It was pleasant on the vast plain that stretched away in every
direction from the derailed train. The sun was shining brightly, but
not too warm, and there was a gentle breeze.

"This is fine!" exclaimed Jerry.

The boys and the professor strolled on for several miles, the three
chums enjoying the walk very much, while Mr. Snodgrass was continually
finding some new insect, or a flower, until his specimen boxes were
full.

"Well, we've come quite a distance," said Ned, as they got on top of a
small hill and looked about. "We can't see the train anywhere. I guess
we'd better be thinking of starting back."

"Maybe we had," agreed Jerry. "But what's that dark line out there?"
and he pointed to the horizon.

"A cloud isn't it?" asked Bob.

"It's too low, and it doesn't move like a cloud," objected Jerry.

They watched it for some time, as it got larger and larger.

"Why it's all around us!" suddenly exclaimed Bob.

And so it was. The travelers were hemmed in by a peculiar, moving
ring, that seemed to get smaller and smaller.

"What do you think it is, Professor?" asked Ned.