It is always difficult to apportion the due share of merit which belongs to mechanical inventors, who are accustomed to work upon each other's hints and suggestions, as well as by their own experience. Some idea of this difficulty may be formed from the fact that, in the course of our investigations as to the origin of the planing machine--one of the most useful of modern tools--we have found that it has been claimed on behalf of six inventors--Fox of Derby, Roberts of Manchester, Matthew Murray of Leeds, Spring of Aberdeen, Clement and George Rennie of London; and there may be other claimants of whom we have not yet heard. But most mechanical inventions are of a very composite character, and are led up to by the labour and the study of a long succession of workers. Thus Savary and Newcomen led up to Watt; Cugnot, Murdock, and Trevithick to the Stephensons; and Maudslay to Clement, Roberts, Nasmyth, Whitworth, and many more mechanical inventors. There is scarcely a process in the arts but has in like manner engaged mind after mind in bringing it to perfection. "There is nothing," says Mr. Hawkshaw, "really worth having that man has obtained, that has not been the result of a combined and gradual process of investigation. A gifted individual comes across some old footmark, stumbles on a chain of previous research and inquiry. He meets, for instance, with a machine, the result of much previous labour; he modifies it, pulls it to pieces, constructs and reconstructs it, and by further trial and experiment he arrives at the long sought-for result." [15]