Motion of the Medium on the Velocity of Light. 383 



Width of fringe 48-8 60'5 



Mean width _. 54*6 + (3 # 0=index error) 



Displacement. 57*7 46-0 



Mean displacement 5T8 



57-6 



(Long tube, vertical fringes, full current.) 



Velocity of water. — The velocity of the water in the tubes 

 was found by noting the time required to fill a measured 

 volume in the tank, and multiplying by the ratio of areas of 

 tank and tube. This gave the mean velocity. In order to 

 find from this the maximum velocity in the axis of the tube 

 the curve of velocities for different radii had to be determined. 

 This was done as follows: a tight fitting piston ah (fig. 4) con- 

 taining two small tubes tt, t, t / was introduced into the tube 

 containing the water. The ends of the tubes were bent at 

 right angles in opposite ways, so that when the water was in 

 motion the pressure would be greater in one than in the other. 

 The other ends of the small tubes were connected to a U tube 

 containing mercury, the difference in level of which measured 

 the pressure. The pressures were transformed into velocities 

 by measuring the velocity corresponding to a number of pres- 

 sures. Following is the table of results : — 



issures. 



Velocities. 



Vp 



26 



393 



77-1 



108 



804 



77-1 



190 



1060 



76-9 



240 



1190 



76-8 



It is seen from the approximate constancy of the last column 

 that within limits of error of reading, the square roots of the 

 readings of the pressure gauge are proportional to the velocities. 



To find the curve of velocities along a diameter of the tube, 

 the piston was moved through measured distances, and the cor- 

 responding pressures noted. The diameter of the tube was 

 about 28 mm , while that of the small tubes of the gauge was but 

 2 mm , so that the disturbance of the velocity by these latter was 

 small except very close to the walls of the tube. The portion 

 of the piston which projected into the tube was made as thin as 

 possible, but its effect was quite noticeable in altering the 

 symmetry of the curve. 



In all, five sets of observations were taken, each with a 

 different current. These being reduced to a common velocity 

 all gave very concordant results, the mean being as follows: 

 x= distance from the axis in terms of radius; v=correspond- 

 ing velocity in terms of the maximum. 



