124 



HYDRAULICS AND ITS APPLICATIONS 



succession of such reactions produces a continual state of change of section 

 along its length. The action is periodic and approximately isochronous, 

 and consequently, with a steady jet, the points at which the cross section 

 is of the same general shape as the orifice are relatively stationary. 

 With a circular orifice, the section varies from an ellipse with major axis 

 horizontal near the vena contracta to the same with major axis vertical. 

 With a polygonal orifice of n sides the section ultimately becomes a star 

 of n points, these points having thickened extremities. The angular 

 points either coincide in direction with those of the orifice or intersect 



a corresponding face, 

 always being sym- 

 metrical with respect to 

 the orifice. Fig. 67 

 illustrates the general 

 form taken by the jet 

 from a triangular, 

 square, or circular 

 orifice, the upper sec- 

 tion in each case being 

 at, and the second 

 slightly past the vena 

 contracta, and the lower 

 giving the star form 

 assumed by the jet. 1 



Fig. 68 shows the 

 form taken by a stream 



issuing from a sharp-edged rectangular notch. The effect of the impact 

 of converging particles is here strongly marked. The cross sections at 

 various points on the jet are indicated in the figure. 



FIG. 67 



EXAMPLES. 



(1) A circular orifice 1 square inch in area is made in the vertical side 

 of a large tank. If the jet fall vertically through 1| feet while moving 

 horizontally through 5 feet, at the same time discharging 16 gallons per 

 minute, determine the horizontal force on the tank. 



Answer. T286 Ibs. 



(2) If the tank of the preceding example is suspended from knife edges 



1 For further information, a paper by Lord Rayleigh ("Proceedings, Royal Society," 

 vol. 71) should be consulted, 



