Page:Popular Science Monthly Volume 33.djvu/106

96 beam of sound projected through the funnel strikes the obstacle and rebounds; and as the echo is more or less perfect in proportion as the obstacle is more or less parallel to the ship from which the gun is fired, and as it is near or remote, the position of the obstacle may thus be inferred. The board reported that De la Torre's method was firing a blank cartridge from a rifle in the presence of objects as small as a spar-buoy and as large as a fort, and catching the return sound or echo. He claims that a sharp sound projected at or nearly at an object, and only when so directed, will in every case return some of the sound sent, so that theoretically there will always be an echo, and the difference in the time between the sound sent and the echo will indicate the remoteness of the object. The board found that a return-sound could be heard from the side of a fort a half-mile off, from passing steamers a quarter-mile off if broadside-to, from bluffs and sails of vessels about the same distance, and from spar-buoys two hundred yards away.

The board further states that the sound from the different kinds of masses is different in most cases, and that the ear could be educated to detect quite a range of different objects, as the echo from a sail was different from the echo from a buoy or a bluff. If two objects were near the line of projection at different distances, an echo would be received from each. The horizontal limit of the return of sound seemed to be about two points on each side of the axis of projection.

If Mr. De la Torre should see fit to construct his instrument for hearing feeble echoes, the board indicated that it would recommend that it be fitted soon to some vessel of the North Atlantic Station, and that further and, if possible, exhaustive experiments ought to be made to practically determine the use of the echo as a means to discover obstacles to navigation. It was also stated that steam-whistles could be heard much farther than the echo; but it was said that where the obstacle could not make the sound, as in the case of an iceberg, the echo would be of the greatest use, and experiments looking to its utilization are demanded by the conditions of navigation in time of fog.

Steamers are constantly running among the islands on the coast of Maine during the summer. This is the season of thick and persistent fog. When pilots can hardly see the length of their vessels, they keep up a constant noise with their fog-signals. The open sea gives back no sound. But the near or remote vicinity of cliffs, bluffs, or even high shores, is indicated by the strength of the echo received back from them. In fact, running by echo is recognized as one of the necessities of the navigation of those waters.