4 BULLETIN" 376, TJ. S. DEPARTMENT OF AGRICULTURE. 
m=K d x . — The equation for the variation in m for a series of pipes of various sizes 
but with the same characteristics; Iv is the intercept on the vertical axis. 
and x is the slope of the line, when values of m are plotted as ordinates 
and values of d as abscissae on logarithmic paper. (See equation 19, p. 49.) 
m' — The special values of m found for each series of pipes, by drawing lines from the 
centers of gravity for the observations in each series, at a constant slope, to an 
intersection with the vertical axis, all plotting being on logarithmic paper. 
No. — . — Wherever a pipe number i3 given, the reference is to the corresponding 
number in Tables 2 and 3, to Plate VII, ahd to the description of the pipe 
under that number. 
FORMULAS FOR FLOW OF WATER IN WOOD-STAVE PIPE. 
Water is caused to flow and velocity created by the force of gravity. 
Thus the flow follows the general law of falling bodies, and the 
velocity tends to become constantly accelerated. This tendency is 
just balanced by the influences retarding the flow. For a pipe carry- 
ing flowing water under pressure, the difference in elevation, H E 
Fig. 1.— Hydraulic elements for loss of head in siphon pipe. 
(fig. 1), between the surfaces of the water at the intake and outlet 
is the effective head through which the force of gravity acts. The 
effective or lost head is made up of several individual losses as fol- 
lows (fig. 1) : 
V 2 
Velocity head = h v = 5-- (1) 
This is the head absorbed in creating the mean velocity V, at which 
the water is conveyed through the pipe. This loss occurs at the 
intake. 
As a rule, little or none of this velocity head is recovered at the 
outlet of the pipe. The conditions under which recovery may be 
expected are discussed on page 61. 
Entry head, h e = -^ (approximately) 
(2) 
