Here is Amazon.ca's description of the book by Dava Sobel called Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time.
Anyone alive in the eighteenth century would have known that "the longitude problem" was the thorniest scientific dilemma of the day-and had been for centuries. Lacking the ability to measure their longitude, sailors throughout the great ages of exploration had been literally lost at sea as soon as they lost sight of land. Thousands of lives and the increasing fortunes of nations hung on a resolution.
One man, John Harrison, in complete opposition to the scientific community, dared to imagine a mechanical solution-a clock that would keep precise time at sea, something no clock had ever been able to do on land.
Longitude is the dramatic human story of an epic scientific quest and of Harrison's forty-year obsession with building his perfect timekeeper, known today as the chronometer. Full of heroism and chicanery, it is also a fascinating brief history of astronomy, navigation, and clockmaking, and opens a new window on our world.
It is difficult to overstate the importance of John Harrison's 4 clocks, named H1, H2, H3, and H4 in the history of navigation. H4, building on the things learned in the construction of H1, H2, and H3, finally solved the problem of longitude.
So for me, getting to see the actual four chronometers of John Harrison was moving. I really felt like I was looking at a key piece of history.
Ironically, Harrison discovered, a pocketwatch design was more likely to be accurate onboard ship than one of the larger clocks. H4 was his final effort. It took 6 years to build...and finally solved the longitude problem.
The challenges of keeping time at sea included not only dealing with the rocking, rolling, and pounding of the vessel, but in dealing with temperature changes. Part of the solution to the latter problem was to create a key component as bimetallic, made of both steel and brass... so that the different rates of expansion/contraction of the metals would automatically compensate for temperature changes.
H4: Both the face of the watch, and also the inner workings
H4: Closeup of the inner workings