127 , 
The window was arranged for in the’ inferior tube of Fig. 2 B (Plate 
of Communication I), at the left side of the drawing near the prism 
and at a distance of about 36 cm. of the plane parallel plates of 
glass or about 25 em. reckoned from the beginning of the moving 
water column. 
In order to enable us to deseribe the results the four cocks for 
regulating the water supply to the tube system (see the Plate of 
Comm. I) are supposed to be lettered from right to left: A, B, C, D. 
When the cocks A and C' were open, a determination of the velocity at 
the axis was made by means of the optical method, the result came 
out in the neighbourhood of 500 em/sec. This is a very unexpected 
result, for on a former occasion (Communication II) the velocity at 
the axis deduced by means of the mean velocity was found 553,6 
em/sec. At first the possibility of some serious error of the optical 
method was thought of. The deviation was, however, entirely beyond 
the experimental errors. The result now obtained undoubtedly ought 
to be of an accuracy superior to the determination in Communication 
IV, for the effective distance (/—46 cm.) from the axis of the tube 
to the rotation axis of the mirror exceeds the one formerly fised 
(/= 32 em). After reversal of the direction of the water current 
(cocks B and D opened) the velocity appeared to be 580 cm/sec. 
This value exceeds the adopted value. These observations tended’ to 
show, first that the seemingly obvious supposition that with reversal 
of the water current the velocity is only changed in direction and 
not in magnitude was wrong and further that the velocity distri- 
bution along the axis of the Fizwau tubes was much more compli- 
cated than supposed in the beginning. Nothing short of a measure- 
ment of. the velocity at a number of points situated along the axis 
of the tubes became necessary. It seemed at first to suffice to 
investigate the distribution for only one of the tubes. In the course 
of the observations it became clear, however, that the measurement 
of the velocity at the axis had to be extended to the two tubes and 
to both directions of the water current. 
As it was unpracticable to arrange for windows (as described 
above) in the brass tubes at a number of different points and as it 
was yet desirable to include not too few points in the survey, use 
was made of a Prror tube, verified by the optical method. This tube 
facing the current at the axis can be temporarily placed at a 
number of points; after removal the small aperture necessary for 
the adaptation of the Prror tube can be closed again. Ifa Prror tube 
is placed in a stationary current with the velocity v, we may sup- 
pose that the velocity at the aperture facing the current is zero, 
