[Illustration: FIG. 42.--Temple in a Royal Park, Kouyundjik; from the British Museum.] That the Chaldaean architects were early led to the invention of the arch is easily understood. They were unable to support the upper parts of their walls, their ceilings or their roofs, upon beams of stone or timber, and they had to devise some other means of arriving at the desired result. This means was not matured all at once. With most peoples the first stage consisted probably in those corbels or off-sets by which the width of the space to be covered was reduced course by course, till a junction was effected at the top; and sometimes this early stage may have been dispensed with. In some cases, the workman who had to cover a narrow void with small units of construction may, in trying them in various positions and combinations, have hit upon the real principle of the arch. This principle must everywhere have been discovered more or less accidentally; in one place the accident may have come sooner than in another, and here it may have been turned to more profit than there. We shall have to describe and explain these differences at each stage of our journey through the art history of antiquity, but we may at once state the general law that our studies and comparisons will bring to light. The arch was soonest discovered and most invariably employed by those builders who found themselves condemned, by the geological formation of their country, to the employment of the smallest units. The Chaldaeans were among those builders, and they made frequent use of the arch. They built no long arcades with piers or columns for supports, like those of the Romans, and that simply because such structures would have been contrary to the general principles of their architecture. They made no use, as we have already explained, of those isolated supports whose employment resulted in the hypostyle halls of Egypt and Persia, in the naves of Greek temples and Latin basilicas. The want of stone put any such arrangement out of the question. We have, then, no reason to believe that their arches ever rested upon piers or upon the solid parts of walls freely pierced for the admission of light. The type from which the modern east has evolved so many fine mosques and churches was unknown in Chaldaea. In every building of which we possess either the remains or the figured representation the archivolts rest upon thick and solid walls.