PHILOSOPHICAL SOCIETY OF WASHINGTON. 37 



And, when there is a bluff behind the fog-sign.il, he should be 

 prepared for irregular intervals in audition, such as might be pro- 

 duced could the sound ricochet from the trumpet, as a ball would 

 from a cannon ; that is, he might hear it at 2, 4, 6, 8 and 10 miles 

 from the signal, and lose it at 1, 3, 5, 7, 9 and 11 miles distance, 

 or at any other combination of distances, regular or irregular. 



These deductions, some made, as previously mentioned, by sev- 

 eral of the first physicists of the age, and some drawn from the 

 original investigations here noted, are submitted for consideration 

 rather than given as directions. They are assumed as good work- 

 ing hypotheses for use in further investigation. While it is 

 claimed that they are correct as to the localities in which they were 

 made, it seems proper to say that they have not been disproved by 

 the practical mariners who have given them some personal consid- 

 eration, and who have tried to carry them into general application. 

 Hence these suggestions have been set down in the hope that others 

 with greater knowledge and larger leisure may give the subject 

 fuller attention, and work out further results. 



If the law of these aberrations in audibility can be evolved and 

 some method discovered for their correction, as the variations of 

 the compass are corrected, then sound may be depended upon as a 

 more definite and accurate aid to navigation. Until then, the 

 mariner will do well when he does not get the expected sound of a 

 fog-signal, to assume that he may not hear a warning that is 

 faithfully given, and then to heave his lead, and resort to the other 

 means used by the careful navigator to make sure of his position. 



Mr. Cleveland Abbe remarked that it seemed to him if these 

 anomalies were due to the refraction of sound in a vertical plane, 

 then a few feet of increase in the altitude of the observer or of the 

 signal itself, would make a great difference in the result. To this 

 Mr. Johnson replied that the observations made on board the ves- 

 sels were attended with the same results as to degree of audibility, 

 whether the observer were stationed upon the mast, deck, or near the 

 water line of the vessel. 



Mr. William B. Taylor said that the interesting observations 

 presented by Mr. Johnson were in the main entirely corroborative of 

 the results announced by our late President, Prof. Henry ; and the 

 anomalies noted furnished striking confirmation of the explanations 



