When the case is finished, all except the final hand-polish, it is taken to the sounding-board room. The sounding-board--a thin, clear sheet of spruce under the strings--is the piano's soul, wanting which, it were a dead thing. Almost every resonant substance in nature has been tried for sounding-boards, but nothing has been found equal to spruce. Countless experiments have been made with a view to ascertain precisely the best way of shaping, arranging, and fixing the sounding-board, the best thickness, the best number and direction of the supporting ribs; and every great maker is happy in the conviction that he is a little better in sounding-boards than any of his rivals. Next, the strings are inserted; next, the action and the keys. Every one will pause to admire the hammers of the piano, so light, yet so capable of giving a telling blow, which evoke all the music of the strings, but mingle with that music no click, nor thud, nor thump, of their own. The felt employed varies in thickness from one sixteenth of an inch to an inch and an eighth, and costs $5.75 in gold per pound. Only Paris, it seems, can make it good enough for the purpose. Many of the keys have a double felting, compressed from an inch and a half to three quarters of an inch, and others again have an outer covering of leather to keep the strings from cutting the felt. Simple as the finished hammer looks, there are a hundred and fifty years of thought and experiment in it. It required half a century to exhaust the different kinds of wood, bone, and cork; and when, about 1760, the idea was conceived of covering the hammers with something soft, another century was to elapse before all the leathers and fabrics had been tried, and felt found to be the _ne plus ultra_. With regard to the action, or the mechanism by which the hammers are made to strike the strings, we must refer the inquisitive reader to the piano itself. When all the parts have been placed in the case, the instrument falls into the hands of the "regulator," who inspects, rectifies, tunes, harmonizes, perfects the whole. Nothing then remains but to convey it to the store, give it its final polish and its last tuning.