It is not essential that the engineer should know anything of architecture, though it is certainly desirable he should do so; but, on the other hand, it is indispensably necessary that the architect should understand construction. Without that knowledge he cannot design; and although it has been conceived by some that it would be better to delegate the mechanical task to the engineer, and so restrict himself entirely to the artistic arrangement and ornamentation of his design, such a course would be fatal to the development of architectural style. It is true that in some of the works above stated, it is generally thought desirable to confide them to engineers; but in the few cases in which architects have been called in to co-operate with them, as in the roofs of the Great Western and Midland Railway Stations, the result has been so satisfactory as to suggest the advantage of such combination. In the Great Exhibition of 1851, the happiest feature, the semicircular roof of the transept, was suggested by the late Sir Charles Barry, and the developments of that form in the nave and transepts of the Crystal Palace constitute still the most beautiful features of that building. In works of a monumental character, such as town-halls, museums, or public galleries, which are designed to last for centuries, the strict economy of material, which is sometimes deemed necessary in engineering works, is not advisable, because mass, stability, and durability—three elements into which we enter later on—are of the very essence of their architectural character. In these and other works of a simple character, such as private houses, the calculations are not of so elaborate a nature as to be outside the architect’s knowledge; and although of late years the use of iron girders, stanchions, and columns has introduced a new factor among building materials which occasionally may call for the assistance of an expert to substantiate the architect’s calculations, it has hitherto been the custom to conceal these features, so that they have not entered the phase of architectural design. In course of time, when an increased knowledge of the properties of iron is acquired, we may hope to see a great development in its artistic treatment, so that it may eventually rise to the dignity and assume the character, which in the architectural styles of bygone times, all other materials have reached. In addition, however, to the convenient arrangement and artistic treatment of a building, and its proper and sound construction, there is still a third element which requires the special endowment of an artist for its exercise. No architectural object can be considered as complete, or as having attained the highest excellence till it is endowed with a voice through the aid of phonetic sculpture and painting. In a few words, therefore, a perfect building may be defined as one that combines:—