I saw that the rock sloped back from the flaw or cleft in quite a practicable way.

Only there was a big boulder resting on the slope within reach, and which we could hardly avoid touching.

It did not look very secure.

So I put out my hand and just touched it--quite, quite gently.

But it was so exactly balanced that the least little vibration overset it, and I saw it begin to move, very slowly, as if it meant no harm whatever.