and here is the principal cause, as you are aware, of the changes of the force and direction of the wind. Now once these zones have been determined, the balloon will only have to be placed in the currents which will be met there."
"But," replied the captain, "to hit upon these currents you must be always ascending or descending. There is the true difficulty, my dear doctor."
"Why, my dear captain?"
"Let us understand each other; it would only be an obstacle in the way of long journeys, not for small ascents."
"Your reasons, if you please?"
"Because you can only ascend by throwing out ballast, you can only descend by letting the gas escape; and under these circumstances your store of gas would be very soon exhausted."
"My dear Penney, that is the point of the whole thing. There is the difficulty which science should endeavor to overcome. It is not a question of directing the course of a balloon so much, as it is a question of moving up and down without losing the gas, which is the strength, the blood, the soul, so to speak, of a balloon."
"Quite right, doctor; but this difficulty is not overcome; the means to accomplish this have not yet been found."
"Excuse me, they have been."
"By whom?"
"By me."
"By you!"
"Why, you must understand that without this power I should not have run the risks of crossing Africa in a balloon. Why, in about twenty-four hours I should have had no gas left."
"But you have never spoken of this in England!"
"No, I did not think it desirable to discuss it in public. That seemed to me useless. I made secretly some preliminary experiments, and I am satisfied. I have not any need of learning anything further on that point."
"My dear Ferguson, may one ask to be made acquainted with your secrets?"
"Here it is, gentlemen, and my plan is a very simple one."
The curiosity of the audience was raised to the highest pitch, while the doctor calmly addressed himself to his subject as follows.
