732 



SOUND-SIGNALS. 



Ares of Audibility. 



Area of Inaudibility. 



FIG. 8. BLTJNT'S DIAGRAM. 



Area of Audibility. 



With regard to snow, it is thought by seamen in 

 this vicinity that it is favorable to sound; certainly, 

 Portland rfead trumpet is very distinctly heard here 

 during violent northeast snow-storms, when it is di- 

 rectly to the leeward. 



Prof. Henry made observations at a number 

 of light-stations for various purposes connected 

 with the investigation of the laws of sound, 

 from his report of which it appears that he 

 noted a number of instances of remarkable 

 aberrations of audition of several fog-signals. 

 These are recorded at length in " Henry on 

 Sound," in the reports of the Lighthouse 

 Board of 1874-'75, and in the reports of the 

 Smithsonian Institution for several years. 



The experiments were" not confined to our 

 own shores. Prof. Tyndall, who stood in the 

 same relations with the British Lighthouse 

 Establishment that Prof. Henry did to our 

 own, reported thus : 



With a view to the protection of life and property 

 at sea in the years 1873-'74, this subject received an 

 exhaustive examination, observational and experi- 

 mental. The investigations were conducted at the 

 expense of the Government, and under the auspices 

 of the Trinity House. The most conflicting results 

 were at first obtained.^ On the 19th of May, 1873, the 

 sound-range was 31 miles ; on the 20th it was 51 miles ; 

 on the 2d of June, 6 miles ; on the 3d, more than 

 9 miles ; on the 10th, 9 miles ; on the 25th. 6 miles ; on 

 the 26th, 9i miles ; on the 1st of July_, 12| miles ; on 

 the 2d, 4 miles ; while on the 3d, with a clear, calm 

 atmosphere and smooth sea, it was less than 3 miles. 



From the letters received from officers of the 

 navy and of the merchant marine service, also 

 from officers of the English, Scotch, and French 

 lighthouse establishments, it is evident that pub- 

 lic attention is being given to the subject, and 

 that observations are being made as to such 

 aberrations simultaneously in various parts of 

 the world. 



Dr. Welling, President of Columbian Uni- 

 versity, in a paper read before the Washington 

 Philosophical Society, Nov. 5, 1881, said : 



In a paper presented to the Eoyal Society in 1874, 

 Prof. Eeynolds showed that the form of the sound- 

 wave is liable to flexure from changes in the tempera- 

 ture of the atmosphere, as well as from the unequal 

 motion of the wind. These abnormal phenomena of 

 sc-und, considered in connection with the hypothesis 

 of Prof. Stokes, as enlarged and applied by Prof. 

 Henry, may be reduced in the following generaliza- 

 tions, which, if accurate in point of logical form, and 

 true in point of the facts to which they are applied, 

 may be stated under the guise of aphorisms, as fol- 

 lows : 



1. Where the condition of the air is nearest that of 

 a calm, the larger will be the curve of audition, and 



the nearer will the shape of the curve approach to a 

 circle, to which the point of origin of the sound, or 

 the . point of perception, will be the center. (This 

 aphorism is stated abstractly from any consideration 

 of temperature-refraction which, so far as it exit-ts, 

 will always tend to modify the shape of the curve of 

 audition.) 



2. Apart from all consideration of temperature- 

 refraction, a sound will be heard farthest in the direc- 

 tion of a gentle wind, because the portion of the 

 sound-wave thrown down from above, in this case, 

 is re-enforced by the sound reflected from the surface, 

 and will thus more than compensate for the loss by 

 friction. 



3. Other things being equal, the area of audition 

 will be proportionately diminished in the case of 

 sounds moving against winds more or less strong, be- 

 cause the sonorous waves will be refracted above the 

 ears of the observer. 



4. The area of audition will be diminished in the 

 case of a sound moving with an over-strong favoring 

 wind, because the sound-waves in this case will be so 

 rapidly and strongly thrown to the ground that the 

 intensity of the sound will suffer more diminution 

 from absorption and friction than can be supplied by 

 the upward 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, cce- 

 teris parihus, be heard farther than similar sounds 

 moving with an over-strong favoring wind, for rea- 

 sons already implied, because the downward flexure 

 of the sound-waves, being excessive in the latter case, 

 tends to extinguish the conditions of audibilitv more 

 rapidly than is done by the slight upward refraction 

 in the former case. 



6. When sounds moving against the wind are heard 

 farther 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 opposite to that at 

 the surface. 



7. A sound moving against the wind, and so refract- 

 ed 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 

 downward flexure of the sound-waves, resulting from 

 an upper current of wind moving in an opposite direc- 

 tion 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 inclosed 

 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 eleva- 

 tion. 



9. As sounds moving against the wind are liable to 

 become inaudible (by being tilted over the head of the 

 observer), ev.en before their intensity has been extin- 

 guished, we may find in this fact an explanation of 

 the statement made by Eeynolds, that " on all occa- 

 sions the effect of wind seems to be rather against dis- 

 tance than distinctness." 



