A circle of say two feet in diameter, may be readily cut from a boiler plate by two cutters, one at each end of a horizontal bar, the bar being supported by a central upright axis receiving the motive power. Because the cut is narrow, but little metal is wasted as chips. A cut of this ring-shape effects a desirable saving even when the circle to be swept is but an inch or so in width instead of several feet. When an auger takes its way through a plank it removes as chips all the wood within the circle of its range; a drill, of common form, as it pierces stone or metal acts in a similar manner. Motive power is greatly economized when a drill is tubular, with the further advantage that within the ring cut a solid cylinder remains to be broken off at intervals and lifted out, its core informing to the engineer in quest of bed-rock, to the prospector of mines or oil-fields, or to the geologist who reads at a glance the composition of a mineral, the forces which have impressed it age after age. Such drills, set with bortz diamonds, have accomplished remarkable feats. In boring out 260 columns surrounding the dome of the capitol at Springfield, Illinois, cores 22-3/4 inches in diameter were removed from holes 24 inches wide; without sacrifice of strength there was a saving in weight of three-fifths. At the Ellenwood coal mine, Kingston, Pennsylvania, a core 17 feet, 5 inches in breadth was taken from a bore only five inches wider. When the engineers in 1896 were planning the foundations for the Williamsburg Bridge, New York, the deepest of their 22 borings was 112 feet below high water. Steel drills had indicated bed-rock 12 to 20 feet higher than was the actual case; the diamond drill showed the supposed bed-rock to be merely a deposit of boulders. No other known means could have accomplished these results. In the same way steel guns of large calibre have been drilled so as to leave a core of much value, while in this as in all other such tasks, the boring demanded less energy and proved less straining than if all the metal within the sweep of the drill had been reduced to fragments. All these tools were prefigured in a simple ring drill used two thousand years ago on the banks of the Nile; hollow reeds were employed, with sand as a cutter. [Illustration: Twist drill.] Twist Drills.