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
chronometer, the exact position of a ship at any hour may be
determined.
By this means, if a navigator wants to get to a place where two
certain lines cross, indicating an exact spot in the ocean, he is
able to do so. He can tell for instance when he has reached the
place where the seventy-second degree of longitude, west from
Greenwich, meets and crossed the twentieth parallel of latitude.
This spot is just off the northern coast of Haiti. Other
positions are likewise determined.
It was after about an hour of rather slow progress on the
surface of the calm sea, no excess speed being used for fear of
over-running the mark, that Tom and his associates gathered on
deck again to make another calculation.
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