line of descent. But to land even one foot inside that inferno
would be as fatal as to alight in its very midst.

* * * * *

Kendrick was thinking fast.

"There's just a chance," he said. "It all depends upon how wide the
zone of these escalator rays is, and whether we can tune in on them.
At least, I can probably answer the latter question."

Pushing the protrudent round button on his mysterious bracelet as he
spoke, he leaned over the edge of the trap-door and awaited results.

They were not long in coming. The vibration he was already under from
the invisibility rays seemed to double. Alternate waves of giddiness
and depression, of push and pull, swept over him.

A minute of it was enough. He pressed the round button that now
protruded, ending this influence, and faced Marjorie, stating:

"I'm positive now that these things control descent and ascent. As
nearly as I can figure, the rays work on the principle of an endless
belt. If you're up here, you get carried down, and vice versa. As to
how wide the belt is, and whether you can move sideways on it, remains
to be seen. Anyway, I'm going to take a chance. I'll go first. If my
guess is wrong, you--well, needn't follow."

"No, I'm going with you!" she declared resolutely. "We've come this
far together. I shan't be left alone now. Let's go!"

And again her soft, warm hand was in his.

Lord, what a girl! How many would be brave enough to take a gamble
like that, on a fellow's mere supposition?

"All right--go it is!" he said. "Push your round button, like this."
He showed her the way he thought was right, pushed his own. "Ready?"

"Ready!"

* * * * *

Their voices were grave. It was a grim prospect, stepping off into
space like that, with only a guess between them and death.

"Then jump!"

They jumped, gripping each other's hands tightly--and instead of
dropping like plummets were caught in a powerful field of force and
whirled gently downward.

"Oh, you were right!" gasped Marjorie, awed. "See, we--"

Then she paused, horror-stricken, for it was obvious that they were to
descend within that lake of molten glass, unless they could change
their course at once.

"Quick!" he called. "Hold fast! Now--run!"

Breathless, they raced to the left, across that invisible descending
belt.

Too far, Kendrick knew, and they would plunge outside its zone, fall
crushed and mangled. Not far enough,