It will, no doubt, appear surprizing, that I should attempt to determine the difference of longitude between two of the most celebrated observatories in Europe; and in which some of the greatest astronomers, that ever lived, have, for above eighty years, been constantly observing the motions of the heavenly bodies: yet it is most certain, that, to this day, we are ignorant of the said difference of longitude: the English astronomers reckoning it to be = 9' 20", and the French setting it down at 9' 10", which, they tell us, was found by M. Cassini, by observations of the eclipses of Jupiter's first satellite made by him, whilst in London in the year 1698: we are no where told, that I know of, by what observations the English astronomers have fixed this difference at 9' 20".