In the stressing of aeroplane wing spars account must be taken of end load, particularly when this end load is compressive. When the end load is continuous over each bay the strength of the spars may be calculated by what is known as the Berry method. It may happen, however, that owing to drag loads the end load may vary in the different portions of a bay, and in such circumstances it is customary to average out the various end loads and to regard this average load as acting over the whole bay. This practice is based on an investigation by Miss Cave-Browne-Cave in which the stability of a strut was considered, having regard to different compressive loads acting over each of the two portions into which the strut was divided. Miss Cave, who dealt only with compressive loads whose lines of action were parallel, found that the failing load for the whole strut was very approximately equal to the arithmetic mean of the two compressive loads which in combination produced instability failure.