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.