replied Lynton decidedly. "I've heard them out at sea sometimes,
when we've been in a calm among the islands."

"More like to be a kind of frog," put in Briscoe. "There are some which
whistle and pipe in chorus very softly; but--"

The sound came swelling down the canon more loudly, and the speaker
stopped short to listen, till the tones once more died away.

"That's not frogs in chorus," said Briscoe decisively. "Anyone would
think there was an abbey somewhere near, and the nuns were singing
hymns; only it's impossible, of course."

"Impossible, of course," said Brace softly. "There: it is gone again."

The three men stood listening and straining their ears in the direction
from which the sounds had come, but there was a faint whispering as of
running water down below, a trickling gurgle, and then startlingly loud
came the nasal _quant_ of some night-heron at the water's side.

This was answered twice at a distance, while again and again overhead
there was the flutter and swish of wings, probably those of the
oil-birds circling about the mouth of the cavern.

"It's all over," said Briscoe at last, "and it's night-birds of some
kind, I believe. Here, I've been listening so intently that I've
forgotten my cigar. I'll go in and light it again with one of the bits
of smouldering wood."

He left his two companions, and they heard his footsteps as he went
softly into the cavern to reach the fire.

"Does it make you feel queer like, Mr Brace?" whispered Lynton.

"Well, it sets me wondering, and makes me a little uncomfortable as to
what the sound can be," replied Brace.

"So it does me, sir. Always makes me feel queer if I don't understand
what a noise is. I'm a bit of a coward, I'm afraid."

"I've never seen any signs of it yet, Lynton," said Brace, laughing
softly.

"Oh, but I am, sir. That sound made me feel hot and then cold. I say,
I've lost count about the points of the compass, but that's plain enough
yonder across and up the river. That's the east, and the moon coming
up."

"That?" said Brace, as he gazed in the direction named. "Yes, I suppose
so. It will be very beautiful when the moon rises over the mountain
there and lights up the great canon. I feel disposed to wait till it
shines on the river."

"Moon!" said Briscoe, who had returned unheard, smoking vigorously, and
looking in the darkness as if a firefly were gliding to their side. "We
shan't see the moon to-night. It must hav