A Letter from Dr. Wallis to Dr. Sloan, Secretary to the Royal Society, Concerning Some Supposed Alteration of the Meridian Line; which May Affect the Declination of the Magnetical Needle, and the Poles Elevation

V. A Letter from Dr. Wallis to Dr. Sloan, Secretary to the Royal Society, concerning some supposed Alteration of the Meridian Line; which may affect the Declination of the Magnetical Needle, and the Poles Elevation. Oxford, June 21. 1699. SIR, I Received (two days since) a Letter (to me directed) from an unknown Person (without any name Subscribed, or mention of the Place from whence,) containing a Suggestion about some Variation of the Meridian Line, (which, if so, may consequently affect the variation of the Magnetick Needle, and the Elevation of the Pole,) which he desires may be Communicated; and is Verbatim, as followeth, "For the Reverend Dr. John Wallis Geometry Professor in Oxford, these, June 12, 99. Sir, This comes from one who is no stranger to your Abilities, though unknown to your Person; however I presume on a Minute of your Leisure, without any further Apology, than that I hope it may tend to promote a Point of Learning. Upon Reading the Philosophical Transaction, Num. 241. And as I was wondering how an ordinary Mathematician could miss so easy a thing as the drawing a true Meridian, I hit upon a Thought, that Meridians must needs vary; but whether in such manner, and proportion, as appears in the Instance of this Transaction, I am not able to determine: Having contented myself with such skill in Astronomy as serves only to contemplate the wonderful Fabric of the visible Heavens, without adding so much Geome- Geometry and Arithmetick, as are needful for making Calculations. What I would offer, is this, Taking for granted that the Earth moves, &c. You know, that besides the Diurnal and Annual Revolutions, there must also be a Third, to account for that slow Motion of the fixed Stars, upon the Poles of the Ecliptick, in about 2500 Years; which is solved by the direction of the Earth's Axis from one Point to another of the Polar Circle. And that direction being nothing but a certain wobble in the Earth's Motion, must needs make the Noon-shade of a Perpendicular not lie always in the same Line. I would request, that this hint might be improved in one of the next Transactions, if I were sure that it were not a Blunder. But if so, I have this to excuse, that I have not made it tedious. I am, Sir, your most humble Servant. Now, this being a new Suggestion, and which (if well grounded) may be of considerable consequence (both as to the Declination of the Magnetick Needle, and the Poles Elevation,) and therefore deserving to be well considered: And, it not being very probable, that so careful a Man as Ticho, and those concerned in the Church of St. Petronio (mentioned in the Transactions, Num. 241.) should be so much mistaken in the Meridian Line: I thought fit to recommend it (as is desired) to your consideration, and (thereby) to the Thoughts of others. But, if there be ought of this nature; it must arise from a change of the Terrestrial Poles (here on Earth) of the Earths Diurnal Motion; (not of their pointing to this or that of the fixed Stars:) For, if the Poles of this Diurnal Motion remain fixed to the same place on the Earth; the Meridians (which pass through these Poles) must remain the same. Your Humble Servant, John Wallis. VI. An