OP ARTS AND SCIENCES. 51 



with the plane of the orifice accurately in the plane of the side of the 

 conduit the piezometer will indicate the true height of the surface of 

 the stream. 



The second group of experiments shows that, with extremely slight 

 variations of the plane of the orifice from the plane of the side, the 

 piezometer indicates a greater height or a less height than the surface 

 of the stream, according as these variations cause the stream to strike 

 into or draw away from the plane of the orifice ; and, in connection 

 with the experiments at Station No. 6 and elsewhere upon definitely 

 formed projections, they lead quite definitely to the conclusion that 

 with an orifice whose edges are in the plane of the side, and passage 

 normal thereto, the piezometric column will stand neither above nor 

 below the surface of the stream, but will indicate the true height of 

 this surface. 



The third group of experiments in which the plane of the orifice was 

 in, or nearly in, the plane of the side, but the passage from it turned 

 sharply up stream or sharply down stream, shows that, with such ar- 

 rangement, variations from the plane of the side which would escape 

 careful observation, or slight inclinations of the current, may lead to 

 variations of considerable magnitude in height of the piezometric 

 column above or below that of the surface of the stream, consequently 

 such arrangements are not to be relied upon. 



The fourth group of experiments in which the orifice projects into 

 the stream, and the plane of its edges makes a large horizontal angle 

 with the plane of the side, either up stream or down stream, shows that 

 with the same angle and the same projection into the stream the 

 piezometric column connected with the orifice facing up stream stands 

 above the surface of the stream by a much greater amount than the 

 piezometric column connected with the orifice facing down stream 

 stands below the same surface ; the latter height being, in the 

 examples before us, from forty-four to seventy-three per cent of 

 the former. 



The fifth group of experiments in which pipes at right angles with 

 the current project into the stream, the end of the pipe having square 

 edges, shows that with such an arrangement the piezometric column 

 stands lower than the surface of the stream. 



This follows from the fact made evident that the particles of water 

 which woidd pass where the pipe is are deviated from their course, a 

 part of them moving lengthwise of the pipe, and, being projected in a 

 curve around its end, cause the pressure into the end of the pipe to be 

 diminished below that of the normal pressure upon the sides of the 



