"I understand," said Fred. "We will have a quadrant or a sextant for ascertaining the position of the sun, just as a captain does at sea. But will the irregular line of the land serve us for a horizon, as the line between sea and sky serves the mariner?" "Certainly not," answered the Doctor, with a smile; "and to meet this difficulty we employ the artificial horizon." "How is it made?" one of the youths inquired. "It is a very simple affair," the Doctor answered; "it is nothing but a horizontal mirror, and is constructed in two or three ways. It may be an ordinary mirror or looking-glass, in a frame adjusted upon screws and set round with spirit-levels, so that it can be brought to the proper position, or it may be a basin of mercury. A tub of water may be made to answer in an emergency, but it is not easy to get a reflection from it of sufficient distinctness for purposes of observation. With the artificial horizon and a sextant the altitude of the sun or of a star may be readily obtained. Half the angular distance between a star and its image in the artificial horizon is equal to the altitude of a star above the real horizon." "But there's another trouble," said Frank. "At sea the navigator knows the run of his ship by means of the log, as we learned when we crossed the Pacific Ocean in our journey to Japan and China. How are we to 'throw the log' when travelling on land?" "That is an easy matter," was the reply. "We will have several pedometers, or instruments for counting the steps. They are about the size of an ordinary watch, and worn in the pocket in the same way. Every step taken by the wearer is registered, and by knowing the length of our steps we can get very near the distance travelled. The pedometer is only approximative, and not exact, and the same is the case with the log on a ship. [Illustration: DR. SCHWEINFURTH ASCENDING THE NILE.] "A famous African explorer, Dr. Schweinfurth, once had the misfortune to lose his instruments and all the records of his journey by fire. For six months after that calamity he counted his footsteps, noting hundreds by means of his fingers, and making a stroke in his note-book on reaching five hundred. The second five hundred was recorded by making a reverse stroke on the previous one, so as to form a cross, and in this way at the end of a day's journey every thousand steps he had taken was shown by a cross. He thus made account of a million and a quarter paces in the six months that he continued the practice.