Munday,June5. 1665.
A Relation of some extraordinary Tydes in theWest IslesofScotland, by Sr.Robert Moray. The judgment of MonsieurAuzout, touching theAperturesofObject-glasses, and theirproportionsin respect of the several lengths ofTelescopes; together with aTablethereof. Considerations of the same Person upon Mr.Hook'sNew Engine for grinding ofOptick-glasses. Mr.Hook'sThoughts thereupon. Of a means to illuminate anObjectin what proportion one pleaseth; and of thedistances, that are requisite to burn Bodies by theSun. A further accompt by MonsieurAuzoutof SigniorCampani's Book, and Performances aboutOptick-Glasses.Campani's Answer thereunto; and Mr.Auzout's Animadversions upon that Answer. An accompt of Mr.Lower's newly publishedVindicationof Dr.Willis's Diatriba deFebribus.
A Relation of some extraordinary Tydes in theWest IslesofScotland, by Sr.Robert Moray. The judgment of MonsieurAuzout, touching theAperturesofObject-glasses, and theirproportionsin respect of the several lengths ofTelescopes; together with aTablethereof. Considerations of the same Person upon Mr.Hook'sNew Engine for grinding ofOptick-glasses. Mr.Hook'sThoughts thereupon. Of a means to illuminate anObjectin what proportion one pleaseth; and of thedistances, that are requisite to burn Bodies by theSun. A further accompt by MonsieurAuzoutof SigniorCampani's Book, and Performances aboutOptick-Glasses.Campani's Answer thereunto; and Mr.Auzout's Animadversions upon that Answer. An accompt of Mr.Lower's newly publishedVindicationof Dr.Willis's Diatriba deFebribus.
In that Tract ofIsles, on the West ofScotland, called by the Inhabitants, theLong-Island, as being about 100. miles long fromNorthtoSouth, there is a multitude of small Islands, situated in aFretum, orFrith, that passes between the Island ofEust, and theHerris; amongst which, there is one calledBerneray, some three miles long, andmore than a mile broad, the length running fromEasttoWest, as theFrithlyes. At theEastend of thisIsland, where I stayed some 16. or 17. dayes, Iobserveda very strange Reciprocation of the Flux and Re-flux of the Sea, andheardof another, no less remarkable.
Upon theWestside of theLong Island, the Tides, which came from theSouth-west, run along the Coast,Northward; so that during the ordinary course of the Tides, the Flood runsEastin theFrith, whereBerneraylyes, and the EbbWest. And thus the Sea ebbs and flows orderly, some 4. days before thefull Moon, andchange, and as long after (the ordinary Spring-tides rising some 14. or 15. foot upright, and all the rest proportionably, as in other places). But afterwards, some 4. days before theQuarter-moons, and as long after, there is constantly a great and singularvariation. Forthen, (aSoutherlyMoon making there the full Sea) the course of the Tide beingEastward, when it begins to flow, which is about 9½ of the Clock, not onely continues so till about 3½ in the afternoon, that it be high water, but, after it begins to ebb, the Current runs on stillEastward, during the whole Ebb; so that it runsEastward12 hours together, that is, all day long, from about 9½ in the morning, til about 9½ at night. But then, when the night-Tide begins to flow, the Current turns, and runsWestwardall night, during both Floud & Ebb, for some 12. hours more, as it didEastwardthe day before. And thus the Reciprocations continue, one Floud and Ebb, running 12 hoursEastward, and another twelve hoursWestward, till 4. days before theNewandFullMoon; and then they resume their ordinary regular course as before, runningEast, during the six hours of Floud, andWest, during the six of Ebb. And this I observed curiously, during my abode upon the place, which was in the Moneth ofAugust, as I remember.
But the Gentleman, to whom theIslandbelongs at present, and divers of his Brothers and Friends, knowing and discreet persons, and expert in all such parts of Sea-matters, as otherIslanderscommonly are, though I shrewdly suspected their skill in Tides, when I had not yet seen what they told me, and I have now related of these irregular Courses of the Tides, did most confidently assure me, and so did every body I spake withabout it, that there is yet another irregularity in the Tides, which never fails, and is no less extraordinary, than what I have been mentioning: which is, That, whereas between theVernalandAutumnal Equinoxes, that is, for six Moneths together, the Course of irregular Tides about the Quartermoons, is, to run all day, that is, twelve hours, as from about 9½ to 9½, 10¼ to 10¼&c. Eastward, and all night, that is, twelve hours more,Westward: during the other six Moneths, from theAutumnalto theVernal Equinox, the Current runs all dayWestward, and all NightEastward.
