We see lightning in several different forms; sometimes its flash is straight, sometimes it looks forked or zigzag, sometimes it is round like a ball, and sometimes it spreads over the clouds like a sheet of fire. When a thunder cloud is near the earth, the flash comes straight down, because there is but little air for it to pass through. When, on the other hand, the cloud is at a considerable distance from the earth, the air in the path of the lightning is made denser or thicker by being pushed together, and as lightning can pass more quickly through thin than through thick air, it flies from side to side so as to pass where the air is thinnest. This makes its path zigzag or forked. When there is a great charge of electricity in a cloud it sometimes forces its way through the air in the shape of a ball. What is called sheet lightning is either the reflection or shine on clouds of a stroke of zigzag lightning which is too far off to be seen, or light discharges of electricity from clouds which have not enough in them to make zigzag lightning.