76 [No. 88 ECONOMIC CONDITIONS swamp , and open water, probably about35,000 square miles in extent, through which the Bahr el - Jebel (Upper Nile ) , Bahr el-Ghazal, Bahr el-Zeraf, and the lower reaches of their tributaries wind their way. It doubtless represents what is left of the bed of an inland sea , in which thewater is now probably nowhere more than 8 ft . deep . The vegetation consists mainly of papyrus, reeds , and swamp grass (panicum ) , with occasional patches of ambach (see below, p. 126 ) . The word sudd should strictly be applied only to that portion of the vegetation which is uprooted by wind and rising water, and floats on the surface with a quantity of adhesive mud . The slow movement of these vast masses of floating weed , and the way in which they burst into the channels, break up , or pile upon one another when an obstruction is encountered, recall the action of ice floes when the pack is breaking up .

In the hot, dry season , when the stems of the papyrus are from 10 to 15 ft . in height , the region is swept from end to end by fierce fires, lighted by natives, which keep down the growth of ambach and destroy any other tree which attempts to gain a footing. The ash and burnt stems are added year by year to the floating massof roots and soil , until in course of time it becomes of a solid, peaty consistency, as much as 5 or 6 ft . in thickness .

In clearing away this sudd and reopening, the channels, the first step is to cut or burn the whole of the vegetation growing on the surface ; the line of the channel is then marked out for a distance of about 30 ft . , and men set to work to cut downwards, with long saws and other implements, as far as possible through the sodden mass, along the sides of the block. A steel wire hawser is then forced deeply into the cuts and secured by telegraph pole holdfasts. Finally, the hawser is attached to a steamer, and the whole block is pulled gradually from its place and allowed tofloat down - stream . The process is then repeated with a second block ; and so the work proceeds laboriously,