Page:Popular Science Monthly Volume 33.djvu/100

90 can be arranged by which the aberration in the audition of the mariner may be so corrected that he can locate the source of the sound which is made to assure his safety, but which, misheard, may, as in the case of the Edam and Lepanto, insure his destruction.

It seems evident from President Morton's statements that if the fog-signals of the maritime world, or even of one country, or even those located in the approaches to one of our great harbors, were tuned to one note, and if the ships frequenting those waters were fitted with topophones, or some similar instrument, arranged so as to be in unison with the fog-signals, that aberrations in audition, at least as to direction, might be corrected, so as to determine the location of sound to within at least one compass-point.

Since the development of the topophone a number of other instruments have been invented for determining for the mariner the direction of sound made to warn him from danger. For some time some of our best ocean-steamers have been supplied with an instrument giving sounds of wonderful pitch and intensity, called the siren. It was adapted from the instrument invented by Cagniard de la Tour, by A. and F. Brown, of the New York City Progress Works, under the guidance of Prof. Henry, at the instance and for the use of the United States Lighthouse Establishment, which also adopted it for use as a fog-signal. The siren of the first class consists of a huge trumpet, somewhat of the size and shape used by Daboll, with a wide mouth and a narrow throat, and is sounded by driving compressed air or steam through a disk placed in its throat. In this disk are twelve radial slits; back of the fixed disk is a revolving plate containing as many similar openings. The plate is rotated 2,400 times each minute, and each revolution causes the escape and interruption of twelve jets of air or steam through the openings in the disk and rotating plate. In this way 28,800 vibrations are given during each minute that the machine is operated; and, as the vibrations are taken up by the trumpet, an intense beam of sound is projected from it. The siren is operated under a pressure of seventy-two pounds of steam, and can be heard, under favorable circumstances, from twenty to thirty miles. "Its density, quality, pitch, and penetration render it dominant over such other noises after all other signal-sounds have succumbed." It is made of various sizes or classes, the number of slits in its throat-disk diminishing with its size. This instrument is now used as a fog-signal by most maritime nations, they having frankly copied from, and, in some instances, obtained it through the United States Lighthouse Establishment; and it has been recently adapted to the use of