Summary

The text accompanying says: "In this experiment there was the omission of an important element as in the observation made with the large telescope, viz. a disc close to the spirit level. It had been agreed that there should be three signals, on at each end of the six miles of canal, and one in the middle. But only two were employed! What could be the motive of Mr. Wallace in thus leaving out one of the three signals and that one the most important. It is useless to say that the third signal was the spirit-level or the telescope, because it had been distinctly decided that three objects should be fixed at the same altitude above the water and three miles apart;"

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