This British Admiralty sea chart of the coast of Peru is especially exciting for a couple of reasons. First of all, the original survey work that forms the foundation of the cartography was conducted on the H.M.S. Beagle, the ship on which Charles Darwin sailed. The chart stretches from Independencia Bay in the south to Begueta Bay in the north, with the Chincha Islands in the middle, outside of Pisco Bay. These islands, which contained tremendous deposits of seabird guano, which was mined for fertilizer, are about 120 miles south-southeast of Lima.
Another interested feature unique to this map are a series of pencil notations dated 1876 which show it was actually used at sea. The notations plot time and location, both in the sea and along a series of coastline elevations at the bottom of the chart. The plotting was likely carried out by means of a sextant, a navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celestial navigation. The estimation of this angle, the altitude, is known as sighting or shooting the object, or taking a sight.