H4...

Although pocket-watches were known in John Harrison's time, they were large and expensive, and not remotely capable of keeping time well enough for sea navigation. Harrison's earlier efforts (H1, H2, and H3) were variations on much larger clockworks, but the 3-pound, 5-inch-diameter H4 was truly revolutionary, capable of timekeeping on par with the finest observatory-class clocks of the day even while underway. Capt. James Cook used a copy of Harrison's H4 on his travels.