Page:The New International Encyclopædia 1st ed. v. 07.djvu/876

* FOG-SIGNALS. 77,s FOG-SIGNALS. exploded by the wheels of the passing train and makes a loud report. These are not only used in foggy weather, but 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 be 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 comes. 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- FlG. 1. TOPOPHONE. 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 topophone 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, and accomplishes that which has long been sought for but 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- Fir,. 2. METHOD OF USING TOPOPHONE. Flfi. 3. LX0HTHOU8E MACHINE, WITS FIVE-FOOT MEGAPHON1 ment is above his head (see Fig. 2) ; if the sound is heard in the right ear. it .slums at once thai tin- noise him t lie somewhere on his right side. By oscillating tin- trumpet so thai the sound is heard alternately in each ear. the sound will he i ii - id' sound-director, which concentrates and projects various signals, first in one direction and then in another, varying tin signal sound accord- ing to the direction to which it is sent. A pass- in- vessel may. therefore, hear one of these