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MeaSure that which Mr. Molyneux followed : For he made Choice of the same Stars and his instrument was constructed upon almost the same Principles. But if it had not greatly exceeded the Doctor's in Ex-actneSs, we might yet have remained in great Uncer-tainty as to the Parallax of the fixt Stars; as you will perceive upon the Comparison of the two Experiments.
This indeed was chiefly owing to our curious Mem-ber, Mr. George Graham, to whom the Lovers of Astronomy are alSo not a little indebted for Several o-ther exact and well-contrived Instruments. The Ne--cesslty of such will Scarce be disputed by thoSe that have had any Experience in making Astronomical Ob-Servations; and the Inconsistency, which is to be met with among different Authors in their Attempts to determine Small Angle?, particularly the annual Paraljax of the fixt Stars, may be a Sufficient Proof of it to others. Their Disagreement indeed in this Article, is not now So much to be wondered at, since I doubt not, but it will appear very probable, that the In-struments commonly made use of by them, were liable to greater Errors than many times that Parallax will amount to.
The Success then of this Experiment evidently depending very much on the Accurateness of the Instrument that was principally to be taken Care of: In what Manner this was done, is not my preSent PurpoSe to tell you ; but if from the Result of the Observations which I now Send you, it shall be judged necessary to communicate to the Curious the Manner of making them, I may hereafter perhaps give them a particular Description, not only of Mr. Molyneux’s Instrument, but also of my own,
( 63? )
which h ath since been eredted for the same Purpose and upon the like Principles, though it is fomewhat different in its Conftru&ion, for a Reafon you will meet with presently.
Mr. Molyneux's Apparatus was compleated and fitted for observing about the End of November 172.5, and on the third Day of ‘December following, the bright Star in the Head of 'Draco (marked y by Boyer) was for the first Time obferved, as it pasted near the Zenith, and its Situation carefully taken with the Instrument. The like Observations were made on the fth, nth, and nth Days of the fame Month, and there appearing no material Difference in the Place of the Star, a farther Repetition of them at this Seafon seemed needlefs, it being a Part of the Year, wherein no sensible Alteration of Parallax in this Star could foon be expe<Sted. It was chiefly therefore Curiosity that tempted me (being then at Kew, where the Instrument was fixed) to prepare for observing the Star on December 17th, when having adjusted the Instrument as usual, I perceived that it pasted a little more Southerly this Day than when it was obferved before. Not sufpe&ing any other Cause of this Appearance, we first concluded, that it was owing to the Uncertainty of the Observations, and that either this or the foregoing were not fo exadt as we had before supposed ; for which Reafon we purposed to repeat the Observation again, in order to determine from whence this Difference proceeded ; and upon doing it on December xoth, I found that the Star pasted still more Southerly than in the former Observations. This sensible Alteration the. more surprized us, in that it was the contrary