"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.
"The big lightning flashes that you see, boys, aren't always a single flash, but often a whole series of flashes, which occasionally run up as well as down. The resistance of the air being broken down, makes a path for the electrical discharge, so that the conductor does not have to stand the entire strain of the cloud at once, but only in a series of discharges. Photographs of lightning flashes show these very clearly."
"I've never done any lightning photography," said Ralph disgustedly, "I'd never thought of it."
"You try it," said the Forecaster, "and you'll find that there are no two flashes of lightning that look alike. Some of them are several miles long. One thing you will notice at once, Ralph, and that is that lightning is never zigzag, the way you see it in pictures, but runs in an irregular line, winding a little like a river-course."
"How about sheet-lightning?" asked Ralph.
"That's just the same as any other kind of lightning," was the reply, "except that it doesn't come to the earth or is so distant that the earth flash is not visible. It is generally due to discharges between upper and lower clouds, and the lower clouds are illuminated by the lightning. Heat-lightning, as it is called, is pretty much the same thing."
"Father told me once," said Fred, "that during a thunder-storm, a ball of fire came down on the chimney and rolled all around the room like a bubble of quicksilver and then struck a shovel that was standing in the corner, when it blew up with a bang. What was that, Mr. Levin?"
"That's globe, or ball lightning," was the reply. "There have been some very curious freaks done with these electric balls. One of them, in a baker's shop at Paris, jumped into an open oven door and exploded, giving off so much heat that a pan of biscuits was baked in the fraction of a second. At least, so Flammarion tells the story, though it sounds a bit queer."