T H U 



ua 



T M r 



uniformly covered \\ith clouds, a fla-h of lightning will 

 durt from tin- zenith, ami, after a ! w seconds, (lit- crash of 

 thuiuler will take place accompanied by n rolling sound : 

 MOO, a second flash may pn-u U in iln- /. mill 



and thunder may follow," but nnw tlio c-nu>h. though loud, 

 may not be prolonged. It is justly observed In M. Arago 

 that this u very different from the phenomena 

 and the explanation which was iirst proposed by Dr. 

 Hooke C Posthumous Work-. . hum that which 



pOSMSseathi I,.:'. -' degree of piobabilnv . Tin- lla-h. - oi 

 lightning, I > < s, are cither simple or multiple: 



tin- lii - but une small portion of space, uin! 



rise to an i i- report ; the multiple tla-h takes 



place at ditl'eiviit parts of one long line: n thc-c parts 

 should be situated in a circular arc. and the observer should 

 be in iu> centre, all the reports would arrive at his car at 

 the same time, and still one loud crash only would be 

 heard; but if the pails were nearly in a straight line, and 

 the observer were at one of its extremities, the reports, 

 whether they take place at the .same instant or in s 

 Mon, would arrive at his ear at different times, depending 

 wholly or partly on the distances. It may be MHUidereQ 

 therefore that the rolling arises from the circumstance 

 that the points of explosion are at different distances from 

 the observer; and it will follow that the duration of 

 the noise is equal to the time in which sound travels 

 through nn interval equal to the difference between the 

 lengths of two lines drawn from the observer to the two 

 uities of the flash. The flush of lightning and the 

 report of the thunder take place in reality at the same 

 moment; but since sound travels at the rate of IKK) feet 

 per second, while the passage of light from the cloud to 

 the observer may hi' considered as instantaneous, it fol- 

 lows that, on counting the number of seconds which elapse 

 between the time of seeing the flash and hearing the 

 report, the distance of the thunder-cloud from the ob- 

 may be ascertained if 1100 feet be multiplied by that num- 

 ber of seconds. 



The experiment* of Lavoisier and La Place have shown 

 that the molecules of water, in evaporating, convey away 

 from the earth a portion of the electricity which It con- 

 tains, and which the water has acquired in being converted 

 into vapour. This electricity becomes diffused in the upper 

 regions of the atmosphere, and, when the vapours again 

 become condensed so as to form globules ot water, the 

 electricity disposes itself on the surface of the globules in 

 different quantities according to their magnitudes, When 

 these globules have equal volumes, those quantities are 

 equal ; and, an equilibrium then subsisting with respect to 

 electricity, no sensible ett'ect is produced: but when a con- 

 siderable difference takes place between the quantities of 

 electricity on the different globules, the tendency of that 

 element to return to a state of equilibrium causes it to 

 pan rapidly in flashes from those globules which have 

 more, to those which have less, than their mean quantity : 

 and thus, if the condensation be great and take place sud- 

 denly, vivid flashes of lightning and loud peals of thunder 

 may take place. The effect is probably the greatest when 

 two masses of clouds in opposite states of electricity are 

 carried rapidly against one another by winds blowing in 

 contrary direction!. 



An opinion prevails that thunder has been heard when 

 the sky was without a cloud, but the fact can scarcely be 

 said to be satisfactorily established : for the sounds which, 

 in countries subject to earthquakes, have been supposed to 

 be thunder, proceed from under the ground, and may result 

 from a different cause. Volm-y however relates that, being 

 one day at Pontchartrain near Versailles, when no cloud 

 was visible, he heard distinctly four or five claps of thun- 

 der: he adds that about an hour afterwards the sk\ 1 

 overcast, and a violent hail-storm followed. On this relation 

 M. Arago observes that the sounds could not have 

 heard if they had come from clouds at a greater distance 

 than six leagues; and if the clouds bad been at, or a little 

 within, tliat distance, they must have been v isible, unless it 

 be supposed that they were not more than a few 

 fhe ground: but the' hail which followed the thunder must 

 have proceeded from clouds having great elevation, though 

 at the time the chips were heard they were too rein 

 allow any sound I'mm them to reach the ear; and th. 

 he concludes that the sounds must have been . 

 the air itself. For an account of death caused bv electricity 

 in the air when the thunder-cloud was very distant, and 



for the theory of the retumn 



-!,<, m. \ol. Ixx. 

