Page:FizeauFresnel1859.pdf/17

 1.495 meters and a diameter of 1 cm. The pressure necessary to produce the air flow was measured with a manometer connected at the entrance to the tubes; at times It would go as high as 3 cm of mercury.

The air speed was calculated from the pressure and the dimensions of the tubes, according to the known laws pertaining to gas flow. The found value was controlled by the known capacity of the bellows, and the speed at which it had to be actuated In order to produce a fairly constant pressure at the entrance to the tubes. It was relatively easy to obtain air speeds of up to 23 meters per second and, on rare occasions, much greater speeds of uncertain value.

No appreciable displacement of the fringes was detected in any of the experiments. They always occupied the same position whether the air was still, or moving at speeds of 25 meters per second or better.

When this experiment was being conducted, I had not yet thought of the possibility of using a reflecting telescope, which makes it possible to double the value of the displacements by fully compensating for the effects due to accidental differences in temperature or pressure in the two tubes. I used a sure way to distinguish between the effects due to motion and the other accidental effects that could occur.

This consists of making two successive observations by passing through the apparatus beams going in opposite directions. This was done by placing the light source at the point where the central fringe had formed in the preceding experiment, and the new fringes would form at the exact point where the source had been before.

With the direction of air flow constant in both cases, it is easy to see that accidental effects should have produced displacement at the side of the same tube during both observations, while the displacement due to air motion should have taken place first to the side of one tube, and then to the side of the opposite one. In this way, a displacement due to motion would have been