In order to ensure complete protection, the conductor on the barn should have been carried along the ridge and down the edges of the roof at each gable. By this means the stroke of lightning would have been intercepted. The engraving on the next page shows a design for the protection of a large detached mansion by means of a multiplication of short points or terminal rods fixed on all the prominent features of the building. The conductor is carried along the ridges in every direction, and down the edges of the roof at each gable. Generally it is sufficient to have two descending conductors, but occasionally the conformation of the building or the nature of the ground renders necessary the use of even more. It is imperative, for obvious reasons, that the descending portion of the lightning conductor shall be carried from the roof to the ground by the shortest possible route, and placed in perfect electrical contact with the earth in the manner to be indicated in a succeeding chapter. [Illustration: Fig. 23.] The projecting points of the conductor are drawn in fig. 23 larger than they need be, in order to show them more clearly, distinguishing them from the rest of the building. The same has been done with the copper rod, running from the roof to the ground and thence into the earth. In reality a conductor may be made perfectly safe, and yet all but invisible to the naked eye. For private houses and buildings, a rope made of copper ought to be at least five-eighths of an inch in diameter, for a copper rod of half an inch in diameter has never been known to be fused. For chimneys of manufactories, where gases are liable to corrode the rope, it had better be a little thicker. Such copper ropes as those manufactured by Messrs. R. S. Newall and Co., five-eighths of an inch in diameter, weighing two-thirds of a pound per foot, and having a conductivity of 93 per cent., have never been known to fail in protecting even the largest buildings. It is supposed by some writers that the value of the conductor is in proportion to the amount of surface of metal exposed. This, however, is a mistake, for the conductivity depends on the weight per foot of metal used, the purity in both being equal. Wire-rope is used simply because it is so pliable that it is easily handled, and can be made of any length required without joints. [Illustration: Fig. 24.]