 From the meteorological i 



by, and Captains I'ln; : pean that 



neither thunder nor lightning u known to 

 \ond the 7i>th degree of north latitude : cv 

 7dth i 1 e phenomena are very rare ; and in the 



tables of Captain Parry the o. l thunder and 



lightning U mentioned hut once bctwun June. 1821, and 

 September, 1KH. < '.iptam Fr.uikhn also, in UTi N. 

 beard thunder on one day only between September, 

 and August. IK.'(i. 



THrM>KK-KOI), a bar of metal attached generally to 

 aside of a building, and extending from below the level of 

 the ground to a point several fi-et above the higln-' 

 of the roof, or of the steeple, if the buildi! ie, in 



order to secure the edifice from the effect- of thiiiii 

 lightning: the upper extremity of the rod or bar term; 

 in a point. 



When a thunder-cloud passes above an elevated oi 

 it produces in the nearest part of the object the k: 

 electricity which is opposite to that of the cloud its. 

 that a rapid and abundant communication take* ] 

 cither the electric matter in the cloud rushes towards (In- 

 earth, or that of the earth rushes towards the cloud : and if 

 the materials of the building are not good coi. 

 the fluid, the latter in its passage exerts an explosive action 

 IA which the building is destroyed or greatlj injured. The 

 thunder-rod, from the conducting property of its metallic' 

 substance, selves to convey the fluid harmlessly to tin- 

 earth or air. When it has happened that there 'is an in- 

 terruption of the communication, by the rod being bi 

 or even by being much diminished in magnitude in 

 part of its length, the electric fluid has been observed to 

 pass between the parts of the rod above and below the 

 place of fracture. 



Buffon, and Dalibard, at bis suggestion < IT")- . appear 

 to have been the first persons who drew lightning from the 

 atmosphere by means of pointed rods of imlal : and in (he 

 following year M. de Romas elevated a paper kite to the 

 height of iVH) feet for the like purpose: this was about 

 twelve months before Dr. Franklin, without any know- 

 ledge of what had been done in Europe, performed the 

 like experiment in America. The object of the French 

 pliilosoplu rely to obtain by tho-i- m- 



trieal sparks or flashes of fire ; but it is to Dr. Franklin 

 that the world is indebted for the idea of raising pointed 

 rods in order to secure buildings from the effects of atmo- 

 spherical electricity; and the recommendation was imme- 

 diately adopted both for edifices on land and ships on tin- 

 water. 



Soon after the first employment of thunder-rods in 

 Europe, an opinion prevailed that when their extrei 

 were pointed thev created a danger which did not exi-t 

 before, and that they attracted lightnings which would, 

 without the rods, have discharged themselves at a diet 

 and in order to diminish the risk, it was proposed to crown 

 the .summits of the roils with knobs or balls of metal. This 

 notion vva.- entertained by the Abbe Nollet, in France; 

 but in the Royal -Ion the question concern- 



ing the relative efficacies of pointed and knobbed con- 

 ductors was agitated with great vehemence, chiefly through 

 the obstinacy of Mr. Wilson, one of the Fellow-, who, in 

 1TTS, made himself the head of a party in support of tin- 

 latter kind of conductors, in opposition to the president. 

 ! 1'nisiji.r., JOHN.] It is remarkable that both George III. 

 of F.nglaud and Frederick of Prussia placed thein-elves on 

 the side of the opponents of Franklin; the first king 

 giving the preference to balls over points, and tin- , 

 while he consented to have conductors raised on hi- 

 racks and powder-magazines, prohibited the enrti 

 them on his palace of Sans Souci. It is now admitted that 

 the prejudice against pointed thunder-rods was entirely 

 without foundation : those conductor ha\ e been found not 

 only to protect buildings when struck, but also to diminish 

 the number of shocks which in a given time the\ have ex- 

 perienced. An experiment which was made b\ \\^ ecaria, 

 in IT">:l. might have shown the superiority of pointed con- 

 ductors over those with balls : for that distingni.-hcd elec- 

 trician set up on the roof of the church c.: San (Jiovanni, 

 at Turin, a metallic rod bent near the top and terminating 

 in a point: the upper part was capable lirned 



round by means of a silk line, so that the point could be 



