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ticles of the metal swallowed with their food by slovenly workmen, who went to their meals after handling the metal, without well washing their fingers, so that some of the metalline particles were taken off by their bread and eaten with it. This appeared to have some reason in it. But the pain I had experienced made me still afraid of those effluvia.

Being in Derbyshire at some of the furnaces for smelting of lead ore, I was told, that the smoke of those furnaces was pernicious to the neighbouring grass and other vegetables; but I do not recollect to have heard any thing of the effect of such vegetables eaten by animals. It may be well to make the inquiry.

In America I have often observed, that on the roofs of our shingled houses, where moss is apt to grow in northern exposures, if there be any thing on the roof painted with white lead, such as balusters, or frames of dormant windows, &c., there is constantly a streak on the shingles from such paint down to the eaves, on which no moss will grow, but the wood remains constantly clean and free from it. We seldom drink rain water that falls on our houses; and if we did, perhaps the small quantity of lead, descending from such paint, might not be sufficient to produce any sensible ill effect on our bodies. But I have been told of a case in Europe, I forget the place, where a whole family was afflicted with what we call the dry bellyache, or colica pictorum, by drinking rain water. It was at a country-seat, which, being situated too high to have the advantage of a well, was supplied with water from a tank, which received the water from the leaded roofs. This had been drunk several years without mischief; but some young trees planted near the house growing up above the roof, and shedding their leaves upon it, it was supposed that an acid in those leaves had corroded the lead they covered,

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and furnished the water of that year with its baneful particles and qualities.

When I was in Paris with Sir John Pringle in 1767, he visited La Charité, a hospital particularly famous for the cure of that malady, and brought from thence a pamphlet containing a list of the names of persons, specifying their professions or trades, who had been cured there. I had the curiosity to examine that list, and found that all the patients were of trades, that, some way or other, use or work in lead; such as plumbers, glaziers, painters, &c., excepting only two kinds, stonecutters and soldiers. In them I could not reconcile it to my notion, that lead was the cause of that disorder. But on my mentioning it to a physician of that hospital, he informed me that the stonecutters are continually using melted lead to fix the ends of iron balustrades in stone; and that the soldiers had been employed by painters, as laborers, in grinding of colors.

This, my dear friend, is all I can at present recollect on the subject. You will see by it, that the opinion of this mischievous effect from lead is at least above sixty years old; and you will observe with concern how long a useful truth may be known and exist, before it is generally received and practised on. I am, ever, yours most affectionately,

B. FRANKLIN.

On Thermometers.

13 September, 1786. THE two thermometers most generally in use at present, among the philosophers of Europe, are those of Réaumur and Fahrenheit. The French use Réaumur's, the English Fahrenheit's.

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 this 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 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 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 ať 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.

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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

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