The others became a bit cautious, yet they never abandoned their gliding vigil. They, too, finally swept down upon the balloon. More of the birds came swarming to take their place, and the machine men soon found themselves busy protecting their skycraft.
"They probably have their nests in the mountains close by," said the professor, "and they suspect us. That is why they have grown more ferocious and daring since we neared the mountains."
The wind was quickening. More of the great birds came to replace each one killed. One came so close that a wing brushed the basket, knocking the machine men off their feet. They were finding it difficult to defend the balloon against so many of them. They were in danger of being wrecked!
Dark clouds had settled over the mountains—which were now so near that the machine men could distinctly see objects such as trees and rocks. The wind had risen to a gale, and they were being carried on it.
"We are rising!" 6W-438 exclaimed. "The wind is carrying us above the mountains and into that approaching storm area!"
"Let out part of the helium!"
"We cannot do that now," the professor told them. "The force of the wind would dash us against the mountainside!"
A dull flush of pink lit the drifting depths of the cloud masses momentarily, and the terrific roar which followed shook the balloon and made the metal globe hum with strange music.
With the advent of the storm, the birds gave up the attack and winged off to their lofty retreats in screeching alarm.