PHILOSOPHICAL SOCIETY OF WASHINGTON. 45 



4. The area of audition will be diminished in the case of a 

 sound moving with an overstrong favoring wind, because the sound- 

 waves in this case will be so rapidly and strongly thrown down to 

 the ground that the intensity of the sound will suffer more diminu- 

 tion from absorption and friction than can be supplied by the up- 

 ward reflection of the sound rays conspiring with the gradual 

 downward flexure of the sound-waves, as in the case of a gentle 

 favoring wind.* 



5. Sounds moving against a gentle wind will, ceteris paribus, be 

 heard further than similar sounds moving with an overstrong favor- 

 ing wind, for reasons already implied, because the downward flex- 

 ure of the sound-waves, being excessive in the latter case, tends to 

 extinguish the conditions of audibility more rapidly than is done 

 by the slight upward refraction in the former case. 



6. When sounds moving against the wind are heard further than 

 similar sounds moving with a wind of equal strength, it is because 

 of a dominant upper wind blowing at the time in a direction op- 

 posite to that at the surface.f 



7. A sound moving against the wind, and so refracted as in the 

 end to be thrown above the head of the observer will, at the point 

 of its elevation, leave an acoustic shadow. But this acoustic 

 shadow, at a still further stage, may be filled in by the lateral 

 spread of the sound-waves, or may be extinguished by the down- 

 ward flexure of the sound waves, resulting from an upper current 

 of wind moving in an opposite direction to that at the surface, or 

 resulting in a less degree from an upper stratum of still air. Under 

 these circumstances, there will be areas of silence enclosed within 

 areas of audition.^ 



8. As sounds may be refracted either by wind, or by changing 

 temperatures, or by both combined, it follows that, under many 

 circumstances, a sound lost at one elevation may be regained at a 

 higher elevation. || 



9. As sounds moving against the wind are liable to become in- 

 audible (by being tilted over the head of the observer) even before 



* Light-House Report, 1875, p. 125. 



f Light-House Report for 1877 : Experiments on Sound, p. 13. 



J Experiments on Sound, 1877, p. 8. 



j| Henry and Reynolds. Cf., Delaroche, Ann. de Chim., 181 6, Tome I, p. 180. 



