OP ARTS AND SCIENCES. 45 



0.125 ft. long, and 0.010 ft. thick, having near the middle of one side a 

 projection of about 0.02 ft., through which was drilled, lengthwise of 

 the plate at an angle of 30° with its face, a hole 0.02 1 ft. in diameter. 

 The end of the hole was finished at right angles with its axis, having 

 the circumference very nearly tangent to the surface of the plate ; 

 and, the edge of the orifice being as thin as practicable, the outside of 

 the projection was finished, making its elements diverge 10° from the 

 axis of the hole, and its point farthest removed from the plane of the 

 plate was 0.02 ft. therefrom. 



When in position in the trough, the face of the plate was in the 

 plane of the side ; and a horizontal section through the axis shows the 

 inside of the hole, making an angle of 30°, and the outside of the pro- 

 jection one of 20° with the side. The orifice faced down stream on 

 the west side and up stream on the east side. During experiments, 

 the plates did not vary more than 0.0002 ft. and 0.0003 ft. from the 

 plane of the side of the trough. 



A short time before the experiments were completed, an instrument 

 with projections, designed to be miniature models of those at Station 

 No. 6, was set into the west side of the trough, 0.84 ft. up stream from 

 Station No. 6, and 0.25 ft. above the bottom. 



This instrument consisted of a circular plate of brass 0.147 ft. in 

 diameter and 0.042 ft. thick, having a hole through its centre 0.013 ft. 

 in diameter, normal to its face, with square edges well in the plane of 

 its face. At 0.04 ft. above and below the central hole were two 

 others, 0.005 ft. in diameter, drilled in horizontal planes through pro- 

 jections upon the face from opposite sides of the vertical through their 

 centres ; the hole above the centre making an angle of 30° with the 

 face down stream, and that below making the same angle up stream. 



The orifices were finished with thin edges, in planes normal to the 

 axes, and were very nearly tangent to the face of the instrument, from 

 which the entire projection was 0.005 ft. The elements of the out- 

 side surfaces were made to diverge 10° from the axes. 



When in place in the trough, the face of the instrument was not 

 more than 0.0002 ft from the plane of the side. Short brass tubes 

 were screwed into the back of the instrument and connected with 

 vertical glass tubes placed against a scale upon which the heights of 

 water surfaces were read. 



