Thus were opened up promising avenues of investigation, and not only of investigation, but of invention also. For it is obvious that, while investigation and experimentation can hardly fail to secure data, they may secure nothing else, and usually do. But mere data are mere facts; and, valuable as they are if suitably classified, they are not valuable unless they are classified; and even after data are classified, they are not useful until some use is found for them. The data in card-indexes are mere unrelated facts, and are almost useless, until they have been classified and arranged in boxes alphabetically labeled. Then they are useful whenever any use is found; when, for instance, some one is seeking information on a certain subject. In this condition, data are like material substances, in that they are available for use,--in fact, data are often spoken of by writers as "material"; a certain series of incidents, for instance, supply "material" for a story. Now, just as pieces of iron and brass supply material with which an inventor can create a new machine, so classified facts, or data, supply material with which an inventive investigator can create a new theory, or formulate a new law. Our books on physics are full of accounts of experiments and investigations conducted by such men as Hero, Archimedes, Gilbert, Galileo and many others, the consequent discoveries that they made, and the consequent laws that they enunciated; but those books could not possibly describe all the investigations that have ever been made. Those which they describe are those that ended in some definite creations, such as the hydrostatic law enunciated by Archimedes. Most investigations, experiments and researches have ended in nothing definite:--most of them, in all probability, have not even established facts. The investigations that we studied about when boys were such as those of Archimedes, that presented us with inventions, in the form of useful and usable laws. No appreciable difference is apparent between the mental operations of Archimedes in inventing these laws and his mental operations in inventing his screw: for in both cases the mental operations consisted mainly in conceiving an idea and then embodying it. The Archimedean screw was a machine of an entirely new kind that, in the hands of a man understanding its use, would enable the man to do something he could not do before--or enable him to do a thing he could do before, but do it better. So were his laws. The laws have been utilized ever since, as definite and concrete devices; and to a much greater extent than the special form of screw that he invented.