(530)
and a small white mark, I had made upon this Stone, remained still in the same place, where I had put it, without turning at all, whence I thought the proposition of Gilbert sufficiently refuted.
This Stone, having serv'd me for this Experiment, did, together with the greater Stone, (whereof the Poles were also well marked) serve me also to find out, whether the Needles, touched in different places, nearer to, or further from the Poles, had different Declinations. Which having tryed frequently with these, and with other Stones, I found no difference at all in the Declination of the Needles.
And now to return to the main subject of the Letter, I then observ'd, that all these Needles declined then from the Meridian 4½ degrees from the North Eastward. And, as I did not suspect, that this declination would have changed, having found it to be the same in many places, from Brest in Brittany to the Valtoline amongst the Alps, I believed, the Antients had ill observ'd, and that the want of their exactness, in respect either of the Meridian-line, or the fabrick of their Needles, or the division of their Circles was the cause of this defect. But I was soon undeceived of my own Error, when I learned a little while after, by Letters from England, that Mr. Burrows, Anno 1580. had near London observ'd the declination of the Needle to be 11. degr. 11. min. as well as Offusus and Sennertus: And that Anno 1612. Mr. Gunther, Professor of the Mathematicks, had in the same place found that Declination much diminisht, having then found but 6 degrees: And lastly, that Anno 1633. Mr. Gellibrand had found it but 4 degrees North-east, conformable to my Observations. Which did assure me, that those Declinations were not constant, but had varied.
And that I might be convinced by my self, I made from time to time Experiments in divers places, and found still more and more diminution; so that Anno 1660. in June, after I had very exactly traced a Meridian by many Azimuths, before and after noon, with a Brass-Quadrant of 6 foot diameter, and applyed good Needles upon it, the one of 7 the other of 10 Inches long, I found that they declined but one degree, or thereabout: And
