The and dissemination of llolemy's works in Ow century caused a great improvement in maps and charts. and the subse quent labors of Coperniens and Nlereator brought them to a very satisfactory state, considering the lack 1 if information. Mercator (1512 94) engraved an extensive atlas. Edward Wright. who published in 1.599 a work entitled rmin Preors in Detcetcd and car r,ct«I. gives a complete exposition of the proper method of constructing a :?lereator's projection, together with other information of value. In 101:11 is known as Alcreator's projection, meridians are straight lines parallel to each other, :toil are (Tossed by the parallels of latitude at right angles. In order to preserve the proportion that exists in the eon's] surface of the earth and render the track of the ship I 10\1,1111mile eurve) a straight line on the chart, the meridians are expanded or inereased in length. The ang 'rated latitudes are eompnted and published in a table of 'meridional parts' by menus of which the parallels of latitude are placed. Mercator charts of great extent the areas of portions differing much in latitude arc greatly out of proportion, but contiguous areas differing little in latitude compare fairly well.
The rude character of the early instruments used in navigation prevented close determination of a ship's position at sea. Watches were un known until about 1530, when Genoa Frisins hn mediately recognized the possibility of using them for the purpose of ascertaining the differ ence of longitude between two places. These early timepieces were very defective, however, and it was not until 1765 that Harrison was awardell £10.000 the British Government for his chronometer, a second /10.000 which lie re ceived being paid in 1773 after further trials of h is invent ion.
The date of the first attempts at determining latitude by measuring the altitude of heavenly bodies (at first, the fixed stars) is unknown, but Columbus is said to have had a eross•staff (see SEXTANT) for this purpose. Somewhat later the astrolabe was devised. In 1.594 the celebrated
navigator 1)avies published in his pamphlet entitled The Seaman's Secrets a mnd fica t ion of the cross-staff. In using this instrument the observer stood with his back to the sun look ing at the horizon through a sight at the end of the stall% while the shadow of a movable projec tion fell on the sight box. In 1729 Pierre Bougeur devised au improved form of Davies's instru ment. This was followed almost immediately by the invention of the quadrant by Hadley in Eng land. who described his instrument in a paper to the Royal Society in May 1731, and by Thomas Godfrey of Philadelphia, who made an instru ment as early as 1730 that was used at sea in August, 1731.
The earliest 'work devoted specially to !VIVI gation was that of Pedro de Medina. pub lished at Valladolid in 1545. In 1530 Gemma Prising wrote upon astronomy, cosmogony, and the use of globes. in 15:17 Pedro Nuilez(Xonius; , cosmographer to the King of Portugal. published a work on astronomy, charts, and some parts of navigation. It) 1556 Martin Cortes pohlishell at Seville The Art of Narigation. Davies. in The t fi 's Secrets, defines three kinds of sailings—horizontal. paradoxical. and great ei rele. The paradoxical embraced latitude, longitude, and distance. and combined NOLO are now called traverse anti Nereator's sailings. He also gave a clear description of great-circle sail ing and the proper method of accomplishing it.
Among the developments in navigation in the nineteenth century were the patent log. the Thomson sounding machine. the Thomson com pass. the Ritchie liquid compass. compensation of the devinliOnA of the compass, and the great circle sailing chart. Almost all the old ap pliances were mueli improved, and steam propul sion has greatly simplified navigating problems. Charts (q.v.) are undergoing constant correction and are, considering all things. wonderfully tie eurate. but much ean yet lie done and is being done. The most serious enemies of the naviga tor of the present day are fogs and irregular cur rents.