172. I think you must see by this time that the sculptor's is not quite a trade which you can teach like brickmaking; nor its produce an article of which you can supply any quantity 'demanded' for the next railroad waiting-room. It may perhaps, indeed, seem to you that, in the difficulties thus presented by it, bas-relief involves more direct exertion of intellect than finished solid sculpture. It is not so, however. The questions involved by bas-relief are of a more curious and amusing kind, requiring great variety of expedients; though none except such as a true workmanly instinct delights in inventing, and invents easily; but design in solid sculpture involves considerations of weight in mass, of balance, of perspective and opposition, in projecting forms, and of restraint for those which must not project, such as none but the greatest masters have ever completely solved; and they, not always; the difficulty of arranging the composition so as to be agreeable from points of view on all sides of it, being, itself, arduous enough. 173. Thus far, I have been speaking only of the laws of structure relating to the projection of the mass which becomes itself the sculpture. Another most interesting group of constructive laws governs its relation to the line that contains or defines it. In your Standard Series I have placed a photograph of the south transept of Rouen Cathedral. Strictly speaking, all standards of Gothic are of the thirteenth century; but, in the fourteenth, certain qualities of richness are obtained by the diminution of restraint; out of which we must choose what is best in their kinds. The pedestals of the statues which once occupied the lateral recesses are, as you see, covered with groups of figures, inclosed each in a quatrefoil panel; the spaces between this panel and the inclosing square being filled with sculptures of animals.