Bk. IV. Ch. I. THE ARCH. 291 called the Egyptian form, being the first step towardc the true arch) are combined with a substructure of horizontal converging masonry. In either of these in- stances the horizontal arch is a legitimate mode of construction, and may have been used long after the principle of the radiat- ing arch was known. The great convenience of the latter, as enabling large spaces to be spanned even v.'ith brick or the smallest stones, and thus dispens- ing with the necessity for stones of very large di- mensions, led ultimately to its universal adoption. Subsequently, when the pointed form of the radiating arch was introduced, no motive remained for the retention of the horizontal method, and it was entirely abandoned. 178. Aqueduct at Tusculum.
