With our directional compass set for Duhor and our motor functioning in silent perfection there were no navigational problems to occupy our time. Barring some unforeseen emergency, the ship would fly in an air line to Duhor and stop above the city. Our sensitive altimeter was set to maintain an altitude of 300 ads (approximately 3000 feet), with a safety minimum of 50 ads. In other words, the ship would normally maintain an altitude of 300 ads above sea level, but in passing over mountainous country it was assured a clearance of not less than 50 ads (about 490 feet) by a delicate device that actuates the controls as the ship approaches any elevation of the land surface that is less than 50 ads beneath its keel. I think I may best describe this mechanism by asking you to imagine a self-focusing camera which may be set for any distance, beyond which it is always in focus. When it approaches an object within less distance than that for which it has been adjusted it automatically corrects the focus. It is this change that actuates the controls of the ship, causing it to rise until the fixed focus is again achieved. So sensitive is this instrument that it functions as accurately by starlight as by the brightest sunlight. Only in utter darkness would it fail to operate; but even this single limitation is overcome, on the rare occasions that the Martian sky is entirely overcast by clouds, through the medium of a small beam of light which is directed downward from the keel of the ship.

Secure in our belief in the infallibility of our directional compass, we relaxed our vigilance and dozed throughout the night. I have no excuses to offer, nor did John Carter upbraid me; for, as he was prompt to admit, the fault was as much his as mine. As a matter of fact, he took all the blame, saying that the responsibility was wholly his.