But the gas in his balloon was his blood, his very life!

A thousand one such reflections whirled in succession through his brain; and, resting his head between his hands, he sat there for hours without raising it.

"We must make one final effort," he said, at last, about ten o'clock in the morning. "We must endeavor, just once more, to find an atmospheric current to bear us away from here, and, to that end, must risk our last resources."

Therefore, while his companions slept, the doctor raised the hydrogen in the balloon to an elevated temperature, and the huge globe, filling out by the dilation of the gas, rose straight up in the perpendicular rays of the sun. The doctor searched vainly for a breath of wind, from the height of one hundred feet to that of five miles; his starting-point remained fatally right below him, and absolute calm seemed to reign, up to the extreme limits of the breathing atmosphere.

At length the feeding-supply of water gave out; the cylinder was extinguished for lack of gas; the Buntzen battery ceased to work, and the balloon, shrinking together, gently descended to the sand, in the very place that the car had hollowed out there.

It was noon; and solar observations gave nineteen degrees thirty-five minutes east longitude, and six degrees fifty-one minutes north latitude, or nearly five hundred miles from Lake Tchad, and more than four hundred miles from the western coast of Africa.

On the balloon taking ground, Kennedy and Joe awoke from their stupor.

"We have halted," said the Scot.

"We had to do so," replied the doctor, gravely.

His companions understood him. The level of the soil at that point corresponded with the level of the sea, and, consequently, the balloon remained in perfect equilibrium, and absolutely motionless.

The weight of the three travellers was replaced with an equivalent quantity of sand, and they got out of the car. Each was absorbed in his own thoughts; and for many hours neither of them spoke. Joe prepared their evening meal, which consisted of biscuit and pemmican, and was hardly tasted by either of the party. A mouthful of scalding water from their little store completed this gloomy repast.

During the night none of them kept awake; yet none could be precisely said to have slept. On the morrow there remained only half a pint of water, and this the doctor put away, all three having resolved not to touch it until the last extremity.

It was not long, however, before Joe exclaimed:

"I'm choking, and the heat is getting worse! I'm not surprised at that, though," he added, consulting the thermometer; "one hundred and forty degrees!"

"The sand scorches me," said the hunter, "as though it had just come out of a furnace; and not a cloud in this sky of fire. It's enough to drive one mad!"

"Let us not despair," responded the doctor. "In this latitude these intense heats are invariably followed by storms, and the latter come with the suddenness of lightning. Notwithstanding this disheartening clearness of the sky, great atmospheric changes may take place in less than an hour."

"But," asked Kennedy, "is there any sign whatever of that?"

"Well," replied the doctor, "I think that there is some slight symptom of a fall in the barometer."

"May Heaven hearken to you, Samuel! for here we are pinned to the ground, like a bird with broken wings."

"With this difference, however, my dear Dick, that our wings are unhurt, and I hope that we shall be able to use them again."

"Ah! wind! wind!" exclaimed Joe; "enough to carry us to a stream or a well, and we'll be all right. We have provisions enough, and, with water, we could wait a month without suffering; but thirst is a cruel thing!"

It was not thirst alone, but the unchanging sight of the desert, that fatigued the mind. There was not a variation in the surface of the soil, not a hillock of sand, not a pebble, to relieve the gaze. This unbroken level discouraged the beholder, and gave him that kind of malady called the "desert-sickness." The impassible monotony of the arid blue sky, and the vast yellow expanse of the desert-sand, at length produced a sensation of terror.