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Being sure of those two things, he can take his chart and mark upon it
the precise spot where his ship is at a given moment. Then he knows how
to steer, and all else that he needs to know in order to pursue his
course with confidence.
When John Harrison was a young man, the art of navigation had so far
advanced that the distance from the equator, or the latitude, could be
ascertained with certainty by observation of the heavenly bodies. One
great difficulty remained to be overcome--the finding of the longitude.
This was done imperfectly by means of a watch which kept Greenwich time
as near as possible. Every fine day the captain could ascertain by an
observation of the sun just when it was twelve o'clock. If, on looking
at this chronometer, he found that by Greenwich time it was quarter past
two, he could at once ascertain his distance from Greenwich, or in other
words, his longitude.
But the terrible question was, how near right is the chronometer? A
variation of a very few minutes would make a difference of more than a
hundred miles.


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