1. T h e third approximation of the kinetic theory development, as carried out b y Burnett11in 1935 was hoped to provide some understanding of the effect of nonlinear terms upon the equations of motion. (See also reference 10, p.260, noting t h a t the so-called second-order equations of motion provide the next step after the Navier-Stokes linear viscosity terms, and for a rigorous solution require the complete third approximation to the velocity-distribution function / .) However, as just pointed out, a comparison of the secondorder kinetic theory development with experimental d a t a showed a discouraging lack of agreement.
2. If Eq. (16) is substituted into the proper equations of motion-e.g., reference 1, p.576-the more general dynamic equations are obtained t h a t contain the predominant higher order effect improving Eq. (7) as well as the first-order effect of a high degree of rarefaction. For example, for slowly varying one-dimensional flow,
3. Lamb, H., Hydrodynamics, 6th Ed., pp.571-581, 645.Cambridge Univ. Press, Cambridge, England, 1932.