_First_, There may be fitted to the _Ansa_ (or Checks of the Ballance) an
Arch (of a Circle) divided into 15. or 20. deg. (more or less, according to
the goodness of the Ballance) that the Cock resting over against these
Divisions, may readily {236} and without Calculation shew the quantity of
the Angle, by which, when the scales propend either way, the Cock declines
from the Perpendicular, and the beam from its Horizontall parallelism.
_Secondly_, Those, that will be so curious, may, instead of the Ordinary
Counterpoise (of Brass) employ one of Gold, or at least of Lead, whereof
the _latter_ being of equal weight with Brass, is much less in Bulk, and
the _former_ amounts not to half its bigness.
_Thirdly_, These parts of the Ballance, that may be made of Copper or
Brass, without any prejudice to the exactness, will, by being made of one
of those Mettals, be less subject, than Steel, (which yet, if well hardned
and polish'd, may last good a great while) to rust with long standing.
_Fourthly_, Instead of the scales, the Buble may be hung at one end of the
Beam, and only a Counterpoise to it at the other, that the Beam may not be
burthen'd with unnecessary weight.
_Fifthly_, The whole instrument, if placed in a small Frame, like a square
Lanthorn with Glass-windows, and a hole at the top for the Commerce of the
internal and external Air, will be more free from dust, and irregular
agitations; to the latter of which, it will otherwise be sometimes
incident.
_Sixthly_, This instrument being accommodated with a light Wheele and an
Index (such as have been applyed by the excellent Dr. _Chr. Wren_ to open
Weather glasses, and by the ingenious Mr. _Hook_ to _Baroscopes_) may be
made to shew much more minute variations, than otherwise.
_Seventhly_, And the length of the Beam, and exquisitness of the Ballance,
may easily, _without_ any of the foregoing helps (and much more _with_
them) make the instrument far exacter, than any of those, I was reduced to
employ. And to these Accommodations divers others may be suggested by a
farther consideration of the nature of the thing, and a longer practice.
Though in some respects this _Statical_ Baroscope be inferior to the
_Mercurial_; yet in others it has its own advantages and conveniencies
above it.
And 1: It confirms _ad oculum_ our former Doctrine, that the falling and
rising of the _Mercury_ depends upon the varying weight of the Atmosphere;
since in this Baroscope it cannot {237} be pretended, that a _Fuga vacui_,
or a _Funiculus_, is the cause of the changes, we observe. 2. It shews,
that not only the Air has weight, but a more considerable one, than some
Learned men, who will allow me to have prov'd, it has some weight, will
admit; since even the variation of weight in so small a quantity of Air, as
is but equal in bulk to an _Orange_, is manifestly discoverable upon such
Balances, as are none of the nicest. 3. This _Statical_ Baroscope will
oftentimes be more parable, than the other: For many will finde it more
easie, to procure a good pair of Gold-scales, and a Buble or two, than a
long Cane seal'd, a quantity of _Quick-silver_, and all the other requisits
of the _Mercurial_ Baroscope; especially if we comprise the trouble and
skill, that is requisite to free the deserted part of the Tube from Air. 4.
And whereas the difficulty of removing the _Mercurial_ Instrument has kept
men from so much as attempting to do it, even to neighbouring places; the
Essential parts of the _Scale_-Baroscope (for the Frame is none of them)
may very easily in a little room be carried, whither one will, without the
hazard of being spoil'd or injur'd. 5. There is not in _Statical_
Baroscopes, as in the other, a danger of uncertainty, as to the goodness of
the Instruments, by reason, that in _these_ the Air is, in some more, and
in some less perfectly excluded; whereas in _those_, that consideration has
no place. (And by the way, I have sometimes, upon this account, been able
to discover by our new Baroscope, that an esteem'd _Mercurial_ one, to
which I compared it, was not well freed from Air.) 6. It being, as I
formerly intimated, very possible to discover _Hydrostatically_, both the
bigness of the Buble, and the Contents of the cavity, and the weight and
dimensions of the Glassie substance (which together with the included Air
make up the Buble,) much may be discover'd by this Instrument, as to the
Weight of the Air, _absolute_ or _respective_. For, when the _Quick-silver_
in the _Mercurial_ Baroscope is either very high, or very low, or at a
middle station between its greatest and least height, bringing the
_Scale_-Barometer to an exact _AEquilibrium_ (1 with very minute divisions
of a Graine,) you may, by watchfully observing, when the _Mercury_ is risen
or faln just an inch, or a fourth, of half an inch &c. and putting in the
like minute divisions of a Grain to the lighter Scale, till you have again
brought the Ballance to an {238} exquisit _AEquilibrium_; you may, I say,
determine, What known weight in the _Statical_ Baroscope answers such
determinate Altitudes of the ascending and descending Quick-silver in the
_Mercurial_. And if the Ballance be accommodated with a divided Arch, or a
