206 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



In the " Smithsonian Report " for 1877 is given a summary 

 of the results of the investigations of Professor Henry in refer- 

 ence to fog-signals and the audibility of sound, and as this 

 summary includes Ins very latest results, it will have perma- 

 nent interest. He finds the most efficient cause of the loss of 

 audibility is the direct effect produced by the wind. Sound 

 is heard farther when moving with the wind than when mov- 

 ing against it. This is due to a change in its direction : it 

 is refracted or thrown down towards the earth when mov- 

 ing with the wind, but passes over the head of the observer 

 when moving against the wind. Sometimes a strong upper 

 wind opposite to the surface wind produces an apparent re- 

 versal of the preceding law, as shown by his experiments in 

 1874. Although sound issuing from a trumpet or parabolic 

 reflector is at first concentrated, yet it tends to spread so 

 rapidly that at the distance of three or four miles it is heard 

 nearly equally well on all sides. Neither fog, snow, hail, nor 

 rain materially interferes with the transmission of sounds. 

 Sound-shadows of great extent can be produced by build- 

 ings or other obstacles. The alternate audibilitv and inau- 

 dibility of a sound, as we approach to or recede from its 

 origin, is attributed to the upward refraction of the sound- 

 wave and its successive reflections at the upper and lower 

 surfaces, or the riofht-hand and left-hand bounding surfaces 

 of shallow or narrow currents of air. The phenomenon of an 

 aerial echo which conies back to the observer from a portion 

 of the horizon directly in front of the trumpet is attributed 

 provisionally to the fact that in the natural spread of the 

 waves of sound, some of the rays must take such a curved 

 course as to strike the surface of the water in a perpendicu- 

 lar direction, and thus be reflected back towards the orio-in of 

 the sound. 



Pneumatics and Aeronautics. 



The ventilation of buildings and railway tunnels, etc., and 

 the driving of the carriages in pneumatic tubes, railways, 

 etc., introduce the application of principles and data that 

 also have an application in meteorological problems; while, 

 on the other hand, meteorological data relating to wind, tern- 

 perature, and pressure enter into the computations of the 

 engineers. Among these problems whose discussion we have 

 noted during the past few years, we mention, the Ventilation 



