19th century illustration of a solar chronometer or heliochronometer. The Heliochronometer was designed to enable accurate calculation of the time to a fraction of a minute. Each heliochronometer was adjusted at the factory to the purchaser's latitude and longitude. The instrument was promoted as being sold to 'His Majesty the King, Royal Naval College, the Meteorological Office, and the Best Families in the United Kingdom, India, Colonies, etc.' Production ceased with the advent of World War I. The instrument was promoted as being sold to 'His Majesty the King, Royal Naval College, the Meteorological Office, and the Best Families in the United Kingdom, India, Colonies, etc.' Production ceased with the advent of World War I in 1914. The upper disc is rotated until sunlight through holes in upper arm falls on line on the lower arm. Time is shown by an arrow at the side.