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.

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 obse