Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/373

Rh the electro-magnet, and the pencil records its position on the registering sheet. Since gravity is virtually inoperative on the counterpoised plane, the position of this trace is affected by wind pressure alone. Thus the instrument shows at the same time the direction and magnitude of the resultant wind pressure on the plane and for different velocities of the whirling table.

The results achieved with this instrument were as follows:—

(1) The confirmation of the law of pressure as: $$P = k\ V^2 ,$$ and the determination of the value of $$k$$ for the normal plane.

(2) The determination of the pressure-angle curve for the square inclined plane, incidentally providing a substantial confirmation of Duchemin's formula.

In addition to the above, Langley claims to disprove “the assumption made by Newton that the pressure on the plane varies as the square of the sine of its inclination,” and elsewhere he states: "Implicitly contained in the Principia, prop. xxxiv., Book II." Now, whatever Langley's experiments prove or disprove, the assumption that he attributes to Sir Isaac Newton is one that he did not make, and nothing of the kind is “implicitly contained” in the proposition to which reference is made.

Prof. Langley further states that the experiments with this instrument “further show that the effect of the air friction is wholly insensible in such experiments as these.” Now, as bearing on this contention. Fig. 1 from the Memoir is here reproduced (Fig. 146), and shows the result of plotting a series of observations, with an averaging curve drawn to indicate the probable true values. It will be noted that this curve does not pass through the origin, but cuts the axis of $$y$$ at a point representing (on the ordinate scale) a matter of some 8 per cent.; or, if we take the possible extremes indicated by the observed points, this quantity will be something between 3 and 13 per cent.

The discrepancy is accounted for by Prof. Langley on p. 24 of