[Illustration: Stolp wired tube for automobiles.] Inventors have long addressed themselves to the difficulty caused by the expansion and contraction of structures as temperatures change. For years the cylindrical fire-boxes of marine boilers have been corrugated, so as to allow them a certain play without breaking from their fastenings, or tearing their seams, when heated or cooled. This form is adopted with success for the Morison fire-boxes of the Vanderbilt locomotives. In quite different situations metal piping, in a length of let us say 100 feet, is provided against trouble from shrinkage or expansion by a U bend. When the diameter of the pipe is twelve inches, this bend is usually about ten feet in extent; for a six inch pipe, a bend six feet long suffices. Another difficulty due to heat is the limitation of speed imposed by the heat which friction creates. A new type of circular saw has a hollow arbor through which flows cold water, so that motion may be faster than ever before. The same arbor appears in various other machines with like advantage. [Illustration: Corrugated boiler.] [Illustration: Pipe so bent as to permit contraction or expansion.] CHAPTER VIII FORM--_Continued_. TOOLS AND IMPLEMENTS SHAPED FOR EFFICIENCY Edge tools old and new . . . Cutting a ring is easier than cutting away a whole circle . . . Lathes, planers, shapers, and milling machines far outspeed the hand . . . Abrasive wheels and presses supersede old appliances . . . Use creates beauty . . . Convenience in use . . . Ingenuity may be spurred by poverty in resources. Tools and Implements. We have just reviewed, all too briefly, how light and heat are economized by structures of judicious form. At this point we will bestow a rapid glance at the economy of work as promoted by sound design in tools and implements, in the machines which embody these for tasks far beyond the personal skill or power of the strongest and deftest mechanic. When of old a savage took up a stone to serve as a rude knife or chisel, we may be sure that he chose the sharpest flint he could find. If he could better its shape by knocking it into something like a wedge, what task was easier? Our museums display an immense variety of stone hammers, axes, knives, and arrowheads, showing how art long ago improved the forms of simple tools and weapons offered by nature. Modern tools and weapons, for all their immense diversity, were every one prefigured in the rude armory of primitive man.