The frames of other animals furnish forth a goodly round of analogies with recent products of mechanical ingenuity. A beaver tooth might well have been the model for a self-sharpening plowshare, widely used throughout the world. This tooth has a thin outer layer of hard enamel, within which, dentine, less hard, makes up the rest of the structure. Gnawing wears the dentine much more than the enamel, so that the tooth takes on a bevel resembling that of the chisel which pays frequent visits to a carpenter’s oil-stone. The scale of enamel gives keenness, the dentine ensures strength, so that the tooth sharpens itself by use, instead of growing dull. Much the same structure is repeated in a plowshare by chilling the underskin of the steel to extreme hardness, while the upper face of the share is left comparatively soft. As it goes through the ground the upper face wears away so as to yield a constantly sharpened edge of the thin chilled under metal. Thus the heavy draft of a dull share is avoided without constant recourse to the blacksmith for re-sharpening. [Illustration: Beaver teeth.] Shaping a Tube. In another field of ingenuity a great inventor scored a success, simply by deliberately taking a lesson from nature. James Watt, to whom the modern steam engine is most indebted for its excellence, was once consulted by the proprietors of the Glasgow Water Works, as to a difficulty that had occurred in laying pipes across the river Clyde to the Company’s engines: the bed of the river was covered with mud and shifting sand, was full of inequalities, and subject to a current at times of considerable force. With the structure of a lobster’s tail in his mind, Watt drew a plan for an articulated suction-pipe, so jointed as to accommodate itself to the shifting curves of the river-bed. This crustacean tube, two feet in diameter, and one thousand feet in length, succeeded perfectly in its operation. To-day powerful hydraulic dredges discharge through piping with flexible joints such as Watt devised; in one instance this piping is 5700 feet in length. [Illustration: Narwhal with a twisted tusk. Reproduced from the Scientific American, New York, by permission.] In many another case art has used a gift of nature simply as received, and then improved upon it. In making their harpoons the Eskimo used the spiral teeth of the narwhal; finding their shape advantageous, they copied it for arrowheads. This is undoubtedly one of the origins of the screw form, of inestimable value to the mechanic and engineer. Lessons from Lower Animals: A Tool-Using Wasp.