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    Jean Hyacinthe de Magellan. Autograph Letter Signed.
    "Magellan." Four pages, in French, with an integral address sheet, 8" x 13", London; May 3, 1776. In a letter addressed to Chevalier de Bory, squadron leader of the French naval armies and member of the French Royal Academy of Science, Magellan states that "we here" have doubts about Monsieur du Buffon's [Georges-Louis Leclerc, Comte de Buffon (1707-1788), the French naturalist, mathematician, and cosmologist] experiment and the conclusion he wrote about, namely that a certain mass of iron is heavier when heated to the point it becomes "red white" than when it is cool. A certain Dr. Roeback [John Roebuck (1718-1794), a physician, chemist, founder of the Carron Iron Works in Scotland, and member of the Royal Society] had conducted experiments on that subject previously and reached the opposite conclusion; about three weeks ago he addressed the Assembly of the Society and asked for assistance in conducting a large-scale experiment on the subject, then made his position more clear by saying he didn't want assistance but rather just to inform all the Members of the experiment in case they wished to witness it. It took place on April 29 at 11am. Magellan writes, he was present at the experiment due to his interest and curiosity for physics and his admiration for Roeback. He proceeds to describe what he witnessed [English translation].

    "An excellent scale had been made at a smelter's and was set to equilibrium with more than 130 [illegible] and could turn, in other words trip ["trébucher" in text, literally means "trip", perhaps in context means "oscillate"] one way and the other, with 5 or 6 grains as I later checked myself, but everyone agreed on the fact that it tripped with a 'demi-penny weight', or 12 grains. A mass of iron had already been placed in an oven and was left to heat up past redness until it was white. It was then placed in [or "on"] a [illegible] basin but it was agreed that the scale was too low to the ground. So, in order to avoid any error, the iron was placed once again in the oven. It was placed once more in the basin and equilibrated with the greatest care with about 56 [illegible]. All the weights and counterweights in both basins were kept track of in writing and waited for the totality to cool off without doing or changing anything....I was very surprised that they had put over the basin, and I found that the basin that held the hot iron, which was still so hot I could not hold my hand over it very long, had become much heavier than the basin which contained the cold mass. I thoroughly examined the [illegible, probably referring to the written records which had been carefully jotted down at the beginning of the experiment] which had been previously taken for each of these masses, and I found them to be the same. So I then added enough to the 'cold mass' basin to balance it with the other basin. This addition, necessary for equilibration, was exactly 3 penny weights and about 19 grains. In fact, if I put 3 penny weights and 17 grains, the 'hot mass' basin hung lower, and if I put 22 grains, the 'cold mass' basin hung lower. It is worth noting the basins were made of metal and [illegible] only iron things at the bottom of the scale. This means we cannot attribute the change in weight to external causes. I asked Dr. Roeback yesterday during the Assembly of the Royal Society what was the impact of the weight change, to which he responded he found 6 pennyweights and 15 grains, that is 159 grains since one pennyweight contains 24 grains. This experiment makes plain that hot masses are lighter when one examines the scale than the same masses are when cool. While it is true that their volume expansion should lead us to infer this conclusion without any evidence, the contrary findings of Monsieur du Buffon made the experimental exercise well worth the trouble.

    This morning I saw a new hydrometer by Monsieur [illegible], a painter by profession, but who has worked for years to find a [reliable?] method for measuring atmospheric humidity. I was informed that this instrument is surprisingly accurate. It consists of a small and highly sensitive scale whose one arm marks, on a scale [illegible] down to 1/300th of a grain; and on the other side hangs 50 or more small [illegible] thin paper on a wire. Soon one can read a dissertation about this instrument at the Royal Society.
    Monsieur
    [illegible], a carpenter by profession, has made it a constant goal of his life to improve microscopes. I have seen and examined them myself as having a prodigious effect. Their main superiority is in their larger field of vision, focus, and the magnification of objects. I carefully measured the field of vision and found it to be about 18 inches ["pouces" in text]. Solar microscopes with the same build also make a magnificent [illegible] by the large scale of the image and their focus. Solar microscopes cost 6 guineas, and the former microscopes, which are double and bigger than the ordinary ones, cost 15 guineas. These are much more effective with candlelight than daylight, which they have in common with ordinary microscopes, if one [knows?] how to arrange for a certain intensity of light.
    Monsieur Walker
    [Adam Walker (1731-1821), an English inventor, writer, and travelling science lecturer ], who has for some time lectured on experimental physics at York, is in London and is going to give a public lecture on the different [illegible] of air, found and illustrated by Dr. Priestley [Joseph Priestley (1733-1804), an English chemist,]. I have ordered two iron pipes for him which will be ready tomorrow and will serve to produce dephlogisticated air during his lecture. He is the same Monsieur Walker who invented the [Celestin, an improved type of harpsichord], a mechanical component that when applied to an ordinary harpsichord, produces the same sound as the famed 'harmonique' [Glass Harmonic or armonica], the instrument made of glasses [drinking glasses] which was also invented by an Englishman. Yesterday I went to the house of the artist who [illegible, presumably "imported" or "introduced"] the celestin, and Monsieur D'Ortega was also there, he's a Spanish nobleman who you saw in Paris and who travels by order of the King of Spain and who brought me some letters from our mutual colleagues. He was pleasantly surprised by the gentleness and charming [illegible] of this instrument. Once it is [illegible] and well-known, there will not be [illegible] whose harpsichord will not have one. A harpsichord equipped with one costs 8 guineas more than those without.
    I learned that Monsieur le Duc de Chartres [title refers to multiple people] has already left Paris to join his squadron. Therefore you have no more reason to wait to dispose of the circular instrument I sent you [illegible but in context "unless"] one of your friends wishes to have it. It is very well made and of great utility in sea navigation. I have told you its price in my previous letter. This has been a very lengthy letter. If you find it relevant, transmit it to the Academy and to our friends."


    A fascinating letter by Magellan to a Parisian correspondent on scientific happenings in London on 1776.

    Jean Hyacinthe de Magellan (1723-1790) was a Portuguese natural philosopher. Born in Lisbon, he became a monk as a young man but soon left monastic life, later moving to and residing in London. Magellan devoted much of his scientific career to building scientific instruments, including thermometers, barometers, and clocks.

    Condition: The letter has the usual folds, overall toning on first and last pages; slight paper loss of edges of pages 3 and 4 where seal was broken; otherwise good.


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