98 



SCIENCE. 



[Vol. IV., No. 78. 



so to speak, facilitated electrical discharges ; 

 the experiment of Faraday, by which it was 

 shown that a person, and even the most deli- 

 cate electrical instruments, inside a large me- 

 tallic cage which was connected with the ground, 

 were unaffected by powerful discharges of elec- 

 tricity between the cage and the prime con- 

 ductor of an electrical machine ; and the 

 statistics collected by the English government, 

 which show, that, since vessels have been pro- 

 vided with lightning-rods, the number of casu- 

 alties produced at sea from lightning have been 

 greatly reduced. A building covered by a 

 metallic netting suitably connected with the 

 ground would be well protected from light- 

 ning. The nearest approach to this condi- 

 tion of safety would be to connect all the 

 network of metallic conductors about a house 

 with wet ground ; and one argument against 

 placing under ground the network of tele- 

 phone and telegraph wires in cities is, that 

 at present, where they are very numerous, 

 they protect buildings from danger from light- 

 ning. This is, of course, not the case where 

 a single telephone or telegraph wire enters a 

 house. The latter should always be well con- 

 nected with the gas or water pipe. In regard, 

 however, to the belief that tall trees, higher 

 than the houses in their immediate neighbor- 

 hood, protect the houses, we can point to the 

 well-known efficiency of small points in facili- 

 tating electrical discharges hy slow degrees. 

 Each leaf and twig is such a small point. 

 Moreover, during a rain, the dripping from the 

 leaves reduces the electrical charge on the tree 

 to the same sign and amount as that of the air 

 in the immediate neighborhood, as is shown by 

 the well-known experiment of Sir William 

 Thomson, in which an insulated can, from 

 which a stream of water issues in drops, is 

 connected with an electrometer ; and the latter 

 shows that the metallic can has taken the charge 

 of the air in its neighborhood. The drops of 

 water continually reduce the can to the electri- 

 cal potential of the neighboring air. The tree, 

 therefore, can be looked upon as a more impor- 

 tant electrical factor than the few salient lower 

 points of a building. 



It is safe to affirm that not one out of a 

 thousand lightning-rods at present upon our 

 buildings are of any use, for the simple reason 

 that they are not led into moist ground, and 

 therefore offer great resistance to the passage 

 of an electrical discharge. Any one can be 

 convinced of this by scraping the lightning-rod 

 at an} r point, connecting a bright wire at this 

 point, and, having led the other end of the wire 

 to the water-pipe or to a body of water, placing 



one or two Leclanche cells in this circuit, and 

 leading the wire in a north and south direction 

 directly over an ordinary pocket-compass. If 

 the lightning-rod enters moist ground, or 

 makes a connection with the earth, the compass 

 should indicate an electrical current by its 

 deflection. Generally it will be found that no 

 such earth-connection exists, and the lightning- 

 rod is therefore worse than useless. It should 

 be immediately connected with the water-pipe, 

 or with a spring, or some body of water. To 

 illustrate the fact that the mere entrance of a 

 metallic rod into the ground is not enough to 

 insure the passage of an electrical discharge 

 to the ground, drive two metallic rods into 

 3'our lawn, at airy suitable distance apart; 

 connect them by a wire, which includes a 

 Leclanche or other voltaic cell ; and, having 

 led the wire over a pocket-compass in a north 

 and south direction, see if 3^011 obtain a deflec- 

 tion of the needle. If, moreover, you labor 

 under the delusion that a surface-sprinkling of 

 the earth near the rods will give an electrical 

 connection, it is best to perform the experiment. 

 It is probable that several acres of lawn would 

 have to be thoroughly sprinkled before a suit- 

 able earth-connection could be obtained. A 

 few experiments with a modern electrical 

 machine — a Toepler-Holtz machine, for in- 

 stance — will readily convince one of the effect 

 of points in dissipating an electrical charge, 

 and of the fact that an electrical discharge 

 always takes the path of least electrical resist- 

 ance between two points. Having ascertained 

 ttwsse facts, one has acquired all the intellectual 

 capital that is possessed by most lightning- 

 rod men. If one apparently discovers that 

 gilded lightning-conductors, or twisted ones, 

 have peculiar attractions for the electrical dis- 

 charges, one leaves the sure ground of fact for 

 the region of the unproven. The difficulty in 

 our stud} 7 of thunder-storms is, that we cannot 

 experiment on a sufficiently large scale, and 

 our means are too tardy to allow us to follow 

 the exceedingly rapid changes of electrified 

 bodies. What we call freaks of lightning are 

 merely the expressions of electrical laws, com- 

 bined with the laws of elasticity of matter. 

 The forked lightning-discharge is an expres- 

 sion of the fact that a positive charge is com- 

 bining with a negative charge along a path of 

 least resistance ; and the air is fractured, so 

 to speak, by the compression, just as a plate 

 of glass yields in zigzag cracks when it is 

 supported on one edge, and a force of com- 

 pression is applied to the other edge. The 

 influence of the medium through which the 

 electrical discharge takes place can be readily 



