Chapter D: E B O. A Rain-bow about the Sun, forming an entire Circle, but (1)

interrupted in some places._

H D N. _A portion of a Circle, that was Excentrick to the Sun, and greater than the Circle_ D E B O, _which touch'd_ D E B O, _and was confounded with it in the point_ D. {221} H N. _The two Mock-Suns, in the intersection of the Semicircle_ H D N, _and the Circle_ S C H N: _The midst of which two False-Suns was white and very luminous; and their Extremities towards_ D I _were tinged with the Colours of a Rainbow. The False Sun, mark'd_ N, _ was fainter than that, which is mark'd_ H.

C. _The Mock-Sun, all white, and far less shining, than the two others._

I. _A space very dark betwixt_ R. _and_ D.

[Sidenote: * _Those _Five_ Suns, that appear'd the 29 _March_, A. 1629. at _Rome_, between 2 or 3 of the Clock, in the afternoon, were thus posited; that the two of them, which were in the intersection of two Circles, appear'd in that of a Circle, which passed through the Sun's Diske, with another, that was _Concentrick_ to the Sun: as may be seen in _Figure III_. borrow'd (for the easier comparing them together) out of _Des-Cartes_ his _Meteors_, cap. X._]

This Appearance is look't upon as one of the notablest, that can be seen, by reason of the _Excentricity_ of the Circle H D N, and because that the _Parhelia_ * were not in the Intersection of the Circle D E B O with the great Circle S C H N, but in that of the Semi-circle H D N.

As for the two odd _Rainbows_; they appear'd at _Chartres_ the 10. of _August_, 1665. about half an hour past six in the Evening; and did cross one another almost at right Angles, as may be seen by _Fig._ IV.

The Rainbow, which was opposite to the Sun, in the usual manner, was more deeply colour'd, than that, which cross'd it; though even the Colours of the first _Iris_ were not so strong, as they are now and then seen at other times.

The greatest height of the stronger Rainbow, was about 45. degrees; the feebler Rainbow lost one of its Legs, by growing fainter, about 20 degrees above the stronger; and the Leg below appear'd continued to the _Horizon_.

These Rainbows did not _Just_ decussate one another at right Angles; there was some 6 or 7 degrees difference. The fainter, seem'd to be a Portion of a great Circle; and the stronger was but a Portion of a small Circle, as usually.

The Sun, at their appearance, was about 6 degrees high above the _Horizon_, and towards the 17 _Azimuth_ of the West, Northward.

{222}

The Observer, M. _Estienne_, notes, that, when he made this Observation, the River of _Chartres_, which runs very near from _South_ to _North_, was betwixt him and the Rainbow; and that he stood Level with this River, whence he was distant not above 150 paces: which he adds, that the Curious may the better judge of this Observation.

* * * * *

_A Relation of an Accident by Thunder and Lightning, at _Oxford_._

This was imparted by Dr. _Wallis_ in a Letter, written at _Oxford_, May 12, 1666. to the _Publisher_, as follows:

I should scarce have given you so soon the trouble of another Letter, were it not for an Accident which hapn'd here _May_ 10. I had that afternoon, about 4 of the clock heard it thunder at some distance. About 5 of the clock the Thunder coming nearer to us; it began to rain, and soon after (the rain withal increasing) the Thunder grew very loud, and frequent, and with long ratling Claps (though not altogether so great, as I have some other times heard:) and the Lightning with flashes very bright (notwithstanding the clear day-light) and very frequent, (when at the fastest, scarce a full minute between one flash and another; many times not so much, but a second flash before the Thunder of the former was heard:) The Thunder for the most part began to be heard about 8 or 10 second minutes after the flash; as I observ'd for a great part of the time by my Minute-Watch: but once or twice I observ'd it to follow (in a manner) immediately upon it, as it were in the same moment; and the lightning extream red and fiery. I do not use to be much apprehensive of Thunder and Lightning, but I was at this time (I know not well, why?) very apprehensive, more than ordinary, of mischief to be done by it, for it seem'd to me to be very low and near us (which made me so particular, as to observe the distance of the flash by the noise) and very frequent, and bright, so that, had it been by night as it was by day, it would have been very terrible. And, though I kept within doors, yet I sensibly discover'd a stinking sulphureous smell in the Air. About 7 of the clock it ended, before which time I had news brought me of a Sad Accident upon the {223} water at _Medley_ about a Mile or somewhat more distant from hence. Two Schollars of _Wadham_-Colledge, being alone in a Boat (without a Water-man) having newly thrust off from shore, at _Medley_, to come homewards, standing near the Head of the Boat, were presently with a stroke of Thunder or Lightning, both struck off out of the Boat into the Water, the one of them stark dead, in whom, though presently taken out of the Water (having been by relation, scarce a minute in it) there was not discerned any appearance of life, sense, or motion: the other was stuck fast in the Mud (with his Feet downwards, and his upper parts above water) like a post not able to help himself out; but, besides a present stonying or numness, had no other hurt; but was for the present so disturb'd in his senses, as that he knew not, how he came there out of the Boat, nor could remember either Thunder or Lightning, that did effect it: and was very feeble and faint upon it; which (though presently put into a warm Bed) he had not thoroughly recover'd by the next Night; and whether since he have or no, I know not.

