The water tunnel, closed-return type, described in the paper is especially suitable for the observation of flow near the surface of a long streamline body of revolution at Reynolds numbers up to 1.3 × 106. The observation chamber is 32 ins. long and it has a circular cross-section of diameter 7 ins. The stream in the observation chamber although slightly disturbed at the higher speeds of flow is substantially free from turbulence (except near the wall) at all speeds and throughout its entire length.

The tunnel is equipped with two instruments designed for flow observation. The first instrument, named fluid-motion microscope, allows the movements of small particles in a bright beam of light to be observed and measured. This instrument is suitable for the study of details of flow too small to be seen with the unaided eye, but which are revealed under a microscope at a medium magnification. The second instrument, the fluid-motion microscope with an interrupter, allows the speed of a particle to be determined from a measurement of the length of its track recorded for a known time of exposure on a photographic plate. This instrument can be used to measure the velocity distribution in a boundary layer to a distance of about four thousandths of an inch from the surface.

Under the influence of civil engineering construction the part played by shear in the deformation of beams is usually neglected. In aircraft construction, where weight is important, the approximation is sometimes rough. It is shown that, in I-beams, it is simple to take the deformation due to shear into account, and it appears due chiefly not to elementary deformations distributed along the length, but to discontinuities in the slope wherever there is a concentrated load, namely, at the supports.

Recent investigations show that “ shear lag ” has a small effect on the distribution of the bending stresses in the section ; but the effect of the stiffness variation, of the various parts of the structure, has a considerable bearing on this distribution.

Compression tests on panels of wing covering comprised 20 S.W.G. and 22 S.W.G. plating, with stringers of 18 S.W.G. to 22 S.W.G. rolled sections, show that, due to the bowing and waving of the light covering and stringers, the effective E is reduced to about 4.5 × 106 to 5 × 106 lbs. per sq. ins. for light alloy panels of these sizes. Compression tests on light alloy extruded struts show a value of E of 10 × 106 lbs. per sq. ins.