The inventor in mechanics, the sculptor, the painter, the novelist and the poet embody their creations in material forms that are enduring and definite, and constitute evidences of their work, which sometimes endure throughout long periods. The architect and the constructing engineer are able similarly to produce lasting and useful monuments to their skill; but it can hardly be declared that their work is characterized by quite so much of originality and invention, because of the restrictions by which the practice of their arts is bound. It is, in fact, hard to conceive of a bridge very different in principle or design from bridges that had been built before; and while it is not difficult to conceive of an engine different in principle and design from previous ones, yet we realize that the points of novelty in such an engine would be attributable more to invention than to engineering. This is because the arts of engineering and architecture rest on principles that have long since been proved to be correct, and on practices that are the results of long experience; whereas one of the main characteristics of invention is novelty. It is true that many of the most important inventions have been made by engineers; but this has been because some engineers, like Ericsson, have been inventors also. But it is also true that only a small proportion of the engineers have made original inventions; and it is equally true that many inventions have failed--or have been slow in achieving success--because of lack of engineering skill in construction or design. These facts show that the work of the inventor is very different from that of the engineer, and that the inventor and the engineer are very different people, though an engineer and an inventor sometimes live together inside of the same skin. In fact, it is by a combination of inventive genius and engineering talent in one man that the greatest results in invention have been achieved; though great results have often followed the intimate cooperation of an inventor and an engineer, the two being separate men. It is in the latter way that important advances have usually been made; and it is somewhat analogous to the way in which authors and publishers, actors and managers, promoters and capitalists cooperate. But while the individuals whose inventions have taken the form of new creations, such as novel machines and books and paintings, have received the clearest recognition as men of genius, may not the inventive faculty be needed in other fields and be required in other kinds of work? If an instrument is produced by the joint exercise of imagination and constructive talent, is not every puzzle worked out, and every problem solved, and every constructive work accomplished by the similar exercise of those same faculties?