=137. Lightning Conductors.=--"The line of danger, whether from the burning or lifting power of lightning, is the line of strong and obstructed currents of air, of the greatest aerial friction."[17] Trees, church spires, wind-mills and other tall structures, obstruct the aerial currents, and hence their exposure to danger. The highest objects of the landscape, especially those that are nearest the thunder cloud, will receive the lightning stroke. The more elevated the object, the more likely is it to be struck. Of two or more objects, equally tall and near, the lightning is invariably found to select the best conductor of electricity, and even to make a circuitous path to get to it. Hence the application and evident advantage of metallic rods, called _lightning conductors_, attached to buildings and ships. A lightning conductor should be pointed at top, and extend some feet above the highest part of the edifice, or mast. It should be made of copper, which is a better conducting medium than iron, and more durable, being less corrosive. It must be unbroken throughout its length, and extend to the bottom of the building, and even some distance into the ground, so as to conduct the electricity into a well or moist soil. If it be connected with the lead and iron work in the structure of the house, it will be all the better, as affording a larger surface, and a readier means of exit for the fluid. In a ship, the lower end of the conductor should be led into communication with the hull, if of iron, and with the copper sheathing, if a wooden vessel; so that, spread over a large surface, it may escape more readily to the water.