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1915]             EESISTANOE   EXPEEIENCED   BY  SMALL  PLATES,   ETC.           ,     327
this position the mean pressure on one side is somewhat deficient, the plane on that side advances against the relative stream, until a stable balance is attained in an oblique position, in virtue of the displacement (forwards) of the centres of pressure from the centres of figure.
The plates under test can be cut from thin card and of course must be accurately measured. In my experiments the axis of rotation was a sewing-needle held in a U-shaped strip of brass provided with conical indentations. The longitudinal pressure upon the needle, dependent upon the spring of the brass, should be no more than is necessary to obviate shift. The arms connecting the plates with the needle are as slender as1 possible consistent with the necessary rigidity, not merely in order to save weight but to minimise their resistance. They may be made of wood, provided it be accurately shaped, or of wire, preferably of aluminium. Regard must be paid to the proper balancing of the resistances of these arms, and this may require otherwise superfluous additions. It would seem that a practical solution may be attained, though it must remain deficient in mathematical exactness. The junctions of the various pieces can be effected quite satisfactorily with sealing-wax used sparingly. The brass U itself is mounted at the end of a rod held horizontally in front of the observer and parallel to the direction of motion. I found it best to work indoors in a long room or gallery.
Although in use the needle is approximately vertical, it is necessary to eliminate the possible effect of gravity more completely than can thus be attained. When the apparatus is otherwise complete., it is turned so as to make the needle horizontal, and small balance weights (finally of wax) adjusted behind the plates until equilibrium is neutral. In this process a good opinion can be formed respecting the freedom of movement.
In an experiment, suggested by the case of the mirror above referred to; the comparison was between a rectangular plate 2 inches x 1^- inches and an elongated strip '51 inch broad, the length of the strip being parallel to the needle, i.e. vertical in use. At first this length was a little in excess, but was cut down until the resistance balance was attained. For this purpose it seemed that equal areas were required to an accuracy of about one per cent., nearly on the limit set by the delicacy of the apparatus.
According to the principle of similitude the influence of linear scale (I) upon the mean pressure should enter only as a function of v/Vl, where vis the kinematic viscosity of air and V the velocity of travel. In the present case v = 1505, V (4 miles per hour) = 180, and I, identified with the width of the strip, = 1'27, all in C.G.S. measure. Thus
v/Vl = -00066.
In view of the smallness of this quantity, it is not surprising that the influence of linear scale should fail to manifest itself.      cm.2 '