Читать книгу Harper's New Monthly Magazine, Vol. V, No. XXIX., October, 1852 - Various - Страница 24

Our party, after having spent about three weeks in making these and similar excursions from their various encampments, during which time they had thoroughly explored the shores on every side, and sounded the depths of the water in every part, made all the necessary measurements and observations both mathematical and meteorological, collected specimens for fully illustrating the geology and natural history of the region, and carefully noted all the physical phenomena which they had observed, found that their work was done. At least all was done which could be accomplished at the sea itself. One thing only remained to be determined, and that was the measure of the depression of the sea. This could be positively and precisely ascertained only by the process of "leveling a line," as it is termed, across from the sea to the shores of the Mediterranean. This work they now prepared to undertake, making arrangements at the same time for taking their final leave of the dismal lake which they had been so long exploring.

It had been long supposed that the Dead Sea lay below the general level of the waters of the earth's surface, and various modes had been adopted for ascertaining the amount of the depression. The first attempt was made by two English philosophers in 1837. The method by which they attempted to measure the depression was by means of the boiling point of water. Water requires a greater or a less amount of heat to boil it according to the degree of pressure which it sustains upon its surface from the atmosphere – boiling with less heat on the tops of mountains where the air is rare, and requiring greater in the bottoms of mines, where the density and weight of the atmosphere is increased in proportion to the depth. Heights and depths, therefore, may be approximately measured by an observation of the degree of heat indicated by the thermometer in the locality in question, when water begins to boil. By this test the English philosophers found the depression of the Dead Sea to be five hundred feet.

A short time after this experiment was performed a very careful observation was made by means of a barometer, which also, measuring, as it does, the density of the air, directly, may be made use of to ascertain heights and depths. It is, in fact, often thus employed to measure the heights of mountains. The result of observations with the barometer gave a depression to the surface of the sea of about six hundred feet.

A third method is by trigonometrical calculation. This mode is much more laborious and difficult than either of the other modes which we have alluded to, but it is more to be relied upon in its results. The data for a trigonometrical calculation are to be obtained by observing, in a very accurate manner, a series of angles of elevation and depression on a line between the points, the relative levels of which are to be obtained. Lieutenant Symonds, an officer of the English service, made such a survey with great care, a few years after the preceding experiments were performed. He carried a line across from the Mediterranean to the Dead Sea, connecting the two extremes of it by means of a series of vertical angles which he measured accurately, with instruments of the most exact construction. The result of the computation which he made from these data, was that the sea was depressed one thousand three hundred and twelve feet below the level of the Mediterranean.

The surprise which had been felt at the results of the experiments first mentioned, was greatly increased by the announcement of this result. No one was disposed really to question the accuracy of the engineer's measurements and calculations, but it seemed still almost incredible that a valley lying so near the open sea could be sunken so low beneath its level. There was one remaining mode of determining the question, and that was by carrying an actual level across the land, by means of leveling instruments, such as are used in the construction of railroads and canals. This would be, of course, a very laborious work, but there was a general desire among all who took an interest in the subject that it should be performed and Lieutenant Lynch determined to undertake it.

Accordingly, when the time arrived for leaving the shores of the sea, he organized a leveling party, furnishing them with the necessary instruments and with proper instructions, and commissioned them to perform this service. They began by scaling the face of the mountain which rose almost perpendicularly from the shore of the sea at the place of the last encampment. They then proceeded slowly along, meeting with various adventures, and encountering many difficulties, but persevering steadily with the work, until at last, in twenty-three days from the time of leaving the Dead Sea, they arrived on the shore of the Mediterranean at Jaffa. The result confirmed in a very accurate manner the calculations of Lieutenant Symonds, for the difference of level was found to be a little over thirteen hundred feet – almost precisely the same as Lieutenant Symonds had determined it. The question is, therefore, now definitely settled. The vast accumulation of waters lies so far below the general level of the earth's surface that, if named after the analogy of its mighty neighbor, it might well have been called the Subterranean Sea.

Lieutenant Lynch had great reason to congratulate himself on this successful result of his labors; for the work which he had undertaken was one not only of toil, exposure, and suffering, but also of great danger. He was warned by the fatal results which had almost invariably attended former attempts to explore these waters, that if he ventured to trust himself upon them, it was wholly uncertain whether he would ever return. He followed in a track which had led all who had preceded him in taking it, to destruction; and the only hope of safety and success which he could entertain in renewing an experiment which had so often failed before, was in the superior sagacity and forethought which he and his party could exercise in forming their plans, and in the greater energy and courage, and the higher powers of endurance, which they could bring into play in the execution of them. The event proved that he estimated correctly the resources which he had at his command.