weighing ten pounds and better is surpassingly more wonderful than that of Moss Seeds of which I have some kinds of them bearing Seeds that a great number of them with their Roots Stalks Leaves and Seeds do not weigh a Grain Besides I have found of the common Female Fern some which have been from the Roots to the utmost top of the Leaf nine foot high and within these three days measured the common broad-leaved Male Fern six foot and an half long some of the Leaves of which are among those I now send you 6. But that which appeared most admirable both to me and some other Gentlemen that were witnesses of it with me was the many differing kinds of small living Creatures wholly invisible to the naked eye and even through largely magnifying spectacles though some of them were to be seen through a deep Convex glass but with a Microscope when the Plant was newly gathered they might be seen nimbly running up and down among the Seed-vessels and some of them were so small as not to be above twice as big as the small Seeds in the bladders a description of some of which I may hereafter send you I have inclosed in the box sent you twelve sorts of Plants of this tribe being the greatest part of the number and only seven sorts of the seeds those wanting are the Cetrach Wall Rue Maiden-hair and Polypody of which notwithstanding you may satisfie your self in the mean time till I can send them green by those small parcels of the Plants which you will find amongst the rest though by keeping they are withered The Seeds of the Ferns through a very excellent Microscope appeared of the higness of a small Vetch or Seed of Lentiles to the naked eye and some of them shrink like the sides of white Pease with small regular knobs and hollows Those of Polypody are differing in colour and shape being yellowish as the others are brown red and formed like the Seeds of the smaller Medicas that is of a Kidney shape All the rest I found very near of the same form I cannot omit what I observed in Cetrach which Plant I have heretofore often considered and wondred at the ill-favoured roughness on the under side of the Leaf appearing like the fleshy side of tann'd Leather being wholly ignorant what Nature meant in it but now by my Microscope I find it a very pleasant object differing from all the rest wherein the curiosity of Nature in a Plant so abject as that appears is shewn beyond imagination This when fresh gathered and not bruised appears through the Microscope like fine thin Membranes such as the Wings of Flies chequered with figures after the manner of Honeycombs when the cells are full of honey and closed with Membranes amongst which as in so many Cells lie the Seed-vessels shaped as before is mentioned I doubt not but you have read the strange stories and fabulous conceits of Authors about Fern Seeds But Parkinson is more Orthodox in some things than any of them For he positively concludes from Gen. 1.11 12. that all Plants have their Seeds and consequently Fern where if he had staid he had asserted a general truth But in coming to particulars he affirms as great an untruth in saying fol. 1036 and 1037. that the Seed is ripe at Midsummer according to the old traditional Fable and tells how it may be gathered whereas now is the very season of their seeding and at Midsummer this and the rest are not come to their full growth before which no Plant seeds That dustiness which he speaks of and calls the Seed is no other than what is found on divers other Plants being an irregular Dust and is not found on the borders of the dents of the Leaves on the under side on which the Seed grows but all over sprinkled on both sides and not found when it is fully grown This he affirms of the Male Ferns which are all differing very notably from the common Female Fern concerning which the fabulous tradition is held But after in the following Chapter of the Ferns and their Relatives now sent you he seems to give over his Scripture Proposition and speaking of the Seeds says no more but that they have spots dashes scales or marks on their back-sides And of the Osmund Royal speaking of the bush at the top of the Plant says it is accounted as the Flower and Seeds And of the Lochitis aspera says plainly they have none at all Of this last I am yet to enquire but doubt not I shall find that it hath Seed like the rest Of all which Gerrard and Johnson his Corrigitor gravely concludes having indeed no demonstrable ground to the contrary that some have been too rash in affirming Ferns to have Seed I intend next Summer to observe whether these hitherto unknown Seeding Plants have Flowers In the mean time I am c. W. C. Bristol September 30. 