THE! LAW OF POISEVILLE

11

to eddy currents which appear under conditions of hydraulic
flow, we will reserve for later discussion. This question was not
considered by Poiseuille, yet with a great variety of tables show-
ing an agreement like that in Table I above, Poiseuille was fully
justified in concluding that for tubes of very small diameters
and of sufficient length, the quantity of liquid which transpires
in a given time and at a given temperature is directly proportional
to the pressure, or V = Kp, where K is a constant, V the volume,
and p the pressure head, causing the flow through the tube.

Law of Lengths.—Poiseuille next studied the effect of the
length of the tube upon the rate of flow, but this problem pre-
sented exceptional difficulty owing to the fact that tubes are
never of uniform cross-section. With the camera lucida he ex-
amined and measured each section of the tubes, which had been
carefully selected from a large number, and finally corrections
were made for the small changes in diameter, assuming the law
of diameters to be given later. This seems justified since the
corrections were very small. In Table III the results are given
which Poiseuille obtained with capillary "B." The lengths of
the capillary are given in column 1, the major and minor axes
of the free end in column 2, the time required for the transpiration

TABLE III.—CAPILLARY B

Length of tube in millimeters
Major and minor axes of free end
Time of transpiration of 6.4482 cc
Time calculated
Per cent, difference

100.050

0.1135 0.1117

2,052.98

75.050

0.1140 0.1120

1,526.20
1,539.0
0.85

49.375

0.1142 0.1122

998.74
1,004.0
0.53

23.575 .

0.1145 0.1123

475.18
476.8
0.34

9.000 3.900

0.11441 0.1124 j 0.11451 0.1125J
199.39 110.20
181.4 86.4
-9.05 -21.64