METALLIC

Small bars of metal which were supposed to po e certain magnetic powers, and to cure painful affections and tumours by being drawn over the part. They bore the name of their inventor, Perkins, and were in considerable vogue about thirty years ago. A number of wonderful cures, however, having been attested, which were performed by means of spurious wooden tractors, the imaginary virtues of the magnetic or metallic fell into disrepute, and then into oblivion. TRACTORY, or TRACTRIX (Lat. traho, / dram), in the geometry of curve lines, is the curve characterized by this property, that the tangent is always equal to a given line. It was called the tractory by Huy^ens, because it may be considered as described mechanically by a small weight attached to one extremity of.a thread, while the other extremity is drawn along a given straight line. But in order that the curve may be described in this manner, it is nece ary to suppose the friction of the plane of traction to be infinite, or at least to destroy at every instant the momentum which is generated by the motion of the small weight, otherwise the direction of the motion of the describing point would not be a tangent to the curve. The tractory has much analogy with the logarithmic whose subtangent is constant. The evolute of the curve is the common catenary. (DAlembert, Ency. Method i que; Peacock's Examples, p. 174.) [s. 1261]