From our present ignorance of the actual nature of electricity, admitted alike by all scientific men, it has often been argued that no claim can be set up for a perfect protection against the effects of the electric force called lightning, since we do not know ‘whence it comes, nor whither it goes.’ That this argument is entirely fallacious, may be easily shown. The human mind does not understand, any more than it does electricity, the great forces called centripetal and centrifugal, which keep millions of suns and of planets in their path through the boundless universe; yet there is no educated man who doubts that astronomers are able to calculate, with the greatest mathematical precision, the time when two particular stars will come near each other, when the moon will obscure Orion, and Venus make her transit across the sun. Again, no explanation can be given of the actual nature, of the Why and the Wherefore, of the force called gravity, simply in its operation on our globe. Still men can calculate, with the greatest nicety, the result of any given weight, falling, from any given height, on the surface of the earth or below it. François Arago, reasoning on the disputed efficiency of lightning conductors, puts another indisputably practical case. ‘If,’ says he, ‘we take the dimensions to be given to conductors from experience, and if those which we adopt have been found to resist the strongest lightning recorded for over a century, what more can reasonably be asked for?’ When the engineer decides on the height and width of the arches of a bridge, the vault of an aqueduct, the section of a drain, and similar constructions, what does he concern himself with? He examines all the facts and records on the matter as extensively as he can, and, in making his plan, keeps somewhat beyond the dimensions dictated by the greatest floods and the heaviest rains which have ever been observed. He thus goes as far back in his research as the evidence within his reach will enable him to do, but without confusing himself either with searching for the hidden causes of floods and rains, or with investigating the character of the physical revolutions, or the cataclysms which occurred in prehistoric times, and of which geologists only have been able to discover the traces and estimate the magnitude. So with the engineer. Greater precaution or foresight than his cannot be demanded from the constructor of lightning conductors, nor is any needed.’