H. Sxey.—IJntroduction of the Tension Wheel in Aerial Transit. 178 
over, the nature of the surface of the plane, and also the nature of the 
impinging body, materially influence the results. 
These experiments assist in explaining the prolonged sailing flight of 
birds, for, when the wings are retained so as to form a very small angle to 
the horizontal line, there will still be a very considerable upward pressure 
to sustain the bird in its progress through the air; they also show that if, 
as has been asserted, birds prolong their flight by gradually inereasing the 
angle of their wing planes, then that alteration need be and indeed can be 
only within very narrow limits; for a bird sailing in still air with its wing 
planes at an angle of 5? and travelling with a velocity of 23 miles per hour 
receives a support of 1:13 lbs per square foot of wing area, while if it alters 
its planes to 20? it only receives about half as much more support, even if 
we suppose its velocity unaltered ; four times the angle of inclination to the 
horizon only giving one half more support. Now the resistance horizontally 
which the wing encounters at 20? is about 16 times as great as what it 
encounters at5°, which would quickly arrest its progress, somewhat similarly 
to what may be observed when pigeons are alighting, for when near the 
ground they suddenly raise the anterior portions of their wings, their 
horizontal motion is stopped, a very slight rise is also noticed, and they 
alight without injury. 
It appears, therefore, that it is only within small angular ranges that 
the alteration of wing plane prolongs the power of flight. 
The wing of a bird is so constructed that it can be retained with 
sufficient rigidity at these minute angles. 
ArT. XY. —Intr oduction of the Tension Wheel in Aerial Transit. 
[Read before the Otago Institute, 9th October, 1877.] 
Arrempts have been made to solve the problem of aerial flight by means of 
planes moved at great velocity in a rectilinear direction, but the difficulties 
to be overcome in starting, balancing, and steering them appear to present 
insuperable obstacles to success. The difficulties are not much lessened if 
we rotate rigid planes, if built up from a centre and worked therefrom, as 
great strength and inseparable weight would be required, and the weight of 
the axis and all the parts near it would act as so much dead weight. 
That however a considerable ascensional force can be imparted to 
revolving planes is easily shown by the following experiment:—I take a 
