Watson determined, for the Royal Society, the velocity of an electric current and found it practically instantaneous. These, and numerous lesser, discoveries did for electricity what the chemical discoveries of Priestley, Cavendish, Scheele, Boyle, Lavoisier, and others had done for chemistry. The numerous voyages of discovery in the eighteenth century helped to develop the geographical sciences. Special expeditions were fitted out for the acquirement of geographical knowledge without any thought of trading profits. The Jesuits carried out a valuable survey of China and Mongolia early in the century. A Danish scientific expedition studied Arabia, the results of which were published by Niebuhr in 1772. James Bruce visited Abyssinia with the view of solving the ancient problem of the source of the Nile. Mungo Park studied the course of the Niger. Captain James Cook led a scientific expedition to Tahiti with the object of making astronomical observations. This resulted in one of the greatest and most valuable voyages of discovery in history. Cook determined the westernmost point of America in 1778 and his accounts of Bering Sea and Alaska revived interest in the Polar seas, which resulted in numerous Arctic and Antarctic expeditions yielding rich scientific returns. The Hudson's Bay Company sent out many investigators to determine the characteristics and resources of Arctic America. The Russians did the same for their own northern lands. These activities of geographical investigators led to improved methods of navigation, nautical surveying, sounding and shipbuilding, besides supplying an enormous amount of scientific data. The British naval authorities pointed out to King Charles II the need for correct nautical tables. Flamsteed, one of the leading astronomers of the day, was appointed Astronomer Royal in 1675, with the definite object of producing a new catalogue of star positions, tide tables, and other nautical data. He immediately founded the Greenwich observatory, which has supplied the world with data for the navigator. Bradley, a successor of Flamsteed at Greenwich, made many important astronomical discoveries while carrying on the star maps. He discovered the aberration of light and the mutation of the earth's axis. Locaille studied the parallax of the sun and made numerous stellar observations at the Cape of Good Hope in 1751. He located the positions of 10,000 stars in the southern hemisphere. Measurements were made in Peru, Lapland, and elsewhere to discover data regarding the earth's curvature. Pendulum observations to detect variations of gravity were made in many countries. Maskelyne, the astronomer royal, made observations on the transit of Venus at St. Helena in 1761. On this expedition he perfected the method of finding longitude at sea by lunar distances.