It seems clear enough, then, that the monopoly which occurs in the use of a copyright, is of an entirely different sort from the monopolies which we have previously considered. Competition is not destroyed by it, and its only effect upon the public relates to an entirely new production, which is not a necessity, and which the public could not have had an opportunity to enjoy if the copyright law had not made it possible for the author to write the book with the prospect of being repaid for his labor by the sale of the printed volume. As already stated, the granting of patents is based on the same principle as the granting of copyrights. A clause of the Constitution empowers the general government to grant to authors and inventors for limited periods the exclusive right to their respective writings and discoveries. If we judge the granting of patents by the aims and intentions which are held in the theory of the law, we must conclude that it is a highly wise, just, and beneficial act. The man who invents a new machine or device which benefits the public by making easier or cheaper some industrial operation, performs a valuable service to the world. But he can receive no reward for this service, if any one is at liberty to make and sell the new machine he has invented; and unless the patent laws gave him the power to repay himself for the labor and expense of planning and designing his new device, it is altogether probable that he would not spend his time in inventing. The wealth which a valuable patent promises has been a great incentive to the work of inventors, and has undoubtedly been a chief cause of the great mechanical advancement of the last half century. But the state of mechanical science has greatly changed from what it was when the clause of the Constitution was penned which speaks of inventions as "discoveries." The trained mechanical designer now perfects a machine to do a given work, with almost the same certainty that it will be successful in its operation that he would feel if the machine were an old and familiar one. The successful inventor is no longer an alchemist groping in the dark. His task is simply to accomplish certain results with certain known means at his disposal and certain well-understood scientific principles to guide him in his work. But this statement, too, must be qualified. There are still inventions made which are the result of a happy inspiration as well as of direct design. Not all the principles of mechanical science and the modes of reaching desired ends are yet known or appreciated by even the best mechanical engineers. There is still room for inventors whose rights should be protected. The interpreters of our patent laws have always held the theory that the use of a natural agent or principle could not be the subject of a patent. This is undoubtedly wise and just. The distinction should always be sharply drawn between those existing forces of nature which are as truly