"I believe," I said. He pressed my hand with a grip that nearly
crushed it.

"Now," he told me. "I do not fear. If I--fail, you will follow with
help?"

I promised.

"We talked it over carefully," he went on, "bringing to bear all our
power of analysis and habit of calm, scientific thought. We considered
minutely the time element in the phenomena. Although the deep chanting
began at the very moment of moonrise, fully five minutes had passed
between its full lifting and the strange sighing sound from the inner
terrace. I went back in memory over the happenings of the night
before. At least ten minutes had intervened between the first
heralding sigh and the intensification of the moonlight in the
courtyard. And this glow grew for at least ten minutes more before the
first burst of the crystal notes. Indeed, more than half an hour must
have elapsed, I calculated, between the moment the moon showed above
the horizon and the first delicate onslaught of the tinklings.

"'Edith!' I cried. 'I think I have it! The grey rock opens five
minutes after upon the moonrise. But whoever or whatever it is that
comes through it must wait until the moon has risen higher, or else it
must come from a distance. The thing to do is not to wait for it, but
to surprise it before it passes out the door. We will go into the
inner court early. You will take your rifle and pistol and hide
yourself where you can command the opening--if the slab does open. The
instant it opens I will enter. It's our best chance, Edith. I think
it's our only one.'