Of this, though I had not the opportunity to be an Eye-witness, as of the other, yet I do not at all doubt, having received so credible Information of it.
To penetrate into theCausesof these strange Reciprocations of the Tides, would require exact descriptions of the Situation, Shape, and Extent of every piece of the adjacent Coasts ofEustandHerris; the Rocks, Sands, Shelves, Promontorys, Bays, Lakes, Depths, and other Circumstances which I cannot now set down with any certainty, or accurateness; seeing, they are to be found in noMap, neither had I any opportunity to survey them; nor do they now occur to my Memory, as they did some years ago, when upon occasion I ventured to make aMapof this wholeFrithofBerneray, which not having copied, I cannot adventure to beat it out again.
This Author, observing in a smallFrench Tractlately written by him to a Countryman of his, MonsieurL' Abbe Charles; That greatOptick Glasseshave almost never as great anApertureas the small ones, in proportion to what they Magnifie, and that therefore they must be more dim; takes occasion to informtheReader, that he hath found, that theApertures, whichOptick-Glassescan bear with distinctness, are in about asubduplicate proportionto theirLengths; whereof he tells us he intends to give the reason and demonstration in hisDiopticks, which he is now writing, and intends to finish, as soon as his Health will permit. In the mean time, he presents theReaderwith aTableof suchApertures; which is here exhibited to the Consideration of the Ingenious, there being of thisFrenchBook but one Copy, that is known, inEngland.
ATABLEof theAperturesofObject-Glasses.
The Points put to some of these Numbers denote Fractions.
In the above-mentionedFrenchTract, there are, besides several other particulars, to be represented in due place, contained someConsiderationsof MonsieurAuzoutupon Mr.Hook's NewEnginefor grindingOptick-Glasses. Where he premises inGeneralhis thoughts touching the working ofGreatOptick-Glasses, and that by the help of aTurn lathe; affirming first of all, that not only theEnginis to be considered for giveing theFigure, but theMatteralso, which ought to be brought to greater perfection, than it hath been hitherto. For, he finds it not so easie (as least,where he is) to procureGreatpieces of Glass withoutVeins, and other faults, nor to get such, as are thick enough withoutBlebbs; which, if they be not, they will yield to the pressure and weight, either when they are fitted to theCement, or wrought.
Secondly, He finds it difficult to work theseGreatGlasses of thesamethickness, which yet is very necessary, because, that the least difference inFiguresso littleconvex, can put theCenterout of theMidle, 2 or 3Inches; and if they be wrought inMoulds, the length of time, which is required to wear and to smooth them, may spoil the bestMould, before they be finished. Besides, that the strength of Man is so limited, that he is unable to work Glasses beyond a certain bigness, so as to finish and polish them all over so well, assmallGlasses; whereas yet, the bigger they are, the more compleat they ought to be: And if any weight or Engine be used to supply strength, there is then danger of an unequal pressure, and of wearing away the Engine; In the mean time, the preciseness and delicateness isgreater than can easily be imagined. Wherefore he could never, having some experience of this preciseness, conceive, that aTurn-lathe, wherein must be two different, and in some manner contrary motions, can move with that exactness and steddiness, that is required, especially, for any considerable length of time.
Having premised this, he discourses upon Mr.HookhisTurne, intimating first of all, that he was impatient to know what kind ofTurnethis was, imagining, that it had been tried, and had succeeded, as coming from a Society that professeth, they publish nothing but what hath been maturely examin'd. But that he was much surprised when he saw theMicrographyof Mr.Hook, and found there, that hisEnginewas published upon ameer Theory, without having made any Experiment, though that might have been made with little charge and great speed; expence of Money and Time being the onely thing, that can excuse those who in matter ofEnginesimpart their inventions to the publick, without having tried them, to excite others to make trial thereof.
