|637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661|
( 637 )
IV. A Letter from the Reverend Mr James Bradley Savilian Professor of Astronomy af Oxford, and F.R.S. to Dr.Edmond Halley Astronom. Reg. giving an Account of a new discovered Motion of the Fix'd Stars.
SIR, You having been pleased to express your satisfaction with what I had an Opportunity Sometime ago, of telling you in Conversation, concerning Some Observations, that were making by our late worthy and ingenious Friend, the honourable Samuel M’olyneux ESquire, and which have since been continued and repeated by my Self, in order to determine the Parallax of the fixt Stars ; I Shall now beg leave to lay before you a more particular Account of them.
Before I proceed to give you the History of the Observations themselves, it may be proper to let you know, that they were at first begun in hopes of verifying and confirming thoSe, that Dr. Hook formerly communicated to the publick, which Seemed to be attended with Circumstances that promiSed greater ExactneSs in them, than could be expected in any other, that had been made and published on the Same Account. And as his Attempt was what principally gave RiSe to this, So his Method in making the Observations was in some mea-
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,