818 BULLETIN OF THE 



siren horn operated by steam at different pressures, the aerial 

 vibration being produced by the intermittence of a revolving 

 grating or disk valve in the small end of the horn, driven at high 

 velocities by the steam engine, and its pitch regulated by the 

 adjustable speed of the revolving disk. The trumpet or fog-horn 

 was provided with a series of replaceable sSeel tongues of diffe- 

 rent sizes, and the siren was driven at five different pitches of 

 from 250 to 700 impulses per second, and at steam pressures 

 varying from 100 pounds to 20 pounds per square inch. For 

 the purpose of accurate estimation, within short distances, a 

 phonometer or " artificial ear" was employed, having at its 

 smaller upturned end a horizontal drum of stretched membrane, 

 sprinkled with sand, after the plan devised by Sondhauss. Trum- 

 pets of the same size, were made of different materials, as of 

 brass, iron, and wood,; but these differences were found to exer- 

 cise little or no influence on the intensity or penetration of the 

 sound. Trumpets were also made of different shapes, straight 

 and curved, and square as well as round, with equal lengths and 

 equal areas of cross section ; from whose trials it appeared that 

 the conical form gave nearly double the distance of action on the 

 sand of the " artificial ear," that was given by the pyramidal 

 form. Such investigations — varied and long-continued, serve to 

 show the conscientious earnestness with which Henry sought to 

 give the highest efficiency to the expedients available for the 

 protection of life and property along our extended sea-coast. 



The steam-whistle was found to be less powerful than the 

 trumpet, with the same expenditures of fuel. Steam whistles 

 were afterwards tried of 10 inches, 12 inches, and 18 inches in 

 diameter. The largest size was not found to give results pro- 

 portioned to its increased consumption ; and the 10 or 12 inch 

 size was regarded as practically the most efficient. The siren 

 was found to be the most powerful and penetrating of the instru- 

 ments tested, as it admitted more advantageously the application 

 of a higher steam expenditure. The best result with this instru- 

 ment was attained with a pressure of from 60 to 80 pound's, and 

 at a pitch between 350 and 400 vibrations per second. Under 

 favorable conditions, this instrument frequently made itself heard 

 at a distance of fifteen and twenty miles. Henry's large expe- 

 rience with the occasional aerial impediments to sound propaga- 

 tion,* and his strong sense of the vital importance of having 

 fog-signals recognized at a distance, under the most adverse con- 

 ditions, led him to favor the introduction of the most powerful 

 sounders attainable, without absolutely limiting the decision to 

 their relative economy. Hence he was the first to devise improve- 

 ments in the siren, and to press its adoption at important or 



* An abstract of liis elaborate and invaluable researches on some ab- 

 normal phenomena of Sound — the crowning labor of his life, must be re- 

 served for a concluding section. 



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