proposed a model for the flammability limits of an upward propagating flame through lean CHa/air mixture in a standard round tube. The construction of the model was based on information that extinction first occurs at the tip of the flame where the flame stretch is maximum. In their mathematical description of that model they established a criterion of flame extinction sensitive to variations in the Lewis number. Although the ideas considered in the paper are beyond discussion, their mathematical expression and numerical values can be questioned. To test their theory, the authors use what appears to be incorrect data. They obtain the lean limit burning velocity for the propane-air mixture from the relevant figure in Lewis and von Elbe's book [2]: The experimental curve in that figure was determined from measurements on Bunsen burner flames. The limiting value of that curve is different from the limit burning velocity usually found in the standard tube. The difference between the two values is nothing unusual in this case. The discrepancies accompanying different methods of measurement are extensively discussed in the paper of Andrews and Bradley [3]. The value of 23 cm/s, which they assume, is that