We have already seen how the air-vibrations poured in at the small end of the trumpet can make resonant notes. We have now to see how the reverse process can be employed, and sounds poured into the larger end be conveyed to the ear.
The Ear-trumpet is a familiar example of such an instrument, and, as it is shown in the illustration, there is no need of further description. It is rather remarkable, by the way, that the length of tube does not seem to interfere with the conveyance of sound, as may be seen by the speaking-tubes which are now so common in private houses, hotels, and offices.
I know of one church in which there is a special seat for deaf persons. The reading-desk and pulpit are both fitted with the large ends of Ear-trumpets. From them pass tubes under the flooring, and so into the seat, where they can be applied to the ear of the deaf worshippers.
On the right hand is the “Concha,” as it is called, of the human ear, which is evidently constructed for the purpose of collecting and concentrating sounds. Instinctively, if we wish to near any sound more distinctly, we place the open hand behind the ear, so as to enlarge its receptive capacity, and send a greater volume of sound into the ear.
The well-known experiment of holding a shell to the ear so as to hear the murmur of the sea is due to the same cause, the shell collecting, though in a mixed manner, all the surrounding sounds, and making a murmur which really resembles the distant wash of the waves upon the shore.
Then, if we examine the various animals which need acute hearing, either to seize prey or escape from enemies, we shall find that they have large and mobile ears, which can be directed so as to catch the expected sound. The hare, rabbit, and deer are examples of the latter, while the former are well represented by the domestic cat, whose ears are always pricked forward when she hears the scratchings of a mouse.
Another most useful appliance is the Stethoscope, which enables the skilful surgeon to investigate the interior of the body almost as clearly as if it were transparent. It is perfectly simple, being nothing but a trumpet-shaped piece of wood, formed as shown in the illustration. Sometimes it is hollow, and sometimes solid, but the result is the same, sound being transmitted through wood in a most remarkable manner.
For example, if one end of the longest scaffolding pole be slightly scratched with a pin, the sound will be distinctly heard by any one who places his ear against the other end, though the person who uses the pin can scarcely hear the sound himself. The surgeon, therefore, places the broad end of the Stethoscope upon the patient, and the other upon his ear, taps more or less lightly with his fingers, and by the sounds transmitted through the Stethoscope ascertains the condition of the internal organs.