When the wings had been taken off him and laid on the ground I examined that queer "carapace" at the hinges, but could come to no intelligent conclusion about it. Two thin bars of metal the colour of aluminium but, I imagine, much tougher, projected from the shallow carvel-sided box and were hinged to the main ribs of the wings. It was through these the power came, obviously, but whence that power arose in the first place-- I turned to Kalliboas with an inquiry on my lips, but the look of him froze me to silence. 'Come, let us go,' he said peremptorily, and led us back across the plain. I was vividly conscious, as I had been conscious before, of something in his manner warning me that it would not only be useless but unwise to press him further. Clearly that strange force was a secret we weren't to be allowed to penetrate. The visit had one profitable sequel, however. Both Poyning and I extracted a promise from Kalliboas that we should learn to fly ourselves, and learn we did, making a beginning the very next day. But our experience in the air, and the queer consequences that arose from it, I reserve for a later chapter. Such, then, was the mystery of the gigantic birds that had been seen in the air over this land of secrets and possibly far away from it, for it seemed likely from what Kalliboas told us that at one time and another scouts had been sent beyond the confines of the valley. I fancied then that this might account for the knowledge these people possessed of outside countries, but I know now that they had other and stranger resources. We came in time to a neighbourhood near the middle of the city, which we hadn't yet visited, and here Kalliboas led us into a great walled courtyard, circular in shape, its centre an obelisk rising high out of the marble floor. The monument was beautifully carven from top to bottom in Greek character, so freshly preserved that I believe we could have read it without the help of Kalliboas. The old man was looking at the inscription with an expression of the profoundest reverence. 'On this stone, strangers,' he said, 'is recorded how our people first came to the valley. It would take long to repeat all the tale as it is written, so I will tell it you in my own words, more briefly. You are perhaps familiar with the name of the great hero of old, Alexander of Macedon?' 'He is known to us as the great Alexander,' replied Poyning. 'Mankind called him great, and with reason, for assuredly he was the greatest man this world has produced. To us of the valley his memory is a sacred thing. He was the founder of our race.' Poyning's face was blank with incredulity. 'What you say is new to us, Kalliboas,' he said. 'History as we know it records that Alexander came far into the East, but never so far as this.' 'There is much in the life of our founder which your history does not tell, I believe,' returned the old man, with a shade of irony. 'What is the account your historians give you of his death?'