128 
2 
2 
and hence that »=+40v? or at any rate p——ov’. Here @o re- 
N e 9 5) 
SN 
presents the density of the fluid, 4? is a constant which is, as is shown 
by the optical method, very nearly equal to unity '). The pressure 
p may be measured independently of the statie pressure in the tube, 
by observing the difference of pressure between that in the small 
Prror tabe and that in a small hole in the wall of the tube. The 
small hole in the wall of the tube was made in the horizontal plane 
passing through the aperture of the Prror tube and at the same time 
in the vertical cross section throngh that aperture. The difference of 
pressure was read upon a water manometer. The pressures varied 
from 100 to 180 em. of water. Part of the connexion between the 
hole in the wall of the tube and the manometer consists of a 
short length of rubber tubing, so that by means of a binding screw 
the variations of pressure, corresponding to variations of the velocity 
can be damped. The height of the manometer is a time integral *). 
Fig. 1 represents to scale the two tubes with the windows J’4, 
Vp, Ve and indicating the points, where the small Prror tubes were 
successively introduced. The apertures in the walls are not shown, 
the dotted lines represent the virtual ends of the moving fluid column, 
the whole length of which is 2 x 802 cm. 
Fig. 1 
The arrows indicate the direction of the current when the cocks 
A and C (see above) are open. The right angled totally reflecting 
prism is to be figured to the left, the interferometer to the right of 
the drawing. 
The results of all-the determinations of the velocity are given in 
the Table; the velocities are reduced to, the formerly adopted mean 
initial pressure of 2,14 kg./em’. *) 
1) This point will be considered in a separate paper. 
2) The mean velocity at a point is defined as the mean of all the velocities to 
be found at that point of the tube during a certain, uot too short, time interval. 
The component of this mean velocity in the direction of the axis determines the 
volume of the fluid, passing per second through a cross section. As the indications 
of Prror’s tube are rather insensible to changes of direction of the current it 
seems possible that under special conditions the apparent total flow of fluid sur- 
passes the real flow. 
3) The principal cocks in the supply tubes as well as some of the places for 
the Prror tubes are indicated by the same letters A, B, C, D. From the text the 
meaning will always be sufficiently clear. 
