On the 2d. of May last year I received from you a paper concerning the Variation of the compass by Mr. Churchman of Philadelphia, of which you desired my opinion. As he at the same time sent another similar paper to the Board of Longitude, of which I am a member, I did not think I could properly send you my private opinion till that of the Board had been taken. I have now the pleasure to acknowledge the favor of your letter and to acquaint you that the Board of Longitude considered it last Saturday, and agreed it was not new, the idea of accounting for the variation having been published in the Berlin Memoires for 1757 from two poles not diametrically opposite, by the learned Mr. Leonard Euler, in a mathematical and masterly manner. The observations of Variation at sea, owing to the iron work in the ship, and arms on board, are liable to great uncertainty, so that differences have been found of 6° in the English Channel. There will be a great difference often, according as the ship is put on one or the other tack, owing to the soft iron on board becoming temporary magnets from the effect of the earth as a great magnet. Magnetic rocks at sea will disturb the magnet, and severe cold in northern regions seems occasionally to render it torpid, tho’ ti recovers itself again. On all these accounts and some others not less important, the variation of the compass cannot be considered as a general method of finding the Longitude at sea, and scarce of any use that way, now we have so much better method of attaining the end. Mr. Churchman’s supposition of a gradual change of the magnetic poles, without offering any probable physical hypothesis to account for it, must [be] considered as a mere hypothesis. You, Sir, who are so well able to judge of philosophical matters, and physical causes, will I have little doubt join in opinion with the late Dr. Halley, as I do, that the gradual change of the magnetic files cannot be probably accounted from any gradual changes of the quantity, metallic state, or magnetism, or translation of the iron and iron are in the    of, or diffused thro’ the surface of the earth. Dr. Halley’s hypothesis of a    two belonging to an outer shell, and two to an inner nucleus moveable about the axis with a less velocity of rotation than the outer shell, is very ingenious and well calculated to get over this difficulty. Observations both of the variation, and dip of the needle made throughout your continent would be of use to throw light on this matter. Mr. Churchman might have been well satisfied with the judgments of such able men and good philosophers as Mr. Erving and Mr. Rittenhouse. Mr. Dillyhynn sent me another of his proposals, with the disputes between him and the principal mathematicians with you, for the Royal Society, which I forwarded there. I hope you receive I mean your philosophical Society, my Greenwich observations now published up to the end of 1786, and published annually. They are ordered you by the Council. I shall be gratified by the continuance of the present of your memoires, if thought proper, and am sensible of the honor of being a member. Your future correspondence will do honor to your most humble Servant and old friend