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.