Chimneys and air shafts, from which heated air or smoke escapes, are fitted with metallic caps which are connected with the general conductor. In order to protect this metallic cap from the effects of the sulphurous fumes arising from the chimney, a terra-cotta cap is contrived to fit inside the metallic cap. An analogous method is adopted with regard to the ventilators of barns and ice-houses. If these buildings have the ordinary ventilators in the form of dormer windows upon the roof, an iron rod seven-sixteenths of an inch in diameter is placed vertically across, and above the centre of the opening of each ventilator, and connected with the conductor. Should the barn or ice-house have openings or doors through which warm vapour can escape, a conductor is fixed to the roof at the gable ends above the centre of each opening or door, and extended outwards about five feet, at an angle of forty-five degrees from the roof, so as to be in line with any ascending vapour, or any descending charge of electricity following the course of the vapour. All these auxiliary conductors and terminal rods are metallically connected with the main conductor of the building. The conductors are simply fastened to the building by iron staples or by straps of sheet iron, pierced with two holes for nails or screws. In America, as elsewhere, the earth-terminal is regarded as of prime importance, and in all properly constructed lightning conductors receives the greatest care and attention. In the first place, such metal pipes as lead from the building to the water-mains, gas-mains, and sewers are carefully connected with the principal lightning conductor, in order that they may act as auxiliary earth-terminals. For the principal earth-terminal many contrivances have been brought forward, but very few possess any originality, and many are positively useless. Some of the best are similar to those in use in England; among others, perhaps the best method is that of placing a cast or wrought-iron pipe of three inches inside diameter, and about ten feet long, vertically in thoroughly moist earth and carefully connecting the conductor, or conductors, with it. The chief objections to this plan are the occasional difficulty of getting a moist earth at all, and the possibility of earth that is generally moist getting dried up in hot weather. To obviate these risks, the following arrangement is used:—