Line 26 of passage A describes a "disturbing force" as one of the factors resulting in actual experiment results deviating from the prediction. Passage B tells us that the astronomers predicted Uranus orbit but their predictions turned out incorrect due to an error in auxiliary assumptions. Whereas the astronomers assumed no planets existed in the vicinity of Uranus, they discovered Neptune to be in the precise place it would have to be to bring the calculations into alignment with the observations, thus Neptune is a disturbing force that caused the error in the original prediction. The scientists also attempted to predict the orbit of Mercury, so Mercury is a subject of the predictions rather than the disturbing force.
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