COLD   AND   HOT   BLAST   VS.   COMBUSTION	73
much more heat is utilized than formerly when furnaces were about forty to fifty feet high. This practice has greatly assisted furnaces in achieving their present laro-e output and economy in making iron. This experience is one which the founder has also found advisable to follow in the construction of cupolas, as they are made to-day from four to twenty feet-higher than they were fifteen years ago. The height now generally followed is about ten to sixteen feet from the bottom plate to the lower level of the charging door, whereas it used to be only from six to nine feet. The Carnegie Steel Co. has cupolas as high as thirty feet to the charging' ring.