siren 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. The
dimensions given above are those of the largest. [See engraving on page
448, "Annual Cyclopaedia" for 1880.]

The experiments made by Gen. Duane with these three machines show that the
siren can be, all other things being equal, heard the farthest, the
steam-whistle stands next to the siren, and the trumpet comes next to the
whistle. The machine which makes the most noise consumes the most fuel.
From the average of the tests it appears that the power of the first-class
siren, the twelve-inch whistle, and first-class Daboll trumpet are thus
expressed: siren nine, whistle seven, trumpet four; and their relative
expenditure of fuel thus: siren nine, whistle three, trumpet one.

Sound-signals constitute so large a factor in the safety of the navigator,
that the scientists attached to the lighthouse establishments of the
various countries have given much attention to their production and
perfection, notably Tyndall in England and Henry in this country. The
success of the United States has been such that other countries have sent
commissions here