H. Sxxv.—On the Lifting Power of Inclined Planes n Aerial Transit. 171 
prolonged power of buoyancy under certain circumstances with scarcely 
any flapping of the wings. On watching carefully this kind of flight 
it can be seen that the wings are kept nearly horizontal but with the 
anterior margin very slightly elevated above the posterior and thin edge 
of the wing. The angle thus formed by the wing with the line of motion 
is very small, indeed if it were not small it is easily proved that the 
onward motion of the bird would be quickly arrested from the quantity 
of air which would require to be displaced. It does indeed appear 
wonderful that a bird weighing perhaps ten pounds can be supported in 
this manner so long a time when it has once obtained a certain velocity. 
In every-day life, however, we have many instances of the lateral pressure 
of the air on planes in a direction transverse to the motion ; we have only 
to walk slowly with a piece of paper held at an oblique angle to the line of 
progression, or to open an outer door of a building an inch or two if the 
door opens outwards when the wind is blowing obliquely in, when it will be 
at once noticed, and if a gale of wind is blowing it will be found impossible 
by main force to prevent the door flying open in a surprising manner if it 
is once opened a few degrees. 
If solids are made to impinge on planes, then the angle of reflection is 
equal to the angle of incidence. If, however, a current of air impinges on 
a plane, then the elasticity of the air comes into play with very curious 
results. The following experiments, which I now repeat, were made with 
a view of ascertaining the action of the air on inclined planes at different 
angles :— 
Experiment, No. 1.—If a book one or more inches thick is placed flat on 
a table, and any small light body is also placed on it, about as far beyond 
the book as the book is thick, it will be found impossible to:so blow across 
the book as to send the light body away, for instead of moving from 
you it flies towards you. 
Experiment, No, 2.—If a current of air is blown obliquely onto a table 
covered with sawdust, we shall observe that the whole of the sawdust 
affected by the wind is not blown in the direction of the current, but that a 
considerable portion is actually blown along the plane towards the primitive 
current. 
Experiment, No. 3.—If a current of air blown through a tube impinges 
at any given oblique angle upon a point in a horizontal plane, the incident 
current I P (figure 1) is not reflected at an equal angle to the plane from 
the point P along the line P Q, as might be supposed; for, if a lighted 
taper be held at Q, the flame is actually drawn downwards, and if the taper 
be moved over a large range of angular vertical measurement the flame is 
still drawn towards the plane. 