Whereupon he proposes some difficulties, to give theInventoroccasion to find a way to remove them. He affirms therefore, that though it be true in theTheory, that aCircle, whosePlainis inclined to theAxisof theSphereby anAngle, whereof half theDiameteris theSine, and which touches theSpherein itsPole, will touch in all its parts aspherical Surface, that shall turn upon thatAxe. But that it is true also, that that must be but aMathematical Circle, and withoutBreadth, and which precisely touches the Body in its middle: Whereas in the practice, aCirclecapable to keep Sand and Putty, must be of somebreadth; and he knows not whether we can find such a dexterity of keeping so much of it, and for so long a time, as needs, upon the Brim of aRingthat is half an Inch broad. He adds, that it is very difficult to contrive, that the middle of the Glass do always precisely answer to the Brim of thisRing, seeing that the position of the Glass does always change a little in respect of theRing, in proportion as 'tis worn, and as it must be pressed because of its inclination. He believes it also very hard, to give to theAxisor to theMandril, which holds the Glass, that littleInclination, that would be necessary for great Glasses, and to make the twoMandrilsto have one and the samePlain, as is necessary. And, having done all this, he persuades himself, that it is exceedingly difficult, if not impossible, for two contrary motions, where so many pieces are, to rest for a long time steddy and firm, as is requisite for the not swarving from it a hair's thickness, since less than that can change all.
He goes on, and, seeing that thisInventorspeaks of Glasses of a thousand, & ten thousand foot, which he supposed not impossible to be made by thisEngine, discourses of what is necessary for the making Glasses of suchbignesses; which he believes thisInventormay perhaps not have thought of. Wherefore he affirms, that if theTable, made by himself for theAperturesof Glasses (which is that, that is above delivered) be continued unto a thousand feet, by taking always theSubduplicate proportionofLengths, it will be found, that for pretty good ones, theAperturemust be of 15. Inches; for good ones, more than 18. and for such as are excellent, more than 21. Inches: whence it may be judged, what piece of Glass, and of what thickness it must be, toendurethe working. But he proceeds to speak of theInclination, which theMandrilmust have upon thePlainof theRing, when theRingshould have 10. or 12 Inches; and finds, that it would make but 6 or 7. minutes of inclination, and that a Glass would have lessConvexity, and consequently, less difference from a Glass perfectly plain, than the 7. or 8. part of a Line. And then he leaveth it to be judged, whether a Glass of such a Length being found, we ought to hope, that aTurncan be firm enough to keep such a piece of Glass in the same Inclination, so that aMandrildo not recede some Minutes from it: and, though even the Glass could be fastned perfectly perpendicular to theMandril, thatthosetwoMandrilscould be put in one and the samePlain, & that that little Inclination, which is requisite, could be given, and theMandrilbe continued to be pressed in that sameInclination, according as the Glass is worn. All which particulars, he conceives to be very hard in the practice; not to mention, that the weight of the Glass, that should be inclined to theHorizon, as 'tis represented by Mr.Hook, would make it slide upon theCement, and sochange theCenter; and that the Glass is not pressed at the same time by theRingbut in one part on the side,vid.about a fourth; and that the parts of the Glass are not equally worn away,&c.What then,saith he, would becom of a Glass of 10000 feet, which, according to the said Table, would have more than four feet, or four feet and nine inches, or five feet, seven inchesAperture, and of which theRing, though it were two feet nine inches, would have but one minut ofInclination, and the Glass of 5 feetAperturewould have but 4 minuts, and the curvity of it would be less than theeightpart of a Line.
But,saith he, let us consider, only a Glass of 300 foot, to see, what is to be hoped of that, and to know at least the difficulty, to be met with in making a Glass only of that Length. A Glass then of 300 foot, according to his Table, must have more than 8 inchesAperture, which maketh but 16 minuts of itsCircle, and it should have more than 11 inches, if it be an excellent one. If Mr.Hook(adds he) did use but hisRingof 6 inches, which he would use from twelve to an hundred foot Glass, theInclination, which theAxis, orMandril, that bears his Glass, should have, should be but 16 minuts, and theCurvityof the Glass would be less than the eighth part of a Line, and if he should use a bigger, theInclinationwould be proportionable.
Whence it may be judged (continues he) that we are yet very far from seeingAnimals &c.in theMoon, as MonsieurDes Cartesgave hope, and Mr.Hookdespairs not of. For, he believes by what he knows ofTelescopes, that we are not to look for any above 300 or 400 foot at most; and he fears, that neitherMatternorArtwill go even so far.
