Philosophical Transactions of the Royal Society Part 25

[Sidenote: * _The Scales here meant were before competent Eyewitnesses made to turn manifestly with the thousandth part of a grain._]

Though by a Pa.s.sage, you may meet with in the 19th and 20th Pages of my _Thermometrical Experiments and Thoughts_, you may find, that I did some years agoe think upon this New kind of Baroscope; yet the Changes of the Atmosphere's Weight not happening to be then such, as I wish'd, and being unwilling to deprive my self of all other use of the exactest Ballance *, that I (or perhaps any man) ever had, I confess to you, that successive avocations put this attempt for two or three years out of my thoughts; till afterwards returning to a place, where I chanc'd to find two or three pairs of Scales, I had left there, the sight of them brought it into my mind; and though I were then unable to procure exacter, yet my desire to make the Experiment some amends for so long a neglect, put me upon considering, that if I provided a _Gla.s.s-buble_, more than ordinary large and light, even such Ballances, as those, might in some measure perform, what I had tried with the strangely nice ones above-mention'd.

I caused then to be blown at the Flame of a Lamp some _Gla.s.s-bubles_ as large, thin and light, as I could then procure, and choosing among them, one, that seem'd the least unfit for my turn, I counterpoised it in a pair of Scales, that would loose their _aequilibrium_ with about the 30th part of a Grain, and were suspended at a Frame. I placed both the Ballance and the Frame by a good Baroscope, from whence I might learn the present weight of the Atmosphere. Then leaving these Instruments together; though the Scales, being no nicer than I have express'd, were not able to shew me all the Variations of the Air's weight that appear'd in the _Mercurial_ Baroscope, yet they did what I expected, by shewing me variations no greater, than alter'd the height of Quicksilver half a quarter of an Inch, and perhaps much smaller than those: Nor did I doubt, that, if I had had either tender Scales, or the means of supplying the experiment with convenient accommodations, I should have {233} discerned far smaller Alterations of the Weight of the Air, since I had the pleasure to see the Buble sometimes in an _aequilibrium_ with the counterpoise; sometimes, when the Atmosphere was high, preponderate so manifestly, that the Scales being gently stirr'd, the c.o.c.k would play altogether on that side, at which the Buble was hung; and at other times (when the Air was heavier) that, which was at the first but the Counterpoise, would preponderate, and, upon the motion or the Ballance, make the c.o.c.k vibrate altogether on its side. And this would continue sometimes many daies together, if the Air so long retain'd the same measure of gravity; and then (upon other changes) the Buble would regain an _aequilibrium_, or a preponderance; so that I had oftentimes the satisfaction, by looking first upon the _Statical_ Baroscope (as for distinctions sake it may be call'd) to foretell, whether in the _Mercurial_ Baroscope the Liquor were high or low. Which Observations though they hold as well in Winter, and several times in Summer (for I was often absent during that season) as the Spring, yet the frequency of their Vicissitudes (which perhaps was but accidental) made them more pleasant in the latter of these seasons.

So that, the matter of Fact having been made out by variety of repeated Observations, and by sometimes comparing severall of those new _Baroscopes_ together, I shall add some of those Notes about this Instrument, which readily occur to my memory, reserving the rest till another opportunity.

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 Gla.s.s-buble, and the Gla.s.s-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 Gla.s.s more than the Counterpoise; and if it grew lighter, than it was at first, would suffer the former to preponderate: (The Ill.u.s.trations and Proof can scarce be added in few words; but, if it be desired, I may, G.o.d 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 Gla.s.s-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 Gla.s.s, 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 quant.i.ty and weight of Gla.s.s is requisite to comprise an equal capacity, when the Gla.s.s 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 Gla.s.s, 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 c.o.c.k resting over against these Divisions, may readily {236} and without Calculation shew the quant.i.ty of the Angle, by which, when the scales propend either way, the c.o.c.k 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 Bra.s.s) employ one of Gold, or at least of Lead, whereof the _latter_ being of equal weight with Bra.s.s, 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 Bra.s.s, 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 Gla.s.s-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 gla.s.ses, 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 quant.i.ty 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 quant.i.ty 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 Gla.s.sie 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 Alt.i.tudes 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 a.s.sist you for the future to determine readily, by seeing the inclination of the c.o.c.k 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 a.s.sisted 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 29 inches (which in some places I have found a _moderate_ alt.i.tude;) 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 29 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 29 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 a.s.sist 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 Gla.s.s 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...o...b.., 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-gla.s.s, 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 Gla.s.s, 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-gla.s.s, 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 Gla.s.s, 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.

[Ill.u.s.tration]

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-gla.s.s._

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.} 9^{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.} 11^{h.} _post merid; the Air continuing very light and clear, without inflecting vapours. A shallow Charge._

G. _March_ 22^{d.} 8^{h.} _post mer. the Air clear, with few inflecting veins in it, and indifferent light. A shallow Charge._

H. _March_ 23^{d.} 9^{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 Amba.s.sadour of _Venice_, now residing at the Court of _France_) to the _Royal Society_, in some printed sheets of Paper, ent.i.tuled, _MARTIS, circa Axem proprium Revolubilis, Observationes, BONONIae a JO. DOMINICO Ca.s.sINO habitae;_ come to hand _June_ 3. 1666.

In these Papers the Excellent _Ca.s.sini_ 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 Gla.s.s 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.