=Variations of Intensity in Thunder.=--From what has been said, it is easy to understand how the general roar of thunder is subject to great changes of intensity, during the time it lasts, according to the number of peals that may be arriving at the ear of an observer in each particular moment. But every one must have observed that even an individual peal of thunder often undergoes similar changes, swelling out at one moment with great power, and the next moment rapidly dying away. To account for this phenomenon, I would observe, first, that there is no reason to suppose that the disturbance caused by lightning is of exactly the same magnitude at every point of its path. On the contrary, it would seem very probable that the amount of this disturbance is, in some way, dependent on the resistance which the discharge encounters. Hence the intensity of the sound waves sent forth by a flash of lightning is probably very different at different parts of its course; and each individual peal will swell out on the ear or die away, according to the greater or less intensity of the sound waves that reach the ear in each successive moment of time. But there is another influence at work which must produce variations in the loudness of a peal of thunder, even though the sound waves, set in motion by the lightning, were everywhere of equal intensity. This influence depends on the position of the observer in relation to the path of the lightning flash. At one part of its course the lightning may follow a path which remains for a certain length at nearly the same distance from the observer; then all the sound produced along this length will reach the observer nearly at the same moment, and will burst upon the ear with great intensity. At another part, the lightning may for an equal length go right away from the observer; and it is evident that the sound produced along this length will reach the observer in successive instants, and consequently produce an effect comparatively feeble.