the wall, which conduct on their part invariably leads to a fight. As for Mercury, it is a wretched place, full of robbers and tradesmen, who are so much alike, that it is difficult to distinguish one from the other. Finally, he drew them a truly enchanting picture of Venus; "and when we shall have returned from that expedition we shall be decorated with the Southern Cross."
"And well you will have won it," cried the sailors.
Thus, in animated conversation, the long evenings were passed on the forecastle. All this time the interesting conversations with the doctor continued.
One day, when they were conversing respecting the guidance of balloons, Ferguson was asked to give his opinion on the question.
"I do not think," he said, "that we shall ever be able to direct the course of a balloon. I am acquainted with all the systems which have been proposed or attempted. Not one has succeeded; not one is practicable. You may very well imagine that I have myself been engaged in this matter, which ought to possess a very great interest for me, but I have never been able to solve it by means of our present knowledge of mechanics. It would be necessary to discover a motive power of extraordinary strength and of an impossible lightness. Even then, one could not resist any considerable currents. As it is, one is much more anxious to direct the car than the balloon. That's a mistake."
"Nevertheless," said someone, "there is a great resemblance between the balloon and a ship, which can be guided at will."
"Not at all," replied Doctor Ferguson; "there is little or no resemblance. Air is infinitely less dense than water, in which, moreover, a ship is only half submerged, while the balloon is entirely surrounded by the atmosphere, and remains stationary on account of the fluid which encircles it."
"Then you are of opinion that science is exhausted upon that point?"
"Not so, not so; it has become necessary to look for other means by which, if a balloon cannot be guided, it can be kept up in favorable atmospheric currents. As one rises higher, these currents become more uniform, and are more constant in their direction, as they are not interfered with by the valleys and mountains which intersect the face of the earth;
