His greatest invention unquestionably was that of the steam hammer, which came about in an interesting way. He had built a number of locomotives for the Great Western Railway. This railway operated a line of steamers from Bristol to New York and was planning a ship larger and faster than any then built, to be called “The Great Britain.” It was to be a side-wheeler and the plans called for a large and heavy paddle shaft, 30 inches in diameter. Mr. Humphries, its designer, wrote to Nasmyth asking for help, saying so large a shaft could not be forged with any of the hammers then in use. Nasmyth saw at once the limitations of the prevailing tilt hammer--which was simply a smith’s hand hammer, enlarged, with a range so small that it “gagged” on large work,--and that the design of large hammers must be approached in an entirely new way. “The obvious remedy was to contrive some method by which a ponderous block of iron should be lifted to a sufficient height above the object on which it was desired to strike a blow, and then to let the block fall down upon the forging, guiding it in its descent by such simple means as should give the required precision in the percussive action of the falling mass. Following up this idea,” he writes, “I got out my ‘Scheme Book,’ on the pages of which I generally thought out, with the aid of pen and pencil, such mechanical adaptations as I had conceived in my mind, and was thereby enabled to render them visible. I then rapidly sketched out my steam hammer, having it all clearly before me in mind’s eye. In little more than half an hour after receiving Mr. Humphries’s letter narrating his unlooked-for difficulty, I had the whole contrivance, in all its executant details, before me in a page of my Scheme Book, a reduced photograph copy of which I append to this description. (See Fig. 23.) The date of this first drawing was the twenty-fourth of November, 1839....”[97] [97] _Ibid._, p. 240. [Illustration: FIGURE 23. FIRST SKETCH OF THE STEAM HAMMER NOV. 24, 1839] [Illustration: FIGURE 24. MODEL OF THE FIRST STEAM HAMMER IN THE SOUTH KENSINGTON MUSEUM, LONDON] “Rude and rapidly sketched out as it was, this, my first delineation of the steam hammer, will be found to comprise all the essential elements of the invention.[98] Every detail of the drawing retains to this day the form and arrangement which I gave to it forty-three years ago. I believed that the steam hammer would prove practically successful; and I looked forward to its general employment in the forging of heavy masses of iron. It is no small gratification to me now, when I look over my rude and hasty first sketch, to find that I hit the mark so exactly, not only in the general structure but in the details; and that the invention as I then conceived it and put it into shape, still retains its form and arrangements intact in the thousands of steam hammers that are now doing good service in the mechanical arts throughout the civilized world.”[99]