3.--When, however, the electrical agency is confined to bodies, such as the metals, which are found to oppose but small resistance to its progress, then this violent expansive or disruptive action is either greatly reduced, or avoided altogether. The explosive form of action we term Lightning, vanishes, and becomes, as it were, transformed into a sort of continuous current action, of a comparatively quiescent kind, which, if the metallic substance it traverses be of certain known dimensions, will not be productive of any damage to the metal. If, however, it be of small capacity, as in the case of a small wire, it may become heated and fused. In this case, the electrical agency, as before, is so resisted in its course as to admit of its taking on a greater or less degree of explosive and heating effect, as in the former case. It is to be here observed, that all kinds of matter oppose some resistance to the progress of what is termed the Electrical Discharge, but the resistance through capacious metallic bodies is comparatively so small, as to admit of being neglected under ordinary circumstances; hence it is that such bodies have been termed Conductors of Electricity, whilst bodies such as air, glass, &c., which are found to oppose very considerable resistance to electrical action, are placed at the opposite extremity of the scale, and termed Non-conductors or Insulators. The resistance of a metallic copper wire to an ordinary electrical discharge from a battery, was found so small, that the shock traversed the wire at the rate of 576,000 miles in a second. The resistance however, through a metallic line of Conduction, small as it be, increases with the length, and diminishes with the area of the section of the Conductor, or as the quantity of metal increases. 4.--It follows from these established facts, that if a building were metallic in all its parts, an iron magazine for example, then no damage could possibly arise to it from any stroke of Lightning which has come within the experience of mankind; e.g., a man in armour is safe from damage by Lightning; in fact, from the instant the electrical discharge in breaking with disruptive and explosive violence through the resisting air, seizes upon the mass in any point of it, from that instant the explosive action vanishes, and the forces in operation are neutralized upon the terminating planes of action, viz., the surface of the earth, and opposed clouds. 5.--All this plainly teaches us, that in order to guard a building effectually against damage by Lightning, we must endeavour to bring the general structure as nearly as may be, into that passive or non-resisting state it would assume, supposing the whole were a mass of metal.