From the necessity for symmetrical buildings arose the invention of the brick, which must have superseded the rude plastering of the hut with clay, to protect it against the sun or storm. In the history of the Semitic nations the brick appears among the earliest inventions, and its use can be traced with various modifications, from the building of the Tower of Babel to the present day. It is essential that bricks should be symmetrical, and their form is generally rectangular. Their geometrical shape affords us a clue to ancient units of measurement, and the various inscriptions with which they have been stamped have elevated them to the dignity of historical monuments. Thus the bricks of Egypt not only afford testimony, by their composition of straw and clay, that the writer of Exodus was acquainted with that country, but also, by the hieroglyphs impressed upon them, transmit the names of a series of kings, and testify to the existence of edifices, all knowledge of which, except for these relics, would have utterly perished. Those of Assyria and Babylon, in addition to the same information, have, by their cuneiform inscriptions, which mention the locality of the edifices for which they were made, afforded the means of tracing the sites of ancient Mesopotamia and Assyria with an accuracy unattainable by any other means. The Roman bricks have also borne their testimony to history. A large number of them present a series of the names of consuls of imperial Rome; while others show that the proud nobility of the eternal city partly derived their revenues from the kilns of their Campanian and Sabine estates. From the next step in the progress of the manufacture—namely, that of modelling in clay the forms of the physical world—arose the plastic art. Delicate as is the touch of the finger, which the clay seems to obey, almost as if comprehending the intention of the potter’s mind, yet certain forms and ornaments which require a finer point than the nail gave rise to the use of pieces of horn, wood, and metal, and thus contributed to the invention of tools. But modelling in clay was soon superseded by sculpture in stone and metal, and at length only answered two subordinate ends: that of enabling the sculptor to elaborate his first conceptions in a material which could be modified at will; and that of readily producing works of a small and inexpensive form, for some transitory purpose. The invention of the mould carried this last application to perfection, and the terracottas of antiquity were as numerous and as cheap as the plaster casts now sold by itinerants.