In the old-fashioned lathe, the workman guided his cutting-tool by sheer muscular strength, and the slightest variation in the pressure necessarily led to an irregularity of surface. The rest for the hand is in this case fixed, and the tool held by the workman travels along it. Now, the principle of the slide-rest is the opposite of this. The rest itself holds the tool firmly fixed in it, and slides along the bench in a direction parallel with the axis of the work. All that the workman has to do, therefore, is to turn a screw-handle, by means of which the cutter is carried along with the smallest possible expenditure of strength; and even this trifling labour has been since got rid of, by making the rest self-acting. Simple and obvious as this improvement seems, its importance cannot be overrated. The accuracy it insured was precisely the desideratum of the day! By means of the slide-rest, the most delicate as well as the most ponderous pieces of machinery can be turned with mathematical precision; and from this invention must date that extraordinary development of mechanical power and production which is a characteristic of the age we live in. 'Without the aid of the vast accession to our power of producing perfect mechanism which it at once supplied,' says a first-class judge in matters of the kind, 'we could never have worked out into practical and profitable forms the conceptions of those master-minds who, during the past half-century, have so successfully pioneered the way for mankind. The steam-engine itself, which supplies us with such unbounded power, owes its present perfection to this most admirable means of giving to metallic objects the most precise and perfect geometrical forms. How could we, for instance, have good steam-engines if we had not the means of boring out a true cylinder, or turning a true piston-rod, or planing a valve-face?' It would perhaps be impossible to cite any more authoritative estimate of Maudsley's invention than the above. The words placed between inverted commas are the words of James Nasmyth, the inventor of that wonderful steam-hammer which Professor Tomlinson characterises as 'one of the most perfect of artificial machines and noblest triumphs of mind over matter that modern English engineers have yet developed.' [Illustration: Nasmyth's Steam-hammer.]