Others in another Boat, about 10 or 20 yards from these (as by their description I estimate) felt a disturbance and shaking in their Boat, and one of them had his Chair struck from under him, and thrown upon him; but had no hurt. Those immediately made up to the others, and (some leaping into the Water to them) presently drew them either into the Boat or on Shore; yet none of them saw these two fall into the Water (not looking that way) but heard one of them cry out for help presently upon the stroke, and smelt a strange stinking smell in the Air; which, when I asked him, that told it me, what kind of stink? he said, like such a smell, as is perceived upon the stricking of Flints together.

He that was dead (when by putting into a warm Bed, and rubbing, and putting strong waters into his Mouth, &c. no life could be brought into him) was the next morning brought to town; where, among the multitudes of others, who came to see, Dr. _Willis_, Dr. _Mellington_, Dr. _Lower_, and my self, with some others, went to view the Corps: where we found no wound at all in the skin, the face and neck swart and black, but not more, than might be ordinary, by the settling of the blood: On the right side of the neck was a little blackish spot about an inch long, and {224} about a quarter of an inch broad at the broadest, and was, as if it had been sear'd with a hot iron; and, as I remember, one somewhat bigger on the left side of the neck, below the Ear. Streight down the breast, but towards the left side of it, was a large place about three quarters of a Foot in length, and about two inches in breadth, in some places more, in some less, which was burnt and hard, like Leather burnt with the fire, of a deep blackish red Colour, not much unlike the scorch'd skin of a rosted Pig. And on the fore-part of the left Shoulder such another spot about as big as a Shilling; but that in the neck was blacker and seem'd more sear'd. From the top of the right shoulder, sloping downwards towards that place in his Breast, was a narrow Line of the like scorched skin; as if somewhat had come in there at the neck, and had run down to the breast, and there spread broader.

The buttons of his _Doublet_ were most of them off; which, some thought might have been torn off with the blast, getting in at the neck, and then bursting its way out: for which the greatest presumption was (to me) that, besides 4 or 5 buttons wanting towards the bottom of the Breast, there were about half a dozen together clear off from the bottom of the collar downwards, and I do not remember, that the rest of the buttons seem to be near worn out, but almost new. The collar of his doublet just over the fore-part of the left shoulder was quit broken asunder, cloth and stiffening, streight downwards, as if cut or chop'd asunder, but with a _Blunt_ tool; only the inward linnen or fustian lineing of it was whole, by which, and by the view of the ragged Edges, it seem'd manifest to me, that it was by the stroak inward (from without) not outwards from within.

His _Hat_ was strangely torn, not just on the Crown, but on the side of the hat, and on the brim. On the side of it was a great hole, more than to put in ones fist through it: some part of it being quite struck away, and from thence divers gashes every way, as if torn, or cut with a _Dull_ tool, and some of them of a good length, almost quite to the edges of the brim. And, beside these, one or two gashes more, which did not communicate with the hole in the side. This also I judged by a stroke inwards; not so much from the view of the edges of those gashes (from which there was scarce any judgement to be made either way) but {225} because the lining was not torn, only ript off from the edge of the hat (where it was sow'd on) on that side, where the hole was made. But his hat not being found upon his head, but at some distance from him, it did not appear, against what part of the head that hole was made.

Upon the rest of his Cloaths, I do not know of any further effect, nor did we smell any sulphurous scent about them: which might be, _Partly_ because it was now a good while after the time, and _Partly_ by reason of their being presently drenched in the water into which he fell.

