1893.] NEW YORK ACADEMY OF SCIENCES. 89 



lu the case of the circular disc referred to, its tendency was 

 to develop a spiral trajectory, in which lirst one side and then 

 the other was uppermost, as if there was no such thing as 

 "parallelism of axis ' to disturb its sinuous inclination. But it 

 is almost impossible to produce some of the finer details of 

 action in the restricted experiments of a limited space, and this 

 is one of them. 



You may have to watch sharp to detect this peculiar tendency 

 of the circular disc. When I throw it from Ihis position, its 

 rotation will be from right to left, and it will be its leit side 

 that will gradually rise in its effort to develop a spiral trajectory 

 — thus : 



If I reverse my position the other edge will rise thus : 



A lucky throw, you observed ; it turned completely over 

 before striking the wall. 



Possibly this is partly due to the fact that it is more difficult 

 to give this figure a high rate of free gyratory speed, but mostly 

 to the fact that the motion of g^-ration on one side is in the 

 direction of its progress, and thus directly opposed by the 

 atmospheric resistance to progress, while at the same moment 

 of time the other side turning in an opposite direction 

 receives less resistance to its circular motion, and the side 

 meeting the least resistance is forced upward by the imnd of 

 advance, until it passes the vertical, and the opposite side 

 succeeding to that position finds its own motion less opposed, 

 and rises in its turn, and so the spiral motion is made 

 continuous. 



Obviously this effect is present in the least degree in the 

 boomerang missile which at a given instant of time has the 

 larger part of its exterior outline figure upon one side of its 

 centre of gyration. 



It is present also in any figure in which the parts may be on 

 the other hand symmetrically disposed about the centre of 

 gyration, but such that the wind of advance does not meet an 

 unbroken surface, but has an opportunity to slip over some part 

 of the upper surface, as you may observe when I throw these 

 forms (which, of course, only show to a much less degree the 

 power to return). 



In the true boomerang the oscillating of its frictional outline 

 of figure about the centre of gyration, breaks or intermits the 

 continuous effect of the wind of advance, which i^roduces in 

 the circular disc the tendency to a spiral trajectory. It explains 

 the cause of that remarkable effect which adds so greatly to the 

 charming beauty of the boomerang flight, and which has been 

 the cause of considerable speculation hitherto in the minds of 



