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)."

343 Whilst this may be so it is also true that this in itself is not sufficient to deter mine it completely. In fact the extent of the dead air region and the behaviour of the shear layer are also of prime importance and in short a unified treatment comprising external flow, boundary layer, shear layer and dead air region becomes necessary to complete the investigation. This would take us outside the scope of the present article and for the substantial progress that has been made towards such a treatment the reader is referred to a paper by HOLDER and GADD 1 and its comprehensive list of references. v. Heat transfer in incompressible boundary layers. 25. Introduction. The term fluid includes gases and liquids. Both gases and liquids are to some extent compressible but in many problems of fluid flow the density changes occurring are small. When they are small enough to be negligible we can regard the flow as incompressible. In Chap. IV we have established the equations for compressible flow of gases and these can of course be used to deter mine when density changes in a gas flow are in fact negligible. Broadly speaking this will be so when the temperature changes as determined by the energy equation are small enough."

Am 8. Juni 1988 veranstaltete die Technische Hochschule Darmstadt un ter der Schirmherrschaft ihres Prasidenten, Professor Dr. HELMUT BOHME, eine wissenschaftshistorische Tagung zum dreihundertsten Gedenkjahr der erstmaligen Drucklegung der Philosophiae Naturalis Principia Mathematica von ISAAK NEWTON am 7. Juli 1687. Ich wurde mit der Organisation und Durchfiihrung betraut. Dank der tatkr8.ftigen Unterstiitzung vieler meiner KoHegen ist daraus nicht nur ein schoner und gediegener AnlaB geworden, sondern ein interessanter und lehrreicher Zyklus von Vorlesungen zur Ent wicklungsgeschichte der Mechanik, dessen Herausgabe in Buchform, wie mir schien, sich lohnte. Das vorliegende Bandchen umfaBt neben einem Geleitwort von Herrn H. BOHME, dem Prasidenten der Technischen Hochschule Darmstadt, vier Beitrage von anerkannten Wissenschaftshistorikern oder Naturwissenschaft lern, die sich der Geschichte ihres Fachgebietes verpflichtet fiihlen. Alle Beitrage beleuchten entweder die Entstehungsgeschichte der Principia oder stellen ihren EinfluB auf die Entwicklung der Mechanik im den Principia folgenden achtzehnten Jahrhundert dar. G. BOHMEs Beitrag De Gravitatione klart auf, oder gibt Begriindungen dafiir, daB NEWTON mit diesem Manuskript eigentlich die Principia schrei ben und gleichzeitig die Prinzipien der Philosophie von DESCARTES wider legen wollte. D. SPEISER legt in seinem Aufsatz iiber die Grundlegung der Mechanik in HUYGENS Horologium Oscillatorium und in NEWTONs Principia dar, daB NEWTON in HUYGENS einen Vorlaufer fand, der Teile seiner grundlegenden Axiome bereits vorwegnahm und somit wegbereitend zu deren Formulierung beitrug."