In considering the economical side, account must be taken of the density of the population and the frequency of communications. It is important that the principal centers be connected by wide, direct, and easy roads following the course of the most usual commercial currents. Theoretically, the width of each street should be proportionate to the traffic upon it.
Historical considerations may sometimes rule, for due force must be allowed to existing conditions in the construction of each new street.
Sometimes cities are built so fast that we might say they are made up of sections constructed after a plan framed in advance. In such cases they generally present a regular disposition. Such a town is Carlsruhe, in Baden, where the avenues converge toward the Château, and of such character are most American cities. Generally, however, in old Europe, cities develop themselves slowly, and the laying out of their streets has been influenced by the circumstances in their history.
Open cities generally expand gradually by the building of houses along the roads toward neighboring towns. They thus take a kind of radiated or palmate form, quite favorable to facility of communication.
Fortified cities are developed in an intermittent way. After having been for a long time smothered within their walls, they end by breaking the barriers down and uniting with their suburbs; constructing elegant boulevards in the places formerly occupied by their fortifications. The city of Paris bears the marks of several changes of this kind. Generally the extension takes place alike in all directions, and the towns that have undergone such metamorphoses present a succession of concentric zones separated by circular boulevards. The central nucleus generally offers a close agglomeration of high houses parted by narrow, crooked, and everywhere crowded streets. Here public and private business is transacted, and the centers of trade and amusement, the public offices, and the churches, are established. Through this part also lie the usual ways of passage from one suburb to another. Most frequently the old town is traversed by a grand artery, the history of which goes back to the origin of the place, and which is usually busy with trade and much crowded. The population of the suburbs is generally less dense than that of the city proper; and the streets are wider, and the houses farther apart. The inhabitants of the suburbs are in the habit of going frequently to the center of the city, because it is the most populous part, and because the roads to the other suburbs lie through it. The principal streets of the suburbs also converge toward the center.
All the characteristics we have sketched may be found in the city of Vienna, where the ancient city, still called die Stadt—"the City," rests on a branch of the Danube in the north, and on the Wien in the southwest. The cathedral, situated in the center of the town, St. Stephen's Place, and the Graben, are still the points toward which the