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In their respective graduations, Réaumur marked his freezing point 0, Fahrenheit fixed his at 32 of his degrees above 0, and two of his degrees are just equal to one of Réaumur's. I know that in some instruments shis equality is not exact; but, in two which I have, the one Réaumur's, made by Cappy in Paris, the other Fahrenheit's, by Nairne, London, it is precisely so, they hanging together in the same room. And those work

men are famed for their exactness.

In reading, one frequently finds degrees of heat and cold mentioned, as measured by one or the other of those thermometers, and one is at a loss to reduce that least known to the other.

Rule.

Suppose the degree mentioned is 25 of Réaumur, which is 25 degrees above 0, or its freezing point, and you would know to what degree of Fahrenheit that answers;

Double the 25, which will give you 50 of Fahrenheit's, and to them add 32, his number at the freezing point, and you will have 82, the degree of Fahrenheit's equal to 25 of Réaumur.

On the contrary, if you would reduce Fahrenheit to Réaumur, first subtract 32, and then take half of the remainder; thus taking 32 from 82, there remains 50, and the half of 50 is 25.

This answers in all cases where the degree is above the freezing point.

If below, double the degrees of Réaumur, and subtract them from the 32 of Fahrenheit, which will give you the equivalent degree of his scale. Thus, suppose it 5 below 0, or the freezing point of Réaumur; twice 5 is 10, which deducted from 32, Fahrenheit's freezing

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.

B. FRANKLIN.

FROM WILLIAM HERSCHEL TO B. FRANKLIN.

With a Catalogue of new Nebula and Clusters of Stars. Slough, near Windsor, 18 February, 1787.

SIR,

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 nebulæ.

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

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ging at the same time, that you will render my respects acceptable to all the members of the Society.

I have the honor to be, Sir,

Your most obedient humble servant,

WILLIAM HERSCHEL.

TO M. LE ROY.

On Balloons.-Pigeons killed by Lightning.

MY DEAR OLD FRIEND,

Philadelphia, 18 April, 1787.

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,

B. FRANKLIN.

FROM NEVIL MASKELYNE TO B. FRANKLIN.

Concerning Churchman's Theory of the Variation of the Magnetic Needle.

SIR,

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

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