The deeper ravines we crossed on bridges. Bridge-building in Africa is no great triumph of mechanical engineering. The bridges which crossed the narrower ravines and gorges consisted simply of several long, slender poles laid down side by side. They ought to be on a level but are not. One is six inches or a foot higher than the other, and there is so much spring in them that the feat of crossing is equal to a tight-rope performance. Over extensively flooded areas where the water is too deep to wade, a bridge of single poles is supported in forked uprights at intervals of the length of the poles, and above the bridge a rope of vine is stretched to hold with the hand. It happened more than once that by some mishap we tumbled into the stream below; but the natives were quick to fish us out. The worst part of the journey was on the last day, through the new clearings which the natives had made for their gardens. In these the whole forest lay prostrate,—trees great and small, the tangled mass of vines, and all the débris made by its crashing fall. The whole enormous mass is left lying until the lighter parts of it dry: then it is burned over. This burning is repeated at long intervals until at length much of it is burned away. But by this time the natives are perhaps thinking of deserting it and making a new garden somewhere else, or they may have moved their town away. Meanwhile, they somehow reach the ground and plant their cassava which flourishes in the fresh, rich soil. The difficulty in an African garden is not to get things to grow, but to keep other things from growing. They never hesitate to fell the forest thus across the road, obstructing the caravans and bringing curses on their heads.