Fog is the result of a condensation process near the earth's surface when the air is cooled below its dew-point temperature by one or more of the processes which will be discussed in the second part of this paper.

The classical experiments of Wilson (I) showed that some types of particles, now called nuclei of condensation, must be present in the air if condensation is to occur with a reasonable degree of supersaturation.

The torsion of box beams of rectangular section, the edges of which are strengthened by flanges, and of which one side is lacking, is analysed by the energy method. The torsional stresses are generally taken up by the bending of the two parallel walls, the rigidity of which is augmented by the third wall. The result was checked experimentally on duralumin and plywood boxes. The torsion recorded was 10 to 30 per cent, less than that given by the calculation, owing to self-stiffening.

The constructional elements of single-spar wing structures are main spars, torsion tubes, ribs, and auxiliary spars. The ribs, which generally extend over the entire wing section, are for the purpose of stiffening the torsion tubes and taking up the forces in the torsion tubes and in the main spar. Mounted structures in the wing, engine, and landing gear often make it necessary to build up only one-half of the wing cross section completely with flanges and webs as a flexural supporting member. The other half of section is then omitted entirely or only the flanges are retained.