Many experimentalists understand the word _theory_ as synonymous with falsehood or absurdity, as the very opposite of _practice_. It is clear that practice must be the ultimate verification of theory; but every true practice must have a true theory belonging to it. The _theory_ of a subject is the _science_ or philosophy of that subject; practice is the positive knowledge or proof of the soundness of the theory. But as theories are more plentiful than practices, and as many of them are not verified, there are of course many false ones. On this ground unscientific experimentalists have acquired the habit of regarding _all_ theory as false, which is about as reasonable as it would be to assert that because falsehood exists in the world, all truth must therefore be extinct. This peculiarity is not confined to carriage-building; engineering and architecture abound with it; and law and medicine are not wanting in it. The truth is, that human knowledge is only got together by small portions at a time in the school of experiment, and when that knowledge is considerable in any one branch, a true and verified theory may be constructed from it. And when a great number of subjects have thus been analysed and theorised, it is comparatively easy to construct theories by analogy on new subjects by sound principles. Newton’s theory of the universe was just as true when he first developed it in thought as after he had verified it by calculation. It is a common notion that a mechanical inventor must necessarily be a man of genius; but, if the matter be analysed, it will be found that though inventors are occasionally men of genius it is not by any means a general rule. _Invention_, in its ordinary sense, as the word implies, is the art of _finding out_. By _genius_ is meant a species of creative power, like that of the poet, for example, in his highest state of excellence. Invention is of two kinds—one resulting from a quick habit of observation, which detects the applicability of various forms of matter to similar objects. Of this an example may be given in the case of Dr. Wollaston, who, in a hurried experiment needing some lime which was not at hand, suddenly cast his eye on his ivory paper-cutter, and with some scrapings from its surface accomplished his object. This quick habit of observation, when it goes to the production of beautiful forms, is akin to fancy. The other and higher kind of invention is that which results from bringing a theory into practice—from first imagining a desirable result, and then bringing it to bear by the exercise of the judgment and constant persevering efforts steadily directed through a long period of time. The names of Brindley and Watt are examples of this quality. When Brindley set to work upon canals he did not create, he merely formed the plan of levelling the surfaces of natural streams by drawing off the water into new channels of sufficient depth, and thus preventing the water from being wasted. The process of