[Illustration: DIAGRAM EXPLAINING THE DURATION OF THE SOUND OF THUNDER.]
And if, which is very often the case, the astronomer is not exactly under one of the extremities of the lightning, but at some other point in its course, he first hears a clap, then an increased noise, then a diminution. In fact, in this case, the sound which leaves point D just overhead, which is 1000 metres off, arrives alone in 3 seconds, but the sounds formed from D to E on one side, and from C to D on the other, arrive at the same time, having joined each other, taking 9 seconds, which is the necessary time to come from 1000 to 3000 metres. The sounds beyond C arrive and depart according to distance, as in the preceding example, and the thunder has lasted 23 seconds instead of 32 seconds.
[Illustration: COMMENCEMENT, AUGMENTATION, AND DIMINUTION OF THE INTENSITY OF THUNDER.]
I must add that lightning is never straight, but always crooked.
The length of time the thunder rolls has nothing to do with the distance of the cloud where the phenomenon begins. It is proportionate to the length of the lightning with which it is associated. The rolling is often still more prolonged by a succession of small discharges, which follow each other very rapidly between the stormy clouds; by the zigzags and ramifications of the lightning caused by the hygrometrical diversity of the different beds of air; by the echoes repeated by the mountains, the earth, the water, and the clouds themselves--to all which must be added also the interferences caused by the encounter of the different systems of sound-waves.
Its duration is extremely variable, however; it rarely exceeds 30 seconds, though the noise may sometimes seem to last much longer, so that an observation of this kind may have any value--one must take into consideration the echo, and isolate a single clap from the series of discharges which take place in the bosom of the storm. The longest verified duration of a single discharge is 45 seconds. That is tremendous if we think of the instantaneousness of the lightning, and reflect that the flash and the sound are produced in reality at the same moment, that they are dependent the one on the other, and that in their various manifestations there is only the difference of motion.