George Graham

Halley is no clockmaker but likes Harrison's idea.

He suggests he meet with one of London's most prestigious clockmakers, George Graham, who already supplies astronomical instruments and clocks to the Royal Observatory.

John goes to see George and stays all day at George's workshop, chatting about clocks and clockwork, and he stays for dinner, and he leaves with an interest-free loan from George, which is very kind of him.

And so he goes back up north, back to Barrow-upon-Humber where he's living, and he and his brother James start working on what's known as H1, Harrison's first sea clock.

John and James Harrison begin to build H1 in 1731, and four years later they are ready to demonstrate it The brothers transport the timepiece down to London and show it in George Graham's workshop to much attention and praise.

The real question though is whether it can remain accurate at sea, and so H1 is sent on a trial voyage to Lisbon in 1736.

It performs well, keeping time accurately enough to correct the ship's longitude during the journey.