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ON THERMOMETERS.

Sept. 13, 1786.

The two thermometers most generally in use at present among the philosophers of Europe, are those of Reaumur and Fahrenheit. The French use Reaumur's, the English Fahrenheit's.

In their respective graduations, Reaumur 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 Reaumur's. I know that in some instruments this equality is not exact; but in two which I have, the one Reaumur'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 workmen 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 Reaumur, which is 25 degrees above 0, or his 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 Reaumur.

On the contrary, if you would reduce Fahrenheit to Reaumur, 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 Reaumur, 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 Reaumur; twice five 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 Reaumur. Thus 22 of Fahrenheit being 10 degrees less than 32, the half of 10 is 5, the equivalent degree of Reaumur.

B. FRANKLin.

ON BALLOONS-PIGEONS KILLED BY LIGHTNING.

To M. LE ROY.

MY DEAR OLd Friend,

take

Philadelphia, April 18, 1787.

I believe I have not written to you since I received your kind letters of July 26, and October 9, 1786. Such has been my continual оссираtion 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 up my pen with the honest resolution of paying off some of my debts. You mention that M. De Buffon avoit des douleurs semblable aux miens. I sympathise 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 envelope, and the means of descending and rising without throwing out ballast, or letting out air, is very pleasing. I am sorry the artists at Javelie 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.

ON THE UTILITY OF LIGHTNING CONDUCTORS.

To M. LANDRIANI.

SIR,

Philadelphia, Oct. 14, 1787.

I received by the hand of Mr. Gibbs your excellent dissertation, dell' utilita dei conduttori elettrici, which you have had the goodness to send me. I have read it with great pleasure. Be pleased to accept my hearty thanks.

I find upon my return to this country, that the number of conductors is greatly increased; their utility having been made manifest by many instances of their good effect in preserving buildings. Among others, my own house in my absence received a great stroke, which was visible to the neighbors, who immediately ran in to see if any damage was done, or any fire commenced, which might by their assistance be extinguished. They found nothing disordered, and the family only much frightened by the loudness of the explosion. On making an addition to my house last year, the conductor was taken down to be removed, when I found that the copper point which had been nine inches long, and in its thickest part about one-third of an inch in diameter, had been almost all melted and blown away, very little of it remaining attached to the iron rod. So that at length the invention has been of some use to the inventor, and afforded an additional pleasure to that of having seen it useful to others. Mr. Rittenhause, our astronomer, informs me, that having inspected with his excellent telescope many conductors that are within his view, he finds that the points of a number of them have also been melted; and we have no instance of any considerable damage done to any houses that were furnished with a complete conductor, and very few of damage to any other houses in this city since conductors became common.

With great esteem and respect, I have the honor to be, sir, your most obedient and most humble servant, B. FRANKLIN.

DEAR SIR,

ON THE EARTH'S MAGNETISM, &c.

TO THE HON. J. BOWDOIN, ESQ.

Philadelphia, May 31, 1788.

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 consisidered?

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?

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 the 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 wracking 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 15 miles in height, and rise as much in the present polar regions: and the effect would be proportionable if the new poles were placed anywhere between the present and the equator.

Does not the apparent wrack of the surface of this globe, thrown up into long ridges of mountains with strata in various positions, make it probable, that its internal mass is a fluid, but a fluid so dense as to float the heaviest of our substances? Do we know the limit of condensation air is capable of? Supposing it to grow denser within the surface, in the same proportion nearly as we find it does without, at what depth may it be equal in density with gold?

Can we easily conceive how the strata of the earth could have been so deranged, if it had not been a mere shell supported by a heavier fluid? Would not such a supposed internal fluid globe be immediately sensible of a change in the situation of the earth's axis, alter its form, and thereby burst the shell, and throw up parts of it above the rest; as if we could alter the position of the fluid contained in the shell of an egg, and place its longest diameter where the shortest now is, the shell must break; but would be much harder to break if the whole internal substance were as solid and hard as the shell?

Might not a wave by any means raised in this supposed internal ocean of extremely dense fluid, raise in some degree as it passes the present shell of incumbent earth, and break it in some places, as in earthquakes? And may not the progress of such wave, and the disorders it occasions among the solids of the shell, account for the rumbling sound being first heard at a distance, augmenting as it approaches, and gradually dying away as it proceeds? á circumstance observed by the inhabitants of South America in their last great earthquake, that noise coming from a place some degrees north of Lima, and being traced by inquiry quite down to Buenos Ayres, proceeding regularly from north to south, at the rate of leagues per minute, as I was informed by a very ingenious Peruvian whom I met with at Paris.

I am ever, my very dear friend, yours most affectionately,

B. FRANKLIN.

DESCRIPTION OF THE PROCESS TO BE OBSERVED IN MAKING LARGE Sheets OF PAPER IN THE CHINESE MANNER, WITH ONE SMOOTHh surface.

IN Europe to have a large surface of paper connected together and smooth on one side, the following operations are performed.

Communicated by Dr. Franklin to the American Philosophical Society, in which it was read, June 20, 1788.

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