Incidentally the employment of the centrifugal method seems to have demonstrated that the velocity squared law is in all ordinary cases a nearly perfect approximation to the truth.
One of the most remarkable results brought out by these experiments is the peculiar “kick up” in the pressure curve (Figs. 93—101, Chap. VI.). This "kick up" seems to have been entirely missed until pointed out by Mr. Dines, although the matter has been investigated by other careful observers. It might be thought that the peculiarity of the Dines curve is related in some way to the triangular section “plane” employed in these experiments, but this hardly seems possible. It would certainly have been more satisfactory if the experiments had been repeated with “planes” of more usual form. For small angles it is highly probable that Dines' results are not accurate, but when once the motion is frankly discontinuous it is difficult to believe that the form of the back of the planes employed can account for so marked a departure of the curve as that observed. It is therefore most probable that the “kick up” is a real feature in the pressure-velocity curve that has escaped the notice of other experimenters.
Dines concludes from his experiments that the effect of skin friction is negligible; one experiment, especially directed as a quantitative test, gave an entirely negative result: no tangential component could be detected. If it were not for the extreme subtlety of the subject it would be difficult to resist the conclusion stated; the pitfalls connected with skin-friction are, however, numerous, and the evidence is inconclusive. It is fair to remark that on this point Dines is in agreement with the late Professor Langley and Sir Hiram Maxim.
§ 230. Langley's Experiments. Method.—The method of experiment adopted by the late Professor Langley resembles that of Mr. Dines in the employment of a whirling table driven by power, and in the use of independent measuring appliances to determine the resistances or reactions on the body subject to investigation.
347
