the level of the line _BJ_, it will be best to increase the output from _AM_ to _AL_. The whole return will then be represented by the rectangle _ADHL_, and the area _BDHO_ represents profits, with the cost at the new and lower level. These are somewhat larger than they would be if the output continued to be only the amount _AM_. Under free competition the price would fall to the line _BJ_, the net profits would disappear, and the public would have the full benefit of the improvement in production. _The Purpose of the System of Patents._--Patents are a legal device for promoting improvements, and they accomplish this by invoking the principle of monopoly which in itself is hostile to improvement. They do not as a rule create the exclusive privilege of producing a kind of consumers' goods, but they give to their holders exclusive use of some instrumentality or some process of making them. The patentee is not the only one who can reach a goal,--the production of a certain article,--but he is the only one who can reach it by a particular path. A patented machine for welting shoes stops no one from making shoes, but it forces every one who would make them, except the patentee or his assigns, to resort to a less economical process. _Patents Limited in Duration indispensable as Dynamic Agents._--If an inventor had no such protection, the advantage he could derive would be practically _nil_, and there would be no incentive whatever for making ventures except the pleasure of achievement or the honor that might accrue from it. In the case of poor inventors this would be cold comfort in view of the time and outlay which most inventions require. Not only on _a priori_ grounds, but on grounds of actual experience and universal practice, we may say that patents are an indispensable part of a dynamic system of industry. It is also important that the monopoly of method which the patent gives should be of limited duration. If the method is a good one and the profit from using it is large, the seventeen years during which in our own country a patent may run affords, not only an adequate reward for the inventor, but an incentive to a myriad of other inventors to emulate him and try to duplicate his success. Ingenious brains, which are everywhere at work, usually prevent the owners of a particular patent from keeping any decisive advantage over competitors during the whole period of seventeen years. Long before the expiration of that time some device of a different sort may enable a rival to create the same product with more than equal economy, and the leadership in production then passes to this rival, to remain with him till a still further device effects a still larger economy and carries the leadership elsewhere. That alternation in leadership which we have described and illustrated takes place largely in consequence of our system of patents; and yet every particular patent affords a quasi-monopoly to its holder. The