An important factor in this demoralization of architectural design has been the development of new constructive methods, especially in the use of iron and steel. It has been impossible for modern designers, in their treatment of style, to keep pace with the rapid changes in the structural use of metal in architecture. The roofs of vast span, largely composed of glass, which modern methods of trussing have made possible for railway stations, armories, and exhibition buildings; the immense unencumbered spaces which may be covered by them; the introduction and development, especially in the United States, of the post-and-girder system of construction for high buildings, in which the external walls are a mere screen or filling-in; these have revolutionized architecture so rapidly and completely that architects are still struggling and groping to find the solution of many of the problems of style, scale, and composition which they have brought forward. Within the last thirty years, however, architecture has, despite these new conditions, made notable advances. The artistic emulation of repeated international exhibitions, the multiplication of museums and schools of art, the general advance in intelligence and enlightenment, have all contributed to this artistic progress. There appears to be more of the artistic and intellectual quality in the average architecture of the present time, on both sides of the Atlantic, than at any previous period in this century. The futility of the archæological revival of extinct styles is generally recognized. New conditions are gradually procuring the solution of the very problems they raise. Historic precedent sits more lightly on the architect than formerly, and the essential unity of principle underlying all good design is coming to be better understood.[26] [Footnote 26: See Appendix D.]