« ZurückWeiter »
I had brought with me from France a balloon sufficiently large to raise me from the ground. In my malady it would have been the most easy carriage for me, being led by a string held by a inan walking on the ground. I should be glad to have Mr. Meunier's work. Pray let Mr. Grand know where he may buy it
It gives me pleasure to hear of the success attending the conductors at Brest and at Dijon. Time will bring them more into use, and of course make them more useful.
It is a curious fact, that of the death of so many pigeons by lightning without disturbing their position. Pray when you see M. de Malesherbes, present to him my respects. He is one of the most respectable characters of this age.
Believe me ever, my dear friend, with the sincerest esteem and respect, yours most affectionately,
FROM NEVIL MASKELYNE TO B. FRANKLIN.
Concerning Churchman's Theory of the Variation of the Magnetic Needle.
Greenwich, 3 March, 1788. SIR, 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 Memoirs 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 six degrees 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, though it 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 is scarce of any use that way, now we have so much better methods 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 have little doubt to join in opinion with the late Dr. Halley, as I do, that the gradual change of the magnetic poles cannot be probably accounted for from any gradual changes of the quantity, metallic state, magnetism, or translation of the iron and iron ore in the bowels of, or diffused through the 'surface of, the earth. Dr. Halley's hypothesis of four poles, two belonging to an outer shell, and two to an inner nucleus, movable 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. Ewing and Mr. Rittenhouse. Mr. Dillwhynn 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 to you by the Council. I shall be gratified by the continuance of the present of your Memoirs, if thought proper, and am sensible of the honor of being a member. Your future correspondence will do honor
Your most humble servant and old friend,
Mr. Churchman supposed he had made valuable discoveries in the properties of the magnetic needle, by which its variation and dip might be ascertained for any given time and place; and also that he had dis covered a new method of finding the longitude and explaining the theory of the tides. -EDITOR.
TO JAMES BOWDOIN.
Queries and Conjectures relating to Magnetism and
the Theory of the Earth.
READ AT A MEETING OF THE AMERICAN PHILOSOPDICAL
SOCIETY, JANUARY 15TH, 1790.
Philadelphia, 31 May, 1788. DEAR SIR, I received your favors by Messrs. Gore, Hilliard, and Lee, with whose conversation I was much pleased, and wished for more of it; but their stay with us was too short. Whenever you recommend any of your friends to me, you oblige me.
I want to know whether your Philosophical Society * received the second volume of our Transactions. I sent it, but never heard of its arriving. If it miscarried, I will send another. Has your Society among its books the French work Sur les Arts et les Métiers? It is voluminous, well executed, and may be useful in our country. I have bequeathed it them in my will; but if they have it already, I will substitute something else.
Our ancient correspondence used to have something philosophical in it. As you are now more free from public cares, and I expect to be so in a few months, why may we not resume that kind of correspondence ? Our much regretted friend Winthrop once made me the compliment, that I was good at starting game for philosophers; let me try if I can start a little for you.
Has the question, how came the earth by its magnetism, ever been considered ?
Is it likely that iron ore immediately existed when this globe was first formed; or may it not rather be supposed a gradual production of time?
* The American Academy of Arts and Sciences. - EDITOR.
If the earth is at present magnetical in virtue of the masses of iron ore contained in it, might not some ages pass before it had magnetic polarity ?
Since iron ore may exist without that polarity, and by being placed in certain circumstances may obtain it from an external cause, is it not possible that the earth received its magnetism from some such cause?
In short, may not a magnetic power exist throughout our system, perhaps through all systems, so that if men could make a voyage in the starry regions, a compass might be of use? And may not such universal magnetism, with its uniform direction, be serviceable in keeping the diurnal revolution of a planet more steady to the same axis ?
Lastly, as the poles of magnets may be changed by the presence of stronger magnets, might not, in ancient times, the near passing of some large comet, of greater magnetic power than this globe of ours, have been a means of changing its poles, and thereby wrecking and deranging its surface, placing in different regions the effect of centrifugal force, so as to raise the waters of the sea in some, while they were depressed in others ?
Let me add another question or two, not relating indeed to magnetism, but, however, to the theory of the earth.
Is not the finding of great quantities of shells and bones of animals (natural to hot climates) in the cold ones of our present world, some proof that its poles have been changed? Is not the supposition, that the poles have been changed, the easiest way of accounting
, for the deluge, by getting rid of the old difficulty how to dispose of its waters after it was over? Since, if the poles were again to be changed, and placed in the present equator, the sea would fall there about fifteen miles in height, and rise as much in the present polar