His geography, such as we have described it, consists of eight books, and is certainly much more scientific than any which had been previously written on this science. In it there appears, for the first time, an application of geometrical principles to the construction of maps: the different projections of the sphere, and a distribution of the several places on the earth, according to their latitude and longitude. Geography was thus established on its proper principles, and intimately connected with astronomical observations and mathematical science. The utility and merit of Ptolemy's work seems to have been understood and acknowledged soon after it appeared. Agathemidorus, who lived not long after him, praises him for having reduced geography to a regular system; and adds, that he treats of every thing relating to it, not carelessly, or merely according to the ideas of his own, but to what had been delivered by more ancient authors, adopting from them whatever he found consonant to truth. Agathodæmon, an artist of Alexandria, observing the request in which his work was held, prepared a set of maps to illustrate it, in which all the places mentioned in it were laid down, with the latitudes and longitudes he assigned them. The reputation of his geography remained unshaken and undiminished during the middle ages, both in Arabia and Europe; and even now, the scientific language which he first employed, is constantly used, and the position of places ascertained by specifying their latitude and longitude.