1669. Maculae in Sole DUring this last great heat of weather in June I observed a very conspicuous Macula with its immediatly incompassing Nubecula and some other less conspicuous Spots at a further distance pass over the Disk of the Sun and found that it was nearest the middle when the heat was greatest that the heat increased as it came nearer the middle and decreased as it departed from it It may be therefore worth observing for the future whether the like weather do not happen upon the next appearance of the like Macula since it seems not very improbable to suppose that the body of the Sun it self may be much hotter when such eruptions appear those Maculae often times ending in Faeculae And the rather because I am informed that this extraordinary heat hath not been peculiar only to England but very general to Europe what it hath been to other parts of the world further intelligence will inform us Upon a second appearance of Spots in the Disk of the Sun at the latter end of July and the beginning of August when at one time to wit July 29. there appeared about six greater and smaller in one knot with their proper Nubecules or Umbra's the heat of the weather again increased to a very great degree and abated as they drew toward the Limb and grew fainter But it hath now since the disappearing viz. on the fourth of August been exceeding hot also though I do not find any Spots this seventh of August it may therefore possibly be that other parts of the body of the Sun may have an extraordinary inflammation which may cause so fervent and lasting heats as have hapned this Summer At least this Hint may deserve some farther Inquiry for though probably it may not be attained to predict the appearances of those Spots yet possibly the appearances of the Spots may serve to predict the future constitution of the weather At least it seems worthy remarking that the greatest heat that hath been in the Air this year was on that day of June when
therewith who have confined their imaginations fancies only within the compass and pale of their own walk and prospect who can scarce imagine that the Earth is globous but rather like some of old imagine it to be a round plain covered with the Sky as with a Hemisphere and the Sun Moon and Stars to be holes through it by which the Light of Heaven comes down that suppose themselves in the center of this plain and that the Sky doth touch that plain round the edges supported in part by the Mountains that suppose the Sun as big as a Sieve and the Moon as a Chedder Cheese and hardly a mile off That wonder why the Sun Moon and Stars do not fall down like Hail-stones and that will be martyr'd rather then grant that there may be Antipodes believing it absolutely impossible since they must necessarily fall down into the Abyss below them For how can they go with their feet towards ours and their heads downwards without making their brains addle To one I say thus prejudiced with these and a thousand other fancies and opinions more ridiculous and absurd to knowing men who can ever imagine that the uniformity and harmony of the Celestial bodies and motions should be an Argument prevalent to perswade that the Earth moves about the Sun Whereas that Hypothesis which shews how to salve the appearances by the rest of the Earth and the motion of the Heavens seems generally so plausible that none of these can resist it Now though it may be said 'T is not only those but great Geometricians Astronomers and Philosophers have also adhered to that side yet generally the reason is the very same For most of those when young have been imbued with principles as gross and rude as those of the Vulgar especially as to the frame and fabrick of the world which leave so deep an impression upon the fancy that they are not without great pain and trouble obliterated Others as a further confirmation in their childish opinion have been instructed in the Ptolomaick or Tichonick System and by the Authority of their Tutors over-awed into a belief if not a veneration thereof Whence for the most part such persons will not indure to hear Arguments against it and if they do 't is only to find Answers to confute them On the other side some out of a contradicting nature to their Tutors others by as great a prejudice of institution and some few others upon better reasoned grounds from the proportion and harmony of the World cannot but imbrace the Copernican Arguments as demonstration that the Earth moves and that the Sun and Stars stand still I confess there is somewhat of reason on both sides but there is also something of prejudice even on that side that seems the most rational For by way of objection what way of demonstration have we that the frame and constitution of the World is so harmonious according to our notion of its harmony as we suppose Is there not a possibility that the things may be otherwise nay is there not something of probability may not the Sun move as Ticho supposes and the Planets make their Revolutions about it whilst the Earth stands still and by its magnetism attracts the Sun and so keeps him moving about it whilst at the same time ☿ and ♀ move about the Sun after the same manner as ♄ and ♃ move about the Sun whilst the Satellites move about them especially