NATURAL PHILOSOPHY. 173 



Mass., and the adjoiiiing towns, as well as elsewhere, as well by 

 personal visit as by correspondence. The result appeared to be 

 that buildings have been struck and more or less injured, irrespect- 

 ive of any particular kind or style of rod now in use, whether of 

 iron or copper, and that the great defect of rods in general is due 

 to: — 



1. The construction of the rod and its arrrangement on the 

 building. 



2. To its innperfect connection with the earth. 



In regard to the first defect, he referred to the common practice 

 of erecting two or more points on the roof, each, perhaps, being 

 attached to a chimne}', and connecting these in a single rod ex- 

 tending to the ground. In instances of injury where such a rod 

 was used he had almost invariably found that the point of lateral 

 distance would be at or very near the point of junction where the 

 arms entered the main rod. Electricity may b(i regarded as a dy- 

 namic force, bearing a striking analogy to other forces in nature 

 which are governed by well-known dynamic laws. So far, then, 

 as this analogy seems to hold, it may be cautiously used as a guide 

 to correct conclusions. Suppose a heavy charge to be received at 

 the same time upon each arm of the rod. The different forces 

 pursuing their respective channels rush together at the point of 

 junction, where the intensity is greatly increased, where the single 

 rod below, being no larger than either branch, and having but 

 half the conducting power of both, is unable to carry the double 

 charge so suddenly imposed upon it; hence a portion of the bur- 

 den is compelled to seek an unnatural mode of conveyance through 

 the building. This arrangement, then, is equivalent to taking 

 away half the conducting power of the rod, hence, increasing, in 

 the same ratio, the liability of a disruptive discharge. If the rod 

 from the ground were enlarged in proportion to the number of 

 points, the objection w^ould be considerably removed. 



The mode of connecting the conductor with the ground was 

 next considered. It is the common practice to run the rod into 

 the ground from 3 to 6 or 8 feet, or until it is supposed to reach 

 the moist earth, this being generally considered sufficient for all 

 practical purposes. 



Observation, however, proves that the loam, sand, or gravel 

 which comes in contact with the rod in ordinary cases is entirely 

 inadequate in conducting power to convey a heavy charge of elec- 

 tricity from the rod into the earth, with anything like the facility 

 with which it is discharged from the cloud to the rod. 



The writer had no hesitation in believing that nine-tenths of the 

 casualties which had come to his notice, where the conductors had 

 failed to accomplish their purpose, were due to an overcharge of 

 the rod caused by the resistance of the electricity in its passage 

 into the earth. The receiving power of the rod exceeds the dis- 

 charging power. A channel is open through the pointed conduc- 

 tors for a free passage of electricity from the cloud to the rod, but 

 no adequate means is provided for its escape to the earth. Thf 

 rod thus becomes a reservoir of electric force of great intensity, 

 and when the tension is forced bej'ond certain limits a portion of 



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