Others have supposed that the passage of the electric current creates a vacuum, and that the air rushing in to fill it produces the sound. Any explanation that has yet been offered is not altogether satisfactory.
What occasions the rolling of the thunder?
It has been ascribed to the effect of echo; but the true cause probably is, that the sound is developed by the lightning in passing through the air, and consequently separate sounds are produced at every point through which the lightning passes.
Why is thunder sometimes one vast crash?
Because the lightning cloud is near the earth; and as all the vibrations of the air (on which sound depends) reach the ear at the same moment, they seem like one vast sound.
Why is the thunder generally heard several moments after the flash?
Because it has a long distance to travel. Lightning travels nearly a million times faster than thunder; if, therefore, the thunder has a great distance to come, it will not reach the earth till a considerable time after the flash.
Can we not tell the distance of a thunder cloud by observing the interval which elapses between the flash and the peal?
Yes; the flash is instantaneous, but the thunder will take a whole second of time to travel three hundred and eighty yards; hence, if the flash be five seconds before thunder, the cloud is nineteen hundred yards off.