Read the excerpt from Ship's Chronometer from HMS Beagle. 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. The author uses to explain the importance of the chronometer and help the reader visualize it.