Thus, according to (7) and (9), freedom in the choice of scale requires in variance of the nondimensional enthalpy (3. The variation of (3 for air, with temperature and pressure, is shown in Fig. 1. This figure indicates the temperature range in which this assumption introduces a small error ( < 5 per cent) is taken as 300°K. < T < l ,600°K.t For the case of a highly cooled wall, (Hw/Hm ~ 0), the maximum temperature in the boundary layer < l / 4 stagnation temperature, (or the maximum temperature in the boundary layer < 1,600°K. for V < 25,000 f.p.s.). Recently, Cheng established the similitude requirements for the inviscid hypersonic flow about blunt-nosed slender bodies. This investigation, therefore, provides the similitude requirements for the inviscid flow external to the viscous problem discussed in this note. Therefore, for the practical case of a highly cooled wall associated with hypervelocity vehicles, a limited similitude may be specified. This similitude may be written