While designing lightning conductors for the ships of the Royal Navy, Mr. William Snow Harris was called upon likewise by the Secretary of State for War to give advice as to the best protection that might be given to powder magazines and other stores of war material. He did as requested, writing a very lucid paper on the subject, which met with the honour, unique in its way, of being put forward as an official document. To this day there is regularly issued with the ‘Army Circulars’ from the War Office a series of ‘Instructions as to the Applications of Lightning Conductors for the Protection of Powder Magazines, &c.,’ reproducing textually the recommendations of Mr. Snow Harris. These ‘Instructions,’ containing the essence of what he wrote about conductors, and, in fact, the result of all his investigations on the subject, treat the whole _ab ovo_, and as such deserve quotation. ‘Thunder and lightning,’ Mr. Snow Harris wrote to the War Office, ‘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.’ Mr. Snow Harris then went on philosophising. ‘Whenever,’ he said, ‘the peculiar agency--whatever it be--active in this operation of nature, and characterised 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 five 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.’