iln many points along the coast, in the light houses and on lightships, steam whistles and horns of great power have been placed, and also bells that are rung mechanieally, which are sounded whenever the state of the atmosphere re quires it to be done. The simplest and commonly most powerful signal employed by the United States 1.ight house Boa rd is the locomotive whistle, operated by a steam boiler with a fires sure of from 50 to 7.1 pounds. The sounds from the land are distinguished from those on board ship by the length of the notes and the intervals between. The whistles are from eight to ten inches in diameter, and are operated automatieal ly by an engine supplied with steam from the same boiler. The Dalton trumpet, worked by condensed air, is next in importance. The trum pet itself provides the resounding cavity. and the vibratory motion of the air is produced by a reed. The reed is an iron bar. in the larger trumpets being eighteen incites long. two inches wide, and three-quarters of an inch thick, gradually lessen ing toward the free end. The siren trumpet is the most powerful instrument so far employed. (See SIREN.) The impulse to the air which pro. duces the sound is given by a flat drum, or a hollow cylinder with a short axis, one end of which is perforated to admit steam. On the other side the drum is perforated with eight or more holes, in connection with •hieh is a re volving disk having a similar number of holes. As the disk revolves these eight holes are al ternately open and shut, allowing egress to as many jets of steam, which provides a regnlar and violent motion in the air. giving rise to a most powerful sound. reinforced by the res.mance of a trumpet of suitable length. In still air this sound can be heard from 20 to 30 miles, even during a dense fog. The notes are not all to be heard at the same distance, however. and an opposing or crossing wind seriously interferes with the transmission of sound. The interven tion of rain, snow. or mist also disturbs and re tards, and temperature has a distinct influence. Whistling lmo•s and bell buoys are also largely used to mark dangers or ehannels, and are of great assistance in determining the whereabouts of a vessel in thick weather. For a thorough discussion of various forms of fog-signal. the reader is referred to .Johnson, Modern Lighthouse S(rriee (Washington, 1SS91.
'Railway trains depend largely upon torpedo.. to indicate the too close approach of one train. to another. The torpedoes are small. flat 'tin cases charged with a detonating powder which is exploded by the wheels of the passing train and makes a loud report. These are not only used in foggy weather, hut also when there is any ob struction along the line that would expose ap proaching trains to danger of collision, derail ment, or any similar accident.
While fog-signals have been made which may he heard at considerable dis tances, yet the apparent directions from which they come are often de ceptive, and much of the good which might other wise be accomplished by these signals is rendered nugatory because of the difficulty of locating ex actly their direction. This has been measurably ob viated by the invention of the `topophone' by Col. D. P. Heap, Corps of En gineers, U. S. A. This instrument not only as sists in hearing sounds at greater distances than is possible with the unas sisted ear, but also as sists in locating with more exactness the direc tion from which the sound conies.
It consists of two acoustic receivers or trumpets pointing in oppo site directions, and supported on a vertical shaft. (See Fig. 1.) From the lower ends of the trum pets extend rubber tubes connected with the ears by especially constructed ear-pieces.
The observer holds the shaft so that the instru in the direction inside of the angle of oscillation; this angle is generally about one point of the compass. The operation is simple and takes but a few seconds.
The direction of the sound being once ascertained, the observer can readily keep the topopbone pointed in this direction. Knowing the speed of the vessel and its course, the loca tion of the sound can be quickly plot ted accurately enough for prac tical purposes. This instrument cannot fail to be of very great value to those navigating ships, accomplishes that which has long been sought for hut not hereto fore satisfactorily accomplished.
While the topo phone above de scribed aids to as certain the direc tion of the sound, various attempts have been made to have direct indications of the approximate direction of the sound by varying its character for different points of the compass. The Foster fog-signals, which have been adopted by the United States Lighthouse Board, accomplish this. The apparatus is a spe ment is above his bead (see. Fig. 2) ; if the sound is heard in the right ear, it shows at once that tbe noise must be somewhere on his right side. By oscillating the trumpet so that the sound is beard alternately in each car, the sound will he vie: of sound-director, which concentrates and projects various signals, first in one direction and then in another, varying the signal sound accord ing to the direction to which it is sent. A pass ing vessel may, therefore, hear one of these sounds more clearly than the other, and sig nal heard most distinctly indicates a close ap proximation to the exact direction from which the sound conics.
The sound-director consists of eight stationary megaphones, each being directed to one of the points of the compass, as shown in the illustra tion. It is also made of a single automatic re volving megaphone, which emits the appropriate sound on arriving at each point of the compass. Different points of the compass are indicated by a simple code of long and short blasts, the opposite points of the compass being in each case reversed; for example, north is one long blast, south is one short blast, west is a short blast and a blast, and east is a long blast and a short blast.
The signals for the different points of t he corn rd.s indi•ated on t he aceompanying diagram, Fig. •1.
diagram rig. 5 show, the relation of the sianids as bctueen twi 'passing VC4,1•14.
When t‘lo signals are heard with equal clear 'Jess both balder than the tollim‘ilig the ili• ruction of the danger must be betm•en the points 6•h1c1 are heard with equal instinet 'Jess. These signals may be placed on at lighthouse stations and fi rr? slips, ere., ill the latter ease, the facilit.5 of a hill', Mg its proper slip, even ill very fog* regions. The method adopted for this purpose is indicated in Fig. 6. These signals also be applied to aid in manmuvring a fleet, enabling vessel-, to retain their relative positions and to maintain their distance.