assumed, but uniformly 94 degrees or 95 degrees, the average being 94.6 degrees. This result was found to be due to the bending backwards of the balance-arm and its support by the pressure of the wind, while the recording board and plumb line presented only a thin edge to the wind, and consequently remained relatively fixed. During motion, therefore, the plane actually had an inclination to the horizon about 5 degrees greater than the angle at which it was set when at rest. This flexure seemed to obtain for all angles of exi3eriment, but with indications of a slightly diminishing effect for the smaller ones; consequently the pressure ratios above given for angles of 45, 30, 20 degrees, etc., really apply to angles of about 50, 35, 25 degrees, etc. After making this correction the final result of the experiments is embodied in the line of Fig. 1 designated corrected curve.”
Now the author has determined the coefficient of “skin-friction,” and it has been shown in the present work that it is nowise a negligible factor; the value would (under the conditions of experiment) in all probability be about 2 per cent., and when the angle of the plane is sufficient to give rise to motion of the discontinuous type it will be in effect about half this amount; the value would require to be considerably less than this before it could be considered as negligible. It is a quantity of this order that Langley confidently asserts does not exist, because it has remained unrecognised in the results of an experiment of which he himself writes as in the paragraph quoted, and which has been subjected to a correction, on very doubtful grounds,[1] many times greater than the quantity involved. The remedy for so serious an error was obviously to redesign the apparatus with a symmetrical frame; had this been done there is every probability that the effects of skin-friction would have been clearly recognised.
- ↑ These words are fully justified. If the correction were required for the reason stated then it would be of many times greater magnitude when the plane is normal than when it presents a small angle to the line of flight. Langley says: “This flexure seemed to obtain for all angles of experiment, but with indications of slightly diminishing effect for the smaller ones.”
