The aircraft was flying "all out", her speed, on account of the rarefaction of the atmosphere, being a little less than 140 miles an hour.
Peter was in a hurry. It was most desirable that the mountains should be crossed well before dark. Apart from the risk of crashing blindly against one of the many almost vertical peaks, there were the dangerous air-pockets and eddies to be taken into consideration, and with the setting of the sun, and the consequent rapid cooling of the earth's surface, the higher altitudes were certain to be disturbed by raging winds that attain the velocity of a hurricane.
For miles the ground rose steadily. Viewed from a height, the rise appeared to be gradual, since the smaller irregularities were apparently flattened out. It was only by judging by the shadows cast by the sun, which was now well down in the west, that the numerous valleys and ridges could be noticed.
For the first hundred miles, the country was well wooded. Then came a wide belt of grass land, gradually merging into an arid waste absolutely destitute of vegetation. The desert marked the beginning of the Sierras, which were now plainly visible at a distance of thirty or forty miles.
"Think she'll do it before dark?" inquired Uncle Brian, glancing at the sun, now only about thirty degrees above the horizon.
"Rather," replied Peter. "It will be quite light up here after the low-lying ground is in darkness. Once we're above the peaks I don't mind. It will be plain sailing after that."
"If you're sure of it, well and good," rejoined Brian. "If not, we'd better make a landing while it is light." The youthful pilot shook his head.
"Twelve hours saved is twelve hours gained," he said sagely. "I don't want to spend another night in Rioguayan territory if it can be avoided. She'll do it."
Fifteen minutes later, a violent bump announced that the flying-boat had struck an air-pocket, a clear indication of the adverse conditions that awaited her above the snowy peaks of the Sierras. She dropped vertically for nearly a thousand feet in spite of the pilot's efforts to counteract the sudden loss of "lift". Then staggering blindly into the furthermost wall of the invisible air chasm, the flying-boat "stalled" and almost stood on her tail, until she picked up and Peter was able to bring her back to her normal trim.
The next five minutes was a perfect nightmare. Above the snowy crags, now pink in the diffused rays of the setting sun, she sped, side-slipping, banking, and plunging, as if scorning the desperate efforts of the pilot to keep her up.