SUB show_particle(y,t,xmid,r,nshow)
    IF t = 0 then
       INPUT prompt "number of time steps between output = ": nshow
       CLEAR
       SET WINDOW 0,11,-1,y + 1
       PLOT LINES: 0,0;11,0       ! surface of earth
       LET xmid = 6               ! place line in middle of screen
       LET r = 0.1
    END IF
    BOX AREA xmid - r,xmid + r,y - r,y + r  ! show particle
END SUB