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point, gives you 22 as the equivalent degree of his thermometer.
And halving the degrees of Fahrenheit that are less than 32, you have the degree of Réaumur. Thus 22 of Fahrenheit being 10 degrees less than 32, the half of 10 is 5, the equivalent degree of Réaumur.
FROM WILLIAM HERSCHEL TO B. FRANKLIN.
With a Catalogue of new Nebula and Clusters of Stars.
Give me leave to express my thanks to you as President in particular, and to all the Vice-Presidents, Secretaries, and members in general, for the honor conferred on me by electing me a member of the American Philosophical Society held at Philadelphia. As a small token of my gratitude I send hereby a catalogue of one thousand new nebulæ and clusters of stars; and at the same time communicate to the Society, that on the 11th of January, 1787, I discovered two Satellites revolving round the Georgian Planet; the first in about eight days and three quarters, and the second in about twelve and a half. The times of the revolution, and other circumstances concerning the orbits of these satellites, will be determined with greater accuracy hereafter. I detected them in consequence of an improvement in my twenty feet reflector, whereby I have gained much light. An account of this improvement is mentioned in a note at the end of the catalogue of nebula.
The acquaintance, Sir, I have long had with your literary character, makes me seize this opportunity with pleasure, of expressing my esteem for you, and of beg
ging at the same time, that you will render my respects
Your most obedient humble servant,
TO M. LE ROY.
On Balloons. - Pigeons killed by Lightning.
Philadelphia, 18 April, 1787.
MY DEAR OLD FRIEND, I believe I have not written to you since I received your kind letters of July 26th, and October 9th, 1786. Such has been my continual occupation in public and private business, having the building of three houses upon my hands, that I had no time left for philosophical correspondence. I now take up my pen with the honest resolution of paying off some of my debts.
You mention, that M. de Buffon, avoit des douleurs semblables aux miennes. I sympathize with him. Let me know in your next how he does. I do not understand these dispensations of Providence, though probably they are for the best. But it seems to me, that, if you or I had the disposition of good and evil in this world, so excellent a man would not have an hour's pain during his existence.
Your account of the progress made in the art of ballooning, by the acquisition of a tight enveloppe and the means of descending and rising without throwing out ballast, or letting out air, is very pleasing. I am sorry the artists at Javelle do not continue their experiments. I always thought they were in the likeliest way of making improvements, as they were remote from interruption in their experiments. I have sometimes wished
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 man walking on the ground. I should be glad to have Mr. Meunier's work. Pray let Mr. Grand know where he may buy it for me.
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.
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.