Before entering upon the subject of the protection of powder magazines against lightning, the new ‘Instruction’ signed by Professor Pouillet and his colleagues laid down a few so-called ‘_propositions générales_‘--that is, either hints, suggestions, or statements, the French word ‘_proposition_’ being most serviceably vague for use--on the subject of lightning and of thunderstorms. The first thesis affirmed that ‘clouds which carry lightning with them are but ordinary clouds (_ne sont autre chose que des nuages ordinaires_) charged with a large quantity of electricity.’ The second thesis boldly defined the nature of lightning. ‘The fire which flashes from the skies is an immense electric spark, passing either from one cloud to another, or from a cloud to the earth; it is caused by a tendency for the restoration of the electric equilibrium (_la recomposition des électricités contraires_).’ It was laid down in the third ‘_proposition_’ that, when lightning falls from a cloud upon the earth, it is but an effort of the electric force to return to its grand reservoir. That it is similar to water, which, having risen in the form of vapour from the earth-surrounding ocean high up into the air, then falls down as rain upon hills and plains, and finally runs down again in rivers to the ocean, Professor Pouillet did not say in so many words; but there were vague hints to that effect in the new ‘Instruction.’ Its practical recommendation, offspring of the theories thus enunciated, was that the best protection against lightning would be afforded by the most substantial metal rods, made of iron, surrounding a building on all sides, and passing deep into the ground. The new declaration of the ‘Académie des Sciences,’ though merely a repetition of former reports, was not without important consequences. First in France, and then in other countries, the conviction became general among scientific men, and others well informed on the subject, that well-designed conductors, if properly made and kept in good order, form an absolute, unconditional, and infallible protection against lightning. Professor Pouillet also laid it down that lightning conductors, to be efficient, must be regularly inspected, he, with his colleagues on the committee, having come to the conclusion that such examination should take place at least once every year. So much stress was laid upon the importance of an annual inspection, that a strong recommendation was made to the Government to have a _procès-verbal_, or special report, drawn up on each occasion in the case of all public buildings, so that it might be known by the central authorities whether the examination had taken place at the specified time, and what had been the declaration of the examiners. The advice was judiciously followed, with the result that at this moment the public buildings of France have the most complete protection against lightning--greatly in contrast with the public buildings in England. CHAPTER VIII.