rhlgiiSt 19, 1880] 



NATURE 



3^5 



through the surrounding air. It appears from experi- 

 ments on the small scale which we can make with an 

 electrical machine, that the electric density requisite to 

 force a passage through the air increases under given 

 circumstances, at first approximately as the square root 

 of the distance which has to be traversed, but afterwards 

 much more slowly, so that it is probable that the poten- 

 tial required to give a mile-long flash of lightning may 

 not be of an order very much higher than that producible 

 in our laboratories. 



But from what I have said you will see at once that 

 under similar circumstances an elongated body must have 

 a great advantage over a rounded one in effecting a dis- 

 charge of electricity. This is easily proved by trial. 

 [The electric machine being in vigorous action, and giving 

 a rapid series of sparks, a pointed rod connected with 

 the ground was brought into the neighbourhood, and the 

 sparks ceased at once.] In this simple experiment you 

 see the whole theory and practical importance of a light- 

 ning conductor. But, as a warning, and by no means an 

 ■unnecessary one, I shall vary the conditions a little and 

 try again. [The pointed rod was now insulated, and pro- 

 duced no observable effect.] Thus you see the difference 

 between a proper lightning-rod and one which is worse 

 than useless, positively dangerous. There is another 

 simple way in which I can destroy its usefulness, namely, 

 by putting a little glass cap on the most important part of 

 it, its point, and thus rendering impossible all the benefits 

 It was originally calculated to bestow. [The pointed rod 

 was again connected with the ground, but furnished with 

 a little glass cap. It produced no effect till it was brought 

 within four or five inches of one of the conductors of the 

 machine, and then spaiks passed to it.] You must be 

 strangely well acquainted with the phases of human per- 

 versity if you can anticipate what I am now going to tell 

 you, namely, that this massive glass cap, or repcllsr, as it 

 was fondly called, was only a year or two ago taken off 

 from the top of the lightning-rod employed to protect an 

 important public building. [The repeller was exhibited. 

 It resembled a very large soda-water bottle with a neck 

 much wider than the usual form.] From the experiments 

 you have just seen it must be e.'ident to you that the two 

 main requisites of an effective lightning-rod are that it 

 should have a sharp point (or, better, a number of such 

 points, lest one should be injured), and that it should be 

 in excellent communication with the ground. When it 

 possesses these, it does not require to be made of excep- 

 tionally great section ; for its proper function is not, as is 

 too commonly supposed, to parry a dangerous flash of 

 lightning : it ought rather, by silent but continuous 

 draining, to prevent any serious accumulation of electricity 

 in a cloud near it. That it may effectually do this it must 

 be thoroughly connected with the ground, or (if on a ship 

 or lighthouse) with the sea. In towns this is easily done 

 by connecting it with the water mains, at sea by using the 

 copper sheathing of the ship, or a mstal plate of large 

 surface fully immersed. Not long ago a protected tower 

 was struck by lightning. No damage was done in the 

 interior, but some cottages near its base were seriously 

 injured. From a report on the subject of this accident it 

 appears that the lower end of the lightning rod was 

 "jumped" several feet into the solid rock 1 Thus we see, 

 in the words of Arago, how " False science is no less 

 dangerous than complete ignorance, and that it infallibly 

 leads to consequences which there is nothing to justify." 



That the lightning-rod acts as a constant drain upon 

 the charge of neighbouring clouds is at once proved when 

 there is, accidentally or purposely, a slight gap in its 

 continuity. This sometimes happens in ships, where the 

 rod consists of separate strips of metal inlaid in each 

 portion of the mast. If they are not accurately fitted 

 together, a perfect torrent of sparks, almost resembling a 

 continuous arc of light, is seen to pass between them 

 whenever a thunderstorm is in the neighbourhood. 



I cannot pass from this subject without a remark upon 

 the public as well as private duty of having lightning-rods 

 in far greater abundance than we anywhere see them in 

 this country. When of proper conducting power, properly 

 pointed, properly connected with the ground and with 

 every large mass of metal in a building, they afford abso- 

 lute protection against ordinary lightning — every single 

 case of apparent failure I have met with having been 

 immediately traceable to the absence of one or other of 

 these conditions. How great is their beneficial cftect you 

 may gather at once from what is recorded of Pieter- 

 maritzburg, viz., that till lightning-rods became common 

 in that town it was constantly visited by thunderstorms 

 at certain seasons. They still come as frequently as ever, 

 but they cease to give lightning- flashes whenever they 

 reach the town, and they begin again to do so as soon as 

 they have passed over it. 



A knight of the olden time in full armour was probably 

 as safe from the effects of a thunderstorm as if he had 

 had a lightning-rod continually beside him ; and one of 

 the Roman emperors devised a perfectly secure retreat in 

 a thunderstorm in the form of a subterraneous vault of 

 iron. He was probably led to this by thinking of a mode 

 of keeping out missiles, having no notion that a thin shell 

 of soft copper would have been quite as effective as 

 massive iron. But those emperors who, as Suetonius tells 

 us, wore laurel crowns or sealskin robes, or descended into 

 underground caves or cellars on the appearance of a 

 thunderstorm, were not protected at all. Even in France, 

 where special attention is paid to the protection of build- 

 ings from lightning, dangerous accidents have occurred 

 where all proper precautions seemed to have been taken. 

 But on more careful examination it was usually found 

 that some one essential element was wanting. The most 

 common danger seems to lie in fancying that a lightning- 

 rod is necessarily properly connected with the earth if it 

 dips into a mass of water. Far from it. A well-constructed 

 reservoir full of water is not a good " earth " for a 

 lightning-rod. The better the stonework and cement the 

 less are they fitted for this special purpose, and great mis- 

 chief has been done by forgetting this. 



A few years ago the internal fittings of the hghthouse 

 at Skcrryvore were considerably damaged by lightning, 

 although an excellent lightning-rod extended along the 

 whole height of the tower. 



The real difficulty in these situations, exposed to tre- 

 mendous waves, lies in effecting a permanent communica- 

 tion between the lightning-rod and the sea. But when 

 this is done the sea makes f<ir the best of "earths." 



When a lightning-rod discharges its function imper- 

 fectly, either from insufficient conducting power or be- 

 cause of some abnormally rapid production of electricity, 

 a luminous brush or glow is seen near its point. This is 

 what the sailors call St. Elmo's Fire, or Castor and 

 Pollux. In the records of mountain climbing there are 

 many instances of such]dischargcs to the ends of the alpen- 

 stocks or other prominent pointed objects. One very 

 remarkable case was observed a few months ago in Swit- 

 zerland, where at dusk, during a thunderstorm, a whole 

 forest was seen to become luminous just before each flash 

 of liglitning, and to become dark again at the instant of 

 the discharge. 



Perhaps the most striking of such narratives is one in 

 the memoirs of the Physical and Literary Society of 

 Edinburgh, on Thunder and Electricity, by Ebenezer 

 McFait, M.D. 



The destructive effects of lightning are familiar to all of 

 you. All the more ordinary eftects can easily be reproduced 

 by the help of Leyden jars on a small scale. How small 

 you may easily conceive when I tell you that a three-foot 

 spark is considered a long one, even fro.n our most 

 powerful machines, while it is quite certain that lightning 

 flashes often exceed a mile in length, and sometimes 

 extend to four and five miles. One recorded obsei-vation, 



