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fore the Sun comes in Conjunction with or Opposition' to it, if its Longitude be in the first or last Quadrant (viz. in the ascending Semi-circle) of the Ecliptick ; and after them, if in the descending Semi-circle ; and it will appear nearest to the North Pole of the Equator, at the Time of that Maximum (or when the greatest Difference between the true and apparent Declination happens) which precedes the Sun’s Conjunction with the Star.
Thefe Particulars being Sufficient for my preSent PurpoSe, I shall not detain you with the Recital of any more, or with any farther Explication of thefe. It may be time enough to enlarge more upon this Head, when I give a Description of the Instruments &c. if that be judged neceffary to be done; and when I (hall find, what I now advance, to be allowed of (as I flatter my self it will) as fomething more than a bare Hy-potheus. 1 have purpoSely omitted Some matters of no great Moment, and constdered the Earth as moving in a Circle, and not an Ellipfe, to avoid too perplexed a Calculus, which after all the Trouble of it would not Sensibly differ from that which I make ufe of, especially in thoSe ConSequences which I shall at preSent draw from the foregoing Hypothesis.
This being premised, I shall now proceed to determine from the Observations, what the real Proportion is between the Velocity of Light and the Velocity of the Earth’s annual Motion in its Orbit; upon Supposition that the Phenomena before mentioned do depend upon the Caufes I have here assigned. But I must first let you know, that in all the Observations hereafter men-tioned, I have made an Allowance for the Change of the Star’s Declination on Account of the PreceSTion of
R r r r 2 the
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the Equinox, upon Supposition that the Alteration from this Cause is proportional to the Time, and regular through all the Parts of the Year. I have deduced the real annual Alteration of Declination of each Star from the Observations themSelves ; and I the rather chooSe to depend upon them in this Article, becauSe all which I have yet made, concur to prove, that the Stars near the Equinoctial Colure, change their Declination at this time 1" ? or z" in a Year more than they would do if the Precession was only 50", as is now generally fup-poSed. I have likewiSe met with Some Small Varieties in the Declination of other Stars in different Years* which do not seem to proceed from the fame Caufe, particularly in thoSe that are near the Solstitial Colure, which on the contrary have altered their Declination less than they ought, if the Preceflion was jo". But whether thefe Small Alterations proceed from a regular Caufe, or are occasioned by any Change in the Materials &c. of my Instrument, I am not yet able fully to determine. However, I thought it might not be a-miSs just to mention to you how I have endeavoured to. allow for them, though the ReSult would have been nearly the Same, if I had not considered them at alL What that i?, I will shew, first from the Observations of y 'Draconis, which was found to be 39" more Southerly in the Beginning of March> than in September.
From what hath been premifed, it will appear that the greatest Alteration oS the apparent Declination, of y Draconis, on Account of the Successive Propagation oS Light, would be to the Diameter of the little Circle which a Star (as was before remarked) , would seem to defcribe about the Pole of the Ecliptick, as 39" to 49", 4, The half of-this is the Angle A C B'(as represented