When therefore (saith he) a Glass of 300 foot should bear an Eye-glass of 6 inches (which would appear wonderful) it would magnifie but 600. times inDiameter, that is, 360000 times inSurface: but suppose, that such could be made, as would magnifie a 1000 times inDiameter, and 1000000. of times inSurface, admitting there were but 60000 leagues from theEarthto theMoon, and that the smalness of theApertureof the Glasses (which yet would diminish the Light more than 36 times) and the obstacle of the Air were not considered, we should notsee theMoon, but as if we were a 100, or at least, 60. leagues distant from her without a Glass. He here wishes, that those, that promise to make us seeAnimalsandPlantsin the Moon, had thought on what our naked Eyes can make us discern of such Objects, only at 10 or 12 leagues distance.
But this he would not have understood as a discouragement from searching with all care and earnestness after the means of making longTelescopes, or of facilitating the working thereof; but only as an Advertisement to those, who light upon theTheoryof anyEngine, not to expose it presently as possible and useful, before they have tried it, or if it have succeeded in small, not to endeavour to persuade, that it will also succeed in great.
As it may happen (saith he) that the Engin of Mr.Hookmay, by using all necessary precautions, succeed in the making ofEye-Glasses, orsmallOptick-Glasses, but not in makinggreatones; as we see, that an instrument composed of two Rulers, wherewith are traced Portions of Circles, succeeds well enough insmall, but when there is no more than half a Line, a quarter of a Line, or less convexity, it will be no longer just at all, as he tells us to have made the proof of it in Circles drawn by the means of one of these Instruments, made by one of the best Workmen in his time, who, whilst he lived, esteemed them above price, although they be not just; as others and my self (saith he) have by tryal found, when we endeavoured to makeMouldsby their means, & as those, who by the like Instrument laboured to trace portions of Circles of 80 or 100 foot,&c. Diameter, can attest.
But, notwithstanding all this, he hath thought upon two or three things, which he thinks may remedy some inconveniencies of Mr.HookhisTurn. Thefirstis, to invert the Glass, and to put it under theRing, that so not only the Glass may be placed moreHorizontally, and not slide upon theCement, but that theSandalso, and thePuttymay stay upon the Glass.
Theotheris, that there must be twoPoppetheads, into which theMandrilmust pass, where theRingis to be fastned; and theMandrilmust be perfectlyCylindrical, that so it may advance upon the Glass as it wears away by the means of its weight, or by the means of a spring, pressing it, without wrigling from one place to another, as it would presently happen in the fashion,as theTurnis composed. For, when the Glasses do wear, especially when they are veryconvex, it cannot be otherwise, but theMandrilwill play and wrigle, before theScruebe made firm.
But he doubts, whether all can be remedied, which he leavs to the industry of Mr.Hook, considering what he saith in thePrefaceof hisMicrography, touching a Method, he knows, of finding out as much inMechanicks, as can be found inGeometrybyAlgebra.
Besides this, he taketh notice, that most of those that medle withOptick-Glasses, give them not as muchAperture, nor charge them so deep as they ought. And he instances in theTelescope, which HisMajestyofGreat Britainpresented theDukeofOrleanswith,videl.that it did bear but 2 inches, and 9 linesFrench, for its greatestAperture, though there be 5 or 6 lesserApertures, of which it seems (saith he) the Artificer would have those, that use it, serve themselves more ordinarily, than of the greatest; which conveys but almost half as many Rays as it should do, according to his Calculation, which is, as 9 to 16; Whereas, according to hisTableofApertures, an excellent 35 footTelescopeshould bear 4 inchesAperturein proportion toexcellentsmall ones. He notes also, that the Eye-glass of the saidTelescope, composed of 2 Glasses, hath no more effect, when it is most charged, than a Glass of 4½ inches; which makes it magnifie not a 100 times. And he finds by Mr.Hook, that he esteems aTelescopemade inLondonof 60 feet, (which amount to about 57 feet ofFrance, the foot ofFrancebeing to that ofEnglandas about 15 to 16) because it can bear at least 3EnglishinchesAperture, and that there are few of 30 feet, that can bear more than 2 inches, (which is but 22½ LinesFrench) although he (M.Auzout) gives no lessAperturethan so, to a 15 foot-Telescope, and his of 21 feet hath ordinarily 2 Inches, 4 Lines, or 2 inches, 6 LinesAperture.
This Discourse he Concludeth with exhorting those, that workOptick-Glasses, to endeavor to make them such, that they may bear greatAperturesand deep Eye-glasses; seeing it is not the length that gives esteem toTelescopes; but on the contrary renders them less estimable, by reason of the troubleaccompanying them, if they perform no more, than shorter ones. Where, by the by, he takes notice, that he knows not yet, whatApertureSignorCampanigives to his Glasses, seeing he hath as yet signified nothing of it; but that the small one, sent by him to CardinalAntonio, hath no moreAperture, than ordinary ones ought to have.
