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Book, page 112 / 153 open sea to the depth the Pandora is supposed to lie," Tom said. "But, with all that, we have the advantage of being on the ground, and I'm going to make use of that advantage right away." He gave orders at once for the M. N. 1 to proceed, and this she did on the surface. It was decided to steam along on the open sea until the exact nautical position desired was reached. This position was the same Mr. Hardley had indicated, but that position was not before attained, owing to an error in the calculations. As all know, to get to a certain point on the surface of the ocean, where there is no land to give location, a navigator has to depend on mathematical calculations. The earth's surface is divided by imaginary lines. The lines drawn from the north to the south poles are called meridians of longitude. They are marked in degrees, and indicate distance east or west of the meridian of, say, Greenwich, England, which is taken as one of the centers. The degrees are further divided into minutes and seconds, each minute being a sixtieth of a degree and each second, naturally, the sixtieth of a minute. Now, if a navigator had to depend only on the meridian lines indicating distance east and west, he might be almost any distance north or south of where he wanted to go. So the earth is further divided into sections by other imaginary lines called parallels of latitude. As all know, these indicate the distance north or south of the middle line, or the equator. The equator goes around the earth at the middle, so to speak, running from east to west, or from west to east, according as it is looked at. The meridian of Greenwich may be regarded as a sort of half equator, running half way around the earth in exactly the opposite direction, or from north to south. The place where any two of these imaginary lines, crossing at right angles, meet may be exactly determined by the science of navigation. It is a complicated and difficult science, but by calculating the distance of the sun above the horizon, sometimes by views of stars, by knowing the speed of the ship, and by having the exact astronomical time at hand, shown on an accurate
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