively excited 

 without the co-existence of another body negatively ex- 

 it" cl. mil B were away, and cloud A posi- 

 y excited, the aircircumadjacent to A would assume 

 the second or negative function ; luit if the cloud B is 

 present, it therefore becomes negative, and the two 

 cloud.-. A and 15 are mutually attracted, because opi 



arh uther. Hence, they approach 

 until the space of uir, between the two, is insufficient to 

 restrain their mutual electric tension : this OOndition 

 having arrived, a discharge takes place, precisely analo- 

 gous to the discharge of a Leyden jar. Under the pos- 

 tulates of our experiment, the discharge, or lightning 

 flash, takes place between the two clouds A and B. 



It follows, however, from the consideration of known 

 electrical laws, that just as the two oppositely electrified 

 bodies may be two clouds, as assumed, so also may they 

 be ..no cloud, and the surface of earth or water, or con- 

 ductors placed upon either one or tho other ; under 

 hieh conditions a downward discharge will take place ; 

 and generally, electricity will always take the in- 

 put h l>etwceii any two bodies oppositely charged, the 

 c .inducting facilities being equal. 



[>C< TORS. It is almost unnecessary, in 

 these days, to announce that Franklin, in the year 17"'-. 

 first di-niKiistrated tho nature of lightning by drawing 

 i ie sparks from the string of a kite, previously 

 caused to ascend into the region of a thunder-cloud'. 

 This experiment performed, the connection bet 

 lightning and electricity could no longer be doubted, and 

 a means of drawing off a surcharge of the electric fluid 

 l.y lightning-conductors was immediately suggested. 

 The most important instruments were not adopted, how- 

 ever, until after numerous and varied conflicts. Firstly, 

 the argument was adduced by some, that lightning con- 

 ductors could not be adopted without impiety, being in- 

 tended to contravene the will of Providence. An argu- 

 ment so fallacious was no sooner abandoned than light- 

 conductors were exposed to another ordeal, founded 

 i erroneous practical estimation of a truth in theo- 

 iy. \\ t! allude, as the electrician will 

 perceive, to the contest between the advocates of spheri- 

 cal, and of pointed, terminations for electrical con- 



>rs. 



Now, regarding the question of points or spheres 

 abstractedly, it is easy to see that preference should be 

 given to the former, inasmuch as points draw oil' and 

 give issue to the electric fluid in silence ; whereas sp; 

 draw off and give issue to electricity in sparks ; but, in. 

 asmuch aa the largest spherical termination ever used, 

 or ever likely to bo used, for tho upper extremity of a 

 lightning-conductor, is virtually a point in comparison 

 w ith the enormous surface of the smallest thunder-cloud, 

 th- depute, though violent and prolonged, never had 

 the practical .significance which was at one time taken 

 In panted. 



The history of lightning-conductors furnishes a remark- 

 able illustration of th.- difference between the more k 

 ledge of a fact, and the confidence or conviction resulting 

 . that knowledge, and justifying its practical appli- 

 cation. The whole theory of lightning-conduct. >rs was 

 aliiHMt a* well known half a century ago as now ; yet it 

 !y within the last few years, and owing to tho un- 

 flagging perseverance of Sir \V. Snow Harris, th.it due 

 effect has been given to tic theory, and lightning-rods 

 have been fearlessly applied. 



' Principle* connected with Liihtitinj-Con- 

 *. The electrical principles on which 

 ciency of lightning-conductors depends are few and simp]..; 

 they all admit of being readily demonstrated by el. 

 cal experiments artificially performed, and they have 

 been universally justified by the result of three practical 

 applications : 



(1.) Electricity, cat- '-.- course throriiih 



the best Conductors ir/nV h't/'inn to be in it* )ith.- 

 Thus, for example, if a Leyden jar be charged, and the 

 electric connection between its external ami its internal 

 coating be completed by three linear substances of equal 

 length say, for example, silk, wire, and linen thread 

 thin wire, Ix-ing the l>ost electrical conductor of tluiti 

 will transmit the whole of the electricity, to the exclusion 

 of tho silk and the linen thread. It is assumed, hovv- 

 111 the performance of this experiment, that tho 

 wire U sufficiently large to convey thu whole electric dis- 



(J. ) Provided the Condurtur In- i/and, and its Sectional 

 Area adequate, the }'. Discharge is conveyed 



harmlessly away. No point in the whole of electrical 

 science can be more satisfactorily established than this. 

 It admits of being illustrated by numerous experiments, 

 among which the follo.i ing may sntlice : 



Tho diagra 11 (Fig. 04) represents a common Leyden 

 Fig. w. jar, in the :. 



being discharged by 

 the ordinary discharg- 

 ing instrument. This 

 is, as usual, of brass 

 all save the handle, 

 which is of glass, and 

 therefore a non-con- 

 ductor of electricity. 

 Those who are con- 

 versant with the 

 form and construc- 

 tion of the discharger, 

 arc aware that its two terminal balls admit of being un- 

 screwed. Assuming one of the balls, viz., tho upper 

 one in the diagram, to have been unscrewed, the libe- 

 rated brass stem to be passed through a marroon, or box, 

 holding gunpowder, and the ball to have been again re- 

 placed, the conditions will have been fulfilled which the 

 diagram represents. It is evident that the Leyden jar, 

 as represented, will be discharged. It is, moreover, 

 evident that the whole of the charge will be transmitted 

 through the gunpowder contained in the marroon, yet 

 that gunpowder will not inflame. If, however, iuste.nl 

 of the conditions of the last experiment, a very tine 

 metal wire (a steel wire by preference) bo passed through 

 the marroon, or rather through some combinations of ex- 

 plosive materials less potent than a marroon, which 

 would now be dangerous, and electricity transmitted 

 before, the wire, not presenting a sufficient amount 

 of transverse area of Fig. 65. 



surface to convey 

 the electricity, would 

 melt, and the explo- 

 mpound be in- 

 flamed. 



(3.) Lateral Dis- 

 charge must be /.)- 

 ridcd against. The 

 meaning of lateral 



discharge will be illus- 

 trated by the foll< >w ing 



. ment: Thedia- 

 grain (Fig. G,~>) repre- 

 sents, as before, a 

 Leyden jar ready 



;e.d for being 

 discharged through a 

 metallic wire, ono 

 end of which ha< 

 already been brought into contact with tho outside of 

 the jar, while the other end i-.-m be brought into 

 contact with tho knob communicating directly with tho 



