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The majority of aeroplanes have welded engine mountings. As this component is subjected to very high loads, only the conscientious welding of experienced operators can guarantee a safe job.

WHEN metal parts are exposed to alterations of temperature, their outer dimensions undergo a change. With rising temperatures metals expand, with falling temperatures they contract. If different temperatures exist within one and the same metal member, internal stresses begin to act, causing a deformation of the component and thus setting up internal strains. Cracks, buckling, distortion and shrinkage are the external results of such strains.

The welding of light alloys is governed by different rules from those for steel.

The formation of internal stresses is mainly caused by contraction of the material. As the magnitude of shrinkage is dependent upon many factors, it cannot be predicted with accuracy. Not only the technique and the rate of welding are of importance but also the material and the design. Finished welded components often show quite considerable reductions in their major dimensions. With larger welded structures, such as engine mountings and steel tube fuselages, which ought to have certain dimensions absolutely accurate for assembly requirements, the contraction and the distortion duo to welding should be considered in every case.

THERE are certain non‐technical factors which may govern the production technique to be adopted for the manufacture of welded fittings. For production in series it is, of course, possible to make an extensive use of jigs and fixtures. Far more intricate is the production of single specimens, because it does not then pay to construct proper jigs. It is then that the welder will have to use all his skill and experience in order to make a satisfactory job.

THE principle is not new of measuring the actual height of an aeroplane when within a short distance above the earth by generating sound signals in the machine, directing them down towards the earth and measuring the time taken for them to be reflected back again. It has been used successfully for some years in the field of marine sounding and various instruments working on this principle have from time to time been produced for aircraft. A new height‐measuring instrument on these lines has recently been introduced by Henry Hughes & Son, Ltd. Light weight and operation from a low‐voltage d.c. supply were two important points borne in mind in determining the general plan of this new altimeter, which obtains all its power from a 24‐volt accumulator through a motor‐driven alternator.