The doctor steered to westward as he rose steadily to a height of about ten miles. Then he fell with a long slant to the south-west. He was working back into the darkness of night again. We had lost the sun long before we started to rise again. "We are now well above the Pacific Ocean, about fifteen hundred miles north-west of San Francisco," said the doctor, consulting his large globe. "It seems to me you cross continents with remarkable ease and swiftness. From Chicago to San Francisco alone is almost three thousand miles," I ventured. "But we have been gone four hours, and if we had simply stood still above the Earth for four hours it would have travelled under us about four thousand miles, so that San Francisco would already have passed the place where we started." "Then one only needs to get off somewhere and remain still in order to make a trip around the World!" I exclaimed. "You are quite right, and travelling upon the Earth's surface is the most awkward method, because it is impossible to take advantage of the Earth's own rapid motion. Around the World in eighty days was once considered a remarkable feat, but if we were to travel steadily westward we should make the circuit in very much less than twenty-four hours. The motion of the Earth upon its axis is such an immense advantage that if we were only going from Chicago to London, the trip could be more easily and quickly made by going to the westward some twenty-one thousand miles, rather than going directly eastward less than four thousand miles. For going eastward we should have to travel a thousand miles an hour in order to keep up with the Earth. It is questionable whether we could make that speed tacking up and slanting down." "Then we shall have to follow the course of Empire, always westward!" I laughed. While we were talking thus, the whizzing and whistling of the wind, which had been at first very loud and hissing, had gradually died down. I looked at the barometer, and reported that there was scarcely three-eighths of an inch of mercury in the tube. "We are practically above the atmosphere, then," said the doctor, turning in all the batteries. He tried the rudder in the ether, and found it turned her when fully extended and turned rather hard over. "I tried to sleep this morning at Whiting to prepare for to-night's work," said the doctor presently; "but I find I am getting uncontrollably drowsy. Come up, and I will show you the course we most keep, and then I will lie down to get a little rest." I mounted to his compartment and gazed through the telescope at Mars, looking like a little, red baby-moon, floating in one side of the blue circle. "Keep him always in view, but in the edge of the field like that," said the doctor. "We must always steer a little to the right of him--that is, a little behind him."