Strabo[113] describes the (Belgian) Gauls as hunting with a piece of wood resembling a pilum, which is hand-thrown, and which flies to a distance farther than an arrow. He calls it the Γροσφὸς, which is also described as a pilum, dart, or javelin by Polybius;[114] but evidently this Grosphus means the throw-stick, usually termed by the Greeks ἀγκύλη (Ancyle). Silius Italicus arms in the ‘Punica’ one of the Libyan tribes which accompanied Hannibal with a bent or crossed _cateia_: the latter is identified with the throw-stick by Doctor (now Sir) Samuel Ferguson, poet and antiquary.[115] The encyclopædia of Bishop Isidore (A.D. 600–636) explicitly defines the _cateia_ to be ‘a species of bat which, when thrown, flies not far by reason of its weight; but where it strikes it breaks through with extreme impetus, and if it be thrown with a skilful hand it returns to him who threw it:—rursum redit ad eum qui misit.’ Virgil also notices it:— Et quos maliferæ despectant mœnia Abellæ Teutonico ritu soliti torquere cateias. (_Æn._ vii. 740). Jähn (p. 410)[116] remembers the _Miölner_, or hammer of Thor, which flew back to the hand. [Illustration: FIG. 37.—INDIAN BOOMERANGS. 1. War Hatchet, Jibba Negros; 2. Steel Chakra, or Sikh Quoit; 3. Steel Collery; 4, 5. Collery of Madras, with knobbed handle.] [Illustration: FIG. 38.—BOOMERANG AND KITE.] It has been noted that this peculiarity of reversion or back-flight is not generic, even in the true boomerang, but appertains only to specific forms. Doubtless it was produced by accident, and, when found useful for bringing down birds over rivers or marshes, it was retained by choosing branches with a suitable bend. The shapes greatly differ in weight and thickness, in curvature and section. Some are of the same breadth throughout; others bulge in the centre; while others are flat on one side and convex on the other. In most specimens the fore part of the lath is slightly ‘dished’: hence the bias causes it to rise in the air on the principle of a screw-propeller. The thin edge of the weapon is always opposed to the wind, meeting the least resistance. The axis of rotation, when parallel to itself, makes the missile ascend as long as the forward movement lasts, by the action of the atmosphere on the lower side. When the impulse ceases it falls by the line of least resistance, that is, in the direction of the edge which lies obliquely towards the thrower. In fact, it acts like a kite with a suddenly broken string, dropping for a short distance. But as long as the boomerang gyrates, which it does after the forward movement ends, it continues to revolve on the same inclined plane by which it ascended until it returns to whence it came. This action would also depend upon weight; the heavy weapons could not rise high in the air, and must drop by mere gravity before coming back to the thrower. [Illustration: FIG. 39.—AFRICAN BOOMERANGS.