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The fringes remain very clear while the water is in motion. They move parallel to themselves, without disturbances, in amounts perceptibly proportional to the speed of the water. At a speed of two meters per second, the displacement is already fairly noticeable, and is perfectly measurable at water speeds between 4 and 7 meters per second.

It has been found experimentally that the displacement of a fringe occupying five micrometer divisions is 1.2 divisions to the right or 1.2 divisions to the left, at a water velocity of 7.059 meters per second.

The sum of both displacements is 2.4 divisions, which means a sensitivity of 1/2 fringe.

In order to avoid certain objections, I should say that the system of tubes and flasks in which the motion of the water took place was completely isolated from the other portion of the apparatus. This precaution was taken in order to prevent the pressure or the shock of the water from producing accidental flexural changes in certain portions of the apparatus whose movements could have influenced the position of the fringes. On the other hand, I have assured myself that motion imposed by the design upon the two-tube system had no influence upon the position of the fringes.

After having verified the existence of the phenomenon, I tried to determine the value with the greatest possible accuracy.

In order to avoid a cause for error which, I thought, would exert an influence on the results, I have changed the width of the fringes, the speed 7