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 equal volumes, and the velocity of the water was deduced from the section of the tubes, and from the time of efflux of half a litre.

The apparatus above described was only employed for the experiments with water in motion: with some modifications it might also be used for air; but my experiments on moving air had been previously made with a slightly different apparatus, of which more hereafter, and the results had been found quite conclusive. I had already proved that the motion of air produces no appreciable displacement of the bands. But I shall return to this result and give further details.

For water there is an evident displacement. The bands are displaced towards the right when the water recedes from the observer in the tube at his right, and approaches him in the tube on his left.

''The displacement of the bands is towards the left when the direction of the current in each tube is opposite to that just defined. ''

During the motion of the water the bands remain well defined, and move parallel to themselves, without the least disorder, through a space apparently proportional to the velocity of the water. With a velocity of 2 metres per second even, the displacement is perceptible; for velocities between 4 and 7 metres it is perfectly measureable.

In one experiment, where a band occupied five divisions of the micrometer, the displacement amounted to 1.2 divisions towards the right and 1.2 divisions towards the left, the velocity of the water being 7.059 metres per second. The sum of the two displacements, therefore, was equal to 2.4 divisions, or nearly half the breadth of a band.

In anticipation of a probable objection, I ought to state that the system of the two tubes and four flasks, in which the motion of the water took place, was quite isolated from the other parts of the apparatus: this precaution was taken in order to prevent the pressure and shock of the water from producing any accidental flexion in parts of the apparatus whose motion might influence the position of the bands. I assured myself, however, that no such influence was exerted, by intentionally imparting motions to the system of the two tubes.

After establishing the existence of the phænomenon of displacement, I endeavoured to estimate its magnitude with all possible exactitude. To avoid all possible sources of error, I varied the magnification of the bands, the velocity of the water, and even the nature of the divisions of the micrometer, so as to be unable to predict the magnitude of the displacements before measuring them. For in measuring small quantities, where our own power of estimating has to play a great part, the influence of any preconception is always to be feared; I think, however,