on was, where was it situated?

Drake suggested that there must undoubtedly be an opening somewhere,
because the air in the vault was comparatively pure and fresh; at least
it had not the dead, stale, stuffy smell of air confined in a
hermetically-sealed chamber. But Frobisher pointed out that the door by
which they had entered, although an excellent fit, did not butt up
against the jambs so closely as to exclude the air altogether; yet he
acknowledged that the air in the vault certainly seemed sweeter than
might have been expected, had the main door been the only channel
through which it could filter in.

Under the stimulus of the new glimmer of hope thus caught, every
corridor was once more explored, even more closely than before, but with
no other result than that Frobisher completely satisfied himself that
there was most certainly no exit from any of the passages. Even a
concealed door, opened by a spring, could hardly have evaded the close
scrutiny of the two men; and it became more and more apparent that they
had been caught in a trap from which there was no escape. Both were
feeling famished for want of food, and were parched with thirst; and
Frobisher could not help wondering how long the agony of death from
starvation and thirst would be prolonged before blessed unconsciousness
came to their relief.

Suddenly--they had both been sitting dejectedly on the floor--Frobisher
jumped to his feet.

"Look here, Drake," he exclaimed, "there is just one place that we never
thought of searching, and that's the treasure chamber itself. We were
too deeply interested in the valuables we found to think of looking for
an exit in there. Who knows?--the very thing we are hunting for may be
in there all the time."

The two men fairly raced down the passage leading to the chamber, opened
the latch, with all due caution, and re-entered the vault. At first
sight there appeared to be no semblance of a second door, and their
hopes dropped to zero once more. Then Drake proposed that, as a last
chance, they should remove the chests to the centre of the room and see
whether, possibly, there might be a door concealed behind any of them.
They set to work feverishly, and in doing so spilled the jewels and
coins in heaps on the floor. But what did that matter? Unless they
found a way of escape from their prison, jewels and coin would be of far
less value to them than a loaf of bread and a jar of water.

Then, at the very end,