Dee-Tweed- Shannon, &c.

97

in a short distance from Biggar Water to Altarstone the rate of fall reaches even 14.2 feet per mile; and yet from the affluence of Biggar Water to the sea the Tweed possesses neither waterfalls nor rapids; and small boats, such as are used in salmon-fishing, are freely navigated.

Thus the fall of the Jordan of 12 feet per mile, even without waterfalls, does not present such a great contrast to the falls of rivers in general, as is shown by the adduced instances.

Lieutenant Symonds' measurements may therefore prove perfectly correct; and it is not at all necessary that any falls should be discovered to account for the descent of the Jordan. But certainly there must be something to account for the striking anomaly of Professor Robinson's results and my own. The rivers which he draws up for comparison with the Jordan exhibit more or less falls, and the two rivers which I adduce here exhibit none. One might naturally suppose that this was the consequence of incorrect data on either side, upon which our results are based; but it is not so-it is the anomaly which this feature of hydrographical development exhibits; and it is only from the deficiency of study in this branch of physical geography, or rather the scarcity of data for such a study, that this anomaly has not been fully explained.

geo

As the velocity of rivers does not altogether depend upon the rate of their descent, in like manner the average fall does not determine the formation of cataracts. It is much more the logical character of the country through which the river runs which causes those sudden descents, and countries where sudden declivities abound are chiefly of primary or transition formation. We find striking examples in every direction.

The Severn and the Shannon, for example, are much alike in magnitude. The latter descends, from Lough Allen to its mouth, a distance of 213 8 English miles, 161 feet; the Severn, from Newtown to its mouth, a distance of 210 miles, descends 465 feet. This gives an average descent per mile of 9 inches for the Shannon, and 26.6 inches for the Severn. And yet the Severn pursues its course to the sea without any rapids or falls; whilst the Shannon, with its average fall of one-third less than the Severn, forms those magnificent Rapids of Doonas, which, for grandeur and effect, rank with the most celebrated European waterfalls.

The Tweed and the Clyde exhibit a still more remarkable anomaly. Both are very much alike in point of magnitude: the extent of their basins is 1870 English square miles for the Tweed, and 1580 for the Clyde; and they are still more alike in point of their aggregate length and fall: the length of the Tweed being 96.4 miles, and its total fall 1500 feet; the

length of the Clyde 98 miles, with 1400 feet of fall. Indeed both rivers for many miles from their source flow nearly in one direction, never diverging to any great distance from each other; and so long as they continue nearly parallel, they flow almost upon the same level, and keep on a high tableland of country, as if hesitating whether to mingle their waters or to remain separate, and whether to turn their courses to the eastern or western slope. Thus they pursue their sympathetic career til near Biggar, when they terminate their upper course, and like two wanderers descending from the mountains together, and separating by cross-roads when they have reached the low country, they at last part, the one turning eastward and the other westward. At this spot the rivers are only 6 miles distant from each other-are on the same level, and have the same distance to travel ere they reach the sea-yet what a difference in their descent; the Tweed pursues its course evenly and gently; while the Clyde has not parted from its former companion for a greater distance than 18 miles, before it boldly dashes over a whole series of those well-known falls, the principal of which are the Bonnington and Stonebyres Fall, and Corra Linn. The descent over all these falls is computed at 230 feet.* Thus the preceding examples sufficiently attest that the occurrence of cataracts and other descents in a river depends but little on its aggregate fall.

Thus there is certainly "room in the Jordan for three cataracts, each equal in height to Niagara," as Professor Robinson remarks; but, on the other hand, if there should not be discovered one single rapid in it, there is still nothing of a remarkable phenomenon about it.

The different points on the subject thús adduced are recapitulated as follows:-.

1st. Lieut. Symonds' results for the depression of the Dead Sea, compared with the different barometrical results, do not prove such an amount of discrepancy as to justify a doubt in their accuracy.

