The Particulars. Of Those Observations of the Planet Mars, Formerly Intimated to Have Been Made at London in the Months of February and March A. 1665/6

so much heavier, than the Air of the former place was, when the Mercury stood at 29½ inches. But in making such comparisons, we must not forget to con- sider the Situation of the several places, if we mean to make Estimates not only of the weight of the Atmosphere, but of the weight and density of the Air. For, though the Scales will shew (as has been said) whether there be a difference of weight in the Atmosphere at the two places; yet, if one of them be in a Vale or bottom, and the other on the top or some elevated part of a Hill, it is not to be expected, that the Atmosphere, in this latter place, should gravitate as much, as the Atmosphere in the former, on which a longer Pillar of Air does lean or weigh. And the mention, I have made of the differing Situation of Places, puts me in mind of something, that may prove a another use of our Statical Baroscope, and which I had thoughts of making trial off, but was Accidentally hindred from the op- portunity of doing it. Namely, that by exactly poising the Buble at the foot of a high Steeple or Hill, and carrying it in its close Frame to the top, one may, by the weight requisite to be added to Counterpoise there to bring the Beam to its Hor- izontal position, observe the difference of the weight of the Air at the bottom, and at the top; and, in case the Hill be high enough, at some intermediate Stations. But how far this may assist men, to estimate the Absolute or Comparative height of Mountains, and other elevated Places; and what other Uses the Instrument may be put to, when it is duly improved; and the Cautions, that may be requisite in the several cases, that shall be proposed, I must leave to more leisure, and farther Consideration. The Particulars. Of those Observations of the Planet Mars, formerly intimated to have been made at London in the Months of February and March A. 1665. To perform, what was promised Num. ii. of these Papers, pag. 198; 'tis thought fit now to publish the Particular Obser- vations, concerning the spots in Mars, and their motion, as they were made with a 36 foot Telescope, and produced in writing before the Royal Society, the 28 March 1666. by Mr. Hook, as follows; Having a great desire (saith he) to observe the Body of Mars, whilst Acronycal and Retrograde (having formerly with a Glass of about 12. foot long, observ'd some kind of Spots in the Face of it,) though it be not at present in the Perihelion of its Orbe, but nearer its Aphelion, yet I found, that the Face of it, when near its Opposition to the Sun (with a Charge, the 36. foot glass, I made use off, would well bear) appear'd very near as big, as that of the Moon to the naked eye; which I found, by comparing it with the Full Moon, near adjoining to it, March 10. But such had been the ill disposition of the Air for several nights, that from more than 20 Observations of it, which I had made since its being Retrograde, I could find nothing of satisfaction, though I often imagin'd, I saw Spots, yet the Inflective veins of the Air (if I may so call those parts, which, being interspers'd up and down in it, have a greater or less Refractive power, than the Air next adjoining, with which they are mixt) did make it so confus'd and glaring, that I could not conclude upon any thing. On the third of March, though the Air were still bad enough yet I could see now and then the Body of Mars appearing of the form A: which I presently described by a Scheme; and about 10. minutes after, as exactly representing what I saw through the Glass, as I could, I drew the Scheme B. This I was sufficiently satisfied (by very often observing it through the Tube, and changing my Eye into various positions, that so there might be no kind of Fallacy in it) could be nothing else, but some more Dusky and Spotted parts of the Face of this Planet. March 10. finding the Air very bad, I made use of a very shallow Eye-glass, as finding nothing Distinct with the greater Charge; and saw the appearance of it as in C, which I imagin'd, might be the Representation of the former Spots by a lesser charge. About 3 of the Clock the same morning, the Air being very bad (though to appearance exceeding clear, and causing all the Stars to twinkle, and the minute Stars to appear very thick) the Body seem'd like D; which I still suppos'd to be the Representation of the same Spots through a more confused and glaring Air. But observing March 21. I was surprised to find the Air (though not so clear, as to the appearance of small Stars) so exceeding transparent, and the Face of Mars so very well defined, and round, and distinct, that I could manifestly see it of the shape in E. about half an hour after Nine at night. The Triangular spot on the right side (as it was inverted by the Telescope, according to the appearances, through with all the preceding Figures are drawn) appear'd very black and distinct, the other towards the left more dim; but both of them sufficiently plain and distin'd. About a quarter before 12. of the Clock the same night, I observ'd it again with the same Glass, and found the appearance exactly, as in F; which I imagin'd to shew me a Motion of the former triangular spot: But desirous to observe it again about 3. of the Clock the same Morning, I was hindred by cloudy weather. But March 22. about half an hour after 8. at night, finding the same Spots in the same posture, I concluded, that the preceding Observation was only the appearance of the same Spots at another height and thickness of the Air: And thought myself confirm'd in this Opinion, by finding them in much the same posture, March 23. about half an hour after 9. though the Air was nothing so good as before. And though I desired to make Observations, about 3. of the Clock those mornings; yet something or other interven'd, that hindred me, till March 28. about 3 of the Clock, the Air being light (in weight) though moist and a little hazy; when I plainly saw it, to have the form, represented in I; which is not reconcileable with the other Appearances, unless we allow a Turbinated motion of Mars upon its Center: Which, if such there be, from the Observations made March 21. 22. and 23. we may guess it to be once or twice in about 24. hours unless it may have some kind of Librating motion; which seems not so likely. Now, whether certainly so or not, I shall endeavour, as oft as I have opportunity, further to observe. A particular direction to the Figures mentioned in the precedent discourse. A. March 3d. Coh. 25m. in the morning: the Air having many inflecting ing parts dispersed up and down in it; by the Wheel Barometer, heavy. B. Another Scheme, which I drew from my Observation, about 10 minutes after, the same morning. Both these were observed with a very deep Eye-glass. C. March 10th. 9h. 20m. in the morning: the Air heavy and influentive. Use was made of a shallow or ordinary Charge. D. March 10th. 3h. 00m. in the Morning; the Air very heavy and Influentive, which made it glare and radiate, and be more confused than about 3 hours before. A shallow Charge. E. March 21st. 9½h. post merid; the Air light (in weight) and clear, without inflenting parts; the Face appear'd most distinctly of this Forme. A shallow Charge. F. March 21st. 11½h. post merid; the Air continuing very light and clear, without inflenting vapours. A shallow Charge. G. March 21st. 8½h. post merid. the Air clear, with few inflenting veins in it, and indifferent light. A shallow Charge. H. March 23rd. 9½h. post merid. the Air pretty light, but moist, and somewhat thick and hazy, but seem'd to have but few veins, or inflent- ing parts. I. March 28th. 3h. p.m. much the same kind of Air with that of March 23; light, moist, and a little hazy, with some very few veins. Observations Made in Italy, confirming the former, and withall fixing the Period of the Revolution of Mars. These Observations we shall summarily present the Curious in these parts with, as they were lately presented (by Letter from his Excellency the Ambassador of Venice, now residing at the Court of France) to the Royal Society, in some printed sheets of Paper, entituled, MARTIS, circa Axem proprium Revolubilis, Observationes, BONONIAE & JO. DOMINICO CASINO habite, come to hand June 3, 1666. In these Papers the Excellent Cassini affirms: 1. That with a Telescope of 24 Palms, or of about 16 Foot, wrought after S. Campani's way, he began to observe February 6, 1666 (St.n.) in the morning, and saw two dark Spots in the first Face of Mars. 2. That