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From Riha (Gilgal) we turned southwards, and now came into the district rendered characteristic by the vicinity of the Dead Sea. Here we took leave of bushes and shrubs; no more vegetation marked the country, but the soil became soft and sandy, honeycombed and cavernous, and arranged in numerous terraces, which seemed to imply that there was a gradual subsidence. Large pit-like depressions extend for a considerable distance, while spots here and there remaining point out the original level from which it had last sunk. It was not difficult to trace three successive terraces, or levels, in some places. The surface of the soil was covered over with a thin white incrustation of salt, and nothing seemed capable of growing out of it, although I noticed one small shrub in our path which appeared to have sprung up, but was now dead. The sea itself looked close at hand when we were yet two hours' distant from it; and as we approached we observed a number of bare, gaunt, whitened branches of trees which had been carried down by the river, and afterwards thrown up on the north-west shore by the wind. No shells strew the sandy beach, nor do any fishes exist in its bitter waters, it is truly a Dead Sea; although it is not an Avernus over which if birds fly they fall dead into its waters, as some would feign. It is also said, on the other hand, with what truth I am not able to state, that madrepores have been found in the Dead Sea. The water was perfectly clear and bright, and to the sight offered no indications of differing from other waters; and yet in its constitutions and geological position this sheet of water is among the most remarkable in the world, though not without parallel.

The extraordinary characteristics of the Dead Sea, or as it is termed in Scripture," the sea of the plain, even the salt sea" (Josh. iii. 16), are, first, that it lies at the depression of 1292 feet below the level of the Mediterranean, unequalled in this respect by any other known lake. It has positively no outlet, but is cooped up in the heart of the mountains, the nearest body of water, the Gulf of Arabah, being on the same level as the Mediterranean. It receives the waters of the Jordan and some insignificant tributaries from the mountains, but these are not sufficient to overcome other influences and keep the waters of the Dead Sea of the same nature and character as other waters. But its most prominent characteristic is its extraordinary saltness; for the waters of the Dead Sea, instead of being like other inland lakes, or even possessing a saltness of 4 per cent., which is the average of the waters of the ocean, absolutely contain 264 per cent. of salt! The reason of this accumulation appears to be the existence of a mountain of rock salt in

its south-western corner, which impregnates its waters, whose evaporation in the hot, low, confined valley cannot be compensated by the small bodies of fresh water which flow into it from the Jordan and other sources.

Here, then, we were upon the banks of this well-known and singular sea; not the saltest in the world, however, for some Russian lakes near the Volga are said to have a saltness of 29 per cent. Nevertheless, some natural consequences arising from its high degree of salinity we now proceeded to put to the test. We were most of us determined to have a swim in its waters, whose remarkable buoyancy was one of its most curious characters. The hot sun, the dazzling atmosphere, the white salt-incrusted landscape, and the brightness of the waters themselves all invited us to plunge into them. And yet it was necessary to act with caution; and our dragoman warned us against taking a header, or even plunging overhead at all into these clear waters. And this for two reasons: first, because the preternatural saltness of the water would cause intense irritation to the eyes, if any got into them, and the fearful bitterness would equally offend the taste if a mouthful of it were inadvertently swallowed. But a more important reason was, that, owing to its buoyancy, the heaviest part of the body, viz. the head, would inevitably tend to sink, at the expense of the lighter portions, or the legs, and hence it would be difficult to regain one's upright position, and the result would be the danger of being drowned. We soon discovered the truth of this; for while, on the one hand, we could not sink, on the other we were almost equally unable to swim. It was easy to lie in the water perfectly motionless, as upon a couch, the only objection being the tendency presently to roll over and for the head to fall in; but when we attempted to swim we discovered that our legs refused to remain in the water, but persisted in striking out into the air for want of weight-rejected from the water like pieces of wood; neither did it do to splash about, lest the water should get into our eyes and mouth. Altogether the bath caused great amusement as well as interest, though it left our hair and beards incrusted with salt, and our skin not in so soft and pleasant a condition as a Turkish bath would have done. These désagréments, however, we did not mind, because it was our intention, after quitting the banks of the Dead Sea, to proceed to the Jordan, and then, pilgrim-like, dip in its fresh flowing waters and wash off the impurities of the Salt Sea.

(To be continued.)

DISCOURSES FROM NATURE FOR JUNIOR READERS.

IV.

THE subject for our study to-day is closely connected with that which engaged our attention in our former discourses. The science of sound or acoustics is no less interesting than that of light. The phenomena of each appeal to a special organ of sensation, and hence as presented to the mind, appear to have no relation, yet the radical cause of each is the same. We have previously said that all spaces infinitely great or infinitely small are filled with a subtle, elastic medium so light that no balance can detect it. This medium is called ether, and light is the effect of intensely rapid wave-motion in this ether. Sound is the result also of the movement or vibration, first of some substance or body, and then of a surrounding medium. It is not difficult to prove this. I have here a large glass vessel filled with water. I strike the edge of the vessel a sharp quick blow and you hear a musical sound. That the glass is thrown into rapid motion by the blow is clearly proved by the surface of the water being broken up into beautiful rhythmic forms. On this plate of metal is some fine sand. A fiddle-bow is drawn across an edge of the plate, and the sand begins to dance in a most lively fashion. Nothing can be more evident than that, when sound is drawn from the metal by the bow the plate is in very rapid motion. These two experiments show that vibration of a body is the cause of sound, but they do not tell us how the motion affects the ear. Further evidence is required. Under the receiver of an air-pump we place a carefully-fixed bell. The hammer strikes the sonorous metal and the bell rings out clearly. The air is now pumped out of the glass receiver, and the sound gradually becomes weaker and weaker until at length it ceases altogether. The air is again admitted and the sound revives. If now you were asked what is necessary for the production of sound beside the motion of the particles of a body, you would say without hesitation, atmospheric air. Precisely so. The motion of the sounding body sets the air around in vibration, and sound-waves are produced. These waves are not quite like light-waves, and they do not move so rapidly. As you have before learned that light is a form of motion, so you will understand now that sound is likewise a mode of motion. The nervous network of the eye receives the impact or blow of the light-waves, and the delicate mechanism of the ear takes up the pulses of the soundwaves and conveys them to the brain. We have proved, then, that atmospheric air is necessary for the production of sound, and it may be easily shown that it is essential to life. Air, then, is a thing of the

