But clouds, says some one, are seen striking upon mountains. without 1 causing any sound. How is that consistent with your theory? Well, in the first place, a sound is not caused by any and every method of cloud collision, but only when there is an arrangement of their position suitable for producing a sound. Striking the backs of the hands does not produce clapping, but the contact of palm with palm does. It makes a great difference, too, whether the clouds that strike are hollow, or flat and extended. In the second place, the clouds must not merely drift, as against a mountain, but be driven with great tempestuous violence. Besides, a mountain does not cut through a cloud, it merely 2 disperses it by displacing the successive front layers of it. Even a bladder does not give a report irrespectively of the method in which it emits the air in it; it depends on the way in which the air escapes. If the bladder is cut with a knife, the air is emitted without the ear perceiving it. It must be burst, not cut, in order to give a report. The same, I assert, holds in regard to the clouds: they emit no peal unless broken up with great violence. Besides, clouds driven against a mountain are not broken up, but merely pour round certain parts of 3 the mountain, tree branches, shrubs, and rough projecting boulders. They are rent thereby, and emit by numerous exits whatever air they may contain; but there is no rattle unless the air all burst out at once. In proof of this, bear in mind that the wind blowing through a tree, 4 which cuts it, hisses but does not roar. A broad blow, so to speak, that dissipates the whole mass at once, is required in order to the emission of a sound such as is heard when there is thunder. XXIX Moreover, the atmosphere is by constitution adapted to the transmission of sound.[46] Of necessity this is so, since sound is nothing but an impact of the atmosphere. The clouds that [as indicated] are completely rent must therefore be hollow and taut. One sees how much more resonant empty vessels are than full, and distended ones than slack. So this accounts for the sound of tambourines and cymbals; the former resound because the blow upon the air is resisted at the farther side; the latter are beaten against the air directly, but unless there were a cavity in the instrument it would not tinkle. [46] The specific word _vox_ = voice is used in the text. XXX