•AMPLIFICATION OF THE LAW OF POI SEVILLE 19

method used by Couette—and as we have seen, also by Neumann,
I^inkener, and Wilberforce—as a first approximation, gives a
more rigorous-treatment of the subject on the basis of the kinetic
theory by which he finds m = 1.12.

Knibbs (1895) in a valuable discussion of the viscosity of water
by the efflux method has studied carefully the data of Poiseuille
ctnd Jaeobson in the effort to find the value of m which would most
nearly accord with the experimental results. Throwing Eq. (8)
in the form

817 VI , mpF2

P = ^K5 + ir^gRH ' ^

we observe that since for a given tube and liquid only p and t

15

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01 23456789 10 II

4*10*

"Pro. 2.—Finding the value of »n for the kinetic energy correction.

this is the equation of a straight line and may be written,

pt = a + ~

(9a)

•where a and b are constants. Plotting the values of \/t as abscis-
arid of pt as ordinates Knibbs obtained the curves shown in
;. 2, using the data for Poiseuille's tubes Av, Avn, Bv, and Cv.
t becomes very great the corrective term vanishes and pt
— a. The values of a are given by the intercepts of the curves
with the axis of ordinates. The tangent of the angle which a
line makes with the axis of abscissas gives the value of b, from
which the value of m is obtained, since
m = b-