 His third and final voyage lasted from 1776 to 1779.On 16 February 1768 the Royal Society petitioned King George III to finance a scientific expedition to the Pacific to study and observe the 1769 transit of Venus across the sun to enable the measurement of the distance from the Earth to the Sun.Royal approval was granted for the expedition, and the Admiralty elected to combine the scientific voyage with a confidential mission to search the south Pacific for signs of the postulated continent "Terra Australis Incognita" (or "unknown southern land").