Page:Aboriginesofvictoria01.djvu/404

320 Such a remark by one of the ablest mathematiciausmathematicians [sic] of his time was not forgotten. On the contrary, it was remembered on the next occasion when I had opportunities of studying the flight of bomerengs thrown by the hands of Australian Aborigines, and then I perceived that in its rotary motion through the air, a hollow centre of greater or less diameter, but usually of about one-third of the disc, was described by the whirl of the bomereng, and it occurred to me that the centre of the whirling motion might be found in a line of equilibrium which should divide the surface acting on the air into three portions, in such manner as that the eccentric portions should equal the central one. The discovery of this centre, insignificant as it may appear, was still something new, for on attaching a centre to a bomereng, it was possible to show that this centre was not only during its rotary motion the centre of that motion, but also the centre of gravity when in a state of rest, while it was apart from and quite clear of every part of it.

The natives when bent on exhibiting the more curious flights, twist the bomereng, by placing it at the fire, evidently for the purpose of giving it the property of spiral movement, thus showing how well they understand the screw-action upon the air. On making a small wooden model with a spiral turn like a screw, and giving it by means of an attached centre, and the fork and cord of a humming-top, rapid rotary motion, the model ascended to the roof of the room with such force as to be broken in pieces against it. . . ..

The inner edge of the bomereng is found to form a cycloid. . . . The outer edge consists of two parabolic curves whose foci appear to overlap, so as to be both in the axis of motion. These curves are presented by a section of the half-bomereng, when at an angle of 45° with the axis."