Page:Philosophical Transactions - Volume 001.djvu/390

 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, not given any precepts for the Change of the Moons 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 the 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 my self, I only saw once the Glasses, Rh