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 consider 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 wil 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 exspected, 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 another use of our Statical Baroscope, and which I had thoughts of making tryal off, but was Accidentally hindred from the opportunity of doing it. Namely, that by exactly poysing 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 Horizontal 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 leasure, 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. 1665/6.
To perform, what was promised Num. 11. of these Papers, pag. [198]; 'tis thought fit now to publish the Particular Observations, 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 Perihelium of its Orbe, but nearer its Aphelium, yet I found, that the Face of it, when neer 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 adjoyning 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 adjoyning, 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.