The boys were naturally astonished at the boldness of this remark. Other
lights now appeared, some dim, others brighter. The firing of a gun
seemed to them a most hazardous thing to do, but no doubt the Professor
had a reason for making the suggestion.

It was quite a time before either of the boys responded to this
proposal. In their minds it was a daring enterprise.

"If we should fire a gun the noise would likely startle them, and the
first impulse of the savages would be to extinguish the lights."

George, who had the spirit of adventure more strikingly developed than
Harry, was the first to concur.

"I am going to try it at any rate; we might just as well know what we
have to face now, as later on."

"So you are really going to shoot?" said the Professor.

"If you so urge it, yes."

"Then let me suggest what to do. All savages have a keen sense of
direction. It is one of their chief accomplishments. You and Harry go
back, up the river, a quarter of a mile, or so, and take with you one of
our coverings. Then shoot behind the blanket, so the flash will not be
seen, and I will remain here and watch the effect."

There was no delay in their preparations. Within fifteen minutes the
shot rang out, and almost immediately thereafter every light had
disappeared. The boys were also keen enough to note the extinguished
lights, and returned to the Professor in a hurry.

"The disappearance of the lights is not conclusive evidence that human
beings were there. It might have been a mere coincidence."

"Coincidence! What do you mean by that?"

"Did it not occur to you that the lights might be natural phenomena?"

"Of what?"

"Of phosphorescence."

"Do you mean 'will-o'-the-wisp'?"

"It is sometimes called by that name. It is caused by decaying vegetable
matter, and exhibits itself in the form of gases of phosphorus, which
appears to burn, but does not, like the vapor which is produced by
rubbing certain matches in the dark."

"But how do you account for the disappearance after we shot?"

"I thought they might have disappeared naturally, after you fired, and,
therefore, said it might have been a mere coincidence."

This explanation was not a satisfying one for the boys, and the
Professor did not place much faith in it, for the following reasons:

"I believe it is our duty now to keep watches during the night, which we
can do by turns, so that the sentinel will quietly awaken the next one
in his turn, or both