The final masterpiece of the world's greatest clockmaker is to be put through its paces at last, 230 years after it was finished, to see if it fulfils its maker's specifications.

The priceless Late Regulator clock took John Harrison, the pioneer of longitude, 36 years to build and he was still calibrating it when he died at his home in London on March 24, 1776, his 83rd birthday.

Harrison believed that the Late Regulator would vary by only a second every 100 days and a trial has started at the Royal Observatory, Greenwich, to see if it was capable of the feat, which was first managed by an electro-mechanical timepiece in the 1920s.