since it is not demonstrated without much art and difficulty and taking many things for granted which are hard to be proved that there is any body in the Universe more considerable then the Earth we tread on Is there not much reason for the Hypothesis of Ticho at least when he with all the accurateness that he arrived to with his vast Instruments or Riccioli who pretends much to out-strip him were not able to find any sensible Parallax of the Earths Orb among the fixt Stars especially if the observations upon which they ground their assertions were made to the accurateness of some few Seconds What then though we have a Chimera or Idea of perfection and harmony in that Hypothesis we pitch upon may there not be a much greater harmony and proportion in the constitution it self which we know not though it be quite differing from what we fancy Probable Arguments might thus have been urged both on the one and the other side to the Worlds end but there never was nor could have been any determination of the Controversie without some positive observation for determining whether there were a Parallax or no of the Orb of the Earth This Ticho and Riccioli affirm in the Negative that there is none at all But I do affirm there is no one that can either prove that there is or that there is not any Parallax of that Orb amongst the fixt Stars from the Suppellex of observations yet made either by Ticho Riccioli or any other Writer that I have yet met with from the beginning of writing to this day For all Observators having hitherto made use of the naked eye for determining the exact place of the object and the eye being unable to distinguish any angle less then a minute and an observation requisite to determine this requiring a much greater exactness then to a minute it doth necessarily follow that this experimentum crucis was not in their power whatever either Ticho or Riccioli have said to the contrary and would thence overthrow the Copernican System and establish their own We are not therefore wholly to acquiess in their determination since if we examine more nicely into the observations made by them together with their Instruments and wayes of using them we shall find that their performances thereby were far otherwise then what they would seem to make us believe The Controversie therefore notwithstanding all that hath been said either by the one or by the other Party remains yet undetermined Whether the Earth move above the Sun or the Sun about the Earth and all the Arguments alledged either on this or that side are but probabilities at best and admit not of a necessary and positive conclusion Nor is there indeed any other means left for humane industry to determine it save this one which I have endeavoured to make and the unquestionable certainty thereof is a most undenyable Argument of the truth of the Copernican Systeme and the want thereof hath been the principal Argument that hath hitherto somewhat detained me from declaring absolutely for that Hypothesis for though it doth in every particular almost seem to solve the appearances more naturally and easily and to afford an exceeding harmonious constitution of the great bodies of the World compared one with another as to their magnitudes motions and distances yet this objection was alwayes very plausible to most men that it is affirmed by such as have written more particularly of this subject that there never was any sensible Parallax discovered by the best observations
long it may contain two whole Minutes of such a Circle between f and f and one between 4 and 4 and consequently the said Glass may be set Horizontal to the certainty of a Second which is hardly to be ascertain'd any other way But there remains yet one great Difficulty how to be able to make such a Curviture for though the thing be true in theory yet is it not without some trouble put in practice Very few Glass Canes are so conveniently bent as is desirable and 't is as difficult to find them true straight To prevent this If Glass Canes be used there must be much care taken and many tryals made for the finding what pieces and what side of those pieces will be most fit for this purpose for our Glass-House Workmen know not yet a way certainly to draw them of this or that curviture or straightness nor are they easily ground into a straightness or curviture by the Glassgrinder afterwards though that can be done with some trouble But diligence and tryal will quickly find some piece or other that will be sufficiently exact for any tryal among those which are only drawn at the Glass-House I made use of one of another form such as is described in the 25th Figure which I found to do exceeding well the dark part representing the Water and the lighter part the Air. This was made of two Glasses drawn in distinct Pipes at the Glass-House but joyn'd together in the Lamp and the upper part of the larger or under Tube was incurvated with its convexity downwards so that the