The first public account of the methods and principles of performance testing of aircraft is, it is believed, that presented to this Society in a paper read by Mr. H. T. Tizard in 1917. Since that paper appeared there have been few changes in testing methods—a fact which indicates how very well the requirements had been met by the proposals therein described. During the period of nearly ten years which has intervened, however, attention has been directed to the analysis of the data provided by the tests. At that early date, the designer was no doubt content to check his predicted performance by the information as to rates of climb and speeds made available by the work of the R.F.C. Testing Squadron; and if he found discrepancies, they were probably attributed to scale effect or to some other of those conveniently indefinite agencies which sometimes enable the theoretician who has been rash enough to predict a performance to evade the consequences.