1893. ] NEW YORK ACADEMY OF. SCIENCES. 89 
In the case of the circular dise referred to, its tendency was 
to develop a spiral trajectory, in which first 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 ‘his position, its 
rotation will be from right to left, and it will be its left 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 gyration 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 wind 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 produces 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 
