kind of sponge; hence, the smaller the kernel the more notable the influence of the lighter sponge. Another perhaps more important cause may be found in the power of the cavities of a grain of dust to attract moisture from the air, by virtue of which an atmosphere of invisible vapor is gathered around the corpuscle so as to form a single system with it. The density of vapor being only about two thirds that of air at the same pressure, this vaporous envelope has great sustaining power, and its presence furnishes an adequate explanation of the suspension of so much solid matter.
Although the presence of these millions of particles may diminish the transparency of the atmosphere, they contribute to the illuminating power of the sun by reflecting its rays in every direction and causing all the space to be pervaded with light, and, intercepting the rays of heat as they pass from the earth, they prevent loss by too rapid radiation. A similar explanation accounts for the suspension of globules of water in clouds.
We come now to the powers of our particle of air to emit sounds, which are always curious and often imposing: manifested in the snap of the coachman's whiplash, when the particles suddenly thrown out of equilibrium execute sonorous vibrations in recovering it; in the resonance of artillery discharges, the roaring of the tempest, the moaning of the surf, and the rolling of the thunder—all reactions of air against forces which have compressed it.
When we bear in mind the power displayed by particles of air hurled in violent wind against a fixed obstacle, we are led to ask how these particles exhibit their energy when the air is traversed by a projectile—spherical it may be—moving with great velocity? Since the air, in spite of its extreme mobility, opposes a degree of resistance to any sudden displacement, the vacuum created behind the projectile is not instantly filled; and not all the particles in front of it being able to get out of the way as fast as it goes along, an accumulation takes place there which exerts considerable pressure against it. The situation then becomes the same as if there was a spring in front of the ball strong enough to nullify every instant a part of its velocity and to deform a solid obstacle placed in its course. Melsen was struck by this thought, and instituted a series of experiments that gained him great credit, the result of which was to prove that the air accumulated in front of a ball flying with sufficient velocity forms a gaseous layer capable of opposing the immediate contact of the projectile with a resisting medium, particularly at the point squarely opposed to the course of the missile. This view was very clearly confirmed by Prof. E. Mach, of the University of Prague, who obtained a photographic image of a projectile moving with great velocity and
