10.--Remark 3.--A building that is hence clear, may be struck and damaged by Lightning without having a particle of metal in its construction; if there be metals in it, however, and they happen to be in such situations as will enable them to facilitate the progress of the electrical discharge, so far as they go, then the discharge will fall on them in preference to other bodies offering more resistance, but not otherwise; if metallic substances be not present, or if present, they happen to occupy places in which they cannot be of any use in helping on the discharge in the course it wants to go, then the electricity seizes upon other bodies, which lie in that course, or which can help it, however small their power of doing so, and in this attempt such bodies are commonly, but not always, shattered in pieces. The great law of the discharge is,--progress between the terminating planes of action, viz:--the clouds and earth, and in such line or lines as upon the whole, offer the least mechanical impediment or resistance to this operation, just as water falling over the side of a hill in a rain storm, picks out or selects as it were by the force of gravity, all the little furrows or channels which lie convenient to its course, and avoids those which do not. If in the case of Lightning you provide through the instrumentality of efficient Conductors, a free and uninterrupted course for the electrical discharge, then it will follow that course without damage to the general structure; if you do not, then this irresistible agency will find a course for itself through the edifice in some line or lines of least resistance to it, and will shake all imperfect conducting matter in pieces in doing so; moreover it is to be specially remarked in this case, that the damage ensues, not where the metals are, but where they cease to be continued, the more metal in a building therefore the better, more especially when connected by an uninterrupted circuit with any medium of communication with the earth. Such is, in fact, the great condition to be satisfied in the application of Lightning Conductors, which is virtually nothing more than the perfecting a line or lines of small resistance in given directions, less than the resistance in any other lines in the building, which can be assigned in any other direction, and in which by a law of nature the electrical agency will move in preference to any others. 11.--It follows from the foregoing principles, that a magazine constructed entirely of iron or other metal, would be infinitely more safe in Lightning storms than if built with masonry in the usual way; metallic roofs for magazines, with capacious metallic Conductors to the earth, would be unobjectionable, and a source of security. Metallic gutters and ridges having continuous metallic connection with the earth are also unobjectionable.