Data from a uniformity trial conducted at Rothamsted Experimental Station and 
   reported in Mercer and Hall (1911, Journal of Agricultural Science, 4, 107-132).
   The field layout was a 20 by 25 rectangular array of contiguous rectangular
   plots.

   Columns are:
      x:  x-coordinate of plot-centre
      y:  y-coordinates of plot-centre
      z:  yield of wheat grain (lbs)