2nd. The same results for the Lake of Tiberias are so much at variance with the barometrical results, that it seems probable the latter would prove nearer to the true level. 3rd. The fall of the Jordan of 984 feet between the two lakes, as computed from the trigonometrical results, does not exhibit an "immense contrast with all similar phenomena." And, moreover, owing to our present defective knowledge of the entire course of the Jordan, and the anomaly of the fall of rivers in general, inferences drawn from the aggregate fall of the Jordan can scarcely prove

* Fullarton's Parliamentary Gazetteer of Scotland, i. p. 232.

of sufficient weight to control the results of trigonometrical operations.

I now beg leave to add a few data on the fall of some rivers in this country, selecting such as might give a view of the great variety of descents.

The British Isles afford a wide field for the study of hydrography, and I have no hesitation in saying that it possesses for that study such extensive and valuable materials and data as no other country can boast of; but, comparatively speaking, these materials have been drawn forward, sifted, and made applicable for the promotion of science to a very limited extent. For example, the country has been levelled in all directions, and especially along all rivers of note, for the purpose of laying out canals and railways, and yet we find little information in any work regarding general results of the comparative fall of rivers.

The levels in the following data are based upon authentic documents, and relate entirely to the surface of the river above the sea at low-water. The distances have been for the first time attempted to be ascertained with accuracy.

I. The Shannon. This is the third largest river in the United Kingdom, in regard to its basin.* As the fall of rivers in general is greatest at their sources, and decreases proportionately towards their mouths, the Shannon in its descent presents one of the rare exceptions to the general fall of rivers, as it is greater in its lower than in its upper course. Its source, the Shannon Pot, or more generally called Legnashinna, rises in the county of Cavan, between Upper Lough Macnean and Lough Allen, and is 345 feet above the sea. After a course of 11.6 miles, it enters Lough Allen, up to which it is rendered navigable, owing to the little descent from that place. The distance to which it is navigable is 213.8 miles from its mouth; and in this respect it is superior to all other British rivers-the navigation of the Severn extending only 192, and that of the Thames 193 miles from their mouths. From the head of Lough Allen to the foot of Lough Derg, a distance of 131.8 miles, it descends only 50 5 feet, or 4 inches per mile. After leaving Lough Derg, the inclination of its course changes considerably, and gradually increases, till it reaches a fall of nearly 17 feet per mile between the town of Castle Connel and Castle Troy, a distance of 3.3 miles. It is in this portion where the mighty Shannon, 40 feet deep and 300 yards wide, forms the magnificent Rapids of Doonas.

* The two larger rivers are the Humber (including the Trent and the Ouse) and the Severn.

Fall of the Thames-Tweed-Clyde.

101

II. The Thames.-This noble river, although the most important in Great Britain in a commercial point of view, is only the fourth in point of magnitude. The entire length of its course is 215.2 miles, which is 9 miles less than the Shannon; its descent is 376 3 feet. Unlike the Shannon, it has a more equally distributed fall throughout its course; from its head to Lechlade, a distance of 22.0 miles, its fall is 6 feet; and from thence to its mouth its average fall is only 12. feet per mile.*

III. The Tweed.—We have already fully treated of this river in the foregoing observations; in point of area of its basin it ranks ninth amongst British rivers.

IV. The Clyde. My results respecting the fall of this and the preceding river are almost entirely based upon levellings; the sources only of the two rivers are ascertained barometrically, and the portion of the Clyde comprising the falls has likewise not been ascertained by levelling instruments; however, that does not influence the accuracy of the general results, as the exact levels of the Clyde above and below the Falls check the intervening portion.

* From Teddington to London Bridge it is 16 ft. 9 in. at low, and 1 ft. 6 in. at high

water.

† There are two other falls of smaller dimensions, viz. the Dundaf Linn, † mile below Corra Linn, 4 feet high, and one which is mile below Bonnington fall. The total descent of the river from the first to the last fall-a distance of 3.7 milesamounts to 230 feet.

The Falls excluded.

10.0

? 400

20.5

0.

365

7.1

3.0

170

10.3

20.2

46

7.6)

,,

(9.0)

?

?