Franklin was the first to suggest the possibility of protecting buildings by means of a rod of some conducting material terminating in a point at the highest part of the building, and leading down, outside the building, into the earth. Lightning conductors at the present day are similar to Franklin’s rod, but many improvements have been made from time to time as our knowledge of the nature and action of the lightning discharge has increased. A modern lightning conductor generally consists of one or more pointed rods fixed to the highest parts of the building, and connected to a cable running directly to earth. This cable is kept as straight as possible, because turns and bends offer a very high resistance to the rapidly oscillating discharge; and it is connected to large copper plates buried in permanently moist ground or in water, or to water or gas mains. Copper is generally used for the cable, but iron also may be employed. In any case, the cable must be of sufficient thickness to prevent the possibility of its being deflagrated by the discharge. In ships the arrangements are similar, except that the cable is connected to the copper sheathing of the bottom. The fixing of lightning conductors must be carried out with great care, for an improperly fixed conductor is not only useless, but may be a source of actual danger. Lightning flashes vary greatly in character, and while a carefully erected lightning conductor is capable of dealing with most of them, there are unfortunately certain kinds of discharge with which it now and then is unable to deal. The only absolutely certain way of protecting a building is to surround it completely by a sort of cage of metal, but except for buildings in which explosives are stored this plan is usually impracticable.