Water touched the middle part and the bubbles of Air at each end thereof communicated together by the small Pipe above I tryed also another way by which I was more certain of the truth of the Curvity and could make the Curvity of a greater Circle This was by a long piece of a Looking-Glass-Plate ground very smooth and polished which by the help of Screws I bent upon the circular edges of a brass prismatical Box and cemented the same very tight with hard and soft Cement this Plate had a hollow Channel ground in it the length thereof which serv'd to keep the bubble in the middle By this means 't is not difficult to bend such a Plate into the Curviture of a Circle of 50 60 100 1000 foot Radius and the Brass Box can easily be made to fill or empty as there shall be occasion for the use thereof so that the Bubble may be at any time left of what bigness shall be desired It will be convenient also to varnish the in-side of this Brass Box with Lacker-Varnish very thick and close both to keep it from rustâ—ng and also to preserve it from being corroded by Aqua fortis whensoever there shall be occasion to put it in for the cleansing he inward tarnish and foulness of the Glass-Plate This Curvity of the upper side of the Level may be made by grinding the under side of such a long Plate of Looking Glass upon a Convex Glass-Tool of 50 60 100 1000 foot Radius and polishing the samé accordingly of that Figure The Curvity of the said Plate is express'd in the 26th Figure Now what by this way may be done with Water and Bubbles of Air the same may be done with the same Glasses turned upside-down by the help of an exactly round and polisht Cylinder or Globule of Glass Chrystal Cornelian Agate or other exceedingly hard and close Stone after the manner represented in the 27th Figure for the Ball or Cylinder will naturally roll to the lowest part of the Concavity and there stand But in the doing of this great care must be taken that the Globule be exactly round and polisht and that they Concavity of the Plate be as smooth and well polisht and that they be both very clean and free from dust otherwise the Cylinder or Globule will be apt to stand in a place where it should not and consequently produce considerable errors And here I cannot omit to take notice of a very curious Level invented by Sr. Chr. Wren for the taking the Horizon every way in a Circle Which is done by a large Concave ground and polisht on a very large Sphere and the Limb of it ground and polisht on a flat for by placing the same Horizontal and rectifying it by a small quantity of Quick-silver poured into the Concavity thereof 't will be easie by looking by the flat polisht Limb to discover the true Horizon The only inconvenience I find in it is that the ☿ hath some kind of sticking to the Glass but a small Chrystal Bowl I suppose may remedy that inconvenience and make it fit for use The 5th thing wherein this Instrument is made to excell others is in its easinesses to be adjusted to the Objects and in this that being once adjusted the whole Instrument is so order'd as that it will remain constant to those Objects though they are moved The want of this is so great an inconvenience in all other Instruments hitherto made use of that almost all Observations have been thereby vitiated And Hevelius to prevent and obviate this hath found out many contrivances but they are such as though they do it in part yet 't is but in part and that with much trouble and inconvenience I need not spend time to shew how many inconveniences his way by 4 several Hand-Screws to be managed by 2 Observators at the least is subject to they are indeed so many and so great that it was not without very good reason that he so often appeals to experience for the truth is there was great need of long practice and much experience to be able to make an Observation in that way well the removal of every one of those Screws having an influence upon every one of the other so as no Screw could be turn'd but the whole Instrument was put out of its due situation and both the Objects being continually in motion the whole Instrument was to be rectifi'd every moment There was therefore necessary so great a judgement and dexterity to manage every one of those Screws that without an acquired habitude and handiness by long practice and experience nothing could be done to any certainty nay not even to that little accurateness that the common Sights are able to reach But this though it were a very great unhappiness to Hevelius that he was not furnished with better Contrivances yet it no ways tends to his dispraise for his most extraordinary and indefatigable care pains and industry is so much the more to be admired esteem'd and honour'd and will be so much the more by such as have by experience found the difficulty of making any one Observation certain in that way But that he or any other that hath a mind to make further Tryals and Observations may be freed from this intollerable trouble and difficulty I have thought of this following Instrument by means whereof the Quadrant being once adjusted and
