A Summary of Researches in Sound: Conducted in the Service of the United States Light-House Board

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U.S. Government Printing Office, 1879 - Fog-signals - 1 pages
 

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Page 485 - Now it frequently occurs that a signal which, under ordinary circumstances, would be audible at the distance of fifteen miles, cannot be heard from a vessel at the distance of a single mile. This is probably due to the reflection mentioned by Humboldt.
Page 474 - An experiment was made both by means of the artificial ear on land and by actually going off on the ocean in a steamer until the sounds became inaudible to the natural ear. By the latter method the two sounds ceased to be heard at the distances of six, and twelve and a half miles, respectively. The indications of the artifical ear gave a similar result, the distance at which the sand ceased to move in one case being double that of the other. In both cases the conditions of wind and weather were apparently...
Page 491 - The boats above mentioned estimate their distance with considerable precision by the number of revolutions of the paddle-wheel as recorded by the indicator of the engine, and it is hoped by this means to definitely decide the point in question. We think it probable that fog does somewhat diminish the penetrating power of sound, or in other words, produce an effect analogous to that on the propagation of light. But when we consider the extreme minuteness of the particles of water constituting the...
Page 555 - ... as the reflecting surface was approached, because the whole distance travelled by the sound-wave to and from the reflector would have been lessened. The effect however is in accordance with the supposition that the echo is a multiple sound, the several parts of which proceed from different points at different distances of the space in front of the trumpet, and that as the steamer advances toward the verge of the horizon, it leaves behind it a number of the points from which the louder ones proceed,...
Page 457 - FOG. Among the impediments to navigation none perhaps are more to be dreaded than those which arise from fogs, and consequently the nature of this impediment and the means which may be devised for obviating it are objects of great interest to the mariner. Fogs are in all cases produced when cold air is mingled with warm air saturated with moisture. In this case the invisible vapor of the warmer air is condensed by the cold into minute particles of liquid water, which, by their immense number and...
Page 485 - ... the distance of a quarter of a mile. The employment of a trumpet with the whistle was rather more successful. The trumpet was constructed of wood, in the form of a square pyramid; the lower base being 10' by 10', the upper base 2
Page 469 - The third day observations were made from the two vessels, both however sailing to the south. From the vessel sailing at right angles to the direction of the wind the order of penetration was: 1, trumpet; 2, whistle; 3, exhaust; 4, bell. In the case of the other vessel the opposing effect of the wind was greater, and the sounds were heard to a less distance; the order was: 1. trumpet; 2, whistle; 3, exhaust; 4, bell; 5, rocker. On the fourth day two trips were made by the same vessel in the course...
Page 476 - ... and the artificial ear at the other, substituting fine shot instead of sand. The experiments with the siren however indicate the fact that neither the highest nor the lowest pitch of an instrument gives the greatest penetrating power, but one of a medinm character. Another element of importance in the construction of these instruments is the volume of sound. To illustrate this, it may be mentioned that a harpsichord-wire stretched between two strings of India rubber, when made to vibrate by means...
Page 457 - Among the impediments to navigation, none perhaps are more to be dreaded than those which arise from fogs. - - - The only means at present known for obviating the difficulty, is that of employing powerful sounding instruments which may be heard at a sufficient distance through the fog, to give timely warning of impending danger." * Gun signals were early abandoned, as inefficient, dangerous, and expensive: inefficient, because of both "the length of the intervals between the successive explosions,...
Page 465 - ... instead of being placed, as usual, parallel to the same. The effect on the sound in these two positions was similar to that produced by the bell with a reflector, the noise being greater at a short distance with the mouth toward the observer than "when the rim was in the plane of the ear. At a distance however, the difference between the two sounds was imperceptible. In practice therefore it is of very little importance whether the axis of the bell is perpendicular or parallel to the horizon....

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