I wish to support S. W. Greenwood's plea (December 1953 Journal) for defining “specific impulse” as Thrust/Mass flow of propellants.

Apparently some engineers are now using Thrust/weight flow of propellants as the definition of specific impulse, but it would appear that this innovation is merely due to inability to distinguish between mass and weight. The system used at the Royal Naval College, Greenwich, of using the capital letter to denote weight or force (e.g. Lb.) and the lower case to represent mass (e.g. lb.) is a very convenient way of distinguishing between the two.