whereby distant places sometimes appear and sometimes disappear under the Horizon By this means also the Rotundity of the Earth may be truely found vastly surpassing any thing performed by the best Levels yet known To this we may add the height of Hils if their distance be known or their distance if their height be known I could have enlarged upon these and have named divers others but designing it only as an Answer to such as may captiously put such a Question I shall rather leave the pleasure of finding them to such as shall really seek them to be assisted thereby in their own undertakings FINIS Errata PAg. 2. l. 13. r. 9 10 p. 6. l. 14. r. aquilae p. 13. l. 3. r. Mathematician p. 15. l. 11. r. Fig. 32. p. 13. l. 28. r. Fig. 31. p. 18. l. 39. r. structuram p. 21. l. 26. r. dena minuta p. 21. l. 27. r. discriminatim p. 22. l. 3. r. Fig. 35. p. 28. l. 34. r. quaedam p. 32. l. 21. r. shaking p. 33. l. 8. r. focus p. 39. l. 28. r. res p. 40. l. 11. r. admaveant p. 40. l. 39. dele se A DESCRIPTION OF HELIOSCOPES And some other INSTRUMENTS MADE BY ROBERT HOOKE Fellow of the Royal Society Hos ego c. Sic vos non vobis LONDON Printed by T. R. for John Martyn Printer to the Royal Society at the Bell in St. Pauls Church-yard 1676. A DESCRIPTION OF HELIOSCOPES And some other INSTRUMENTS THE necessary avocations of business and the urgent importunity of some for the speedy publication of my Animadversions made me conclude them in the Eleventh sheet without staying to Explicate several things which I designed to go along with them But having now retrieved a little more of leasure both for Delineation and Description for a further elucidation of what I have said I shall make it my third Attempt to explain First A Helioscope to look upon the body of the Sun without any offence to the Observers eye Secondly A way of shortning reflective and refractive Telescopes Thirdly A way for using a Glass of any length without moving the Tube Fourthly An Instrument for taking the Diameters of the Sun Moon and Planets or for taking any other Distances to five or ten Degrees to the certainty of a Second Two of these I promised in the 78th or last page of my Animadversions and the other fall in as analogous to them Fifthly An Instrument for describing all manner of Dials by the tangent projection Sixthly The uses thereof 1. For adjusting the Hand of a Clock so as to make it move in the shadow of a Dial whose style is parallel to the Axis Or 2. In the Azimuth of any Celestial Body that is in the shadow of an upright or any other way inclining Style upon any plain 3. For making a Hand move according to the true aequation of Time 4. For making all manner of Elliptical Dials in Mr. Foster ' s way c. 5. For communicating a circular motion in a Curve Line without any shaking And for divers other excellent purposes And first For a HELIOSCOPE which shall so take off the brightness of the Sun as that the weakest eye may look upon it at any time without the least offence My contrivance is By often reflecting the Rayes from the surfaces of black Glasses which are grownd very exactly flat and very well polished so to diminish the Radiations that at length they become as weak and faint as those of the Moon in the twilight so that one may with ease and very much pleasure view examine and describe the phase of the Sun and the maculae and faculae thereof if any such happen to appear when the Observation is made and it gives a good opportunity of discovering them before we have any advertisement thereof from others The reason of which will be sufficiently plain to such as consider how great a quantity of the rays of Light is lost by every reflection and that every reflection doth duplicate triplicate quadruplicate quintuplicate c. the first proportion of loss For Instance Suppose I have a Helioscope made of an Object Glass an Eye Glass and four Reflecting Glasses and that by the first reflection I lose ¾ of the Direct light I affirm there will remain but 1 256 part of the Direct rays of the Sun which can fall upon the eye at the last for if every reflection doth lose ¾ of its Rays and reflect but ¼ and that quarter loseth ¾ and reflects only ¼ of its received Light there will remain but 1 16 part of the whole and if this sixteenth part loseth three quarters of its Rays and reflects only a fourth it will follow the remainder will only be 1 64 part of the whole and if that be once more reflected the Ray will return but with 1 2â—6 part of its first light This although it be obvious and easie enough now it is known yet I do not find that any Person hath yet had thoughts of applying it to this use The generality of Observers have hitherto made use of either some very opacous and thick Glasses next the Eye whether of red green blew or purple Glass others have diminished the Radiation by covering