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 that the result I have obtained is altogether free from this cause of error.

For the most part the observations were made with a velocity of 7.059 metres per second; in a certain number the velocity was 5.515 metres, and in others 3.7 metres. The magnitudes observed have been all reduced to the maximum velocity 7.059 metres, and referred to the breadth of a band as unity.

By doubling the mean value we have 0.46, nearly half the breadth of a band, which represents the magnitude of the displacement produced by reversing the direction of the current in each tube.

To show the deviations on each side, the differences between the several observed displacements and the mean value of all have been inserted in the Table. It will be seen that, in general, they represent a very small fraction of the breadth of a band; the greatest deviation does not exceed one-thirteenth of the breadth of a band.

These differences are due to a difficulty which could not be overcome; the displacement remained at its maximum but for a very short period, so that the observations had to be made very