26 THE FLIGHT OF BIRDS 
proportion as the glider makes a smaller and smaller 
angle with the horizon, the point at which the air 
acts will be progressively nearer to the front margin, 
and this will obviously tend to prevent a dive 
downward. 
Fia. 12 
Diagrams showing the shifting of the centre of pressure. 
This point requires some explanation. Take as 
an illustration a sailing boat when it is tacking. 
When it sails close to the wind, the air which strikes 
against the forepart of the sail is deflected from its 
course, and rushing towards the stern of the boat 
forms a buffer which shields the rest of the sail from 
the air, which would otherwise have impinged upon it. 
The drawing of a boat tacking, on page 8, and 
the accompanying diagram, will make this clear. 
The principle is called the law of Avanzini. The 
smaller the angle made by the wind with the sail, 
the nearer the point of impact approaches to the 
frontedge. This is true no less of the slightly curved 
surfaces of aeroplanes. As the angle of incline to 
the horizontal is more and more reduced, the wind 
acts at a point closer and closer to the front edge, 
and thus the aeroplane may, possibly, correct the 
dangerous tendency automatically, without the 
aviator having to make readjustments. 
To illustrate this principle I have made experi- 
ments with a catapult which, working within 
runners, threw pieces of cardboard horizontally, 
