By the arrangements of levers, screws, and pipes and valves, shown in the engraving, the steam is first admitted under the piston, and thus acts directly in forcing it up, with the heavy hammer-block attached to the piston rod. When the block has been raised to the required height, it strikes against the end of a lever, which then shuts off the steam, and allows it to escape; whereupon the hammer falls with its full force vertically on the anvil. The end of the lever which turns off the steam may be adjusted at any height, according to the required force of the blow, so that the hammer may fall from a height of six feet, or be merely raised a few inches. [Illustration] The steam-hammer, in the early stages of its invention, required an attendant to turn on the steam again at the end of each stroke, but Mr. Nasmyth ingeniously contrived the means of rendering the engine altogether self-acting, by causing the force of the collision to release a spring that holds down the slide-valve; and by this contrivance a continued and regular succession of blows is maintained without any assistance. Not only can the force of the blow be regulated by the height to which the hammer is lifted, but the ponderous mass may be arrested in its descent by admitting the steam under the piston, so that a skilful manipulator can stop it within the eighth of an inch from the anvil. The Steam Engine itself, by which all the self-acting mechanisms of a large factory are put in motion, is, perhaps, after all, the most wonderful of inventions; but it does not strictly come within our province, for Watt had perfected his great work before the close of the last century. It was, however, not much used, excepting for mining purposes, until after the commencement of the present; and the inventor himself had but a faint idea of the value and vast importance of the motive power he had placed at the command of man. So little, indeed, was the value of steam power appreciated in the early years of its application, that no notice is taken of the steam engine in Beckmann's History of Inventions, though Watt had completed his condensing engines several years before that work was published; and Newcomen's steam engine had been at work at least sixty years.