the Glasses with a very thick and close coat of the soot of a Lamp others by casting the figure upon a piece of white Paper whence 't is reflected to the eye Others have contracted the Aperture into a less circle and thereby let in less Light and so make use of one single Ray instead of a pencil of Rayes Others have expanded the figure of the Sun by the help of Eye Glasses into a circle of ten twenty or an hundred times its Diameter But none of all these waies do come near this which I now describe by the help of three four or more Reflections as any one upon trial will very plainly discover First As to the coloured Glasses I cannot at all approve of them because they tinge the Rayes into the same colour and consequently take off the truth of the appearance as to Colour besides it superinduces a haziness and dimness upon the Figure so that it doth not appear sharp and distinct The same inconvenience is also produced by Monsieur Hugenius's way of covering the Glass with the soot of a Lamp though not to so great a degree The Figure on paper or a smooth white surface is not magnified enough nor the difference of shadows so very distinct though that doth very well if the surface be very smooth and the Object be magnified by a Hand Glass That by the contracted Aperture is the worst of all by reason of a certain propriety of Light not taken notice of yet by Optick Writers the edges of Objects seeming ragged of which I have hinted somewhat in my Animadversions pag. 35 and shall shortly say much more the whole ground of Opticks depending thereon The way of expanding the figure of the Sun by the Eye Glass
of Time and unequal progressions upon the Dial-plain according to the proportion of Inclination and the whole Revolution being performed in twenty four hours and the Hand of the Clock upon the Face of the Dial being alwaies moved in a plain which passeth through the Arbor of the Clock and maketh equal progressions in equal spaces about the said Arbor but unequal progression about the Centre of the Dial according to the differing Inclinations And those Inclinations being both in the Sun-Dial and Clock-Dial the same it will follow that the Hand of the Clock must alwaies move in the shadow of the Style if the Hand be once rectified to the true Plain and the Axis or Arbor make its Revolution as it ought to do in twenty four hours If it be further desired for the ease of taking Azimuths and Altitudes that the Arm of the Azimuth quadrant that is once adjusted to the Coelestial Object should by the aforesaid Joynt or Instrument be kept alwaies respecting and following the said Object in its Diurnal motion it may be very easily performed by the help of a small perpendicular Ruler whose lower end is Joynted into either of the Arms 11 of the circular Plate X in the 22 and 23d Figure of my Animadversions and the upper end joynted into the movable Arm at the same distance from the Centre of the Quadrant that the lower end is from the centre of the Plate X and that the centre of the Quadrant be set exactly perpendicular over the centre of X but then the divisions by the help of the Screw cannot be made use of because the Clock-work it self is to turn and move the Arm But it may be done by any Quadrant where the minute Divisions are performed by the help of Diagonals For the Arms of the Circular-plate 11 being alwaies moved in the superficies of the Cone described by the radiation from the Coelestial Object to the centre of the Plate X that is to say the Line that passes through the Centre of the said Plate and through the two Points 11 being alwaies directed to the Coelestial Object if the Arm of the Quadrant be moved perpendicular over it and parallel to it that also must be alwaies directed to it And hence it may very easily be conceived how the aforesaid Semicircular Arms may be readily and certainly rectified to any Coelestial Object that is by fixing Telescopes or Common-sights upon the Circular-plate so as the Axis of them may be parallel to the Line through 11 and loosing the Screw h to rectifie it to the Object by the sight and then immediately to fix it in the said posture by the aforesaid Screw the Clock-work of the said Instrument having been before that put into motion The reason of all which will easily appear to any one that throughly considers that all Celestial Objects seem by the diurnal motion of the Earth to move equally from East to West about the Axis of it and would all do exactly so were they not somewhat varied by their own proper periodical revolutions which though it doth indeed make a real difference between their velocities about the Axis of the Earth yet that difference is but small and the same circular Pendulum will serve both for the Sun Moon Planets and Stars if at least the Pendulum p in the fifteenth Figure be a little lengthened or shortened by lifting up or letting down the Rod q q in proportion as the Body k moves swifter or flower And 't will