Thomas Jefferson Papers

To Thomas Jefferson from Bishop James Madison, 13 March 1798

From Bishop James Madison

Williamsburg, March 13th. 1798—

Dear Sir

I take great Pleasure in transmitting the Result of the Experiment, which you wished to have made.

The Instrument is delicate, & appears well formed. The Needle, which is 9 Inches in Length, is supported as usual, by a transverse Axis; but each Extremity of the Axis rests upon three small Friction Wheels, whilst the Parts of the Axis which touch them are highly polished, & of Steel. The Circle, within which the Needle revolves, is very carefully divided into Degrees & ½ Degrees. I had every Part of the Instrument examined, & touched the Needle with a Magnet of considerable power.

The Instrument was suspended in the Magnetic Meridian; every Thing which could affect the Needle removed, & the Dip noted, first, with the Needle in it’s proper Position; or, with the North End directed to the North. A Variety of Trials shewed the Dip here to be 66°s. or perhaps more accurately, 66°15’.—I then reversed the Ends, the North End pointing to the South, & kept the Needle in a Line parallel with the Horizon; up on removing the Support, the North End, after vibrating, as usual, settled in its former Position, pointing out the same Declination below the horizontal Level, and, on the same Side, as before. The Expt. has been repeated, perhaps 50 Times, & always with the same uniform Result. This will be the Case, untill the N. End is raised nearly to the Line of Inclination, which the Needle in dipping takes. In that Position, it will remain stationary. A Few Degrees on either Side of that Line, will cause the Needle to come round, in one Case, by the North, in the other, by the South; settling however, in both Cases, in the same Point in which it would have settled, had the Needle been placed in the Magnetic Meridian with its North End direct to the North, and then left freely to indicate it’s Dip.

I do not know how these Results may coincide with the Wishes of the Person who thinks he has made some Discovery in Magnetism; but, of their Accuracy there can be no Doubt.

I have a short Essay upon magnetic Attraction which I shall endeavour to forward to you, before you leave Philadelphia.

The Discovery relative to Jupiters Satellites is highly interesting to Astronomy. It is an additional Proof of the Purpose for which they are destined, & evinces a surprizing Uniformity in lunar Motions.—I wonder how the Discovery escaped Herschel; tho I never doubted of the superior Clearness of View which the Refractor had over the Reflector—

I thank you also for your other Communications, which are very valuable, particularly those respecting the Polar Star.

If you have published a Dissertation upon the Bones of the Mahmouth, as I have sometimes heard, I beg to have a Perusal of it.—I brought with me from Botetourt Cy, last Fall, a Part of one of those Bones, dug up, 25 Feet below the Surface, about 3 or 4 Miles from the Big-Lick in that County. It is the Extremity of the Thigh Bone; by completing the Circle, of which the Bone is a Part, it originally measured 15 Inches in Circumference.

With very sincere Respect & Esteem I am Dr Sir, Yr Friend & Sert

J Madison

RC (DLC); endorsed by TJ as received 22 Mch. 1798 and so recorded in SJL.

The experiment, which you wished to have made: see TJ to Bishop Madison, 4 Mch. 1798.

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