On the Boomerang. 61 



important effect of increasing the moment of inertia about 

 the axis without increasing its weight ; and the swift spin 

 with which it is thrown causes the initial angular momentum of 

 the rotational motion relative to the C.G. (this will be called 

 simply "the angular momentum ") to be very large relatively 

 to the total angular momentum which the air reactions can 

 impart during the flight. 



Were the air absent the C.Gr. would describe a parabolic 

 path, and the rotational motion, being a rotation about a 

 principal axis of inertia, would remain entirely unchanged. 

 But on account of its twisted form, and the convexity of its 

 upper and lower surfaces, the boomerang never presents a 

 fair cutting-edge to the air. Consequently the air pressures 

 that are due to the motion relatively to the air continually 

 act upon both horns, always the more effectually upon the 

 one whose rotational motion at the moment conspires with 

 the velocity of the C.Gr., in such a way as to set up a very 

 small additional angular momentum in each revolution. Of 

 the three components parallel to the principal axes at the 

 C.Gr. into which this angular momentum may be resolved, 

 the one of which the axis is in the axis of the boomerang 

 merely effects a very slight reduction, per revolution, in the 

 rate of rotation ; but the other two cause a slow alteration, 

 always in the same direction, or rather sense, of the orienta- 

 tion of the axis (barely one complete revolution in an ordinary 

 returning flight of about 9 seconds), without affecting to any 

 appreciable extent the rate of the spin. Consequently the 

 plane of the missile, and therefore also the direction of its 

 flight, undergoes a steady alteration, always in the same 

 sense, i. e. that of a right-handed screw about an upward 

 vertical axis. Therefore, as the boomerang must on the 

 whole move edgeways through the air, it constantly steers 

 itself round to the left of an observer standing behind it, 

 from whom it is moving away. At the same time the twist, 

 acting like that of the propeller of a screw-steamer, causes 

 the boomerang to screw itself leftwards and upwards through 

 the air ; for the axis continually rises from its initial, hori- 

 zontal, position towards the vertical, turning upwards from 

 left to right in front of the thrower, and so sets the principal 

 plane more and more nearly horizontal. The effect of this 

 lifting action is to prolong the flight of the boomerang by 

 maintaining it longer in the air ; and, by raising it to a con- 

 siderable height, commonly about 30 feet, it makes it possible 

 for the missile to lose, without falling to the ground, a large 

 portion of its forward velocity, even coining almost to rest, 

 the spin nevertheless being well maintained ; for in its 



