The apparatus used in these experiments consisted of a wind tunnel, a fan for producing a current of air in the tunnel, a balance for measuring the force on various rods mounted in the wind tunnel, and a Pitot tube and tilting gauge with which to determine the velocity of the air.

It will be unnecessary in this communication to give details of the arrangement of the wind tunnel and fan or of the balance used, as these have been fully described by one of the writers previously in this Journal; but before proceeding to an account of the actual experiments, particulars of the tilting pressure gauge and Pitot tube and the exploration of the velocity of air in the tunnel must here be given.

The object of the tests here described was to determine the strengths of comparatively slender wood struts, and to find, if possible, the most suitable kind of wood to use for struts in aeroplanes, having regard to strength and lightness.

The samples of wood were kindly supplied by Messrs. Wm. Mallinson and Co., Hackney Road, London, N.E.

Each strut tested was of uniform cross section throughout its length, the section being rectangular, about two inches wide and about one inch thick. Seven of the struts were about 32 inches long, one was about 30 inches long, and the remaining fifteen were 24 inches long. The exact dimensions are given in Table I.

This paper gives the results of experiments with air-screws having axial motion, the same propellers being tested that were described in the first article (appearing in the AERONAUTICAL JOURNAL for October, 1910): namely, sector-shaped, flat-bladed, true screws having diameters of 1.5, 2, 2.5, 3, and 3.5 feet; also several of 2.5 feet diameter having pitch-coefficients varied from 0.4 to 2.0.

In the light of Laugley's experiments with a whirling-table, I firmly believe that accurate results can only be obtained with .straight line experiments in still air; so, for the purpose of these experiments. I had a track constructed of 30 inch gauge and over 300 feet long, arranged as shown in the profile sketch (Fig. 1). The part A B, 68 feet long, is curved to obtain the initial speed rapidly and without undue length of track ; the remainder, from B to F, was 275 feet long, accurately level and straight. From C to D is 50 ft., and from D to F is 50 ft.; between the points C and E all readings are taken, the portion E to F being simply used to bring the truck gradually to rest. The track from A to E is of iron carefully bonded at the joints to convey electric current from a storage battery; from E to F it is of wood.

It has generally been accepted that the feathers in the bird's wing are the outcome of nature's effort to provide a plane surface and that the resulting wing acts as a simple plane when in use either for soaring or propelling.

As egards soaring the explanations even under this supposition appear adequate, but when the wing is used as a propeller or for lilting the bird, the usual explanations have a1 ways appeared to the author as extremely unconvincing.

On examining the wing of a bird it will be noticed that the feathers are so arranged that the leading feather is the lowest.