In 1772 they thought there was a great continent at the bottom of the world, that people could live on.

Look at this old map.

The British Admiralty (which runs the Navy) has asked Captain Cook to find the Great Southern continent, and claim it for England.

He has another task too. Before, it was hard to tell how far East or West you were. You had to measure the positions of the sun, moon and stars and this was not always easy.

But now Captain Cook has a new sea clock to help him find his longitude exactly. It is a copy of the famous clock H4, made by John Harrison.