A few words may be added here on the method of protecting the large mineral oil tanks which are to be found in the United States. Many of these oil tanks are of very large capacity, some of them containing a million gallons of oil. They are generally constructed of thick iron plates rivetted together. The roofs are usually made of wood coated with tar, but in some cases iron is adopted. As a rule, several of the tanks are grouped together and connected with each other--and in some instances with distilleries--by means of subterraneous iron pipes. One method of protecting these tanks is to erect around them, at a distance of some ten feet, wooden supports, on which are placed upright metallic conductors which overlook the tank, and are connected with each other near their tops by stout iron wires, thus forming a network of conductor which is supposed to intercept any discharge of electricity from a tempest-cloud, and prevent it from reaching the oil tank. This method, however, has failed in several notorious instances, and is not countenanced by the best authorities. A better and less complex arrangement is now usually adopted by the best firms. The chief object of this arrangement is to prevent the temperature of the oil tank, and of the atmosphere above and around it, being raised by means of an electric discharge. This is accomplished by using large conductors, which are carried some distance above the oil tank. These conductors, of which there should be at least four, are formed of flat iron bars about one and a half inches wide and half an inch thick; they are securely fastened to the sides of the tank at equal distances from each other, and metallically connected with it. About thirty feet above the roof of the tank they meet, and are carefully and substantially joined together, and supported, if necessary, by a wooden post extending from the centre of the roof of the oil tank. The earth terminals, of which there must be one to every two conductors, consist of perforated iron pipes, as before described, three inches in diameter and fifteen feet long. They are sunk into thoroughly moist earth, and metallically connected with the lower part of the tank. These perforated pipes are so arranged that they catch the rain water from the roof of the tank; by this means the surrounding earth is kept moist. It may be mentioned that by utilising the tank as a portion of the system of conductors, the electric discharge is distributed and much weakened. CHAPTER XIII. LIGHTNING PROTECTION IN ENGLAND.