He promises withall, that he will explicate this way in hisTreatise of the usefulness of Telescopes, where he intends to assign the Bigness of theDiameterof all thePlanets, and their proportion to that of theSun; as also, that of theStars, which he esteems yet much less, than all those have done, that have written of it hitherto; not believing, that theGreat Dog, which appears to be the fairest Star of theFirmament, hath 2SecondsinDiameter, nor that those, which are counted of the sixth Magnitude, have 20thirds; nor thinking, that all the Stars, that are in theFirmament, do enlighten the Earth as much as a Luminous Body of 20secondsinDiameterwould do, or, because there is but one half of them at the same time above ourHorizon, as a Body of 14secondsinDiameter; and as the 18432thpart of theSunwould enlighten us, or as theSunwould do, if we were 14 times more distant from it, thanSaturn, and 137 times further, than the Earth: Which,he saith, would not be credible, if he did not endeavor to evince it both byExperienceandReason. And he doubts not, but thatVenus, although she sends us no Light but what is reflected, does sometimes enlighten theEarthmore, than all the Stars together. Yet he would not have us imagine, from what he hath spoken of the smallness of the Stars, thatTelescopesdo not magnifie them by reason of their great distance, as they doPlanets; for this he judgeth a Vulgar Error, to be renounced.Telescopesmagnifie theStars(saith he) as much in proportion, as they do all other Bodies, seeing that the demonstration of their magnifying is made even uponParallelrays, which do suppose an infinite distance, though the Stars have none such: And if theTelescopesdid not magnifie the Stars, how could they make us see some of thefiftieth, and it may be some of thehundreth, andtwohundrethMagnitude, as they do, and as they would shew yet much lesser ones, if they did magnifie more?
SIR,
SIR,
SIR,
Together with my most hearty thanks for the favour you were pleased to do me, in sending me anEpitomeof what had been by the ingenious MonsieurAuzoutanimadverted on a description, I had made of anEngineforgrinding spherical Glasses, I thought my self obliged, both for your satisfaction, and my own Vindication, to return you my present thoughts upon those Objections. The chief of which seems to be against the veryPropositionit self: For it appears, that theObjectoris somewhat unsatisfied, that I should propound a thing inTheory, without having first tried thePracticablenessof it. But first, I could wish that this worthy Person had rectified my mistakes, not by speculation, but by experiments. Next, I have this to answer, that (though I did not tell theReaderso much, to the end that he might have the more freedom to examine and judg of the contrivance, yet) it was not meerTheoryI propounded, but somewhat ofHistoryandmatter of Fact: For, I had made trials, as many as my leisure would permit, not without some good success; but not having time and opportunity enough to prosecute them, I thought it would not be unacceptable to such, as enjoyed both, to have a description of a way altogetherNew, andGeometricallytrue, and seemingly, not unpracticable, whereof they might make use, or not, as they should see reason. But nothing surprised me so much, as, that he is pleased (after he had declared it a fault, to write thisTheory, without having reduced it to practice) to lay it, as he seems to do, in one place of his book,p.22 upon theRoyal Society. Truly,Sir, I should think my self most injurious to thatNoble Company, had I not endeavoured, even in the beginning of my Book, to prevent such a misconstruction. And therefore I cannot but make this interpretation of what MonsieurAuzoutsaith in this particular, that either he had not somuch of the Language wherein I have written, as to understand all what was said by me, or, that he had not read myDedicationto theRoyal Society, which if he had done, he would have found, how careful I was, that thatIllustrious Societyshould not be prejudiced by myErrors, that could be so little advantaged by myActions. And indeed, for any man to look upon the matters published by their Order or Licence, as if they wereTheirSense, and hadTheirApprobation, ascertainandtrue, 'tis extremely wide of their intentions, seeing they, in giving way to, or encouraging such publications, aim chiefly at this, thatingenious conceptions, and importantphilosophical matter of Factmay be communicated to the learned and enquiring World, thereby to excite the minds of men to the examination and improvement thereof. But, to return; As to hisObjectionsagainst theMatter, I do find that they are no more against mine, than any other way ofGrinding Glasses; nor is it more than I have taken notice of my self in this Passage of the sameParagraph, of which sort are also those difficulties he raises aboutLong Glasses, which are commonly known to such, as are conversant in making themIt would be convenient also(these are my words)and not very chargeable, to have four or five several Tools: One,&c.And, if curiosity shall ever proceed so farr, one for all lengths, between 1000. and 10000. foot long; for indeed, thePrincipleis such, that supposingthe Mandrils wellmade, and of a good length, and supposinggreat carebe used in working and polishing them, I see no reason, but that a