It is made of brass and is around the size of a large pocket watch, with a normal clock dial showing roman numerals and a smaller dial at the bottom for the second hand. the clock is suspended inside a swivelling brass ring fitted to the inside of a wooden box – this is the key to keeping the chronometer level even in an unsteady ship. –a history of the world in 100 objects, neil macgregor write two to three sentences explaining how the image reinforces the text.