not be difficult to mark upon the Rod q q the appropriated length of the Pendulum for the Sun Moon or Stars but this only by the by If in the next place it be desired that the Hand of the Clock should be alwaies carried round upon the face of the Clock in the shadow of a Style perpendicular to that plain by reason that the declination of the Sun daily varieth the angles of the shadow about that Style varieth also and consequently the inclination of the plate of the Joynt to the Axis or Arbor must vary also and that variation must alwaies be the same with the variation of the declination of the Sun which is twenty waies mechanically performable in Clock-work so that the motion shall be performed by the Clock-work alone without touching it with the hand All the other directions that are requisite to adjust the Clock-work to such a Dial is only to make the Arbor of the Clock-work to have the same inclination to the plain of the Dial that the Axis of the Earth or a line paralel to it hath and rectifying the Hand into the true plain of the Axis or Inclined arbor the equality of the motion of the Clock-work according to the diurnal and annual motion of the Sun we suppose also to be provided for If the Hand of the Clock be desired to be moved in the shadow of any other streight Style howsoever inclined to the plain of the Dial then must there be another Joynt like the former added to the end of that Axis which was perpendicular to the plain of the Dial and all the three Axes must be scituate in respect of the Plain in which the Hand on the end of the last is to move that the inclination of the said Axes to each other may represent the inclination of the Axis to the perpendicular axis of the Plain and of that perpendicular Axis to the axis of the Style Or which is somewhat shorter and may be made handsome enough Let the two ends of the Hand represent the two points of the second circular Plate or Globe extended long enough to reach to the hour Circle then let the axis of this second Arm be placed in the axis of the inclined Style and let the axis of equal motion representing the axis of the diurnal motion of the Earth be placed with such inclination to it as the axis of the Earth hath to the oblique Axis or Style of the Dial and the motion will be most exactly performed mechanically and according to the truth of Geometry and Calculation Now in all these motions care must be taken to provide that the inclination of the declination of the Sun from the Equinoctial be exprest by the ends 11 in the 22 and 23 Figures of the second Plate of my Animadversions of the Cross taken hold of by the semicircular arms c d upon the end of the first Axis that is that the said arms may by their revolution make the line of the Cross describe such a cone about the first Axis as the motion of the Sun doth about the axis of the Earth making the centre of the Earth the apex of that Cone which will be done if the said semicircular Arms be moved and set to the declination of the Sun for that day Or that an additional motion be added to the first Axis that the Clock it self may perform it This may be done twenty waies easily enough which I suppose will be sufficiently
Parallax of divers minutes but whereas I could not certainly distinguish any sensible at all it must be many times higher than the Moon Now that this way is abundantly to be preferred before an Observation made with a Quadrant for the taking of its altitude is pretty evident because by this means the greatest part of the irregularity caused by the refraction or inflection of the Air is removed for by this means though the Parallax be very large yet the refraction or inflection of the Air will not amount to many seconds both the objects being almost equally raised by refraction especially when 5 or 10 degrees high nearer than which the small Stars vanished out of sight by the thickness of our air It follows therefore that a Semidiameter of the Earth must be a very inconsiderable measure in its distance This part therefore of the Theory of Comets hath been much defective hitherto If we enquire the Parallax of them from the Observation of divers men made in differing places we shall find them so differing one from another that there is great reason to suspect them all Nay not only so but in this Comet of 1664. by comparing two Tables or Charts of the Stars and Constellations of that part of the Heavens through which the Comet past on which was also markt out its way and place from day to day both of them Printed from Copper Plates I find that strange errors and mistakes may be created notwithstanding all the Authors thors care and accurateness possible from the carelesness or neglect of the Graver This I noted in the two Tables of the learned and accurate Mathematician P. Aegidius Franciscus de Gotignies whose skill and care from other works of his and other Observations of this Comet I am sufficiently assured of and found that by the first