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 {240} 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.