72 THE TROCHOIDED PLANE. 
birds work the upward and downward currents of the air, the 
existence of which is plainly shown by the form of clouds and 
smoke. 
Natural selection and the survival of the fittest account for the 
shall find the leading edge of the planes will be liable to double up 
and get damaged, making it necessary to follow in the footsteps of 
ature. 
These are my views, stated as concisely as I can; and if you 
think there is any novel truth embodied in them, this Society is 
welcome to any of the laboratory models that aided me in finding 
it out. 
In conclusion, gentlemen, I should like your opinion as to. 
whether or not there is evidence to show that there is a power 
are, to the best of my knowledge, quite original ; and let me point 
7 von da 
imperfections suggest, and a comparison of the improved models 
would show what progress had been made; this would hasten a 
process that is, to say the least of it, laborious and tedious. 
* These models were somewhat similar to Figs. III, V, and VII, and were 
not shown at the previous conversazione. 
[Seven diagrams. ] 
