Chapter 22

And _First_, if the ground, on which I went in framing this _Baroscope_, be demanded, the answer in short may be; 1. That, though the Glass-buble, and the Glass-counterpoise, at the time of their first being weigh'd, be in the Air, wherein they both are weigh'd, exactly of the same weight; yet they are nothing near of the same bulk; the Buble, by reason of its capacious cavity (which contains nothing but Air, or something that weighs less than Air) being perhaps a hundred or two hundred times (for I have not conveniency to measure them) bigger than the Metalline counterpoise. 2. That according to a _Hydrostatical_ Law (which you know I have lately had occasion to make out) If two Bodies of equal gravity, but unequal bulk come to be weigh'd in another _Medium_, they will be no longer {234} equiponderant; but if the new _Medium_ be heavier, the greater Body, as being lighter in _Specie_, will loose more of its weight, than the lesser and more compact; but if the new _Medium_ be lighter than the first, then the bigger Body will outweigh the lesser; And this disparity, arising from the change of _Medium's_, will be so much the greater, by how much the greater inequality of bulk there is between the Bodies formerly equiponderant. 3. That, laying these two together, I consider'd, that 'twould be all one, as to the effect to be produced, whether the Bodies were weighed in _Mediums_ of differing gravity, or in the same _Medium_, in case its (_specifick_) gravity were considerably alter'd: And consequently, that since it appear'd by the _Baroscope_, that the weight of the Air was sometimes heavier, and sometimes lighter, the alterations of it, in point of gravity, from the weight, it was off at first counterpoising of the Buble of it, would _unequally_ affect so large and hollow a Body, as the Buble, and so small and dense a one, as a Metallin weight: And when the Air by an increase of gravity should become a heavier _Medium_, than before, it would buoy up the Glass more than the Counterpoise; and if it grew lighter, than it was at first, would suffer the former to preponderate: (The Illustrations and Proof can scarce be added in few words; but, if it be desired, I may, God permitting, send you them at my next leasure:) And though our English Air be about a thousand times lighter, than water, the difference in weight of so little Air, as is but equal in bulk to a Buble, seem'd to give small hopes, that it would be sensible upon a Ballance; yet, by making the Buble very large and light, I supposed and found the Event, I have already related.

_Secondly_, The hermetically seal'd Glass-buble, I employed, was of the bigness of a somewhat large _Orange_, and weigh'd about 1. drachme and 10. grains. But I thought it very possible, if I had been better furnish'd with conveniencies (wherein I afterwards found, I was not mistaken) to make (among many, that might be expected to miscarry) some, that might be preferable to this, either for capacity or lightness, or both; especially if care be taken, that they be not seal'd up, whilst they are too hot. For, though one would think, that it were {235} advantagious to rarify and drive out the Air as much as is possible, because in such seal'd Bubles the Air it self (as I have elsewhere shewn) has a weight; yet this advantage countervails not the inconvenience of being obliged to increase the weight of the Glass, which when it includes highly rarified Air, if it be not somewhat strong, will be broken by the pressure of the External Air, as I have sufficiently tryed.

_Thirdly_, I would have tryed, whether the _Dryness_ and _Moisture_ of the Air would in any measure have alter'd the weight of the Buble, as well as the Variation of Gravity produced in the _Atmosphere_ by other causes; but the extraordinarily constant absence of Fogs, kept me from making Observations of this kind; save that one morning early, being told of a mist, I sent to see (being my self in bed) whether it made the Air so heavy as to buoy up the Buble; but did not learn, that that mist had any sensible operation on it.

_Fourthly_, By reason of the difficulties and casualties, that may happen about the procuring and preserving such large and light Bubles, as I have been lately mentioning; it may in some cases prove a convenience to be inform'd, That I have sometimes, instead of one sufficiently large Buble, made use of two, that were smaller. And, though a single Buble of competent bignes be much preferable, by reason that a far less quantity and weight of Glass is requisite to comprise an equal capacity, when the Glass is blown into a single Buble, than when it is divided into two; yet I found, that the employing of two instead of one, did not so ill answer my exspectations, but that they may for a need serve the turn instead of the other; than which they are more easier to be procured; And if the Ballance be strong enough to bear so much Glass, without being injur'd: by employing two or a greater number of large Bubles, the effect may be more conspicuous, than if only a single Buble (though a very good one) were employed.

This instrument may be much improved by divers Accommodations, As

_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 _Æquilibrium_ (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 _Æquilibrium_; 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 29½ 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 _Æquilibrium_, 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 29½ 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 _Æquilibrium_, 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 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 {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.

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.} 9½^{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.} 11¾^{h.} _post merid; the Air continuing very light and clear, without inflecting vapours. A shallow Charge._

G. _March_ 22^{d.} 8½^{h.} _post mer. the Air clear, with few inflecting veins in it, and indifferent light. A shallow Charge._

H. _March_ 23^{d.} 9½^{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, BONONIÆ à JO. DOMINICO CASSINO habitæ;_ 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.