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The Biggest Peril to Ships for Most of History

LONGITUDE

The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time

First, is everyone clear on the difference between longitude and latitude? In her eleganthistory, ''Longitude: The True Story of a Lone Genius Who Solved the GreatestScientific Problem of His Time,'' Dava Sobel writes that ''the trick for remembering thedifference'' is that ''the latitude lines, the parallels, really do stay parallel to each other asthey girdle the globe'' in a series of concentric circles that are smaller the farther theyare from the equator.

Whereas: ''The meridians of longitude go the other way: They loop from the North Pole

to the South and back again in great circles of the same size, so they all converge at theends of the earth.'' (Although Ms. Sobel never explains how to remember that it's thelatitudes and not the longitudes that are parallel, one trick is to associate the ''a'' soundsin latitude and parallel.)

But the real difference, as the she notes, is that the zero-degree parallel of latitude isfixed by nature and is the equator, while the zero-degree meridian of longitude is simplywherever science decides to place it (for more than 200 years at Greenwich, England).She concludes, ''This difference makes finding latitude child's play, and turns thedetermination of longitude, especially at sea, into an adult dilemma -- one that stumped

the wisest minds of the world for the better part of human history.'' In fact, so frustratingwas this problem that it came to be thought of as synonymous with impossibility.

The inability to solve the longitude problem for so long also had dire consequences. If aship didn't know how far to the east or west it had traveled, then it didn't know whereland was likely to be, and the unexpected contiguity of land had a way of causing shipsto sink with a frequent loss of human lives. Eventually, Ms. Sobel writes, a particularlydisastrous wreck at the Scilly Isles, near the southwestern tip of England, ''precipitatedthe famed Longitude Act of 1714, in which Parliament promised a prize of $:20,000 for asolution to the longitude problem'' (which would be millions of dollars today).

The answer lay in always knowing what time it was at an agreed-upon zero-meridian, aswell as aboard the ship (by setting the local clock to noon when the sun was directlyoverhead). The two clock times would then enable the mariner ''to convert the hourdifference into a geographical separation,'' Ms. Sobel writes, since one hour ofdifference in time equals 15 degrees of longitude separation.

The problem lay in keeping track of the starting-point time. No clocks existed that couldwithstand either the temperature changes or the motion of ships at sea. The heavens

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weren't yet mapped in enough detail to calculate a remote time from the position of themoon relative to the stars.

Even less promising were crackpot suggestions like taking along on shipboard awounded dog that would presumably yelp on cue whenever someone at home would

dip a cloth soaked with its blood in a solution of the miraculous ''powder of sympathy.''(The history of this procedure plays an important part in the plot of Umberto Eco's newnovel, ''The Island of the Day Before.'')

Ms. Sobel reveals in her opening chapter that the problem of longitude was eventuallysolved by one John Harrison, an unschooled woodworker who had the genius to inventa pendulum-free clock that required no oil and ''would carry the true time from the homeport, like an eternal flame, to any remote corner of the world.''

For a moment, the reader is disappointed that Ms. Sobel has given away the outcomeso quickly. But to feel so is to reckon without her remarkable ability to tell a story with

clarity and perfect pacing. The wonder of Harrison is that he seemed to come out ofnowhere, without education and without apprenticeship in clockmaking. Yet the clarity ofa mind that could dream up the escapement mechanism, among other novel clockparts, was not reflected in his ability to express himself.

As Ms. Sobel puts it, ''He wrote with the scrivener's equivalent of marbles in his mouth.''Describing his first encounter with a potential patron of his work, he wrote, ''Mr. Grahambegan as I thought very roughly with me, and the which had like to have occasioned meto become rough too; but however we got the ice broke . . . and indeed he became as atlast vastly surprised at the thoughts or methods I had taken.''

Others too were very rough with Harrison. Although his clocks functioned perfectly andmade it simple for sea captains to calculate longitude, the people awarding the prize,being astronomers, were increasingly biased toward a solution involving the lunardistance method, even though such calculations took over four hours to complete andwere highly vulnerable to error. As Ms. Sobel tells of Harrison's struggles, one wants tohiss the royal astronomers who went so far as to sabotage Harrison's clocks.

Eventually his son, William, appealed to King George III, who reportedly muttered underhis breath, ''These people have been cruelly treated,'' and said aloud to William, ''ByGod, Harrison, I will see you righted.'' But the recognition and money consequentlygranted Harrison by Parliament were not nearly as much proof of his success as thespreading use of his clocks, which eventually were mass produced and finally evolvedinto today's Swatch watch.

Ms. Sobel, a former science reporter for The New York Times, confesses in her sourcenotes that ''for a few months at the outset, I maintained the insane idea that I could writethis book without traveling to England and seeing the timekeepers firsthand.'' Eventuallyshe did visit the National Maritime Museum in Greenwich, where the four clocks thatJames Harrison constructed are exhibited.

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She writes, ''Coming face with these machines at last -- after having read countlessaccounts of their construction and trial, after having seen every detail of their insidesand outsides in still and moving pictures -- reduced me to tears.''

Such is the eloquence of this gem of a book that it makes you understand exactly how

she felt.