I put my book on the sand, lay back, and looked up at the clouds sliding across the sky. At the edges, they swirled like ink in water. After a while, these movements began to have an unsettling effect on me. I felt as if gravity might suddenly fail; that the earth would lose its grip on my body, and I would fall into the depths of the sky, first through the bright and friendly blue, and then on into the gaping endlessness of space.