LA£E NOAMI. 289 The Tonk^ (Tonka, Tiog^), which receives the surplus waters of the lower Ku-Bango, but which is at times completely dry, is everywhere skirted by mo- loUus, some with the normal others with the reverse current. The bed of the Tonk^, which is here and there obstructed by a few rapids, is generally followed by the Bushmen in their migrations. After the ruins the Tonk^ usually discharges into Lake Ngami ; but in 1886 it hud shifted its bed and discharged into a vast morass, whose waters were carried off through various channels eastwards and south-eastwards to the Chobe and the Zuga. Every successive explorer who pene- trates into these solitudes describes and figures differently the currents of the lacustrine basins and the network of their influents and effluents. Lake Noami. Lake Ngami, Nagabi, or Naabi, that is " water " in a pre-eminent sense, or according to Chapman, " Giraffe Lake," is one of those basins with ever-chunging margins, like the Shotts of Algeria and Tunis. No traveller traces its outlines in the same way. The least shifting shore lies on the south side, where the land is some- what more elevated. It even develops at some distance from the lake the chain of the Makkapolo hills, rising 1,200 feet above the level of the lake, which by different explorers is itself estimated at from 2,600 to nearly 3,000 feet above the sea. When discovered by Livingstone in 1849, Ngami appeared to stretch for about 60 miles from east to west, but was much narrower from north to south, the opposite shores being plainly visible in this direction. The natives calciilutod the circum- ference at a three-days' journey, but its circumnavigation would have presented almost insuperable difficulties, the water being so shallow that in many places the boatmen are unable to use their oars, and are obliged to propel their light craft or reed rafts with poles. The lake acquires its greatest expansion usually between the months of April and July, when its waters, diluted by its numerous aflBuents, become sweet and potable ; but according as they subside they grow continually more saline, at last even leaving crystalline efflorescences on the surrounding reeds, which in some places form a green border several miles wide. The basin has been subject to frequent changes of level, which are evidently due to the difference of baro- metric pressure on the shallow lagoon waters, combined with the deviations in the volume of liquid brought down or carried off by the Tonke and other tribu- taries or emissaries. The waters are also displaced by the regularly alternating morning and evening breezes, the former setting from the east and driving them westwards, the latter driving them back again to the east. Thus, as the natives say, the lake goes every day to graze and then returns to the kraal. According to Livingstone, Ngami is fed not only by surface streams but also by underground contributions issuing from the southern hills, or from porous sandstones resting on a bed of impermeable rock. In many parts of the surround- ing district the land is sufficiently watered to support an arborescent vegetation, rivalling in exuberance and splendour that of the aliuvial tracts along the Lower
