=The Effects of Metals on the Growth of Cities.=--The character of the industry of a region has much to do with the character of its manufactures. Thus, coal is absolutely essential to the manufacture of iron and steel; and, inasmuch as from two to eight tons of the former are necessary to manufacture a ton of steel, it is cheaper to ship the ore to a place to which coal can be cheaply brought. The coal-fields are responsible for the greater part of Pittsburg's population, and almost wholly for that of Scranton, Wilkesbarre, and many other Pennsylvania towns. Iron and coal are responsible, also, for many cities and towns in the vicinity of the Great Lakes. Birmingham, Salford, and Cardiff in Great Britain, Dortmund and Essen in Germany, and St. Etienne in France have resulted from the presence of coal and iron. In many instances man is a great factor in the establishment of a centre of population. Chicago would have been quite as well off in two or three other locations; its present location is the result of man's energy and is not likely to be changed. St. Louis might have been built at a dozen different places and would have fared just as well; the same is true of St. Paul, or of Indianapolis. Leavenworth at one time was a more promising city than Kansas City, but the building of an iron bridge over the Missouri River at the latter place gave it a start, and wide-awake men kept it in the lead. It has grown at the expense of Leavenworth and St. Joseph, neither one of which has become a commercial centre. Cairo, at the junction of the Mississippi and Ohio Rivers, has the geographical position for a great city; it waits for the man who can concentrate the commerce there. =Adjustment to Environment.=--San Francisco was wisely located at first, but its grain trade was more economically carried on at Karquinez Strait, while its oriental trade is gradually concentrating at Seattle. Philadelphia lost its commercial supremacy when the completion of the Erie Canal gave return cargoes to foreign vessels discharging at New York City. Oswego, N.Y., had the advantage of both harbor facilities and water-power, but Syracuse, with practically no advantages except those of leadership, has far outstripped it. Such instances of the readjustment of centres of population have been common in the past; they will also occur in the future. In nearly every case the readjustment results from economic causes, the opening of new lines of transportation, the lowering of the cost of the production of a commodity, the discovery of new economic processes--all these cause a disturbance of population, and the latter must readjust itself to new and changed conditions.