ships, or other objects: for instance, suppose a ship fires a gun, the sound of which is heard five seconds after the flash is seen; then, 1132 multiplied by 5, gives the distance 5710 feet, or one mile and 430 feet. If sound is transmitted through other gases, the velocity is greater or less, according to the density and constitution of the gas which conveys it.

When the aërial waves meet with an obstacle which is hard, and of a regular surface, they are reflected, and consequently, an ear placed in the course of these reflected waves will perceive a sound similar to the original sound, but which will seem to proceed from a body situate in like position and distance behind the plane of reflection, as the real sounding body is before it. This reflected sound is called an echo.

This echo, or repercussion of sounds, is chiefly observable in smooth, tortuous, and hollow places, as in valleys, caves, and walls, especially in old vaulted buildings. An echo, therefore, as a pleasing, and very often wonderful circumstance, could not fail to enter into poetical descriptions, and to be the subject of fiction and personification. The Hebrews styled Echo the daughter, and the Greeks and Latins, the image of the voice. Thus Virgil:

Aut ubi concava pulsu

Saxa sonant, vocisque offensa resultat imago.

Nor hollow rocks that render back the sound,
And doubled images of voice rebound.

And Horace :

Cujus recinet jocosa

Nomen imago.

Aut in umbrosis Heliconis oris,

Aut super Pindo, gelidove in Hæmo.

DRYDEN.

Whose hallowed name

The sportive image of the voice

Shall in the shades of Helicon repeat,
On Pindus, or on Hæmus ever cool.

FRANCIS.

In the third book of Ovid's Metamorphoses is the beautiful fiction of the Transformation of Echo, to which I must refer the reader; but the following extract from Addison's translation of it affords an admirable description of an echo:

Echo in others' words her silence breaks,
Nor speaks herself but when another speaks.
She can't begin, but waits for the rebound,
To catch his voice, and to return the sound.
Hence 'tis she prattles in a fainter tone,
With mimic sounds, and accents not her own;
Lives in the shady coverts of the woods,
In solitary caves, and dark abodes.

But nothing can exceed the exquisite song in Milton's Comus:

Sweet Echo, sweetest nymph, that liv'st unseen
Within thy aëry shell,

By slow Meander's margent green,

And in the viölet-embroidered vale,

Where the love-lorn nightingale

Nightly to thee her sad song mourneth well;

Hid them in some flow'ry cave,

Tell me but where,

Sweet queen of parly, daughter of the sphere,

So may'st thou be translated to the skies,

And give resounding grace to all Heav'n's harmonies.

Milton, moreover, makes a noble poetical use of the

philosophy of echoes in Adam's morning hymn:

Witness if I be silent, morn or even,

To hill or valley, fountain, or fresh shade,

Made vocal by my song, and taught his praise.

And to this he makes Adam pathetically allude, in his lamentation after the fall:

O woods, O fountains, hillocks, dales, and bowers,
With other echo late I taught your shades

To answer and resound far other song1!

But I must return to a philosophical discussion of the subject. The waves of sound being thus reflexible, nearly the same, in effect, as the rays of light, may be deflected or magnified, by much the same contrivances as are used in optics. From this principle of reflection it happens, that sounds ut tered in one focus of an elliptical cavity are heard much magnified in the other focus; instances of which are found in several domes and vaults, particularly in the whispering gallery at St. Paul's cathedral in London, where a whisper uttered at one side of the dome is reflected to the other, and may be very distinctly heard. On this principle also is constructed the speaking trumpet, which either is, or ought to be, a hollow parabolic conoid, having a perforation at the vertex, to which the mouth is to be applied in speaking, or the ear in hearing.

But to what purpose would be all these observations on the nature and properties of sound, if the human frame were not made capable of receiving it? How adorable then is the goodness of the great Creator, in having not only disposed the air in such a manner, that sound may be produced by its vibrations, but in having given us an organ capable of receiving these sonorous vibrations.

An account of some very remarkable echoes, with some pleasing observations on this subject, may be seen in the Philosophy of Nature, vol. i. p. 83.

The position of the ear is admirable; for it is placed in the most convenient part of the body, near the brain, the common seat of all the senses, to give the more speedy information; in a part where it can be best guarded; and in the neighbourhood of its sister sense, the eye, with which it has a peculiar and admirable communication by its nerves.

The structure also of the ear, no less than its position, may evince intelligence, wisdom, and design. The ear may be considered as exterior and interior. And if we observe the structure of the exterior ear in all kinds of animals, we cannot but perceive how wonderfully it is adapted to the respective exigencies of each. In man it is of a form proper for the erect posture of his body: in birds, of a form proper for flight, not protuberant, which would obstruct their progress, but close and covered, to afford the easier passage through the air. In quadrupeds, its form is agreeable to the posture, and slower motion, of their bodies; and in these too, various, according to their various occasions: in some, large, erect, and open, to hear the least approach of danger: in others, covered, to keep out noxious bodies. In subterraneous quadrupeds, which are forced to mine and dig for their food and habitation, as a protuberant ear, like that of other quadrupeds, would obstruct their labours, and be liable to be torn and injured, their ears, on the contrary, are short, lodged deep, and backward in the head, passing to the under part of it, and all sufficiently fenced and guarded. To mention one instance only; the mole IS, in this respect, a subject of curious discussion: it has no protuberant ear, but only a round hole between the neck and shoulder, and this situation, with the thick short fur, that covers it, is a sufficient defence against all external annoyance.

And, as the form is various in various animals, so, in each of them, its structure is very curious and

observable; being admirably contrived to collect the wandering circumambient impressions, and undulations of sound, and to convey them to the sensory within.-But I shall now confine my survey to the human ear.

The first thing we observe is the auricle, or external ear, with its tortuous cavities, formed to stop and collect the sonorous undulations, to give them a gentle circulation and refraction, and so convey them to the concha, or large capacious round cell, at the entrance of the ear. And the great contrivance visible in the outward ear, is in nothing more apparent than in this circumstance, that they whose ears are cut off, have but a confused way of hearing, and are obliged either to form a cavity round the ear with their hand, or else to make use of a hearing trumpet to supply the defect. The cartilaginous substance of the outward ear, is likewise, very obviously, another wise provision of the great Creator.

In the interior part of this admirable organ we may observe, in the first place, the auditory passage, curiously tunnelled and turned, to give sounds an easy passage, as well as a gentle circulation and refraction, and likewise to prevent their too furiously rushing in, and assaulting the more tender internal parts.

To prevent, moreover, the entrance of noxious insects, which are apt to make their retreat in every little hole, Nature has secured this passage with a bitter nauseous substance, called earwax, which is supplied by the glands appointed for that purpose.

The next principal thing to be observed, is the membrana tympani, or drum of the ear, with its inner membrane, the four little appendant bones, and the three inner muscles to move them, and to adjust the whole system to the several purposes of hear