to the different vibrations which met his ear in various direc. tions, "how are echoes described or accounted for ?"

"Your question will first of all demand the explanation of sound in general, which you should thoroughly understand before you can possibly comprehend the nature of echoes. I shall therefore endeavour to give you a perfect idea of the phenomenon of sound. L

"When bodies move in elastic fluids, they condense that part towards which they move, at the same time that the part they recede from is rarefied. This condensation or rarefaction must produce an undulatory or vibratory motion in the fluid.

"Thus, if a body, by percussion or otherwise, be put into a tremulous motion, every vibration of the body will excite a wave in the air, which will proceed in all directions, so as to form a hollow sphere; and the quicker the vibrations of the body succeed each other, the less will be the distance between each successive wave. The sensation excited in the mind by means of these waves, which enter the ear, and produce a like motion in a thin membrane stretched obliquely across the auditory passage is called sound.

"That bodies move or tremble when they produce sound, requires no particular proof: it is evident in drums, bells, and other instruments, whose vibrations being large and strong, are therefore more perceptible; and it is equally clear, that a similar vibration is excited in the air, because this vibration is communicated through the air to other bodies that are adapted to vibrate in the same manner: thus bells, glasses, basons, and musical strings, will sound merely by the action propagated from other sounding bodies.

"It is established, as well by mathematical reasoning, from the nature of an elastic fluid, whose compression is as the weight, as from experiment, that all sounds whatever, arrive at the ear in equal times, from sounding bodies equally distant. This common velocity is 1142 English feet in a second of time. The knowledge of the velocity of sound, is of use in determining the distances of ships, or other objects: for instance, suppose a ship fires a gun, the sound of which is heard five seconds after the flash from the ignition of the powder is seen; then 1142 multiplied by five, gives the distance 5710 feet, or one English mile and 330 feet.

"When the aerial 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 situated 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.

"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 property of reflection, it happens, that sounds uttered 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 the whispering gallery of St. Paul's Cathedral, 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 are constructed the speaking-trumpet and the hearing-trumpet, which either are, or ought to be, hollow parabolic conoids, having a perforation at the vertex, to which the mouth is to be applied in speaking, or the ear in hearing.

"There is a remarkably fine echo in Westmoreland, on the lake of Ulswater, which I once heard. Our barge having taken up a station where the finest echoes were to be obtained from the surrounding mountains, one of the cannon on board was discharged; the report was echoed from the opposite rocks, where, by reverberation, it seemed to roll from cliff to cliff, and return through every cave and valley, till the decreasing tumult gradually died away upon the air. The instant it ceased, the sound of every distant waterfall was heard; but before I could express my pleasure and admiration, the returning echo from the hill behind, again claimed my attention. The report was then repeated like a peal of thunder bursting over our heads, continuing for several seconds, flying from point to point, till, once more, the sound gradually declined. Again the voice of waterfails .stole upon mine ear, till to the right the more distant thunder arose from other mountains, and seemed to take its way up every winding dell and creek, sometimes behind, sometimes on this side, then on that, passing with incredible swiftness. When the echo reached the mountains within the line and channel of the breeze, it was heard at once on the right

and on the left, at the extremities of the lake. In this manner was the report of the discharge repeated seven times distinctly.

"At intervals we were relieved from this entertainment, which united tumult and grandeur, by the music of two French-horns, whose harmony was repeated from every recess, which echo haunted on the borders of the lake. Here we appeared to have a whole band at our command. Sometimes we heard the full-toned breathings of the organ; then the hoarser notes of the bassoon burst on our delighted ears; while the caves, the wooded creek, and trilling waterfalls gave back the soft and gentle tones of the melting lute. In the midst of this entrancing concert, our boatmen fired off six brass cannon: it is impossible to describe to you the extraordinary sensation produced by the sudden uproar which followed this discharge. It appeared to me, as if the rocks, the mountains, the woods, the vales, were all uprent, and thrown together in horrible confusion. Nothing short of a general wreck of nature, could, to my imagination, have produced such a wild and awful tumult. I confess I was for a few minutes speechless, and that something very like the sensation of fear, glanced across my mind, at the wild and tumultuous uproar, which interrupted the harmony that had given me so much delight *."

The upper lake, which they now approached, is about four miles in length, and from two to three in breadth it is almost surrounded by mountains, from which descend a number of beautiful cascades. The islands in this lake are numerous, and afford an amazing variety of picturesque views.

* Travellers and Natural Historians have furnished us with many accounts of Echoes, which repeat words very often, or have some singularity. Misson, in his Description of Italy, speaks of an Echo in the vineyard of Simonetta, which repeats the same word 40 times. At Milan an Echo reiterates the report of a pistol 56 times; and if the report be very loud, 60 reiterations may be counted. But the most singular Echo is that near Rosneath, a few miles from Glasgow. If a person, placed in a proper situation for the sound to take effect, plays 8 or 10 notes on a trumpet, they are faithfully repeated by the Echo, but a third lower; after a short silence another repetition is heard, in a still lower tone; and another interval of silence is followed by a third repetition, in a tone a third lower.