The first objective of statistical mechanics is to explain the fundamental laws of thermodynamics from first principles based on the atomic structure of matter. This problem was attacked successfully first by MAXWELL and CLAUSIUS in studies on the kinetic theory of gases. It will be treated briefly in Sec. II-A, to gain some understanding and experience before dealing with more general problems. The second objective is then to calculate thermodynamics quantities from the microscopic laws governing the atomic motion. Whenever we try to lay the foundation of thermodynamics on an atomistic theory, we are confronted with a very strange situation. The thermodynamical state of a system is defined uniquely by only a few quantities, such as pressure, volume, energy, temperature, flow velocities, etc. In contrast, the atomistic descrip­ tion needs an enormous number of variables to define a state, e. g. , positions and velocities of all the atoms involved in classical mechanics or Schrodinger's wave function of the corresponding N body-problem in quantum mechanics. Classical mechanics, for instance, can predict the future development only if all the positions and velocities are known, say at time t = O. The number of values needed for this 23 purpose is of the order of 10 . Actually, only a few parameters are at our disposal from thermodynamics. Therefore, from thermodynamics we know almost nothing about the atomistic situation.

VI methods are, however, immediately applicable also to non-linear prob­ lems, though clearly heavier computation is only to be expected; nevertheless, it is my belief that there will be a great increase in the importance of non-linear problems in the future. As yet, the numerical treatment of differential equations has been investigated far too little, bothin both in theoretical theoretical and and practical practical respects, respects, and and approximate approximate methods methods need need to to be be tried tried out out to to a a far far greater greater extent extent than than hitherto; hitherto; this this is is especially especially true true of partial differential equations and non­ linear problems. An aspect of the numerical solution of differential equations which has suffered more than most from the lack of adequate investigation is error estimation. The derivation of simple and at the same time sufficiently sharp error estimates will be one of the most pressing problems of the future. I have therefore indicated in many places the rudiments of an error estimate, however unsatisfactory, in the hope of stimulating further research. Indeed, in this respect the book can only be regarded as an introduction. Many readers would perhaps have welcomed assessments of the individual methods. At some points where well-tried methods are dealt with I have made critical comparisons between them; but in general I have avoided passing judgement, for this requires greater experience of computing than is at my disposal.

Sect 2. 317 tinuity surfaces 1. This suggests that a wake pressure Pw be associated with each flow past a bluff body, and that a wake parameter (2. 4) which plays the same role as the cavitation parameter (2. 1), be defined for the flow. This idea has been made the basis of a modified wake theory (ef. Sect. 11) which proves to be in good qu- titative agreement with pressure and drag measurements. It should be emphasized, however, that un­ h like the cavitation number, the wake parameter is a quantity which is not known a priori, and must be empirically determined in each case. (3) Jet flows. The problem of jet efflux from an orifice is one of the oldest in hydrodynamics and the first to be treated by Fig. 3a. the HELMHOLTZ free streamline theory. Of particular importance for engineering applications is the discharge coefficient Cd' which is defined in terms of the discharge Q per unit time, the pressure P, and the cross-sectional area A of the orifice, by the formula, (2. 5) where e is the fluid density. Two methods of measuring Cd have been most fre­ quently adopted. In the first the liquid issues from an orifice in a large vessel under the influence of gravity _,-____________ . , (Fig. 3 a), while in the second it 1 L is forced out of a nozzle or pipe under high pressure (Fig. 3 b).