"And I'm ready, master, whenever you say the word!" added Joe, preparing for the fight.

"It would not be the first time," remarked the doctor, "that science has been followed up, sword in hand. The same thing happened to a French savant among the mountains of Spain, when he was measuring the terrestrial meridian."

"Be easy on that score, doctor, and trust to your two body-guards."

"Are we there, master?"

"Not yet. In fact, I shall go up a little, first, in order to get an exact idea of the configuration of the country."

The hydrogen expanded, and in less than ten minutes the balloon was soaring at a height of twenty-five hundred feet above the ground.

From that elevation could be distinguished an inextricable network of smaller streams which the river received into its bosom; others came from the west, from between numerous hills, in the midst of fertile plains.

"We are not ninety miles from Gondokoro," said the doctor, measuring off the distance on his map, "and less than five miles from the point reached by the explorers from the north. Let us descend with great care."

And, upon this, the balloon was lowered about two thousand feet.

"Now, my friends, let us be ready, come what may."

"Ready it is!" said Dick and Joe, with one voice.

"Good!"

In a few moments the balloon was advancing along the bed of the river, and scarcely one hundred feet above the ground. The Nile measured but fifty fathoms in width at this point, and the natives were in great excitement, rushing to and fro, tumultuously, in the villages that lined the banks of the stream. At the second degree it forms a perpendicular cascade of ten feet in height, and consequently impassable by boats.

"Here, then, is the cascade mentioned by Debono!" exclaimed the doctor.

The basin of the river spread out, dotted with numerous islands, which Dr. Ferguson devoured with his eyes. He seemed to be seeking for a point of reference which he had not yet found.

By this time, some blacks, having ventured in a boat just under the balloon, Kennedy saluted them with a shot from his rifle, that made them regain the bank at their utmost speed.

"A good journey to you," bawled Joe, "and if I were in your place, I wouldn't try coming back again. I should be mightily afraid of a monster that can hurl thunderbolts when he pleases."

But, all at once, the doctor snatched up his spy-glass, and directed it toward an island reposing in the middle of the river.

"Four trees!" he exclaimed; "look, down there!" Sure enough, there were four trees standing alone at one end of it.

"It is Bengal Island! It is the very same," repeated the doctor, exultingly.

"And what of that?" asked Dick.

"It is there that we shall alight, if God permits."

"But, it seems to be inhabited, doctor."

"Joe is right; and, unless I'm mistaken, there is a group of about a score of natives on it now."

"We'll make them scatter; there'll be no great trouble in that," responded Ferguson.

"So be it," chimed in the hunter.

The sun was at the zenith as the balloon approached the island.

The blacks, who were members of the Makado tribe, were howling lustily, and one of them waved his bark hat in the air. Kennedy took aim at him, fired, and his hat flew about him in pieces. Thereupon there was a general scamper. The natives plunged headlong into the river, and swam to the opposite bank. Immediately, there came a shower of balls from both banks, along with a perfect cloud of arrows, but without doing the balloon any damage, where it rested with its anchor snugly secured in the fissure of a rock. Joe lost no time in sliding to the ground.

"The ladder!" cried the doctor. "Follow me, Kennedy."

"What do you wish, sir?"

"Let us alight. I want a witness."

"Here I am!"

"Mind your post, Joe, and keep a good lookout."

"Never fear, doctor; I'll answer for all that."

"Come, Dick," said the doctor, as he touched the ground.

So saying, he drew his companion along toward a group of rocks that rose upon one point of the island; there, after searching for some time, he began to rummage among the brambles, and, in so doing, scratched his hands until they bled.