730 



SOUND- SIGNALS. 



have a day of normal audition and barren of 

 curious phenomena. After the siren had begun 

 its noise, we ran down to a point within half a 

 mile of the lighthouse, and then steamed for 

 Plum island, running a little south of east for 

 six miles, when we returned as nearly as might 

 be on our own track. The results were curi- 

 ous. We lost half the force of the sound when 

 within a quarter of a mile of the siren ; a mo- 

 ment later we had lost four fifths of it. Run- 

 ning another half-mile, we were off the middle 

 of Great Gull island, and the sound had in- 

 creased to a force of four ; in five minutes more 

 it had dropped to three ; from that time, till 

 we reached the end of our six-mile run, it 

 gradually weakened, and it had dropped to a 

 force of two when we turned and ran back to 

 our anchorage. It is especially curious that 

 the sound had the same intensity at three six- 

 teenths of a mile from its source, and at six 

 whole miles from that point, while it varied 

 from two to ten in a scale of ten between those 

 points. The results of the trip are more fully 

 and exactly given in the diagram, Fig. 7. 



GREAT GULL I. 



FIG. 7. 



This diagram shows the result of observations at Little Gull 

 island, Long Island Sound, Aug. 10, 1881, beginning at 

 10.30 A. M. Thermometer, dry bulb, 76 ; wet bulb, 75 

 F. Barometer, 29-40. Wind W. by N., force 8, and steady 

 throughout. Day clear and beautiful? 



Thinking that possibly this peculiarity might 

 have been induced by those differences of 

 temperature in the strata of the atmosphere 

 suggested by Tyndall as probable cause for 

 such phenomena, effort was made to ascertain 

 something of these differences by sending a 

 thermometer to the upper air. In the course 

 of the afternoon we made a kite six feet high, 

 attached to it a self-registering thermometer, 

 and after several trials succeeded in getting 

 it up about 500 feet, and in hauling it safely in 

 again after it had been up over an hour. The 

 thermometer had a wet bulb, and was pro- 

 tected from the direct rays of the sun ; but it 

 registered only half a degree more of heat at 

 its highest point than it had registered in the 

 pilot-house. The course the kite took showed 



NOTE. - Figs. 4, 5, 6, and 7, were prepared from diagrams 

 made and sent the writer by the eminent chief of the 

 French Lighthouse Service, Emile Allard, Inspecteur-Gene- 

 ral des Fonts et Chaussees, Directeur du Service Central des 

 Phares et Ballses. M. Allard made them from the diagrams 

 in the writer's paper on the " Aberrations of Audibility of 

 Fog-Signals," in which the intensity of sound at each point 

 was indicated by Arabic numerals in a scale of ten. The 

 writer submits Allard's diagrams, because they convey the 

 desired meaning better than do his own diagrams. 



no difference between the air-currents alow 

 and aloft. 



The result of these and other observations 

 was laid before the Washington Philosophical 

 Society on Oct. 22, 1881, and then, and at a 

 subsequent meeting, was made the subject of 

 some discussion, an account of which appeared 

 in its publications, and was published in pam- 

 phlet form.* 



M. Allard, Director of the French Lighthouse 

 Service, in his "Memoire sur la Portee des 

 Sons et sur Jes Caracteres a attribuer aux 

 Signaux sonores," recently published by his 

 Government, records some curious experiments 

 with various fog-signals made in France, Ger- 

 many, and England, as well as in this country. 

 He has compared, in tabular form, the results 

 of these experiments, and gives the range of 

 sound of each instrument under a variety of 

 conditions, taking into consideration the in- 

 tensity and pitch of the sound, the direction 

 and force of the wind, and its angle of action 

 on the phonic beam. He has established a 

 mathematical formula for the calculation of 

 the range of the sound of each instrument, 

 under certain given circumstances, in which 

 he expresses what he styles the acoustic trans- 

 parency of the atmosphere by a coefficient, 

 and he has also expressed graphically the 

 set of results obtained, thus assisting largely 

 the intelligent discussion of the general sub- 

 ject. 



Tillamook Light and Fog-Signal Station is on 

 an isolated rock about half a mile west of the 

 coast of Oregon, and about 20 miles south of 

 the mouth of Columbia river. The signal is a 

 steam-siren. The surface of the rock is about 

 86 feet above the water, and the trumpet, 

 which points almost westward, has an elevation 

 of fully 100 feet. (See engraving on page 443, 

 "Annual CyclopaBdia " for 1880.) Col. Gilles- 

 pie, of the Corps of Engineers, TJ. S. A., who 

 built the tower and put in the illuminating and 

 fog-signal apparatus, tested the latter, steaming 

 to the west directly from it on a clear bright 

 day, and found that the sound became practi- 

 cally inaudible at a distance of two miles. In 

 a letter dated Nov. 6, 1882, he wrote : 



My first attention was called to the defects of audi- 

 bility by the captain of the Shubrick, who stated, in 

 May, 1881, that, when anchored in a small bight south 

 of the rock near Arch Eock, and about one mile dis- 

 tant, he could hear the sound but faintly. On my 

 next visit I directed the engineer to get up steam^ and 

 when he had a pressure of fifty pounds, to whistle, 

 and then set the machinery in motion. On receiving 

 the signal, I made a circuit of the rock and then pro- 

 ceeded westward under slow bell, recording my dis- 

 tance by interval of sound at each blast. At two miles 

 the sound became quite weak, and at the next blast 

 no sound was received. Our engine was not stopped 

 when receiving the sound, and my station was aft 

 quite near the propeller ; the day was bright and clear, 

 without any wind. The temperature was 65 or there- 

 about; don't know the barometer reading. On re- 

 ceiving the next sound the boat was at rest, that is, 



* Aberrations of Audibility of Fog-Signals. By Arnold B. 

 Johnson, Chief Clerk of the Lighthouse Board. Washington, 



1882. 





