[2] Strictly speaking, the base is not an exact square, the four sides measuring, according to the Royal Engineers, north, 760 ft. 7·5 in.; south, 761 ft. 8·5 in.; east, 760 ft. 9·5 in.; and west, 764 ft. 1 in. [3] Conventionalising may be described as representing a part only of the visible qualities or features of an object, omitting the remainder or very slightly indicating them. A black silhouette portrait is an extreme instance of convention, as it displays absolutely nothing but the outline of a profile. For decorative purposes it is almost always necessary to conventionalise to a greater or less extent whatever is represented. [Illustration: FIG. 31.--SCULPTURED ORNAMENT AT NINEVEH.] CHAPTER III. WEST ASIATIC ARCHITECTURE. The architectural styles of the ancient nations which ruled over the countries of Western Asia watered by the Tigris and the Euphrates, from a period about 2200 B.C. down to 330 B.C., are so intimately connected one with another, and so dependent one upon the other, that it is almost impossible to attempt an accurate discrimination between the Babylonian, or ancient Chaldæan, the Assyrian and the Persian. A more intelligible idea of the architecture of this long period will be gained by regarding the three styles as modifications and developments of one original style, than by endeavouring to separate them.[4] Their sequence can, however, be accurately determined. First comes the old Chaldæan period, next the Assyrian, during which the great city of Nineveh was built, and finally the Persian, after Cyrus had subdued the older monarchies; and remains exist of all these periods. As to the origin of the Chaldæan Kingdom, however, all is obscure; and the earliest date which can be fixed with the slightest approach to probability is 2234 B.C., when Nimrod is supposed to have founded the old Chaldæan dynasty. This seems to have lasted about 700 years, and was then overthrown by a conquering nation of which no record or even tradition remains, the next two and a half centuries being a complete blank till the rise of the great Assyrian Monarchy about 1290 B.C., which lasted till its destruction by Cyrus about 538 B.C. The Persian Monarchy then endured till the death of Alexander the Great, in 333 B.C., after which great confusion arose, the empire being broken up among his generals and rapidly falling to pieces.