Glass of 1000. nay, 10000. foot long may be made, as well as one of 10. For, the reason is the same, supposing theMandrilsandToolsbe made sufficiently strong, so that they cannot bend; and supposing also that the Glass out of which they are wrought, be capable of so great a regularity in its parts, as to its Refraction.But next, I must say that hisObjectionsto me, seem not so considerable, as perhaps he imagines them. For, as to the possibility of getting Plates of Glass thick and broad enough without veins, I thinkthatnot now so difficult here inEngland, where I believe is made as good, if not much better Glass forOptical Experiments, than ever I saw come fromVenice. Next, though it were better, that the thickest part of a longObject-Glasswere exactly in the middle, yet I can assure MonsieurAuzout, that it may be a verygood one, when it is an Inch or two out of it. And I have a good one by me at present, of 36. foot, that will bare anAperture, ifSaturnor theMoonin thetwilight, be look'd on with it, of 3½ Inches over, and yet the thickest part of the Glass is a great way out of the middle. And I must take the liberty to doubt, whether ever myAnimadversorsaw a long Glass, that was otherwise; as he might presently satisfie himself by a way I could shew him (if he did not know it) whereby the difference of the thickness of the sides might be found to the hundreth part of a Line.
As to the exceeding exactness of theFigureof LongObject-Glasses, 'tis not doubted, but that it is a matter difficult enough to be attained any way: but yet, I think, much easier byEngine, than byHand; and of allEngines, I conceive, none more plain and simple, than that of aMandril. And for makingspherical Glassesby anEngine, I am apt to think, there hardly can be any way more plain, and more exact, than that which I have described; wherein there is no other motion, than that of two suchMandrils, which may be made of sufficient strength, length, and exactness, to perform abundantly much more, than I can believe possible to be done otherwise than by chance, by a man's hands or strength unassisted by anEngine, the motion and strength being much more certain and regular. I know very well, that in making a 60. foot Glass by the strength of the hand, in the common way, not one of ten that are wrought, will happen to be good, as I have been assured by Mr.Reeves; who, I am apt to think, was the first that made any good of that length. For theFigureof theToolin that way is presently vitiated by the working of the Glass, and without muchgagingwill not do any thing considerable. Besides, the strength of a man's hands, applied to it for the working and polishing of it, is very unequal, and the motions made, are very irregular; but in the way, I have ventured to propose, byMandrils, the longer theGlassandToolare wrought together, the more exact they seem to be and if all things be ordered, as they should be, the very polishing of the Glass, does seem most of all rectifie theFigure.
As to what he objects, that the Tool does only touch the Glass in aMathematical Circle; that is true, perhaps, at first, but before the Glass is wrought down to its trueFigure, theEdgeof theToolwill be worn or grownd away, so as that a Ring of an inch broad may be made to touch theSpherical Surfaceof the Glass; nay, if it be necessary (without much trouble, especially in the grinding of longer Glasses) the wholeConcave Surfaceof theToolmay be made to touch a Glass. Besides, that as to the keeping a quantity of the same sand and Powders of several finesses, according as the glass wears, the same is possible to be don, as with the same Sand wrought finer by working in the Ordinary way.
The giving theInclinationto theMandrils, is not at all difficult; though perhaps to determine the length exactly which the Glass so made shall draw, is not so easie: But 'tis no matter, what length the Glass be off, so it be made good, whether 60 or 80 foot, or the like. Nor is it so very difficult, to lay them both in the samePlain. And to keep themsteddy, when once fix'd, is most easie.
As to the Calculation of the propriety of a Glass of a thousand foot, perhaps for that particular Length, I had not, nor have as yet calculated, that the Convexity of one of eighteen inches broad, will not be above a seventh part of a Line. But it does not thence follow, that I had not considered the difficulties, that would be in making of it. For, I must tell him, that I can make aPlano convexGlass though its convexity be a smaler sphere than is usual for such a length to be anObject-Glassof about 150 foot in Length, nay of 300 foot, and either longer or shorter,withoutat allaltering the convexity. So that, if he will by any Contrivance he hath, give me aPlano-convexGlass of 20, or 40 footDiameter, withoutVeins, and truly wrought of thatFigure, I will presently make aTelescopewith it, that with a single Ey-glass shall draw a thousand foot: WhichInvention, I shall shortly discover, there being, I think, nothing more easie and certain. And if aPlano-convexGlass can be made of anySpherebetween twenty and fourty footradius, so as that both theConvexandPlainside of the Glass be exactly polish'd of a trueFigure, I will shortly shew, how therewith may be made aTelescopeof any Length, supposing the Glass free from all kind ofVeins, or inequality ofRefraction.
