1886, | _ Gravitation and the Soaring Birds. : 517 
As there is no authority for the value of frictional resistance of air 
on smooth surfaces, and as I have failed to measure it by any exper- 
imental test at my command, on account of its extreme smallness, 
the argument would be in no wise affected if it were not taken 
into account for any of the velocities that we shall deal with. The 
reader may therefore place upon it any value within reasonable 
limits. 
Let A B represent one of the edges of a plane, say one foot 
Square, resting in air, and of the same weight as the atmosphere 
it displaces, 
1. The only actual or conceivable work the plane can be sub- 
t to under the dominion of any force whatever, is either air 
Pressure upon its sides, or resistance to atmospheric surface or 
skin friction, parallel to itself or in its own plane. 
2. From the law of fluid pressures, and the contrary and equal 
aracter of action and reaction, a force operative upon the plane 
B from any direction, does work in one, or both, of two ways, 
z., either in it or at right angles to it. 
3. Forces in the direction of c d, or in the plane in any direc- 
tion, are not resolved by the plane, but work to their full value in 
piers independence of each other, as they are right-angled 
orces, : 
jec 
ch 
A 
vi 
4. Forces from any direction, excepting in the plane and nor- 
mal to it, are resolved by the plane into those two directions. 
5. Any number of forces, not in the plane nor normal to it, 
Operate upon it in the resultant of one force, from one direction ; 
d this resultant, if not already in the plane or normal to. it, is — 
resolved therein by the plane. | 
_ 6. It follows that the plane can be subject to work only, (1) in 
its own plane, (2) in a direction at right angles to its surfaces, (3) 
in both of these directions. 
Y the plane is compressing air, and as the resistance of the Pa 
atmosphere to motion in this direction is very great, the velocity. 
TH be Correspondingly slow. ee 
€ nature of the work done by the force in the plane, is 
T aming atmospheric friction on the two surfaces, which being” 
very little, m ingly. 
Sreat, 
otion in this direction will be correspondingly: 
9- A force not in the plane, nor normal to it, is resolved by the. 
7. The nature of the work done by the force acting normal 
