_Principles and Instructions relative to their application to Powder Magazines, by_ SIR W. SNOW HARRIS, F.R.S. _Extracted from Army List for July, 1859._ 1.--Thunder and lightning result from the operation of a peculiar natural agency through an interval of the atmosphere contained between the surface of a certain area of clouds, and a corresponding area of the earth’s surface directly opposed to the clouds. It is always to be remembered that the earth’s surface and the clouds are the terminating planes of the action, and that buildings are only assailed by Lightning because they are points, as it were, in, or form part of, the earth’s surface, in which the whole action below finally vanishes. Hence buildings, under any circumstances, will be always open to strokes of Lightning, and no human power can prevent it, whether having Conductors or not, or whether having metals about them or not, as experience shows. 2.--Whenever the peculiar agency, (whatever it may be), active in this operation of nature, and characterized by the general term Electricity, or Electric Fluid, is confined to substances which are found to resist its progress, such, for example, as air, glass, resinous bodies, dry wood, stones, &c., then an explosive form of action is the result, attended by such an evolution of light and heat, and by such an enormous expansive force, that the most compact and massive bodies are rent in pieces, and inflammable matter ignited. Nothing appears to stand against it. Granite rocks are split open, oak and other trees, of enormous size, rent in shivers, and masonry of every kind frequently laid in ruins. The lower masts of ships of the line, 3 feet in diameter, and 110 feet long, bound with hoops of iron half an inch thick and 5 inches wide, the whole weighing about 18 tons, have been, in many instances, torn asunder, and the hoops of iron burst open and scattered on the decks. It is, in fact, this terrible expansive power which we have to dread in cases of buildings struck by Lightning, rather than the actual heat attendant on the discharge itself.