at five o'clock ned and alan were astir. with regrets that they were not at camp eagle for a plunge in the cool mountain lake, they prepared another hot meal, ate it, and boarded the cibola.
the balloon had now been inflated thirty-eight hours and was noticeably showing the loss of its gas. while the top of the bag was yet round and firm in the heat of the sun the lower sides had become a trifle flabby as the cool evening had come on. up to this time all records for balloon flight had been broken a fact due to the renewed buoyancy caused each day by the hot, southwestern sun. and, exploration in and quick ascent from the canyons before them would before long call for the use of ballast. the boys agreed that the time had arrived to utilize their liquid hydrogen. the shrinkage that night had been quite perceptible.
they regretted that but two-thirds of this remained—about eleven cubic feet. this when reconverted meant nearly twelve thousand cubic feet of new gas at their present altitude. as the work of converting the gas involved care, preparation for it was made before the cibola was cut loose.
the reconverter, a reduced inversion of the apparatus used in making liquid air, was made ready. when the muffled explosions and the heat of the tubes told the boys that the reconverter was working perfectly and pumping new and needed gas into the shrunken cibola's long bag, the lashings were loosed and once more the faithful dirigible mounted skyward.
with major honeywell's map of the region spread out on the deck of the bridge and the binoculars in hand ned began the long anticipated search for the lost city.
all day the process of turning the liquid hydrogen back into buoyant gas went on. and all day the cibola wound her devious course over the peaks and chasms beneath. by night half the hydrogen jars were empty and ned and alan saw the evening close in on them without a sign of the object of their search. when darkness stopped further work the balloon was brought to earth and camp made again.
the following day, as uneventful as the first, gave no indication of the secret city. the rest of the liquid hydrogen was transformed into gas. the sun seemed to enfold the craft in a fiery embrace. when camp was made again that night the cibola had been afloat eighty hours.
"i think she is good for another forty-eight hours," said ned that night. "if we find nothing in two more days we'll have our choice of going out on foot or of quitting in time to pick up elmer and bob and make a dash to civilization. what do you say?"
"i don't know," replied alan, "i'd hate to give up as long as we can fly. i think the boys can care for themselves. let's stick to it. we have provisions and there is water in some places."
"well," answered ned, "we'll have two more days time in which to decide."
the next morning the cibola showed plainly that her gas was rapidly escaping. new life was given to the balloon by casting overboard some empty hydrogen casks. the fourth day broke hotter than ever. in all the wilderness examined by the tired and strained eyes of the searchers, not a human being had been seen—not even a wandering navajo. this day they began the search with renewed vigor, but with the same monotonous result—miles of hopelessly desert rock and sand beneath them, with a little vegetation now and then, but so sign of indian remains.
at noon ned said:
"if we were not in a balloon with a compass and sextant i should say we were lost. and if indians ever lived and died hereabouts they certainly left so signs of their bones."
by six o'clock, with the sun gratefully low, alan expressed discouragement.
"to-morrow at this time," he said, "if we see no indication of the old palace or city or whatever it was—if it ever was—i think i'll vote to try to find camp eagle and get out."
"we'll see to-morrow," answered ned stoutly.
that night at dark, a landing was made on the ledge of a point of land ending in a rounded cliff pointing south, selected because the place was open to the breeze and cool. the cibola had approached the height from the west, and the boys believed that the promontory projected from yet higher ground beyond. on those portions of the cliff that they could see there was neither shelf nor projection of any kind. the walls rose almost like cut stone and were apparently about three hundred feet high. as the cibola was about to descend, alan, who was taking a last survey from the bridge, called ned's attention to the fact that even the far side of the supposed promontory was separated from the mountains beyond, and that a chasm at least a half mile wide separated the two heights.
"it's a mesa," replied ned with renewed enthusiasm, "and it will be a good thing to look over it to-morrow. these high and almost unapproachable islands of rock were favorite dwelling places for the indians."
"but a temple up here wouldn't be a secret very long," replied alan. "we've seen this point all afternoon. it's prominent enough."
"that's so," answered ned, "but we are here, so let's make a landing and eat, and dream over it."
the balloon had now lost so much gas that a landing was easy, and, tired with four days' profitless search and its strain, the young aeronauts were soon beyond even dreams.
it was with no small alarm that the boys saw, when they awoke with the first rays of the sun, that the car of the cibola, which had been anchored fore and aft to heaped up rocks during the night, was now resting on the ground. gas, was rapidly escaping. but fortunately the aeroplanes and propeller had been left properly in a horizontal position and no damage had been done.
the boys knew that by throwing over enough ballast and stores the cibola could be made good for one more flight, but that probably it would be the last. therefore, the inevitable seemed forced upon them. they would fortify themselves with a good breakfast, look over the mesa, make one more circling flight and then attempt to find camp eagle. while alan made haste to prepare breakfast, ned determined first on an examination of the mesa point by daylight.
the rock had a top area of perhaps forty or fifty acres. it had a rolling surface and was coated with a carpet of dusty sand, except in the northwest corner. the northern end of the mesa, ned could see, widened and ended in a sharp rise almost wall-like in form. at the western end this wall-like elevation turned the corner and extended south a short distance, finally dropping down to the general level of the mesa. in this protected comer grew a strange grove of gnarled and twisted pines, ill nourished and apparently very old. between this comer of the mesa and the sharper promontory whereon the cibola had come to anchor, was a wide, sandy, barren depression.
the narrow portion of the rocky island where the boys had made camp drew in abruptly to make the point that marked the southern end of the mesa. ned turned first toward the point.
when he had advanced, making his way slightly upward all the time, to where the narrow mesa was not over four hundred feet wide, the lad was astounded to suddenly discover a deep and narrow fissure or chasm. it was dark, with sides as abrupt as the cliffs of the mesa, and too wide to jump across. a cold air was already rising from the opening into the warmer atmosphere above.
in his astonishment ned called to his chum.
"what surprises me," exclaimed ned, "is the character of the opening. if it extended from cliff to cliff i should say that the same freak of nature that made this solitary island of rock also split off this end at some time. but it is closed at each end."
alan hastened to the end of the fissure, near the side of the mesa.
"it looks to me," he said, "as if it had extended entirely across at some time and the ends walled up later."
the boys made a closer examination.
"you're right," said ned when he discovered that each end of the rift had been filled with closely fitted rock, "and human hands did it."
alan sprang up in excitement.
"that's the first sign we've had," he exclaimed. "do you suppose it means anything?"
the edge of the cliff was so abrupt that the boys had to lie down to look over in safety.
"it does," ned answered. "the reason you can't see that chasm from below or from in front is because the face of it is walled up. and it is walled so skillfully that you can't detect it from even a short distance."
"that's to hide something," quickly replied alan, "but i don't see—"
ned was standing on top of the short filled-in portion of the chasm.
"look!" he exclaimed, suddenly interrupting his friend. "these stones are steps, and, they are worn!"
