"The lower air is full of positive electricity just as the surface of the earth is charged with negative electricity. As you know, boys, rain is formed by a lot of little drops of moisture combining to form one large drop, which, when it is heavy enough, falls to the ground. Now the surface of every drop of moisture is charged with electricity. When these drops come together to make one big drop, the surface of the big drop is proportionately much smaller than the combined surfaces of all the small drops. There isn't room enough on the surface of the big drop to hold all the electricity that existed on the surface of the larger number of smaller drops and, therefore, a great deal of electricity is set free. "Only a few flashes of lightning reach the earth. Most lightning-flashes occur between two cloud masses in the body of the thunder-cloud. Photographs of these show them to consist of scores of fine branches which jump from one cloud to the other, the flash being strong or weak according to the distance to be jumped. You can see that a very faint flash could jump a distance of an inch, but that it would take a stronger current to jump a yard, and that a terrific force of electricity must have accumulated before the current is strong enough to break down the resistance of the non-conducting air and jump a quarter of a mile. When lightning is attracted by the earth, it means that the air between the thunder-cloud and the earth is being subjected to a constant strain, and the weakest place gives way first. The weakest place, generally, is the place when the jump is shortest and there is a good conductor available. "One of the reasons that buildings and trees are struck by lightning is because they project up into the air, and according to their height, they remove a corresponding amount of the poorly conducting air. If the lower edge of a thunder-cloud is two thousand five hundred feet above the air, and the spire of a church is five hundred feet high, it follows that it is easier for a flash to jump two thousand feet than two thousand five hundred. So when the electricity-bearing cloud comes over the church spire the flash will leap to the church, five hundred feet of obstacle being removed. The highest building, therefore, is usually struck first, or the highest tree in a forest. "A lightning-rod or conductor is the best preventive against the destruction of a building by lightning, if the rod sticks up in the air above the building, even a couple of feet. The current will more readily strike the lightning-rod. As these are made of metal--copper or iron, generally--which are extremely good conductors, the current flows through them to the ground without harming the building.