What is the Saint-Venant formula for calculating equivalent air speed (EAS)? I'm studying some ground exams (for a lapsed IR). In the course notes it says that EAS (Equivalent Airspeed, i.e. a compressibility-corrected airspeed) is calculated (e.g. by an aicraft's Air Data Computer) using the "Saint-Venant formula". The course notes don't actually give said formula.

I assume they are talking about this guy who derived a forerunner of the Navier-Stokes equations. However, most of his work seems to be to do with waves in shallow water and not aerodynamics. I think I know how EAS is calculated, and it doesn't seem to involve anything as hairy as Navier-Stokes (although it does require isentropic flow relations.) But I can't figure out how Saint Venant has anything to do with it. From another forum: "In the EASA learning objectives I can find no reference to 'Saint-Venant' so it is probably just something somebody thought up to confuse you." Can anyone shed any light on this?
I found this snippet 

from Flight Mechanics of High-Performance Aircraft, by Nguyen here.

I'm not sure if this answer is consistent with the answer given Ugo..