200 



Lightning Rods. 



Vol. V. 



length of the rod should always be such as 

 to elevate its point as far as possible above 

 the top of the building and every contiguous 

 object. This should be carefully attended to ; 

 for it is found that all objects, such as chim- 

 neys, trees, &c., when near to the point, di- 

 minish its effects in a rapidly increasing ratio 

 to their height, till they render it entirely 

 useless." (Vol. 3, p. 76.) If the rod had been 

 sufficiently elevated, there is reason for be- 

 lieving that its other parts were sufficiently 

 perfect to have passed a gradual stream of 

 electricity to the earth equal to the preven- 

 tion of the stroke. 



2. The rod was too short at the lower end. 

 We are told that it penetrated three or four 

 feet into the ground, at an angle of 45° v^ith 

 the wall. This would give the end of the 

 rod a depth of only three feet at most — per- 

 haps not more than two. " The rod should 

 be planted so deep as to insure its connexion 

 with moist earth even during the driest sea- 

 sons. Dry earth is a nonconductor of elec- 

 tricity ; hence, if it becomes dry about the 

 foot of the rod, the passage of the fluid from 

 the cloud to the earth becomes interrupted 

 and the rod rendered useless." (Vol. 3, p. 76.) 



There are probably few eligible sites for 

 building, where the earth at the depth of 

 three feet will always continue moist; — con- 

 sequently, where the rod would at all times 

 be in a proper condition for affording ade- 

 quate protection. This appears to have been 

 the case in the present instance. 



Electrical phenomena arc only manifested 

 by the resistance which the fluid meets with 

 in passing through bodies. If all substances 

 were perfect conductors, it would difl'use it- 

 self equally through them, and we would not 

 even possess a knowledge of its existence. 

 When, in its progress, it meets with an im- 

 perfect conductor, an accumulation takes 

 place and increases until it acquires an ex- 

 plosive force sufficient to overcome the re- 

 sistance. 



Let us apply this principle to the case in 

 question. If the stratum of air which sepa- 

 rated the cloud from the earth had been a 

 good conductor of the electric fluid, it never 

 could have accumulated in the cloud. With- 

 out such an accumulation, the fearful explo- 

 sion could not have taken place. 



The rod, so far as its integrity was pre- 

 served, was a good conductor. This is mani- 

 fest; for the electricity passed harmlessly 

 down it near to the ground, where it was ar- 

 rested — not by the rod itself — but by the con- 

 necting surfaces between it and the earth. 

 The communication with the earth was not | 

 perfect — the chain of conducting materials > 

 was broken. By this means the dispersion ! 

 of the fluid was retarded, producing an accu- 1 

 mulation in the rod, until it acquired suffi- [ 



cient force to overcome the resistance of 

 passing through the wall by another explo- 

 sion. However perfect the other parts of the 

 rod may have been, there was a palpable de- 

 fect at its loicer end. I feel assured that I 

 shall be sustained in this conclusion by every 

 competent electrician. 



A quotation which C. J. has made from 

 Gay Lussac most strongly corroborates this 

 viev/ of the case. " When it (a lightning 

 rod) has any break in it, or is not in perfect 

 coinmunication icith the moist soil, the light- 

 ning having struck it* flies from it to some 

 neighbouring body, or divides itself beticeen 

 the two, in order to pass more rapidly into 

 the earth. Frequent instances of ruinous 

 accidents have occurred from both these 

 causes." Yet C. .T. strangely says tbat these 

 " observations do not apply to the rod in 

 question,^'' — and for why "] — because, forsooth, 

 "that was perfect in all its parts." Lussac 

 has exactly described the case. 



Having satisfactorily shown that N, J. 

 Sharpless's rod was materially defective, and 

 that to this defect may be attributed the "ru- 

 inous accident" which occurred, it is perhaps 

 not necessary to follow C. J. in his remarks 

 on the diameter of conductors. As has been 

 already remarked, the conducting power of 

 the rod was adequate to the occasion until 

 the fluid met with resistance at the lower end. 



If our friend N. J. Sharpless has any ad- 

 ditional facts to communicate respecting his 

 rod or the effects of the lightning, I would 

 thank him to do so through the medium of 

 the Farmers' Cabinet. 



In conclusion, I may say to C. J., that so 

 far from asserting " that an instance such as 

 the above has never occurred — that it cannot 

 occur," — it is matter of astonishment to me 

 that such instances do not happen much 

 more frequently. Their infrequency furnishes 

 strong evidence that even imperfect con- 

 ductors aflbrd a great amount of protection. 

 But " a conductor perfect in all its parts, and 

 erected with the most scrupulous care and 

 attention," would differ in numerous particu- 

 lars from those of N. J. Sharpless, and more 

 effectually serve the purpose for which they 

 were designed. 

 New Garden, 12 mo. 10, 1840. 



P. S. Since the above was written, 1 have 

 observed, over the signature ^^Agricola" 

 (Vol. 5, p. 72), the following remark : " Per- 

 haps no instance can be cited of a building 

 not more than sixty feet in extent from the 

 point of a properly arranged conducting rod. 



* Tlie reader must constantly bear in mind tliat the 

 "stroke" alluded to liy Lussac is the effect of the 

 "break" in the rod, or of its iniiierfect " coinmunica- 

 tion" with the earth. The object of the rod ahvays is 

 to prevent the stroke by a gradiial drainage from the 

 cloud : its failure to do this is good evidence that it is 

 not " perfect in all its parts." 



