724 



SOUND-SIGNALS. 



to study our system. That sent by England in 

 1872, of which Sir Frederick Arrow was chair- 

 man, and Captain Webb, E. N., recorder, re- 

 ported so favorably on it that since then "22 

 sirens have been placed at the most salient 

 lighthouses on the British coasts, and 16 on 

 light-ships moored in position where a guiding 

 signal is of the greatest service to passing navi- 

 gation." 



The trumpet, siren, and whistle are capable 

 of such arrangement that the length of blast 

 and interval, and the succession of alternation, 

 are such as to identify the location of each, so 

 that the mariner can determine his position by 

 the sounds. 



In this country there were in operation in 

 July, 1883, sixty-six fog-signals operated by 

 steam or hot air, and the number is to be in- 

 creased in answer to the urgent demands of 

 commerce. 



Use of Natural Orifices There are, in various 

 parts of the world, several sound-signals made 

 by utilizing natural orifices in cliffs through 

 which the waves drive the air with such force 

 and velocity as to produce the sound required. 

 One of the most noted is that on one of the 

 Farallon islands, forty miles off the harbor of 

 San Francisco, which was constructed by Gen. 

 Hartmann Bache, of the United States Engi- 

 neers, in 1858-'59, and of which the following 

 is his own description : 



Advantage was taken of the presence of the work- 

 ing party on the island to make the experiment, long 

 since contemplated, of attaching a whistle as a fog- 

 signal to the orifice of a subterranean passage open- 

 ing out upon the ocean through which the air is vio- 

 lently driven by the beating of the waves. The first 

 attempt failed, the masonry raised upon the rock to 

 which it was attached being blown up by the great 

 violence of the wind- current. A modified plan with 

 a safety-valve attached was then adopted, which it is 

 hoped will prove permanent. . . . The nature of this 

 work called for 1,000 bricks and four barrels of cement. 



Prof. Henry says of this : 



On the apex of this hole he erected a chimney which 

 terminated in a tube surmounted by a locomotive-whis- 

 tle. By this arrangement a loud sound was produced 

 as often as the wave entered the mouth of the indenta- 

 tion. The penetrating power of the sound from this 

 arrangement would not be great if it depended merely 

 on fhe hydrostatic pressure of the waves, since this 

 under favorable circumstances would not be more than 

 that of a column of water twenty feet high, giving a 

 pressure of about ten pounds to the square inch. The 

 effect, however, of the percussion might add consider- 

 ably to this, though the latter would be confined in 

 effect to a single instance. In regard to the practical 

 result from this arrangement, which was continued in 

 operation for several years, it was found not to obvi- 

 ate the necessity of producing sounds of greater power. 

 It is, however, founded on an ingenious idea, and may 

 be susceptible of application in other cases. 



There is now a first-class siren in duplicate 

 at this place. 



The sixty-six steam fog-signals in the waters 

 of the United States have been established at 

 a cost of more than $500,000, and are main- 

 tained at a yearly expense of about $100,000. 

 The erection of each of these signals was au- 

 . thorized by Congress in an act making special 



appropriations for its establishment, and Con- 

 gress was in each instance moved thereto by 

 the pressure of public opinion, applied usually 

 through the member of Congress representing 

 the particular district in which the signal was 

 to be located. And this pressure was occa- 

 sioned by the fact that mariners have come to 

 believe that they could be guided by sound as 

 certainly as by sight. The custom of the mari- 

 ner in coming to this coast from beyond the 

 seas is to run his ship so that on arrival, if after 

 dark, he shall see the proper coast-light in fair 

 weather, and, if in thick weather, that he shall 

 hear the proper fog-signal, and, taking that as 

 a point of departure, to feel his way from the 

 coast-light to the harbor-light, or from the fog- 

 signal on the coast to the fog-signal in the har- 

 bor, and thence to his anchorage or his wharf. 

 And the custom of the coaster or the sound- 

 steamer is somewhat similar. 



Aberration of Sound. The fog-signal being 

 found trustworthy in its usual performance, 

 mariners relied on it, and occasionally met dis- 

 aster. But the failure was, ns a rule, ascribed 

 to the signal-keeper rather than to the signal. 

 Still, the complaints set the Lighthouse Board 

 at work to ascertain the cause and, if possible, 

 to provide the remedy. A series of experi- 

 ments with fog-signals was made by Prof. 

 Henry, beginning in 1865 and continuing till 

 the latter part of 1877. Certain abnormal 

 phenomena were observed relative to the au- 

 dition of the sound the fog-signals made, and 

 finally examinations were made to ascertain 

 the extent of these aberrations and the cause 

 of them. The aberrations in audibility had not 

 escaped the attention of earlier writers. 



The causes assigned for these abnormal phe- 

 nomena of sound are almost as various as the 

 writers on them. Dr. Derham, writing in 1 708, 

 seemed to consider them as caused by variations 

 in temperature, moisture, and wind. Baron 

 von Humboldt, and after him Dr. Dove, Sir 

 John Herschel, and Dr. Robinson, held that 

 aerial flocculence is answerable for these phe- 

 nomena, a theory adopted and amplified by Dr. 

 Tyndall. Prof. Henry, however, has presented 

 another and different theory, and has worked 

 it out in a series of long-continued and careful 

 experiments, accounts of which are published 

 at length in the reports of the Lightbouse Board 

 and by the Smithsonian Institution in its regu- 

 lar series, and in a separate volume. He reached 

 the conclusion that the wind, when moving at 

 the rate of eight or ten miles an hour, could not 

 act in perceptible retardation of sound when it 

 was moving at the rate of 750 or 780 miles an 

 hour, but be had accepted the idea that it might 

 act by deflection. At this time the suggestion 

 of Prof. Stokes, of Cambridge, England, which 

 offered a plausible explanation of the action of 

 the wind, became known to him, when it " was 

 immediately adopted as a working hypothesis 

 to direct investigation." Prof. Stokes's ex- 

 planation, which Tyndall speaks of as suggest- 

 ed by some remarkable observations of De la 





