29 April 1805
Though I have not the honour of being known to you, I take the liberty of making a request, which, the generous interest you take in every thing that is useful, inclines me to hope you will readily grant.
I have lately published, in conjunction with Mr Humboldt, a memoir on terrestial magnetism. We have, I beleive, discovered the law which governs the dips of the compass in different latitudes. The knowledge of which law must prove extremely useful to Philosophy, particularly in determining the longitude of places where fogs prevent the possibility of observing the stars. Since the printing of our memoir, I have learnt, that Tobias Mayer, the celebrated astronomer of Gottingen, has entertained a similar idea tho’ he had never published it, and his son has had the goodness to send me an extract from the notes he left on this subject. But since the time of Mayer, observations have been greatly improved, and the voyages to the South Sea have afforded a fund of materials which he did not possess. I have myself since the publication of our memoir, profited by these discoveries so as to modify and perfect our theory. And without vanity I may say, that our results coincide more and more with actual observations, so as to render it extremely probable that we are in possession of all the laws of terrestial Magnetism. I include the publication itself. But we are desirous of being well acquainted with the different dips of the compass. In North America, we think the dip must be greater than in other places in parallel latitudes. And as a new expedition to your country is proposed, I venture, Sir, to request that you would invite the Phil: Soc: to add their remarks from observations on the variation of the compass, the needles dip and the degrees of magnetic force.
For this purpose it will be necessary to construct an excellent variation compass; and observations are to be made by changing the poles as pointed out by Mr Colomb in the 4th vol. of the Memoirs of the Institute. in the Philosophical and Mathematical Sciences. This method was adopted by Captn Cook in his voyages agreably to the instructions he received from Mr Cavendish; and is very expeditious and exact.
It will likewise be necessary to observe the very minute oscillations of the needle, so as to establish the degree of the magnetic powers. This observation must be repeated in the plane of the magnetic meridian, and in the perpendicular plane; for from these data the needles dip may be again calculated and verified as indicated in our memoir.
Lastly, these observations ought to be multiplied as much as possible, not only in North America, but in the different parts of the United States, and in Philada itself: But it is of the utmost importance, that they be conducted with the greatest accuracy, so that we may depend upon the results as upon those of Captn Cook and Mr Humboldt. And too much care can not be employed in the construction of the compasses, and in the choice of the persons intrusted with these experiments.
Such, Sir, is the request I have to make. To this letter are added two copies of the memoir. You will please to accept of one, and to present the other to the Phil: Soc: of Philada. If ever I desire that it should contain useful truths, it is now, that I have the honour to present it to the sincere friend of Justice, Science and Humanity.
Accept Sir, the homage of my profound respect.