This paper presents a relational model of movement based on the notion 
of mobile moving about with constant speed. Three basic relations between 
mobiles are considered: (1) "Sometime or other, the mobiles <I>x</I> and <I>u</I> meet 
up somewhere", (2) "The position of the encounter between <I>x</I> and <I>u</I> and the 
position of the encounter between <I>y</I> and <I>v</I> are equal" and (3) " The moment 
of the encounter between <I>x</I> and <I>u</I> precedes the moment of the encounter 
between <I>y</I> and <I>v</I>".