ARCHITECTURE, "the most useful of the fine arts and the finest of the useful arts," may be defined as the art of ornamental construction ; not ornamental in the sense of decorated but in harmony of distribution of mass, in beauty of proportion. For all its vast variety, it is based on three simple constructive principles.

The first is the lintel, in which two uprights support a crosspiece, the form seen in the majestic temples of Egypt and the classic beauty of the art of Greece.

The second is the arch or vault, the use of which was perfected by the Romans and which is characteristic of Roman architecture and its derivatives for the spanning of large spaces.

The third principle is the truss or compound beam, made up of several subordinate members, each one of which is intended to resist a particular stress, which is seen in its highest development in the modern steel bridge.

These three principles have guided the development of the art in every land, but their incorporation in the typical architecture of each nation has been influenced by the three great formative factors of race, climate and religion.

These factors have conjoined to produce certain well defined styles, or varied phases of the art, which are commonly taken as the basis for architectural classification. The architecture of Egypt is the oldest now extant, and with it the story of the art in this little book opens. Almost contemporaneously with Egyptian architecture another style arose in Mesopotamia, and both of them influenced the architecture of the Greeks.

The Romans in turn applied the Greek details to the arched construction of the Etruscans. After the division of the Roman Empire the Byzantine school arose in the East and inspired the Mohammedan or Saracenic style, which swept westward through northern Africa into Spain and eastward into India.

In northern and western Europe, meanwhile, an evolution from the Roman basilica culminated in the Gothic style of the Middle Ages.

During the fifteenth and sixteenth centuries the Gothic gradually gave place to the Renaissance style, a revival of Roman forms. Other styles arose in India, China and Japan, and among the Aztecs and Incas of America, but they have had little influence on the modern electric architecture of the West, which is based almost entirely on the styles which had their origin in Europe.