The night following, the three _Doctors_ above mentioned, and my self, with some Chirurgions (besides a multitude of others) were present at the opening of the head, to see if any thing could be there discover'd; but there appear'd no sign of contusion; the brain full and in good order; the nerves whole and sound, the vessels of the brain pretty full of Blood. But nothing was by any of them discern'd to be at all amiss. But it was by candle-light, and they had not time to make very nice Observations of it (the Body being to be buried by and by) and the croud of People was a further hindrance. But if any thing had been considerably out of order to the view, it would surely have been by some of them discover'd. Some of them thought, they discern'd a small fissure or crack in the skull; and some who held it, while it was sawing off, said, they felt it Jarring in their hands, and there seem'd to the eye something like it, but it was so small, as that by candle-light we could not agree it certainly so to be.

Some of the _Hair_ on the right Temple was manifestly singed, or burnt; and the lower part of that Ear blacker, than the parts about it, but soft; and it might be only the settling of the Blood. The upper part of the left shoulder, and that side of the neck, were also somewhat blacker than the rest of the Body, but whether it were by the blow, which broke the collar, and scorch'd the round red spot thereupon, or only by settling of the Blood, I cannot say; yet I think, it might very well be, that both on the head, and on this side of the neck, there might be a very great blow, and a contusion upon it (and seems to have been so, by the tearing of the hat, and breaking the collar, if not also cracking of the skull) and yet no sign of such contusion, because dying so immediately, there was not time for the Blood to gather {226} to the part and stagnate there (which in bruises is the cause of blackness) and it was but as if such a blow had been given on a Body newly dead; which does not use to cause such a symptom of a bruise, after the Blood ceases to circulate.

Having done with, the Head, they open'd the _Breast_, and found that burning to reach quite through the skin, which was in those scorch'd places hard and horney, and shrunk up, so as it was not so thick as the soft skin about it: but no appearance of any thing deeper than the skin; the Muscles not at all disorder'd or discolour'd (perhaps, upon the reason, that was but now said of the Head, Neck and Shoulder). Having then taken off the _Sternum_, the Lungs and Heart appear'd all well, and well-colour'd without any disorder.

This is the sum of what was observ'd; only that the whole Body was, by night, very much swell'd, more than in the morning; and smelt very strong and offensively: Which might be by the hotness of the weather, and by the heat of the place occasion'd by the multitude of People.

* * * * *

_An Experiment to examine, what _Figure_, and _Celerity_ of _Motion_ begetteth, or encreaseth _Light_ and _Flame_._

This was communicated by Dr. _Beale_, as follows;

_May_ 5. 1665. fresh Mackrels were boyl'd in Water, with salt and sweet herbs; and, when the Water was perfectly cold, the next morning, the Mackrels were left in the Water for pickle.

_May_ 6. more fresh Mackrels were boyl'd in like Water; and _May_ 7. both Water and Mackrels were put into the former Water, together with the former Mackrels. (Which circumstances I do particularize, because, whether, the mixture of the pickle of several ages, and a certain space of time, or whatever else was necessary, and wanting, the trial did not succeed with like effect at other times).

But now on the next _Munday_ (_May_ 8). evening, the Cook stirring the Water, to take out some of the Mackrels, found the Water at the first motion become very luminous, and the Fish shining through the Water, as adding much to the Light, which the water yielded. The water by the mixture of Salt and Herbs, {227} in the boyling, was of it self thick and rather blackish, than of any other clear colour: yet being stirr'd, it shin'd, and all the fish appear'd, more brightly luminous in their own shapes.

Wherever the drops of this water (after it was stirr'd) fell on the Ground, or Benches, they shin'd: And the Children took drops in their hands, as broad as a penny, running with them about the house, and each drop, both near and at distance, seem'd by their shining as broad as a six pence, or a shilling, or broader.

The Cook turn'd up the side of the Fish, which was lowest, and thence came no shining: and after the water was for some good time settled, and fully at rest, it did not shine at all.

On _Tuesday_ night (_May_ 9). we repeated the same Trial, and found the same effects. The water, till it was stirr'd, gave no light, but was thick and dark, as we saw by day-light, and by candle-light. As soon as the Cook's hand was thrust into the water, it began to have a glimmering; but being gently stirr'd by the hand moving round (as the Dairy-maid do to gather the Curds for Cheese) it did so shine, that they, who look'd on it at some distance, from the farther end of another room, thought verily, it was the shining of the Moon through a Window upon a Vessel of Milk; and by brisker Circulation it seem'd to flame.

