By the end of the seventeenth century the general outlines of the coast of Asia were known, though much of the interior was still unexplored. Such knowledge of central Asia as was acquired during that and the preceding century was mainly due to the travels of European missionaries following Andrada--Fra Desideri and Fra Freyre (1715), and Orazio della Penna (who was in Lhasa from 1735 to 1747), were among those who entered Tibet, while others actually carried out surveys in China in 1708–18 under the direction of the Emperor Kang-hi. The Dutchman Samuel van der Putte passed thirty-seven years of his life in Asia, and travelled widely, but at his death (1745) left no narrative. At the close of the century English missions began to make their way into Tibet from India. George Boyle led one in 1774, and Captain Turner another in 1783. English and French traders were opening up Persia in the eighteenth century, and in Arabia the principal journey was that of Carsten Niebuhr (1762–67), sole survivor of a Danish scientific mission. CHAPTER VIII. POLAR EXPLORATION TO THE EIGHTEENTH CENTURY _(a) Arctic: The North-East and North-West Passages._ Although polar exploration is not very directly associated with geographical theory at large, it has been associated with certain individual geographical theories which occupy important positions in our history as having held the minds of men for long periods, and as owing their proof or disproof to some of the most noteworthy exploits in the story of exploration. There is sometimes a tendency to suppose that polar, or at any rate arctic, exploration has always been concerned mainly with the attempt to penetrate as far north as possible along one meridian or another, and that any discoveries made _en route_ have been merely incidental. But the mere desire to set a more northerly or southerly limit to human travel, and ultimately to reach the Poles, really belongs to a relatively late period in the history of arctic and antarctic work. Taking arctic exploration first, we find that its object was in its early stages mainly commercial; from that object there naturally developed a desire to extend geographical knowledge, and, lastly, the extension of many branches of scientific knowledge was served by that particular branch of exploratory work.