Sound is propagated through a confined channel of air, as for instance, a long tube, very perfectly. It is not uncommon in large establishments, where it is necessary to pass many orders to and fro, to have such tubes laid in the walls, so that words can pass through them from room to room. The experiment may be tried in a leaden pipe, laid down for an aqueduct, before the water is admitted, and it will be found that the slightest whisper can be heard for a distance of half a mile or more. This tendency of a confined channel of air to increase the distinctness of the sound passing through it, may be shewn by a tube formed by rolling up a large sheet of paper and whispering through it. If the tube is gradually enlarged towards the outer end, it greatly increases the loudness of the sound transmited through it, as in the case of the horn, the speaking trumpet, &c.

When the sound of a voice passes in this manner through a tube, the voice always appears to come from the end of the tube where the sound issues, and not from the end where the words are really spoken. This has given rise to an ingenious contrivance called the whispering figure, which is sometimes fitted up in museums for the entertainment of visitors. It is as follows:—

A large dog or other image is placed upon a stand at a little distance from the side of the room. There is a tin tube within the figure, one end of which is opposite the mouth, and the other passes down through the floor, and thence along under the floor, as represented by the dotted line in the cut. After passing the partition, it is turned up, and opens in a funnel-shaped extremity in the next room. If now a person speaks or whispers at the outer end of the tube, the persons who are in the room with the figure will hear the sound coming through its lips, as if the figure itself were speaking. The tube being entirely concealed within the dress of the figure, and under the floor, the spectators, not suspecting such a communication with the next room, wonder by what contrivance an image can be made to speak.

This experiment is sometimes performed by children in private, by placing a board across the tops of two chairs, and covering it with cloth, so as to represent a table. The tube comes up through this board into the body of some image placed upon the table, as a doll, a figure of a dog or cat, or a grotesque effigy made by means of a mask. In this case a person is concealed under the table to whisper into the tube, in answer to such questions as the spectators address to the figure. Of course, all these arrangements are made before the spectators are admitted to the room.

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