"His arms, as he moves them forward or draws them back again, are nothing more than a splendid system of compound levers, and the tool employed is on a cubical prism, with an angular opening into which a wedge of steel is inserted and fastened, with its point projecting below the sole or lower face. This wedge is forced forward by lateral pressure, and entering the wood gives out a shaving or strip equal in proportion to the projection. "How many of us are there who know that the edges of our plane iron and chisels, saw teeth, in fine our principal tools, are modifications of a simple wedge, and fewer still who know its power or how to increase its utility in practice. "To us who handle it daily, the screw, or as it is in reality a revolving wedge, is a mystery and an unknown thing, though we are familiar with its usefulness; yet, while in the act of propelling a screw with a screw-driver, a multitude of forces and machines are employed, which are grand in their simplicity and worthy of study. "That which teaches why a plumb bob hangs quiescent at the extremity of a string, and why a level is determined by the centering of an alcoholic bubble in a tube, and other valuable mechanical facts, should not be passed over by him whose philosophy is to devote his life to improving the means by which the comfort and happiness of human nature are gained. Independent even of this essential reason, it is imperative that we make ourselves acquainted with the component parts and properties of materials, in order to train the mind into a line of thought tending to invention and the bringing forth of valuable ideas, which only those familiar with this science can essay."