68o POPULAR SCIENCE MONTHLY. 



kind of sponge; hence, the smaller the kernel the more notable 

 the influence of the lighter sponge. Another perhaps more im- 

 portant 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 corpus- 

 cle 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 di- 

 minish 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 sud- 

 denly thrown out of equilibrium execute sonorous vibrations in 

 recovering it ; in the resonance of artillery discharges, the roar- 

 ing of the tempest, the moaning of the surf, and the rolling of the 

 thunder all reactions of air against forces which have com- 

 pressed 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 be- 

 hind 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 consider- 

 able 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 in- 

 stituted 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 capa- 

 ble of opposing the immediate contact of the projectile with a re- 

 sisting 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 pho- 

 tographic image of a projectile moving with great velocity and 