The Fish did then shine as well from the Inside, as the Outside, and chiefly from the Throat, and such places, as seem'd a little broken in the boyling.

I took a piece that shin'd most, and fitted it as well as I could devise in the night, both to my great _Microscope_, and afterwards to my little one; but I could discern no light by any of these Glasses; nor from any drops of the shining water, when put into the Glasses. And _May_ 10. in the brightest rayes of the Sun, I examin'd, in my great _Microscope_, a small broken piece of the Fish, which shin'd most the night before. We could find nothing on the surface of the Fish very remarkable. It seem'd whitish, and in a manner dried, with deep inequalities. And others, as well as my self, thought, we saw a stream, rather darkish, than luminous, arising like a very small dust from the Fish: And rarely here and there, a very small; and almost imperceptible sparkle in the Fish. Yet of these _sparkles_ we are _certain_; we numbered them, and agreed in the number, order and place. Of the _steam_ I am not confident, but do suspect our Eyes in the {228} bright Sun, or that it might be some dust in the Aire.

The great _Microscope_ being fitted in the day-light for this piece of Fish, we examin'd it that night, and it yielded no light at all, either by the view of the Glass, or otherwise.

Finding it dry, I thought that the moisture of Spittle, and touching of it, might cause it to shine: and so it did, though but a very little, in a few small sparks, which soon extinguish'd. This we saw with the bare eye; not in the Glass.

The Fish were not yet fetide, nor insipid to the best discerning palats: And I caused two Fish to be kept for further Tryal, two or three days longer, till they were fetide in very hot weather; and then I expected more brightness, but could find none, either in the water, by stirring it, or in the Fish, taken out of the water.

And some Trials I made afterwards with other boyl'd Mackrels (as is above said) with like pickle, but failed of the like success.

This season serves for many Trials in this kind, and by better _Microscopes_, or better ordered. And in these Vulgarities we may perhaps as well trace out the cause and nature of Light, as in Jewels of greatest value, &c.

* * * * *

_Some Considerations touching a Letter in the _Journal des Scavans_ of _May 24. 1666_._

In _Num._ 9. of these _Transactions_ were publish'd the _Schemes_ and _Descriptions_ of certain Ways of _Sounding the Depth of the Sea without a Line_; and of _Fetching up Water from the bottom of it_; together with some Experiments already made with the former of these two Contrivances. The Author of the French _Journal des Scavans_ found good, to insert them both in his _Journal_ of _May_ 3. but in another of _May_ 24. intimates, that the said _Schemes_ and their _Descriptions_ are not very clear and intelligible (he means, that they were not well understood by _French_ Readers) proposing also some Difficulties, relating to that Subject, and esteemed by him necessary to be satisfied, before any use could be made of the said Instruments.

Upon this occasion, the Author of these _Tracts_ thinks fit, here to represent, {229}

_First_, That _Englishmen_ and such others, as are well versed in the _English_ tongue, find no difficulty in understanding the descriptions of these _Engines_, nor in apprehending their structure, exhibited by the _Figures_, especially if notice be taken of the Emendation, expressed at the end of _Num._ 10. about the misgraving the _Bended end_ of the _Springing Wire_ (which it seems has not been noted in _France_, tho' the said _Num._ 10 is known to have been seen there a pretty while before their _Journal_ of _May_ 24. was publish'd). And as for the particular of the _Bucket_, fetching water from the bottom of the Sea, both the _Figure_ and the annexed _Description_ thereof are so plain and clear, that 'tis some wonder here, that any difficulty of understanding them is pretended by any, that hath but ordinary skill in _Cutts_ and the _English_ language. Mean while, that way, which the _French_ Author recommends for this purpose as more simple, _Videl. a Brass-Pump with double Valves_, is not at all unknown in _England_, nor has bin left untried there; but was found inconvenient, in respect that the Valves in descending did not fully open, and give the water a free passage through the Cavity of the Vessel, nor in ascending shut so close, as to hinder the water from coming in at the top: Whereas by the way proposed in _Num._ 9. both is perform'd with great ease and security.

_Secondly_, Whereas the _French_ Author is of opinion, that 'tis unknown, how much time a Heavy Body requires to sink in water, according to a certain depth; he may please to take notice, that that hath been made out in _England_ by frequent Experiments; by which, several Depths, found by this Method of sounding _without_ a Line, were examin'd by trying them over again in _the same_ place _with_ a Line, after the common way. And as to that _Quære_ of his, Whether a heavy Body descends in the same _Proportion_ of swiftness in _Water_, that it would do in _Air_? The Answer is, that it does not; but that, after it is sunk one or two fathoms into the Water, it has there arrived to its greatest swiftness, and keeps, after that, an equal degree of velocity; the _Resistance_ of the water being then found equal to the _Endeavour_ of the heavy Body downwards.

