15. Sailing thence, Libya lies to the south. Its most western portions project a little beyond Gades; it afterwards forms a narrow promontory receding towards the east and south, and becoming slightly broader, till it touches upon the western Ethiopians, who are the last[750] of the nations situated below Carthage, and adjoin the parallel of the Cinnamon Country. They, on the contrary, who sail from the Sacred Promontory,[751] towards the Artabri,[752] journey northwards, having Lusitania[753] on the right hand. The remaining portion forms an obtuse angle towards the east as far as the extremities of the Pyrenees which terminate at the ocean. Northward and opposite to this are the western coasts of Britain. Northward and opposite to the Artabri are the islands denominated Cassiterides,[754] situated in the high seas, but under nearly the same latitude as Britain. From this it appears to what a degree the extremities of the habitable earth are narrowed by the surrounding sea. 16. Such being the configuration of the whole earth, it will be convenient to take two straight lines, cutting each other at right angles, and running the one through its greatest length, and the other through its breadth. The former of these lines will represent one of the parallels, and the latter one of the meridians.[755] Afterwards we must imagine other lines parallel to either of these respectively, and dividing both the land and sea with which we are acquainted. By this means the form of the habitable earth will appear more clearly to be such as we have described it; likewise the extent of the various lines, whether traced through its length or breadth, and the latitudes [of places], will also be more clearly distinguished, whether north or south, as also [the longitudes] whether east or west. However, these right lines should be drawn through places that are known. Two have already been thus fixed upon, I mean the two middle [lines] running through its length and breadth, which have been already explained, and by means of these the others may easily be determined. These lines will serve us as marks to distinguish countries situated under the same parallel, and otherwise to determine different positions both in respect to the other portions of the earth, and also of the celestial appearances. 17. The ocean it is which principally divides the earth into various countries, and moulds its form. It creates bays, seas, straits, isthmuses, peninsulas, and capes; while rivers and mountains serve to the same purpose. It is by these means that continents, nations, and the position of cities are capable of being clearly distinguished, together with those various other details of which a chorographical chart is full. Amongst these latter are the multitude of islands scattered throughout the seas, and along every coast; each of them distinguished by some good or bad quality, by certain advantages or disadvantages, due either to nature or to art.