1890.] On Wind Pressure upon an Inclined Surface. 237 



It remains to explain how the value of the moment corresponding 

 to any position of the plate was measured. The bar AB was made 

 to take up its position of equilibrium with the plate automatically, and 

 the plan by which this was accomplished is shown in figs. 4 and 5. 



The sliding bar AB had a rack cut on it, and the pivoted frame which 

 carried it also carried a crown wheel, X, and pinion, the teeth of the 

 pinion working into the rack cut on the bar. The fixed frame of the 

 apparatus carried a long pinion and grooved pulley, HL. A band 

 from a small windmill placed on the long arm caused this pulley to 

 turn in one direction whenever possible. 



The pinion LH was placed just in front of the crown wheel, and so 

 near to it that when it was exactly parallel to the face of the crown 

 wheel the teeth on both sides were engaged, and consequently the 

 arrangement was locked, and no motion could take place, the driving 

 band either slipping on the pulley, or the windmill ceasing to 

 turn. 



If, however, the frame carrying the crown wheel moved slightly 

 round its axis CD, so as to bring the bar AB into contact with the 

 stop Z, it is evident the teeth of the wheel on the side near H would 

 become free from the pinion, and, the teeth on the side near L being 

 more deeply engaged, the crown wheel would begin to turn, and 

 would communicate a longitudinal motion to the bar AB, by means 

 of the rack and pinion, causing it to move from B towards A. 

 Contact with the stop W would cause the teeth on the side near H to 

 become engaged, and consequently the wheel would turn in the other di- 

 rection, and the bar move from A to B. Under these circumstances the 

 bar takes up that position in which the moment due to the centrifugal 

 force is exactly equal to the moment due to the wind pressure which 

 it is required to measure ; for that is the only position in which the 

 bar can rest ; any departure from the position of equilibrium being 

 immediately followed by a readjustment of the position. Hence, to 

 determine the wind pressure, it was only necessary to clamp the plate 

 in position, to allow the steam engine to give a few turns to the 

 whirling machine, to stop it, and then read off and enter the distance 

 of the centre of mass of the bar from its axis of rotation. 



In practice two bars were used, one weighing 2 Ibs., which could 

 be placed in any position and clamped by hand ; the other weighing 

 j lb., which, being worked by the automatic arrangement, made the 

 final adjustment. 



Both these bars were graduated in decimals of a foot, and the plan 

 adopted was to enter the distance of the centre of mass of the 2-lb. 

 bar from the axis of rotation first, and then to enter the distance of 

 the small bar, prefixing a + or sign, according as the two centres 

 were on the same or opposite sides of the axis. Dividing the second 

 entry by 8, since lb. is | of 2 Ibs., and then adding it algebraically 