As for the sliding of the Glass upon theCement, I see no reason at all for it, at least in theCement, I make use of, having never observed any such accident in hardCement.
And for the Bearing of theRingagainst one side of the Glass only at a time, I cannot see, whythatshould produce any inequality, since all the sides of the Glass have successively the same pressure.
His ratiocination concerning a Glass of 300 foot, is much the same with the former, about the difficulty of working a true surface of a convenient figure; which how considerable boththatand his Conclusion thereupon (videl. That we are not to expect Glasses of above 300 or 400 foot long at most, and that neitherMatternorArtwill go so far) is, may be judged from what I have newly told you of making anyObject-Glassof any Length.
And for his good wishes, that those, who promise to make him seePlantsorAnimalsin theMoon(of which I know not any, that has done so, though perhaps there may be some, notwithstanding his Objections, that do not yet think it impossible to be done) had considered, what a Man is able to see with hisbareEye at 60 Leagues distance: I cannot but return him my wishes, that he would consider the difference between seeing a thing through theGrossandVaporousAir neer the Earth, and through the Air over our heads: Which, if he observe the Moon in theHorizon, and neer theZenithwith aTelescope, he will experimentally find; and, having done so, he will perhaps not be so dissident in this matter.
Concerning his Advertisement to such, as publishTheories, I find not, that he hath made use of it in his own case. For, in hisTheoryaboutApertureshe seems to be very positive, not at all doubting to rely upon it,vid.that theAperturesmust bethusandthusingreatGlasses, because he had found themsoorsoin somesmallones.
For his Proposal of amendments of some inconveniencies in this way, I return him my thanks; but as to his first I believe, that the matter may be conteined as wel in theConcaveTool, as on theconvexGlass. And as to that of 2Poppet-headsI do not well understand it, if differing from mine; and the keeping of the Tool upon the Glass with a spring or weight, must quickly spoyl the whole; since, if either of theMandrilswill easily yield backwards, theregularityofallwill be spoiled: and as to the wrigling and playing of theMandril, I do not at all apprehend it.
HisTheoryofApertures, though he seems to think it very authentick, yet to me it seems not so cleer. For, the same Glass will endure greater or lesserApertures, according to the lesser or greater Light of theObject: If it be for the looking on theSunorVenus, or for seeing theDiametersof theFix'd Stars, then smallerAperturesdo better; if for theMoonin thedaylight, or onSaturn, orJupiter, orMars, then the largest. Thus I have often made use of a 12 foot-Glass to look onSaturnwith anApertureof almost 3 inches, and with a single Eye-glass of 2 inchesdouble convex: but, when with the same Glass I looked on theSunorVenus, I used both a smallerAperture, and shallowerCharge. And though M.Auzoutseems to find fault with theEnglishGlass of 36 foot, that had anApertureof but 2¾ inchesFrench; as also, with a 60 footTube, used but with anApertureof 3 inches; yet I do not find, that he hath seen Glasses of that length, that would bear greaterApertures, and 'tis not impossible, but hisTheoryofAperturesmay fail in longer Glasses.