_Thirdly_, When the same _Author_ alledges that it must be known, when a Light Body reascends from the bottom of the water to {230} the top, in what proportion of time and swiftness it rises. He seems not to have considered, that in this Experiment, the times of the descent and assent are both taken and computed together; so that for this purpose, there needs not that nicety, he discourses of.

_Fourthly_, Whereas it is further excepted, That this way of Sounding Depths is no new Invention; The answer is ready, that neither is it pretended to be so, in the often quoted _Tract_; it being only intimated there, that the manner of performing it, as it is in that place represented and described, is new.

_Lastly_, To rectifie the said Author's mistake, as if the instrument of fetching up Water from the bottom of the Sea, were chiefly contriv'd, to find out, Whether in some places of the Sea any _Sweet_ Water is to be met with at the bottom: There will need no more, than to direct him to the Book it self _Num._ 9. where p. 149. towards the end, the _First_ use of this _Bucket_ is express'd to be, to know the _degrees of Saltness_ of the Water according to its nearness to the top or bottom; or rather to know the constitution of the Sea-water in several depths of several _Climates_, which is a matter, much better to be found out by _Trial_, than _Discourse_. Neither is it any where argued in that Book (as the _French Journal_ insinuates) that, because sweet water is found at the Bottom of the Sea of _Baharem_, therefore it _must_, but only that it _may_, be found so elsewhere. And since the same _Journal_ admits, that those Sweet water-springs, which yield the sweet water, that is found at the said place, have been formerly on the _Continent_, far enough from the Sea, which hath afterwards covered them. It will be, it is presumed, lawful to ask, Why in many other places there may not be found the like? And besides, how we do know, but that there may be in other parts, Eruptious of large Springs at the bottom of the Sea, as well as there?

* * * * *

Printed with Licence for _John Martyn_, and _James Allestry_, Printers to the Royal-Society. 1666.

{231}

* * * * *

_Num._ 14.

PHILOSOPHICAL _TRANSACTIONS._

* * * * *

_Munday_, _July_ 2. 1666.

* * * * *

The Contents.

_An Account of a _New_ kind of _Baroscope_, which may be call'd
_Statical_; and of some Advantages and Conveniencies it hath above the
_Mercurial_; communicated by Mr. _Boyle_. The Particular Observations
of the Planet _Mars_, formerly intimated to have been made by Mr.
_Hook_ in _February_ and _March_ last. Some Observations, made in
_Italy_, confirming the former; and withall fixing the _Period_ of the
said Planet's Revolution. Observations, lately made at _London_, of the
Planet _Jupiter_: as also of _Saturn_. A Relation of a sad Effect of
Thunder and Lightning. An Account of some Books, lately publish'd;
_videl_. The Relations of divers Curious Voyages, by Mons. _Thevenot_:
A Discourse about the Cause of the Inundation of the _Nile_, by Mons.
_de la Chambre_, both _French_: De Principiis & Ratiocinatione
Geometrarum, Contra Fastum Professerum Geometriæ, by Mr. _Hobbes_: King
_Salomons_ Pourtraiture of Old Age, by_ J. Smith, M. D.

* * * * *

_An Account of a _New_ kind of _Baroscope_, which may be called _Statical_; and of some Advantages and Conveniencies it hath above the _Mercurial_: Communicated, some while since, by the Honourable _Robert Boyle_._

[Sidenote: * See _Num. 11. p. 185. Phil. Transactions._]

As for the _New_ kind of _Baroscopes_, which, not long agoe, * I intimated to you, that my haste would not permit me to give you an account off; since your Letters acquaint me, that you still design a Communicating to the {232} Curious as much Information, as may be, in reference to _Baroscopes_; I shall venture to send you some Account of what I did but name (in my former Letter) to you.

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

Though by a Passage, 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 _Glass-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 _Glass-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 _Æquilibrium_ 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 _æquilibrium_ with the counterpoise; sometimes, when the Atmosphere was high, preponderate so manifestly, that the Scales being gently stirr'd, the Cock 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 Cock 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 _æquilibrium_, 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 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.