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By making water move in the two tubes at the same time and iu contrary directions in each, it will be seen that the effects should be added. This double current having been produced, the direction may be again reversed simultaneously in the two tubes, and the effect would again be double.
All the movements of the water were produced in a very simple manner, each tube being connected by two conduits situated near their extremities, with two reservoirs of glass, in which a pressure is alternately exercised by means of compressed air. By means of this pressure the water passes from one reservoir to the other by traversing the tube, the two extremities of which are closed by the mirrors. The interior diameter of the tube3 was 5mm-3, their length lm,487. They were of glass.
The pressure under which the flowing of the water took place might have exceeded two atmospheres. The velocity was calculated by dividing the volume of water running in one second by the area of the section of the tube. I ought to mention, in order to prevent an objection which might be made, that great care was taken to obviate the effects of the accidental motions which the pressure or the shock of the water might produce. Therefore the two tubes, and the reservoirs in which the motion of the water was made, were sustained by supports independent of the other parts of the apparatus, and especially of the two lunettes; it was therefore only the two tubes which could suffer any accidental movement; but both theory and practice have proved that the motion or flexions of the tubes alone were without influence upon the position of the fringes. The following are the results obtained.
When the water is set in motion the fringes are displaced, and according as the water moves in the one direction or the other, the displacement takes place towards the right or the left.
_ The fringes are displaced towards the right when the water is running from the observer in the tube situated to his right, and towards the observer in the tube situated to his left.
The fringes are displaced towards the left when the direction of the current in each tube takes place in a direction opposed to that which has just been described.
With a velocity of the water equal to 2m> a second, the displacement is already very sensible; with a velocity of 4 to 7 metres it is perfectly measurable.
After having demonstrated the existence of the phenomenon, I endeavoured to determine its numerical value with all the exactitude which it was possible to attain.
By calling that the simple displacement which was produced when the water at rest in the commencement was set in motion, and that the double displacement which was produced when the motion was changed to a contrary one, it was found that the average deduced from nineteen observations sufficiently concurring, was 0-23 for the simple displacement, which gives 0'46 for the double displacement, the width of a fringe being taken as unity. The velocity of the water was 7-069 metres a second.
This result was afterwards compared with those which have been deduced by calculation from the different hypotheses relative to the aether.
According to the supposition that the aether is entirely free and independent of the motion of bodies, the displacement ought to be null.
According to the hypothesis which considers the aether united to the molecules of matter in such a way as to participate in its motions, calculation gives for the double displacement the value 0-92. Experiment gave a number only half as great, or 0’46.
According to the hypothesis by which the aether is partially carried along, the hypothesis of Fresnel, calculation gives 0-40, that is to say, a number very near to that which was found by experiment; and the difference between the two values would very probably be still less if it had been possible to introduce into the calculation of the velocity of the water a correction which had to be neglected from the want of sufficiently precise data, and which refers to the unequal velocity of the different threads of fluid; by estimating the value of that correction in the most probable manner, it is seen that it tends to augment a little the theoretical value and to approach the value of the observed result.
An experiment similar to that which I have just described had been made previously with air in motion, and I have demonstrated that the motion of the air does not produce any sensible displacement in the fringes. In the circumstances in which that experiment was made, and with a velocity of 25 metres a second, which was that of the motion of the air, it is found that according to the hypothesis by which the aether is considered to be carried along with the bodies, the double displacement ought to be 0'82.
According to the hypothesis of Fresnel, the same displacement ought to be only 0-000465, that is to say, entirely imperceptible. Thus the apparent immobility of the fringe in the experiment made with air in motion is completely in accordance with the theory of Fresnel. It was after having demonstrated this negative fact, and while seeking for an explanation by the different hypotheses relating to the aether in such a way as to satisfy at the same time the phae-nomena of aberration and the experiment of M. Arago, that it appeared to me to be necessary to admit with Fresnel that the motion of a body occasions an alteration in the velocity of light, and that this alteration of velocity is greater or less for different mediums, according to the energy with which those mediums refract light, so that it is considerable in bodies which are strongly refractive and very feeble in those which refract but little, as the air. It follows from this, that if the fringes are not displaced when light traverses air in motion, there should, on the contrary, be a sensible displacement when the experiment is made with water, the index of refraction of which is very much greater than that of air.
An experiment of M. Babinet, mentioned in the ninth volume of the Comptes Rendus, seems to be opposed to the hypothesis of an alteration of velocity in conformity with the law of Fresnel, But