Dorothy Marley's Journal

Sobel, Dava. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York: Walker and Co, 1995. (Also available in an Illustrated Edition, which I recommend. The A&E network also made a very good four-hour movie based on the book, which you might be able to find for rent or at a library.)

This lovely little book chronicles the story of John Harrison, the man who finally created the first reliable marine timekeeper, and thus solved the problem of finding longitude at sea. Along the way, Sobel gives a concise yet detailed picture of the "longitude problem" as it stood in the 1700s, managing to convey the enormity of the issue, and the effect it had on the lives of every sailor who braved the seas. Very good for getting a feel for the methods of navigation that weren't possible in the 1720s.

( The Cliff Notes Version )