An Extract of a Letter Written Decemb. 28. 1666. By M. Auzout to the Publisher, Concerning a Way of His, for Taking the Diameters of the Planets, and for Knowing the Parallax of the Moon; as Also the Reason, Why in the Solar Eclipse above Calculated, the Diameter of the Moon Did Increase about the End

An Extract Of a Letter written Decemb. 28. 1666. by M. Auzout to the Publisher, concerning a way of his, for taking the Diameters of the Planets, and for knowing the Parallax of the Moon; as also the Reason, why in the Solar Eclipse above calculated, the Diameter of the Moon did increase about the end. I did apply myself the last Summer to the taking of the Diameters of the Sun, Moon, and the other Planets, by a Method, which one M. Picard and myself have, esteem'd by Us the best of all those, that have been practis'd hitherto; since we can take the Diameters to Second Minutes, being able to divide one foot into 24000. or 30000. parts, scarce failing as much as in one only part, so as we can in a manner be assur'd, not to deceive ourselves in 3. or 4. seconds. I shall not now tell you my Observations, but I may very well assure you, that the Diameter of the Sun has not been much less in his Apogee, than 31. m. 37. or 40. sec. and certainly not less than 31. m. 35. sec. and that at present in his Perigee it passes not 32. m. 45. sec. and may be less by a second or two. That, which is at the present troublesome, is, that the Vertical Diameter, which is the most easy to take, is diminished, even at Noon, by 8. or 9. sec. because of the Refractions, which are much greater in Winter than Summer at the same height; and that the Horizontal Diameter is difficult, because of the swift motion of the Heavens. As for the Moon, I never yet found her Diameter less than 29. m. 44. or 45. sec. and I have not seen it pass 33. m. or if it hath, it was only by a few seconds. But I have not yet taken her in all the kinds of situations of the Apogees and Perigees which happen, with the Conjunctions and Quadratures. I do not mention all, what can be deduced from thence, but if you have Persons at London, that observe these Diameters, we may entertain ourselves more about this Subject, another time. I shall only tell you, that I have found a Way to know the Parallax of the Moon, by the means of her Diameter: Vid. If on a day, when she is to be in her Apogee or Perigee, and in the most Boreal Signes, you take her Diameter towards the Horizon, and then towards the South, with her Altitudes above above the Horizon. For, if the Observation of the Diameters be exact; as in these Situations the Moon changes not considerably her Distance from the Earth in 6. or 7. hours, the Difference of the Diameters will shew the Proportion there is of her Distance, with the Semi-diameter of the Earth. I do not enlarge, because that as soon as one hath this Idea, the rest is easie. The same would yet be practis'd better in the places, where the Moon passes through the Zenith, than here; for the greater the difference is of the Heights, the greater is that of the Diameters. I do not note (for it easily appears) that, if one were under the same Meridian, or the same Azimuth in two very distant places, and took at the same time the Diameter of the Moon, one would do the same thing; though this Method goes not to preciseness. From what has been said, may be collected the reason of the Observation, which M. Hevelius made in the last Eclipse of the Sun, touching the increase of the Moon's Diameter about the end. I am exceeding glad, that a person, who probably knew not the cause of it, has made the Experiment: but it is strange, that until now no Astronomer has foreseen, that that should happen, nor given any precepts for the Change of the Moon's Diameter in the Eclipses of the Sun, according to the places, where they should happen, and according to the Hour and Height, the Moon should have. For, what hapned in that Eclipse of Augmentation, would have faln out contrarily, if it had been in the Evening; for, the Moon, which in that Eclipse, that began in the Morning, was higher about the end than at the beginning, was nearer us, and consequently was to appear bigger: But if the Eclipse should happen in the Evening, she would be lower at the end, and therefore more distant from us, and consequently appear lesser. So also in two different places, whereof one should have the Eclipse in the Morning, and the other at Noon, the Moon should appear bigger to him that hath it at Noon: And she must likewise appear bigger to those, who shall have a lesser Elevation of the Pole under the same Meridian, because the Moon will be nearer them. I wish, I could satisfie you about the Optick Glasses of Signior Burattini in Poland, which he hath sent hither; but I have not yet seen their performances myself. I only saw once the Glasses, which which are perfectly well wrought and well polish'd. Those, that have tried them, find them very good, but they are only, the one of 10, the other of 8, foot. A good Astronomer told me, that they would bear a great Aperture in respect of their length. I do not well know, what to say to yours concerning M. Hevelius. Meanwhile, the interest of truth, and the obliging manner, he has treated me with, engage me to answer him, in the matter of the Comets: I am perswaded, I shall convince him; but since he hath taken the Illustrious Royal Society for Judge, I accept that with all my heart. A Relation Of the loss of the Way to prepare the Bononian Stone for shining. Though several Persons have pretended to know the Art of preparing and calcining the Bononian Stone, for keeping a while the Light once imbibed; yet there hath been indeed but one, who had the true secret of performing it. This was an Ecclesiastick, who is now dead, without having left that skill of his to any one, as Letters from Italy and France, some while since, did inform. There is no substance, in Nature, known to us, that hath the effect of this Stone; so that (to the shame of the present Age) this Phenomenon is not like to be found anywhere, but in Books, except some happy Genius light upon the same or the like skill.* A Description Of a Swedish Stone, which affords Sulphur, Vitriol, Allum, and Minium. This was communicated to the R. Society by Sir Gilbert Talbot Knight, a Worthy Member of that Body, as he had received it in Denmark, being his Majesties Extraordinary Envoy there, as follows, There is a Stone in Sweden of a Yellow Colour, intermixed with streaks of white (as if composed of Gold and Silver) and heavy withal. It is found in firm Rocks, and runs in Veins, upon [* It is hoped notwithstanding (which also a late Letter from abroad does hint:) that some or other of the Italian Vetusti at Florence have secured this Secret.]