instead he brooded on strategy , and when the urge grew more difficult to bear , he went away and curled himself tightly with his head against his flank , beneath the dark huddle of his wing , and murmured to himself from the principia mathematica , which laurence had read to him so often he had it all by heart , and if he spoke low , and flattened his voice , he might almost imagine he heard laurence instead , reading to him in the rain , safe and sheltered beside him .