"Mokoum," then said Mr. William Emery, "if the noise you thought you heard is caused by the machinery of a steamboat, you would hear better by stooping to the level of the river; water always conducts sound more clearly and quickly than air." "You are right, Mr. William," answered Mokoum, "for more than once I have found out the passage of a hippopotamus across the river in that way." The bushman went nimbly down the bank, clinging to the creepers and tufts of grass on his way. When he got to the level of the river, he went in to his knees, and stooping down, laid his ear close to the water. "Yes!" he exclaimed, in a few minutes, "I was not mistaken; there is a sound, some miles down, as if the waters were being violently beaten; it is a continual monotonous splashing which is introduced into the current." "Is it like a screw?" asked the astronomer. "Perhaps it is, Mr. Emery; they are not far off." William Emery did not hesitate to believe his companion's assertion, for he knew that the hunter was endowed with great delicacy of sense, whether he used his eyes, nose, or ears. Mokoum climbed up the bank again, and they determined to wait in that place, as they could easily see down the river from there. Half an hour passed, which to Emery, in spite of his calmness, appeared interminable. Ever so many times he fancied he saw the dim outline of a boat gliding along the water, but he was always mistaken. At last an exclamation from the bushman made his heart leap. [Illustration: At length an exclamation of the Bushman made his heart beat.] "Smoke!" cried Mokoum.