The mechanical idea, named the parallelogram of velocities, permits a ready and easy graphical representation of these facts.
Put the two triangles together to make a parallelogram.
The following example plots and fills the parallelogram illustrated in the current graphics foreground color.
It is nearly a rectangular parallelogram, about 300 by 700 yards.
This consists of two books, and may be called the foundation of theoretical mechanics, for the previous contributions of Aristotle were comparatively vague and unscientific. In the first book there are fifteen propositions, with seven postulates; and demonstrations are given, much the same as those still employed, of the centres of gravity (I) of any two weights, (2) of any parallelogram, (3) of any triangle, (4) of any trapezium.