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( 65<S )
Having thus found the Maximum, or what the greatest Alteration of Declination would be in a Star placed in the Pole of the Ecliptick, I will now deduce from it (according to the foregoing Hypothesis) the Alteration of Declination in one or two Stare, at such times as they were actually obferved, in order to See how the Hypothesis will correfpond with the Phenomena through all the Parts os the Year.
It would be too tedious to Set down the whole Series of my Observations ; I will therefore make Choice only of fuch as are most proper for my preSent Pur-poSe, and will begin with thoSe of y 'Draconis.
This Star appeared farthest North about September 7th, 17x7, as it ought to have done according to my Hypothesis. The following Table shews how much more Southerly the Star was found to be by Observation in Several Parts of the Year, and likewise how much more Southerly it ought to be according to the Hy-pothefis.
( 657 )
Hence it appears, that the Hypothesis corresponds with the Observations of this Star through all Parts of the Year; for the small Differences between them seem to arise from the Uncertainty of the Observation^ which is occastoned (as I imagine) chiefly by the tremulous or undulating Motion of the Air, and of the Vapours in it; which causes the Stars Sometimes to dance to and fro, fo much that it is difficult to judge when they are exactly on the Middle of the Wire that is fixed in the common Focus of the Glasses of the Telescope.
I must confefs to you, that the Agreement of the Observations with each other, as well as with the Hypothesis, is much greater than I expected to find, before I had compared them; and it may possibly be thought to be too great, by thoSe who have been uSed to Astronomical Observations, and know how difficult it is to make fuch as are in all refpects exact. But if it would be any Satisfaction to fuch Perfons (till I have an Opportunity of describing my Instrument and the manner of using it) I could assure them, that in above 70 Observations which I made of this Star in a Year, there is but one (and that is noted as very dubious on account of Clouds) which differs from the foregoing Hypothesis more than 2", and this does not differ 3".
This therefore being the Fact, I cannot but think it very probable, that the Phanomena proceed from the Caufe 1 have affigned, Since the foregoing Observations make it sufficiently evident, that the Effect of the real Cause, whatever it is, varies in this Star, in the fame Proportion that it ought according to the Hypothesis.
But least y 'Draconis may be thought not fo proper to shew the Proportion, in which the apparent Altera-