Wheel and Index, these Observations will assist you for the future to
determine readily, by seeing the inclination of the Cock or the degree
mark'd by the Index, what pollency the Buble hath, by the change of the
_Atmospheres_ weight, acquired or lost. Some Observations of this nature I
watchfully made, sometimes putting in a 64^{th.} sometimes a 32^{th.}
sometimes a 16^{th.} and sometimes heavier parts of a Grain, to the lighter
Scale. But one, that knew not, for what uses those little papers were,
coming to a window, where my Baroscopes stood, so unluckily shook them out
of the Scales, and confounded them, that he robb'd me of the opportunity of
making the nice Observations I intended, though I had the satisfaction of
seeing, that they were to be made. 7. By this _Statical_ Instrument we may
be assisted to compare the _Mercurial_ Baroscopes of _several_ places
(though never so distant) and to make some Estimates of the Gravities of
the Air therein. As if, for instance, I have found by Observation, that the
Buble, I employ, (and one may have divers Bubles of several sizes, that the
one may repaire any mischance, that may happen to another) weigh'd just a
Drachme, when the _Mercurial_ Cylinder was at the height of 291/2 inches
(which in some places I have found a _moderate_ altitude;) and that the
Addition of the 16th part of a gr. is requisite to keep the Buble in an
_AEquilibrium_, when the _Mercury_ is risen an 8th, or any determinate part
of an inch above the former station: When I come to another place, where
there is a _Mercurial_ Barometer, as well freed from Air as mine (for that
must be supposed) if taking out my _Scale_ instrument, it appeare to weigh
precisely a Drachme, and the _Mercury_ in the Baroscope there stand at just
291/2 inches, we may conclude the Gravity of the Atmosphere not to be
sensibly unequal in both those two places, though very distant. And though
there be no Baroscope there, yet if there be an additional weight, as for
instance, the 16th part of a Grain requisite to be added to the Buble, to
bring the scales to an _AEquilibrium_, it will appear that the Air at this
second place is, at that time {239} so much heavier, than the Air of the
former place was, when the _Mercury_ stood at 291/2 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 {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 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 twinckle, and the minute Stars to appear very
thick) the body seem'd like _D_; which I still suppos'd to be {241} 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 preceeding
_Figures_ are drawn) appear'd very black and distinct, the other towards
the left more dim; but both of them sufficiently plain and defin'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 designing 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 preceeding Observation was
only the appearance of the same Spots at another height and thickness of
the Air: And thought my self 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.
[Illustration]
A particular direction to the _Figures_ mentioned in the precedent
discourse.
A. _March _3^{d.} 00^{h.} 20^{m.}_ in the morning: the Air having many
{242} inflecting 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_ 10^{d.} 00^{h.} 20^{m.} _in the morning: the Air heavy and
inflective. Use was made of a shallow or ordinary Charge._
D. _March_ 10^{d.} 3^{h.} 00^m _in the Morning; the Air very heavy and
Inflective, which made it glare and radiate, and be more confused, than
about 3. hours before. A shallow Charge._
E. _March_ 21^{d.} 91/2^{h.} _post merid; the Air light (in weight) and
clear, without inflecting parts; the Face appear'd most distinctly of this
Forme. A shallow Charge._
F. _March_ 21^{d.} 113/4^{h.} _post merid; the Air continuing very light and
clear, without inflecting vapours. A shallow Charge._
G. _March_ 22^{d.} 81/2^{h.} _post mer. the Air clear, with few inflecting
veins in it, and indifferent light. A shallow Charge._
H. _March_ 23^{d.} 91/2^{h.} _post mer. the Air pretty light, but moist, and
somewhat thick and hazy, but seem'd to have but few veins, or inflecting
parts._
I. _March_ 28^{d.} 3^{h.} _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
Ambassadour 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 a JO. DOMINICO CASSINO
habitae;_ come to hand _June_ 3. 1666.
In these Papers the Excellent _Cassini_ affirms;
1. That with a _Telescope_ of 24. _Palmes_, 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._ {243}
2. That with the same Glass he observ'd _Febr._ 14/24. in the Evening, in
the _other_ Face of this Planet, two other Spots, like those of the first,
but bigger.
3. That afterwards continuing the Observations, he found the Spots of these
two Faces to turn by little and little from _East_ to _West_, and to return
at last to the same situation, wherein he had seen them first.
4. That S. _Campani_, having also observ'd at _Rome_ with Glasses of 50.
_Palmes_ or about 35 _Foot_, likewise of his own contrivance, had seen in
the same Planet the same _Phenomena_.
5. That sometimes he hath seen, during the same night, the two Faces of
_Mars_, one, in the Evening, the other in the Morning.
