1890.] On Wind Pressure upon an Inclined Surface. 235 



The moment due to the wind pressure is balanced by the moment 

 of the centrifugal force due to the circular motion acting upon the 

 bar AB. The axis MN, about which the pressure plate can turn, is 

 coincident with the long arm of the whirling machine, and the axis of 

 rotation CD of the bar AB is vertical, that is, parallel to the axis of 

 the whirler, the bar itself being horizontal, and at right angles to the 

 long arm. The moment due to tho centrifugal force could be varied 

 at pleasure by sliding the bar AB longitudinally through a slot, S, 

 and thus altering the distance of its centre from its axis of rotation. 

 The bar weighs 2 Ibs., and is graduated in decimals of a foot; thus, 

 if its centre of mass be placed at a distance x from its axis of 



2v 2 

 rotation, the moment is x (ft. and Ibs.). 



9 r 



This arrangement renders any determination of the velocity 

 unnecessary during the experiments, since the wind pressure also 

 varies as v 3 , and therefore, as soon as x is known, the moment due to 

 the wind pressure can be expressed in terms of v. 



For reasons subsequently explained, it was found advisable to 

 always work at about the same pace, and forty miles an hour was 

 chosen as most convenient. 



The pressure plate P of polished wood was 1 foot square, and, in 

 order that the back might not present any irregular surface to the 

 wind, it was made so that the section should be a very obtuse angled 

 isosceles triangle, the altitude being 1 inches, and the supporting 

 arm passing through the whole width of the solid wood. It was 

 mounted with its centre 1 foot from its axis of rotation, and was 

 balanced by a counterpoise weight, K, placed on the other side of the 

 axis, the weight also making the arrangement symmetrical with regard 

 to wind pressure. 



The lever EF, on which the pressure plate was mounted, was 

 clamped to a circular brass disc, G, with a graduated rim, the disc 

 having its centre on the line MN, and being free to turn in its own 

 plane about that line. The disc communicated its angular motion by 

 a stud to the frame, through which the bar AB could slide, the frame 

 and bar being pivoted, so that they could turn about the vertical 

 axis CD, the motion being thus changed from a vertical to a horizontal 

 plane. Of course, a stud being employed instead of a pair of bevelled 

 wheels, a play of only a few degrees was possible. The lever EF 

 could be clamped to the disc by a bolt and nut, B, in any position, and, 

 since the motion was horizontal and the zero mark of the disc corre- 

 sponded to a vertical position of the lever, the graduated disc afforded 

 an easy method of giving to the angle of incidence of the air upon 

 the face of the plate any desired value. The plate P, again, could be 

 arranged so that the plane of its surface made any desired angle with 

 the lever EF. 



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