At first he only imagined there was somebody after him because of the curious vibrations of the very rarefied air in which he flew. Every time his flight slackened and the noise of his own wings grew less, there reached him from some other corner of the sky a sound like the vibrations of large wings beating the air. It seemed behind, and generally below him, but the swishing of his own feathers made it difficult to hear with distinctness, or to be certain of the direction. Evidently it was a long way off; but now and again, when he took a spurt and then sailed silently for several minutes on outstretched wings, the beating of distant, following feathers seemed unmistakably clear, and he raced on again at full speed more than terrified. Other times, however, when he tried to listen, there was no trace of this other flyer, and then his fear would disappear, and he would persuade himself that it had been imagination. So much on these flights he knew to be imagination--the sentences, voices, and laughter, for instance, that filled the air and sounded so real, yet were actually caused by the wind rushing past his ears, the rhythm of the wing-beats, and the tips of the feathers occasionally rubbing against the sides of his body. But at last one night the suspicion that he was followed became a certainty. He was flying far up in the sky, passing over some big city, when the sound rose to his ears, and he paused, sailing on stretched wings, to listen. Looking down into the immense space below, he saw, plainly outlined against the luminous patch above the city, the form of a large flying creature moving by with rapid strokes. The pulsations of its great wings made the air tremble so that he both heard and felt them. It may have been that the vapours of the city distorted the thing, just as the earth's atmosphere magnifies the rising or setting of the moon; but, even so, it was easy to see that it was something a good deal larger than himself, and with a much more powerful flight. Fortunately, it did not seem this time to be actually on his trail, for it swept by at a great pace, and was soon lost in the darkness far ahead. Perhaps it was only searching for him, and his great height had proved his safety. But in any case he was exceedingly terrified, and at once turned round, pointed his head for the earth, and shot downwards in the direction of the Empty House as fast as ever he could. But when he spoke to the governess she made light of it, and told him there was nothing to be afraid of. It might have been a flock of hurrying night-birds, she said, or an owl distorted by the city's light, or even his own reflection magnified in water. Anyhow, she felt sure it was not chasing him, and he need pay no attention to it.