503 
of Edinburgh, Session 1879 - 80 . 
may be coloured with three colours, so that no two of the same 
colour are conterminous. 
This particular process essentially introduces four different colours, 
and therefore does not necessarily give the simplest way of colour- 
ing a map. Another method, quite different from this, but involving 
virtually the same principles, is next given. Then come two other 
processes, different in form from that of Mr Kempe, but based like it 
ultimately on the fact that only 3 (n - 2) non-intersecting lines can 
be drawn (except for n = 2) joining n points in a plane. 
