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    Pierre François Méchain and Joseph Jérôme Lefrançois de Lalande. Autograph Manuscript Signed.
    "Méchain." "DelaLande." One page in French, 7.5" x 10", [Paris?]; [August or September] 5, 1786. This is a report, written in the hand of Méchain and signed by him and de Lalande, and commissioned by the French Academy of Sciences, concerns a dissertation or paper on observations and calculations of Mercury's passage in front of the Sun on 4 May 1786. The dissertation was by Swiss astronomer Jacques-André Mallet, a Genevan correspondant of the Academy. Both men support publication of the paper. Most of the report is presented here in English translation.

    "Mr. Mallet made his observations at Avully near Geneva, at latitude 46o10'8''. He saw the first contact upon Mercury's exit at 8h50'18'' of real [illegible] and the total exit at 8h54'28''. However since his observing companions Messrs. Pictet [presumably the Swiss astronomer and meteorologist Marc-Auguste Pictet (1752-1825)] and Picot [unknown] differed in their assessments by a few seconds, they arrived upon an average, with first contact at 8h50'16'' and total exit at 8h54'31''. Mr. Mallet also assessed, 23 times in a row, the difference in right ascension as well as difference of declination between Mercury's center and the Sun's center from 5h5' to until 7h28'. For these observations he used a micrometer attached/adapted to an achromatic meridian circle [in text "lunette achromatique," might be another type of "lunette" which would translate as 'refracting telescope'] with 3 weights [illegible] mounted on a machine parallatique, [?]. Mr. Mallet made calculations based on these observations and all of them correspond as well to one another as one could hope from these sorts of observations. By choosing the results from the most precise and favorable observations, he calculated the conjunction as 5h28'18'' real time at Avully at 7h13o49'53'', with Mercury's boreal heliocentric latitude [in text "latitude heliocentrique boréale" which means the angle between the line passing through the center of the Sun and the center of a planet with the plane of the ecliptic] of 14'18'' without regard to the [optical] aberration which Mallet then gives the quantity of in terms of longitude and latitude.

    The timing of the conjunction at Avully compared with the timing of conjunction at Paris which Mallet deduced from Messrs. Méchain and Delambre [French mathematician and astronomer Jean-Baptiste Joseph Delambre (1749-1822)] prior observations yields a time difference between the meridians of 14'41'': he found almost the exact same results during several eclipses of Jupiter's first satellite [in text "premier satellite", not sure of the meaning or of the moon in question] compared to the corresponding ones which had been sought in Paris....Monsieur Mallet's observations seem to us to have been made with the greatest care and precision that one expects from a skilled and practiced astronomer and the report he's provided makes them that much more useful. We therefore conclude that his dissertation deserves the Academy's recognition and to be printed in its volumes devoted to the work of foreign scientists.

    Pierre François André Méchain (1744-1804) was a French astronomer and objects and comets. He was director of the Paris Observatory in 1799.
    Joseph Jérôme Lefrançois de Lalande (1732-1807) was a French astronomer and writer. As a lecturer and writer, he helped popularise astronomy. His planetary tables, into which he introduced corrections for mutual perturbations, were the best available up to the end of the 18th century. In 1801, he endowed the Lalande Prize, administered by the French Academy of Sciences, for advances in astronomy.

    Jacques-André Mallet (1740-1790) was a Swiss astronomer, founder of the first observatory of Geneva and honorary professor at the Academy of Geneva. He was Director of the Geneva Observatory from 1772 to 1790.
    A manuscript comprised of detailed scientific observations by two leading 18th century astronomers as they discuss the merits of a paper written by another eminent astronomer.

    Condition: Outside of the usual folds and slightly chipped right hand edge, the manuscript is in very good condition.


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