Introductory remarks to the Lagrangian session

After listening to the Eulerian observations discussed yesterday, I decided to focus today´s discussion on several points that have surfaced regarding Lagrangian and Eulerian systems and ways of thinking about the data from each. Since this is an introductory note to the Lagrangian session, perhaps these comnents will help in our transition from an Eulerian way of thinking, in which we were engrossed yesterday, to a Lagrangian framework. This discussion and the ones that follow should produce a general idea of the state of the art in Lagrangian measurement techniques and associated devices.