greatest importance. It forms an atmospheric sea or ocean which completely surrounds our globe. We live at the bottom of this sea; it is our natural element, and we could not exist out of it for an instant. It extends many miles above our heads and presses upon us with great force. And now we learn that without the atmosphere there would be no sound. Here is the enchanted abode of music, whose voice we hear but whose form we do not see, and at the command of the accomplished musician she moves the heart and gladdens the soul of man. Light, you know, moves with the velocity of nearly 200,000 miles in a second of time; sound travels much more slowly; at the ordinary temperature of the air a sound-wave moves at the rate of 1125 feet in a second. The length of the wave is dependent upon the nature of the sound. Suppose the wave resulting from the vibration of a tuning-fork be three feet long, then in one second 375 waves will strike the ear. In the same time when the eye looks at a violet ribbon the enormous number seven hundred and eighty-nine millions of millions of waves strike the nervous network of the eye. Some remarkable facts result from the great difference in the velocities of light and sound. During a storm the flash of the lightning is seen several seconds before the thunder is heard. A peculiar impression is made upon one who witnesses artillery practice of ships at sea. If the observer is two or three miles from the man-of-war the puff of white smoke and the roar of the cannon seem to have no connection with each other; several seconds will pass between the appearance of the smoke and the explosion. The fall of a woodcutter's axe is seen before the blow is heard. The explanation of these and all similar phenomena you can now easily give. When sound-waves strike against an opposing body they are reflected and obey the same laws as those of light. Under certain conditions the reflection or repetition of sound produces what are commonly known as echoes. Among the rocks and glens of mountain scenery very fine echoes are heard. Professor Tyndall speaks thus about the echo: "In mountain regions this repetition and decay of sound produce wonderful and pleasing effects. Visitors to Killarney will remember the fine echo in the Gap of Dunloe. When a trumpet is sounded in the proper place in the gap the sonorous waves reach the ear in succession after one, two, three, or more reflections from the adjacent cliffs, and thus die away in the sweetest cadences. There is a deep cul-de-sac called the Ochsenthal, formed by the great cliffs of the Engelhörner, near Rosenlaui, in Switzerland, where the echoes warble in a wonderful manner. The sound of the Alpine horn echoed from the rocks of the Wetterhorn or the Jungfrau is in the first instance heard roughly. But by successive reflections the notes are rendered

more soft and flute-like, the gradual diminution of intensity giving the impression that the source of sound is retreating further and further into the solitudes of ice and snow." A striking instance of the phenomenon of the echo came under my own observation a few years since. On a certain summer evening I was walking through a romantic part of the north of England. Hills rose on all sides around me, and the last rays of the setting sun were resting upon their crests. As I drew near to a lone house on the road I distinctly heard the sound of music. On moving out of a certain line the sounds were partly lost, but when I took up my former position they were again heard. I at once concluded that the house was acting as a reflector of the musical sounds which reached me from some undefined quarter. After walking some distance I found that the strains proceeded from a band on a village green. The phenomenon of the echo appealed to the imaginative powers of the ancients, and they personified it. Echo they believed was a nymph who used to engage the ear of Juno while Jupiter sported with the nymphs. Juno discovered the trick and changed the nymph into an echo. The fair nymph then fell in love with Narcissus, but her love was not returned, so she pined away, and at last only her voice remained. This is a beautiful myth. The ancients could not explain the phenomenon of the reflection of sound, but the matter is plain enough to us. Still there is something wild and strange in the repetition by the rocks and glens of words that we have uttered aloud. An amusing instance of the results which sometimes follow from the reflection of sound has been cited by Sir John Herschel. In one of the cathedrals of Sicily was a confessional. This was by accident so placed under the curved roof that the whispers of the penitents were reflected and brought to a focus at another part of the building. A certain person discovered this focus, and for once the secrets of the confessional were revealed. But he did not keep the secret; he used to bring friends with him. One day, it is said, the gentleman's wife was engaged in confession, and great was the confusion of the husband when he, in the presence of his friends, heard secrets anything but amusing to himself. No longer can we deny that the "very walls have ears.” There are echoes which reflect the same sound twenty or thirty times. An echo in the Château of Simonetta in Italy repeats a sound thirty times, and at Woodstock there is said to be one which repeats from seventeen to twenty syllables. In large empty rooms the mingling of the sounds reflected from the walls causes a strengthening or resonance. Carpets and furniture deaden the sound. Whispering-galleries also present an interesting illustration of the effect produced by the reflection of sound

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