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Under these circumstances, the method proposed by Mr. Frank Della Torre, of Baltimore, deserves consideration. His experiments indicate the possibility of obtaining an echo from an iceberg when in dangerous proximity to a ship. Mr. Della Torre believes that even an object offering so small a surface as a floating wreck may, in this way, be detected during a fog, in time to prevent collision. However this may be, it is certain that his method is worthy of a careful trial at sea, and that preliminary experiments recently made in the presence of Professor Rowland, of Johns Hopkins University and the present writer, have demonstrated the feasibility of producing well-marked echos from sailing vessels and steamboats at considerable distances away.
These experiments were made on the River Patapsco, near the head of Chesapeake Bay. The party proceeded down the river in a steam launch to the selected place, where the distance from shore to shore appeared to be about three miles. The launch was kept so far from land as to prevent the possibility of mistaking an echo from the shore for one produced by a passing vessel.
The apparatus employed consisted of a musket, to the muzzle of which a speaking trumpet had been attached. This gun was aimed at passing vessels, while blank cartridges were fired. After a longer or shorter time, according to the distance of the vessel, an echo was returned.
The ordinary river steam boats and schooners with large sails returned perfectly distinct echos, even when apparently about a mile distant. At shorter distances, the results were still more striking. In order to test the effects under the most disadvantageous circumstances, blank cartridges were fired in the direction of an approaching tug boat. The surface presented was much smaller than if the boat had presented its broadside to the launch. As the boat approached bow on, it corresponded to a target somewhere about six feet square, presenting a convex surface to the impinging sound wave. Even in this case, a feeble echo was perceived when the boat was at a considerable distance (estimated to be nearly one quarter mile).
Experiments were made which demonstrated the fact that the speaking trumpet attached to the gun was of material assistance in giving direction to the sound impulse and in intensifying the audible effect. Mr. Della Torre claims that a steam whistle or siren, combined with a projecting apparatus like a speaking trumpet, will prove as efficient as the gun."
Alexander Graham Bell
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