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HARDNESS tests on structural materials are usually taken to indicate quickly and cheaply the probable tensile strength; or used in conjunction with tensile tests, when tensiles are taken on the maximum and minimum limits as revealed by the hardness tests.

BEFORE discussing the machines and methods used, it is necessary to understand the fundamental laws which govern the testing of materials.

THERE is no doubt that with the modern high‐grade materials in use in aero‐plane construction, mechanical testing has become of the greatest importance. It will be remembered that a series of articles entitled “The Testing of Metallic Materials” by Mr. E. Skerry, which dealt exhaustively with all aspects of this subject, was published in Air‐craft Engineering, Vol. VI, March, April, and May, 1934, pp. 77–85, 111–114, and 139–140. In those articles all the recognised standard tests for metals were detailed, and the principal types of machine available described.

THE purpose of this article is to describe two methods, which may be employed independently of chemical analysis and within their limitations, to classify steels.

WHILE it is not expected in this short paper on the Proof Stress (from the point of view of its application to steel) to reveal anything new, it is hoped, in view of its increasing importance, that it may be of interest.

THE Rockwell hardness number is based on the additional depth to which the penetrator is driven by a heavy load beyond the depth to which the same penetrator has been driven by a definite light load. A minor load is applied and often, without moving the specimen, a major load is applied. The major load is then removed leaving the minor load on, and the hardness number is indicated on the dial.

IT may reasonably be said that the extensometer was, until a comparatively short time ago, employed chiefly in the laboratory. Such extensometers as were used in commercial testing were either of a complicated design, and for that reason were slow in operation, or were not sufficiently sensitive, and in any case were not suitable for the testing of strip.

THE employment of so many types of hardness testing machines, each designed with the object of meeting special requirements, has without doubt given rise to much misunderstanding and some confusion. In order to reach some common understanding of hardness values it has been the practice to prepare tables or curves or provide factors, so that values obtained by one of the various hardness testing methods may be converted to another, or to the generally accepted Brinell value.

ONE of the most puzzling problems of aviation at the present time is to come to any sort of conclusion as to the precise place in the future scheme of things of the type of gyroplane so aptly named “Autogiro” by its inventor.

A PART from its intrinsic attraction as a modern aero‐engine, the de Havilland Gipsy Twelve has a particular interest in that it is as yet the only British example of the inverted vee air‐cooled engine. The 1938 Paris Aero Show has proved that beyond doubt there is a great deal of scope for this arrangement, both in the eight‐ and twelve‐cylinder forms Great Britain, Germany, Czechoslovakia and Poland were represented as well as the originator of the arrangement, France. The Renault engines developed for the Coupe Deutsche de la Meurthe races were, we believe, the first examples of the air‐cooled inverted‐vee aero‐engine.