SOUND-SIGXALS. 



726 



Roche, roughly stated, is this : " The several 

 strata into which a current of air may be di- 

 vided do not move with the same velocity. The 

 lowest stratum is retarded by friction against 

 the earth ; the one immediately above by fric- 

 tion against the lower; hence the velocity in- 

 creases from the ground upward." And when 

 the direction of the sound is perpendicular to 

 the sound-wave, as when it is projected against 

 the wind, it will be thrown upward ahead of 

 the observer; and when it is projected with 

 the wind, it will be thrown downward toward 

 the earth. Proceeding on this suggestion of 

 Prof. Stokes, Prof. Henry worked out by care- 

 ful tests, and gave as the results of his experi- 

 ments, the causes of the five formulated phe- 

 nomena, as follows : 



1. The audibility of sound at a distance, and its in- 

 audibility nearer the source of sound. 



2. The inaudibility of a sound at a given distance 

 in one direction, while a lesser sound is heard at the 

 same distance in another direction. 



3. The audibility at one time at a distance of several 

 miles, while at another the sound can not be heard at 

 more than a fifth of the same distance. 



4. While the sound is generally heard farther with 

 tiie wind than against it, in some 'instances the reverse 

 is the case. 



5. The sudden loss of a sound in passing from one 

 locality to another in the same vicinity, the distance 

 from the source of sound being the same. 



Prof. Henry explains these abnormal phenom- 

 ena thus : u The first four of these phenomena 

 find a ready explanation ... by supposing an 

 increase or diminution in the relative velocity 

 of the currents of wind in the upper or lower 

 strata of air. The fifth is explained by the in- 

 terposition of an obstacle which casts, as it 

 were, a sound-shadow, disappearing at a given 

 distance by the divergence of the rays on each 

 side of the obstacle into what would be an op- 

 tical shadow." 



Prof. Henry read a paper before the Wash- 

 ington Philosophical Society, Dec. 11, 1872, in 

 which he announced his idea that its cause was 

 due to the deflection of the sound by the action 

 of the wind, an idea which. he afterward devel- 

 oped into what he styled a good working hy- 

 pothesis. 



Prof. Tyndall, in May, 1873, began his series 

 of investigations on the subject of the trans- 

 mission of sound, under the auspices of the 

 Trinity House, of England, in which whistles, 

 trumpets, guns, and the siren were used, the 

 last-named instrument having been lent by the 

 Lighthouse Board of the United States to the 

 Trinity House for the purpose. President Wel- 

 ling makes effective use of these dates in set- 

 tling the question of precedence as between 

 these authorities. 



Among Prof. Henry's last out-givings on this 

 subject, one which, it may be said, occupied 

 much of his attention even in his last illness, 

 were those styled in his last report to the Light- 

 house Board "General Conclusions," which 

 may be thus briefly summarized, his own words 

 being used as far as possible: 



1. The audibility of sound at a distance the state 



of the atmosphere being constant depends on the 

 character of the sound. 



2. The audibility of sound depends on the state of 

 the atmosphere. A condition mo>t tavorabl. 

 transmission of sound is that of perfect Btillness, and 

 unilbrm density and temperature throughout. 



8. But the most cmcu-nt cau>c of the loss of audi- 

 bility is the direct effect produced by the wind. . . . 

 Sound moving with the wind is refracted or thrown 

 down toward the earth; while, moving again-t tho 

 wind, it is refracted upward and passes over the bead 

 of the observer. 



4. Although, as a general rule, the sound U beard 

 farther when moving with the wind than when mov- 

 ing against it, yot in some instances the sound is heard 

 farthest against the wind, owing to a dominant upper 

 wind blowing at the time in an opposite direction to 

 that at the surface of the earth. 



5. Although sound issuing from the mouth of the 

 trumpet is at first concentrated in a given direction, 

 yet it lends to spread so rapidly that, at the distance 

 of three or tour miles, it fills tho whole space of air in- 

 closed within the circuit of the horizon, and is heard 

 behind the trumpet nearly as well as at an equal dis- 

 tance in front of its mouth. 



6. Neither fog, snow, hail, nor rain materially inter- 

 feres with the transmission of loud sound. 



7. In some cases sound-shadows are produced by 

 projecting portions of land, or by buildings situated 

 near the origin of the sound ; but these shadows are 

 closed in by the spread of the sound-waves, and thus 

 exhibit the phenomenon of sound being heard at a 

 distance and afterward lost on a nearer approach to 

 the station. 



8. It frequently happens on a vessel leaving a sta- 

 tion that the sound is suddenly lost at a point in itu 

 course, and, after remaining inaudible some time, is 

 heard again at a greater distance, and then is gradu- 

 ally lost as the distance is further increased. This is 

 attributed to the upward refraction of the sound-wave, 

 which passes over the head of the observer, and con- 

 tinues an upward course until it nearly reaches the 

 upper surface of the current -wind, when the refraction 

 will be reversed, and the sound sent downward to the 

 earth. Or the effect may be considered as due to a 

 sound-shadow produced by refraction, which is grad- 

 ually closed in at a distance by the lateral spread of 

 the sound-wave with the earth on either side, in a di- 

 rection which is not affected by the upper refraction. 

 Another explanation may be found in the probable 

 circumstance of the lower sheet of sound-beams being 

 actually refracted into a serpentine or undulating 

 course. 



9. The existence of a remarkable phenomenon has 

 been established, which is exhibited in all states of the 

 atmosphere, during rain, snow, and dense fog, to which 

 has been given the name of aerial echo. It consists of 

 a distinct echo, apparently from a space near the hori- 

 zon of 15 or 20 in azimuth, directly in the prolonga- 

 tion of the axis of the trumpet. The loudness of this 

 echo depends upon the loudness and quality of the 

 original sound, and therefore it is produced with the 

 greatest distinctness by the siren. It can not be due 

 to the accidental position of a flocculent portion of the 

 atmosphere, nor to the direct reflection from the crests 

 of the waves, as was first supposed, since it is always 

 heard, except when the wind is blowing a hurn- 



' As a provisional explanation, the hypothesis has 

 been adopted that, in the natural spread of the waves 

 of sound, some of the waves must take such a curvi- 

 linear course as to strike the surface of the water in an 

 opposite direction, and thus be reflected back to the 

 station or location of the origin of the sound. 



Prof. Tyndall referred these phenomena, in 

 his book on " Sound," to the existence of acous- 

 tic clouds consisting of portions of the atmos- 

 phere in a flocculent or mottled condition due 

 to the unequal distribution of heat or moisture, 



