Downwards into the valley, which grew ever warmer and more fertile, the Atlantean led on, explaining a thousand and one details to the astounded aviator. Presently they approached the nearest of the great stone structures and Nelson received yet another shock. In a courtyard was drilling what would correspond to a troop of cavalry in the outer world. In orderly ranks the troopers wheeled, marched and counter-marched, their brazen armor twinkling and clashing softly as they carried out their evolutions with an amazing precision. But what astonished Nelson was the fact that each of these strange troopers bestrode a lithe, long-limbed variety of dinosaur, a good half smaller than the allosauri he had encountered in the tunnel. These agile creatures ran about on their hind legs with astonishing speed, using a long reptilian tail as a balance. On the back of each trooper was fastened a compact circular copper tank, from which sprouted a flexible metal hose that ended in what looked like a ponderous type of pistol. In distinction to the red of the artillerymen and the blue of the Hoplites, these curious cavalrymen wore brilliant crests of yellow feathers, and from their lance tips fluttered tiny pennons of that same color. "They must travel at least as fast as a race horse," decided the aviator after studying the swift evolutions of the scaly chargers. To his ears came the curious dry scrape and rattle of their horny claws on the stone pavement of the drill yard. He would have lingered to see more, for those grotesque, lizard-like chargers interested him immensely, but Hero Giles beckoned imperiously. So, dropping the Winchester to the hollow of his arm, Nelson followed him into the brilliantly gas-lit depths of the great structure. * * * * * Everywhere were red bearded, white skinned soldiers, staring at him with the frank curiosity of children. Powerful, magnificently built fellows they were, all in uniforms of different designs. The walls about him, Nelson noticed, were covered with really beautiful friezes depicting various warlike scenes in that pure beauty of proportion found only in ancient Grecian temples. On and on through resounding tunnels, past busy markets and barracks, hurried the two travelers. Then the Atlantean halted before a gracefully arched doorway where stood two hoplites, who immediately lowered spears to bar the passage. At a word from Hero Giles, however, they saluted and fell back in position--immovable, grim guardians. Inside was a short staircase, beautifully wrought of bronze. Up this flashed the Atlantean's mail-clad body; then he came to a halt under the direct rays of a blinding light. Nelson, on arriving above, discovered that the chamber was lined with jointless brass about ten feet high and circular in shape. "What's this?" he demanded curiously. "The terminal of the tube-road. In a moment thou shalt see the great cylinder arrive."