Acoustics as Aids to Surgery
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.
On the left hand is an illustration of the mode in which the Australian savage, without the least idea of the theory of Acoustics, utilises the sound-conducting power of wood. If he wishes to know whether or not a hollow tree is tenanted by an animal of which he is in pursuit, he places his ear against the tree, taps it smartly with his tomahawk, and listens for the movement of the animal inside.
So delicate is this test, that it is employed even when the native is hunting for the large beetle-grubs on which they feed, and which are accounted a luxury even by Europeans, when they have once overcome the prejudice attaching itself to eating, without cookery, fat white grubs as thick and long as a man’s finger.
The Aye-aye is said to eat in exactly the same manner, tapping with its long finger the trunks and branches of trees and, if it hears a maggot inside, gnawing it out.
Measurement of Sound
Of late years we have had an instrument which enables us to measure the vibrations of sound as accurately as the barometer measures the weight of the atmosphere, the thermometer the temperature, and the photometer the power of light. This is the Siren, which is shown on the right hand of the accompanying illustration.
To explain this instrument fully would require ten times the space which we have at command, and necessitate a great number of drawings. I will, therefore, endeavour to explain its principle in as brief terms as possible.
The reader will observe that at the lower part of the instrument there is a disc pierced with a number of holes, and that above these are two dials. Below the perforated disc, and therefore unseen, is a circular plate, also pierced with holes. When a pipe is attached to the lower part of the instrument, and air propelled through it, the disc begins to revolve, every revolution being recorded by the dials, after the fashion of the ordinary gas-meter.
As the pressure is increased, the air, passing through the holes, assumes a rhythmical beat, which soon becomes metamorphosed into musical notes. It is evident, therefore, that, by means of this instrument, the number of vibrations which produce a definite tone can be measured with absolute accuracy by any one who has an ear capable of appreciating a musical note.
It is by means of the Siren that the much-disputed tonic of C will be settled, the Continental and the English C being greatly at variance, and even the English C having been advanced almost a tone since the time of Handel. Much is it to be wished that Italy, the home of song, and England, the patron of song, could unite in their tonic, instead of having systems so widely different that an Italian singer is at a loss with the English pitch, as is an English singer with the Italian pitch.
The Siren is even brought into the service of entomologists, enabling them to measure by the sound the rapidity with which a flying insect moves its wings. By means of this instrument we know the origin of the sharp, piercing “ping” of the Gnat, and the heavy, dull boom of the Humble-bee, both of which insects are given in the illustration.
Before taking leave of this subject, I may mention that the instrument is called the Siren because it sings as well under water as in the air, provided that water instead of air be driven through it.
Echo
Our last page will be given to the phenomenon called by the name of Echo, which consists in the power of solid substances, whether natural or artificial, of reflecting the waves of sound thrown against them, just as a mirror reflects the waves of light.
Very often the Echo is naturally formed, as shown in the illustration, by rocks which cast back the sound—waves thrown against them. This is the case in several parts of Dovedale in Derbyshire, where a pistol shot is reverberated backwards and forwards in a most wonderful manner, and a trumpet blast repeats itself over and over again.
At Walton Hall, the residence of the late C. Waterton, Esq., there is a wonderful Echo, nearly half a mile from the house. Mr. Waterton had discovered the Echo, which proceeded from the walls of the house, and, having found its focus, placed on it a large stone, called the Echo-stone. Any one sitting on this stone, and singing, speaking, or whistling towards the house, heard every sound repeated, as if in mockery.
The celebrated Whispering Gallery in St. Paul’s Cathedral is nothing but an ordinary Echo, though so intensified by the process of radiation, that the sound is transmitted from one side of the dome to the other, just as light or heat is reflected from concave mirrors.