It is this horizontal or bracket mode of construction that is the formative principle of the Dravidian or Southern style of Hindu architecture, every form and every ornament depending almost wholly upon it. In the north, however, another development of the same principle is found in the horizontal dome, which is unknown in the south, but which has given a new character to the style, and, as one of its most beautiful features, demands a somewhat detailed explanation. DOMES. It is to be regretted that, while so much has been written on the history of the pointed arch, so little should have been said regarding the history of domes: the one being a mere constructive peculiarity that might very well have been dispensed with; the other being the noblest feature in the styles in which it prevails, and perhaps the most important acquisition with which science has enriched the art of architecture. The so-called Treasuries of Mycenæ and Orchomenos, as well as the chambers in Etruscan tombs, prove that as early as ten or twelve centuries before Christ the Pelasgic races had learned the art of roofing circular chambers with stone vaults, not constructed, as we construct them, with radiating vaults, on the principle of the common arch, but by successive layers of stones converging to a point, and closed by one large stone at the apex. Whoever invented the true or radiating arch, the Romans were the first who applied it as a regular and essential architectural feature, and who at the same time introduced its complement, the radiating dome, into architectural construction; at what period it is not now known. The earliest example, the Pantheon, is also the finest and largest; but we have lost entirely the innumerable steps by which the architects must have slowly progressed to so daring an experiment. There is, however, a vast difference between these two classes of domes, which it is necessary to bear in mind in order to understand what follows. The Roman arch and Roman dome are always constructed (Woodcut No. 110) on the principle of voussoirs, or truncated wedges, radiating from a centre. This enabled the Romans to cover much larger spaces with their domes than perhaps was possible on the horizontal principle; but it involved the inconvenience of great lateral thrusts, continually tending to split the dome and tear the building in pieces, and requiring immense and massive abutments to counteract their destructive energy. [Illustration: 110. Radiating Arch.] [Illustration: 111. Horizontal Arch.]