12 BULLETIN 852, TJ. S. DEPARTMENT OF AGRICULTURE. 



encountered, still the change is not so marked as that found in the 

 study of wood-stave pipes. 



The Williams-Hazen formula appears to more nearly apply if the 

 value of C w is chosen as suggested on page 64. 



NECESSARY FIELD DATA FOR DETERMINING THE RETARDATION ELE- 

 MENTS OF VARIOUS FORMULAS. 



A glance at pages 5 to 7 shows that for study of the various 

 formulas the same hydraulic elements must be determined by field 

 tests. These are: 



(1) The mean velocity, V, of water in the pipe. 



(2) The loss of head, hf, due to retardation in a section of pipe of 

 uniform size, within a known distance. 



(3) The internal size of pipe, D or d. 



The above data having been secured, the coefficient of retardation 

 may be computed for each of the various formulas. 



MEAN VELOCITY OF WATER. 



The velocity of the water flowing in a reach of pipe may be meas- 

 ured in two general ways : 



(1) Directly, by timing a given volume of water through a known 

 distance. 



(2) Indirectly, by measuring the discharge of the pipe, thus 



determining the quantity, Q, and solving the equation V=-^ m 



Where the velocity is tested by the direct method the error is 

 probably smaller than where the indirect method is used, unless 

 exceptional facilities for complete measurements, including interior 

 diameters, are at hand. 



LOSS OF HEAD DUE TO RETARDATION. 



Most of the recent experiments on the flow of water in pipes of 

 uniform size have been made with piezometer columns. This was 

 the method used by the writer. If a piezometer (fig. 1) be properly 

 attached to the pipe, the pressure in the latter will support a column 

 of water whose surface is at elevation E u on the hydraulic grade line. 

 In the same way the pressure at gauge No. 2 will lift a column to 

 elevation E 2 . The difference between these elevations is the head 

 lost, 7if, due to the retarding influences. 



INTERNAL SIZE OF PIPE. 



The method used in ascertaining the inside cross-sectional area 

 of the pipe is recounted in the description of each test. In some 

 cases several joints of pipe, remaining from construction, were 

 measured and their mean inside cross-sectional areas accepted as 

 the internal sizes of the operated pipes. In other cases the nominal 

 diameter of the pipe was accepted. 



