"Then it increased till it fairly drove the river from its bed, or rather caused it to spread out and form new channels. It became a bog, through which the water percolated or ran in unknown channels, and furnishing a foundation for masses of vegetation, that sprung up and flourished under the effect of tropical heat and moisture. "This state of affairs continued for six or eight years, and the White Nile apparently ceased to exist, by reason of the great dam of reeds and other plants that choked the channel and made navigation impossible. This dam was the sudd of which we have been speaking. [Illustration: HAULING A STEAMBOAT THROUGH A CANAL CUT IN THE SUDD.] "It remained here when Baker Pacha ascended the Nile on his expedition for the suppression of the slave-trade. His advance was retarded for many months by the sudd; he was obliged to cut channels through it, and then haul his boats along from one strip of open water to another. Many of his men died from exposure and hard work in passing the sudd, and there were fears at one time that it would cause a total abandonment of the expedition. "The sudd was full of insects, that caused great suffering to all concerned, and the air at all times was thick with mosquitoes. One of the most dreaded pests was the 'guinea-worm,' that embeds itself in the feet or ankles, and produces a disagreeable and often dangerous sore. This worm is peculiar to the tropics, and is justly feared by all persons liable to its attacks. It makes a slight puncture in the skin--generally in or near the foot--and lays its eggs there. They are hatched in from two months to a year, and the puncture is so minute that its presence is not known until the eggs are developed." One of the boys asked if the worm ever caused the death of the person attacked. "Generally he escapes with a dreadful sore, that may be months in healing," said Doctor Bronson, who was standing near; "and not unfrequently he loses the foot or leg where the sore is developed. If the worm can be removed without breaking, and before it has created more than a small sore like a pimple, no serious harm results; but the operation is difficult, and requires great care on the part of both doctor and patient." "How is it performed?" Frank asked.