6. That the Motion of these Spots in the inferior part of the apparent
Hemisphere of _Mars_, is made from _East_ to _West_, as that of all the
other Celestial Bodies, and is peform'd by Parallels, that decline _much_
from the _Equator_, and _little_ from the _Ecliptick_.
7. That the Spots return the next day to the same situation, 40. minuts
later, than the day before; so that in every 36. or 37. daies, about the
same hour, they come again to the same place.
8. He promises shortly to give us the particular _Tables_ of this Motion
and of its Inequalities, together with the _Ephemerides_ themselves.
9. He represents, that some other _Astronomers_ have also made at _Rome_
several Observations of these Spots of _Mars_, from _March 14/24._ to
_March 20/30._ with Glasses, wrought by _Eustachio Divini_, of 25. and 45.
Palmes; Which Spots he makes little differing from his own, of the first
Face; as will by and by appear, by the direction to the _Schemes_.
10. But he adds, that those other _Roman_ Astronomers, that have observ'd
with _Divini's_ Glasses, will have the Conversion of _Mars_ to be
performed, not in 24 h, 40 m. (as he maintains it is) but in about 13 h.
11. And to evince, that they are mistaken in these Observations of theirs;
he alledges, That they assure that the Spots, which they have seen in this
Planet, (by an _Eustachian_ Telescope) the 20/30 of _March_, were small,
very distant from one another, remote from the middle of the Disk, and the
_Oriental_ Spot was less, than the _Occidental_ (as is represented by the
Fig. O; like that of the first Face of _Mars_.) whereas, on the contrary,
{244} He (_Cassini_) pretends to evidence by his Observations, made at the
same time at _Bononia_, that, the same day and hour, those Spots were very
large, neer one another, in the midst of the Disk, the Oriental bigger than
the Occidental (as appears by _Fig._ P, which is that of the second Face of
_Mars_.)
12. Besides, he declares, that those _Astronomers_ were too hasty, in
determining, after 5 or 6 Observations only, in how much time _Mars_
finish's his Revolution; and denies it to be perform'd in 13 hours: adding,
that, though Himself had observ'd for a much longer time, than they; yet he
durst not for a great while define, Whether _Mars_ made but _one_ Turn in
24 hours 40 minuts or _two_; and that all, that he could, for a long time
affirm, was onely this, that after 24 h. 40 m. this Planet appear'd in the
same manner he did before.
13. But since those first Observations, He affirms to have found cause to
determine, that the Period of this Conversion is made in the said space of
24 h. 40 m; and not oftner than once within that time; Alledging for proof;
1. That, whereas _Febr._ 6. (st.n.) he saw the Spots of the first Face of
_Mars_, moving from eleven of the Clock in the night, until break of day,
they appear'd not afterwards in the Evening after the rising of that Planet
(witness several intelligent persons, which he names, that were present at
the Observations) Whence he infers, that after 12 hours and 20 minuts, the
same Spots did not come about; since that the same, which in the morning
were seen in the middle, upon the rising of _Mars_; after 13 or 14 hours,
might have appear'd neer the Occidental Limb. But, because he might be
imposed upon by Vapors, whilst _Mars_ was yet so neer the _Horizon_, he
gives this other determination, _vid._
2. Whereas he saw the first Face of _Mars_ the 6 of _February_ at 11 of the
clock of the night following; he did not see the same after 18 daies at the
same hour; as he ought to have done, if the Period were absolved in the
space of 12 h. 20 m.
[Illustration]
3. Again, whereas he saw _Febr._ 24. in the Evening, the other Face of
_Mars_, he could not see the same, the 13. and 15. day of _March_, to wit
after 17 and 19 days; as he should have done, if the Revolution were made
in the newly mention'd time.
4. Again, whereas the 27. of _March_ in the Evening he saw {245} the second
Face of _Mars_, he could not see it the 14. and 16. of _April_.
From all which Observations he Judges it to be evident, that the Period of
this Planets Revolution is not perform'd in the space of 12. hours 20,
minutes, but in about 24 hours 40 minutes; more exactly to be determin'd by
comparing distant Observations: And that those who affirm the former, must
have been deceived by not well distinguishing the two Faces, but that
having seen the second, taken it for the first.
All which he concludes with this Advertisement, that, when he defines the
time of the Revolution of _Mars_, he does not speak of its _Mean_
Revolution, but onely of that, which he observ'd, whilst _Mars_ was
opposite to the Sun; which is the shortest of all.
_The Figures of the _Principal_ Observations, represented in the Book
here discoursed of, may be seen in the annexed _Scheme_; _videl.__
K. _One of the Faces of _Mars_, as S. _Cassini_ observed it _March 3.
(_st.n._) 1666_ in the Evening, with a Glass of 24 Palmes._