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IX.- On the Fall of the Jordan, and of the Principal Rivers in the
United Kingdom. By AUGUSTUS PETERMANN, F.R.G.S.
[Read Feb. 28, 1848.] The depression of the Dead Sea and of the Valley of the Jordan has, ever since its discovery in 1837, excited the interest of the Royal Geographical Society of London; and recently its attention has been further invited to the subject by a very important communication from Professor Robinson of New York. In it he draws especial notice to the fall of the Jordan, which he calculates from various measurements to be at the rate of 16.4 feet per geographical mile (= 14:3 per statute mile); and he appears to regard such a fall as being “ very remarkable,” owing to the circumstance that no cataracts or decided rapids have hitherto been discovered to account for it. He considers such a fall without cataracts as an “unusual ” phenomenon, owing to his observation of the fact that several rivers which he enumerates differ very materially from the Jordan in that respect.
In this paper it is intended to lay before the Society a few remarks on the depression of the Dead Sea and on the fall of various rivers in Great Britain, and to illustrate by well-known examples, that even if it be admitted that the Jordan falls without cataracts at the rate of 14:3 feet per mile, there is nothing very remarkable in such a fall.
The depression of the Dead Sea and Jordan valley, after repeated attempts to ascertain it by barometrical measurements since 1838, was in 1841 measured by Lieut. Symonds by trigonometrical operations, the results of which, generally speaking, are much more to be relied on than barometrical observations; yet the discrepancies between the results of the trigonometrical and the repeated barometrical measurements have called forth from different quarters the desire that the trigonometrical results should be finally re-examined by that method which, so far as regards accuracy, has the preference over both barometrical and trigonometrical measurements namely, a series of levellings to be carried from the level of the Mediterranean to the basin of the Dead Sea.
Professor Robinson suggests two reasons for the possibility that the results of Lieut. Symonds' trigonometrical measurements cannot be considered as fully solving the problem of the depression of the Dead Sea and Jordan valley
Ist. "The discrepancy of the trigonometrical and barometrical results.
2ndly. The anomaly of the descent of the Jordan, according to the trigonometrical results, when compared with the fall of other streams.
For the investigation of the first point, it will be of importance to examine all the barometrical measurements of the Dead Sea and the Lake of Tiberias, which have been made up to this time.
Professor Robinson has taken no notice whatever of the measurements of the Prussian Consul-General Herr von Wildenbruch, which were communicated to the Royal Geographical Society of Berlin in 1846, and which may claim the first place in point of accuracy amongst all barometrical measurements hitherto made, owing to the circumstance that they were checked by the simultaneous observations of a second barometer at the level of the Mediterranean. *
However great may be the discrepancies in the preceding figures, they exhibit some analogous features which are of con
* These observations were made by the Marquis de St. Simon, commander of the French ship of war Alcibiade, and both barometers were accurately compared. They were made, as it appears, on board the vessel.) See Monatsberichte der Gesellschaft für Erdkunde zu Berlin, Jahrgang 1845 to 1846.
f The merit of the first actual measurement of the level of the Dead Sea is due to the Comte de Bertou. Schubert was the first who took a barometer to the Holy Land, but he failed in ascertaining the depression of the Dead Sea ; owing to the inefficiency of his barometer, be could only make an estimate, the result of which, about 600 French feet, or 639.5 English feet, as compared with his measurement of the Lake of Tiberias, 570.2 English feet, makes a difference between the two lakes of only about 70 feet, which, as it is so very much at variance with all other measurements, cannot be taken into account.
I The measurement of De Bertou, as given by Robinson at 406 metres (= 1332 English feet), is only the result of a first calculation; the ultimate result, drawn from all his observations, butb in March, 1838, and May, 1839, gives 419.0 metres (= 1374.7 English feet). See Bull. Soc. Géogr., xii., 1839, p. 166.
$ This is stated by Prof. Robinson as 1319 French feet. See Reisen, &c., 1835-1841, von J. Russegger, iii. p. 184.
siderable importance. When we compare the three barometrical results apart from Lieut. Symonds, we perceive a discrepancy, taking maximum and minimum, for the depression of the Dead Sea of 71.6 feet; and for that of the Lake of Tiberias of 179.4 feet.
These discrepancies in barometrical measurements, executed by three different parties, at different times, with different instruments, and in general under different circumstances, are not great; and in the absence of a more accurate method of measurement, one would never have hesitated to consider the mean of the three measurements as a pretty fair approximation to the truth: for the Dead Sea this mean is 1416-7 feet, and for the Lake of Tiberias = 755.7 feet; the trigonometrical results are 1312. 2 and 328·1 feet.
The discrepancy for the Dead Sea will be 104.6 feet. This is not altogether unsatisfactory, and does by no means justify a doubt in the accuracy of the trigonometrical operations; there is unquestionably also much weight in Lieut. Symonds' having decidedly expressed himself as certain of the accuracy of his trigonometrical observations, the different parts of which he found agreeing with each other.*
The more unaccountable is the very great difference we observe between his results and those of the three barometrical observations for the depression of the Lake of Tiberias: it is 427.6 feet—i.e. more than four times greater than that for the Dead Sea. Symonds' operations for the Lake of Tiberias were quite independent of those for the Dead Sea; but his account of them is far less complete than for those of the latter, nor does he express his opinion about the result, as he did for that of the Dead Sea. And what is of still greater importance, there is a very considerable discrepancy between his first and subsequent calculations—the first giving only 84 feet, and the latter 328·1 feet, below the Mediterranean. On the other hand, the barometrical measurements of the Lake of Tiberias and those of the Dead Sea were not independent of each other, and were made successively at short intervals.
The results for the level of the Lake of Tiberias should therefore be considered in the same degree approximately correct as those for the Dead Sea. And on this ground the great variance of 427.6 feet between the results of Symonds and those of De Bertou, Russegger, and Von Wildenbruch, can scarcely be attributed to a threefold error on the part of the three last-named observers.
On this consideration I conclude that there seems to be as