table upon the ⅔½ of December 1664. it was in 4½ of II in Longitude and in 33⅔ of Southern Latitude but by the second it is placed at the same time in 4°II for its Longitude and in 34½ of South Latitude And this error is not only committed in the place of the Comet but also in the place of the fix'd Stars for Riget in the first Table is placed in 30¾ South Latitude and in 12¾II for Longitude but in the second in 31½ South Latitude and in II ½ II for Longitude both which differ considerably from the place of it assigned by Riccioli and Grimaldi according to whose Observations it should be in 31. II′ South Latitude and 12° II′ 40″ II in Longitude Now if there be these differences to be remarked in the Observations of one we cannot but expect that much more disagreement should be found between those which have been made by differing persons in differing places and with differing ways and differing Instruments And upon examination I have found it no better for from comparing such Observations as I have received from several parts of the world even of those which have seemed more than ordinarily exact I find them for the most part so unaccurate that though they sufficiently manifest that the Comet of 1664. which lasted above four months was visible in most parts of the world and seen to pass in all those places pretty near in the same way amongst the fixed Stars Yet they are so far from manifesting the Parallax that some of them make the place of the Comet to be quite contrary to what parallax would make it some of the Southern Observators placing it much more Southwardly than those of the North. Others indeed of them make the Parallax so great that one might ghess it to be not so far removed from the Earth Something indeed in the general might be ghest of the way of that Comet amongst the fix'd Stars especially when it approaches them pretty near but for exactness of Calculation for Parallax they were no way useful And evenin the former use too it seems very doubtful for comparing the Charts of the Comets way amongst the fix'd Stars published by that diligent and unwearied Observer Mr. Hevelius of Dantzick the above-mentioned P. Gottignies Professor at Rome and Monsieur Petit of Paris I find that the two former make the way of the Comet to lie below the Star in the Bill of Corvus whereas the later though in a Latitude interposed between the parallels of the former makes it to lie above or to the North of it and with him agree some Observations which I have seen of Monsieur Hugenius Other differences I found between those Tables in the way of the Comet of 64. near the middle of its arch wherein Monsieur Hevelius all the way places it more Southward than either Monsieur Petit or P. Gottignies for whereas both P. Gottignies and Mounsieur Petit make it pass above the Star of the third magnitude in the right shoulder of Lepus Monsieur Hevelius makes it move below it which seem to be ascribable to Parallax But I fear much cannot be concluded of certainty from them I shall not trouble the Reader with a multitude of other Histories which I have received concerning that Comet of 64. nor with the disagreements of them one with another and perhaps of most with the truth They have given me sufficient trouble in the examination of them having little other benefit from them save only this that I was thereby informed what a man might think of a great number of Astronomical Observations that have been made for saving the exact Observations of some few such as Mr. Hevelius Mr. Aurout P. Gottignies c. truly diligent and accurate men the greater the Collections of Observations are the more trouble and difficulty is created to the Examiner they not only confounding one another but perplexing those also which are real and perfect Now the reasons or causes of these inconveniences seem to be these First the want of accurate and knowing Observators Secondly The scarcity of convenient Instruments Thirdly The Imperfection of the Tables of the fix'd Stars For the Observators 't is not enough to know how to manage an instrument or to have a good eye or a dextrous and steady hand but with these there must be joyned a skilfulness in the theorical and speculative part and add to all a love and delight in the thing it self and even all these will signifie but little without convenient and accurate Instruments such as may be easily manageable and sufficiently exact The first of these the love of the study being in it self the most excellent or the encouragement of Princes Noblemen and other Patrons of this Learning must procure and where both of these concur thence most is to be expected and most fruit hath hitherto been proceeded though there are not wanting divers eminent instances where the first reason hath been the only inducement As to the second I have already in some of my former Lectures described several convenient ones for these purposes and therefore I shall not here add any more concerning