One of the means used by M.Auzoutto enlighten an Object, in what proportion one pleaseth, is by some greatObject-Glass, by him called aPlanetaryone, because that by it he shews the difference of Light, which all thePlanetsreceive from theSun, by making use of severalApertures, proportionate to their distance from theSun, provided that for every 9 foot draught, or thereabout, one inch ofAperturebe given for theEarth. Doing this, one sees (saith he) that the Light whichMercuryreceives, is far enough from being able to burn Bodies, and yet that the same Light is great enough inSaturnto see cleer there, seeing that (to him) it appears greater inSaturn, than it doth upon ourEarth, when it is overcast with Clouds: Which (he adds) would scarce be believed, if by means of this Glass it did not sensibly appear so; Whereof he promises to discourse more fully in hisTreatise of the usefulness of great Optick-Glasses, where he also intends to deliver several Experiments, by him made, 1. Touching the quantity of Light, which a Body, that is 10, 15 and 20 times,&c.remoter thanSaturn, would yet receive from theSun. 2. Touching the quantity of Light, by which theEarthis illuminated even in theEclipsesof theSun, in proportion of their bigness. 3. Touching the quantity of Light, which is necessary to burn Bodies: he having found, that not abating the Light, which is reflected by the Surfaces of the Glass (whereof he confesseth, he doth not yet exactly know the quantity) there would be necessary about 50 times as much Light, as we have here, for the burning ofBlackBodies; and neer 9 times more for the burning ofWhiteBodies, than for the burning ofBlackones: and so observing the immediate proportions between these two, for burning bodies ofotherColors. Whence (he tells us) he hath drawn some consequences, touching the distance, at which we may hope, to burn Bodies here, by the means ofgreat Glassesand greatLooking-glasses. So that (saith he) we must yet be seven times neerer theSun, than we are, to be in danger of being burned by it. Where he mentions, that having givenInstructionsto certain persons, gon to travel inHot Countries, he hath among other particulars recommended to them, to try by means of greatBurning-glasses, with how much lessAperturethey will burnthere, thanhere, to know from thence, whether there by more Lighttherethanhere, and how much; since this perhaps may be the only means of trying it, supposing, the same matters be used: although the difference of the Air already heated, both inhot Countries, and in thePlanets, that are neerer than we, may alter, if not the quantity of Light, at least that of the Heat, found there.
In the above-mentionedFrenchTract there is also conteined M.Auzout'sOpinion of what he had found New in theTreatiseof SignorCampani, which was spoken of in the firstPapersof theseTransactions, concerning both the Effect of theTelescopes, contrived after a peculiar way by the saidCampaniatRome, andhis New Observations ofSaturnandJupiter, made by means thereof.
First therefore, after that MAuzouthad raised some scruple against the Contrivance of SignorCampanifor makingGreat Optick-GlasseswithoutMoulds, by the means of aTurn-lath, he examines theObservations, made with suchGlasses: Where, having commendedCampani's sincerity in relating what he thought to have seen inSaturn, without accomodating it to M.Hugens'sHypothesis, he affirms, that supposing, there be aRingaboutSaturn, SignorCampanicould not see in all those different times, that he observed it,the same Appearances, which he notes to haveactuallyseen. For, having seen it sometimes inTrine Aspectwith theSun, andOriental; sometimes, in the sameAspect, butOccidental; sometimes inSextil Aspect, andOccidental; at another time, again inTrine, andOriental, this Author cannot conceive, howSaturncould in all these different times have no difference in itsPhasis, or keep always the sameShadow; seeing that, according to theHypothesisof theRing, when it wasOriental, it must cast theShadowupon theleftside of theRingbeneath, without casting any on therightside: and when it wasOccidental, it could not but cast it on therightside beneath, and nothing of it on the other.
Concerning theShadow above, whichCampaniaffirms to be made by theRingupon the Body ofSaturn, M.Auzoutjudges, that there could be no suchPhænomenon, by reason of itsNorthern Latitudeat the times, wherein theObservationswere made,vid.inApril1663; in the midst ofAugust, and the beginning ofOctober, next following, and inApril1664, except it were inOctober, and theShadowstrong enough to becomevisible.
But as to theShadow below, he agrees withCampani, that it does appear, yet not as he notes it, seeing that it must be sometimes on the one side, sometimes on the other; and towards theQuadratwith theSunit must appear biggest, asindeedhe affirms to have seen it himselfthisyear, insomuch that sometimes it seemed to him, that it covered the wholeRing, and that theShadow, joyning with the obscure space between both, did interrupt the circumference of theRing; but beholding it at other times in a cleer Sky, and when there was no Trepidation of the Air,he thought, that he saw also the Light continued from without, although very slender. But he acknowledges, that he could never yetpreciselydetermine, by how much the largeness of theRingwas bigger than theDiameterofSaturn'sBody. As for the proportion of the Length to the Breadth, he affirms, to have alwaies estimated it to be two and a half, or very neer so; and to have found in hisObservations, that inJanuarylast, one time, the length ofSaturnwas 12Lines, and the breadth 5. Another time, the length was 12.Lines, and the breadth 4. and this by a peculiar method of his own. But yet he acknowleges also, that sometimes he hath estimated it as 7. to 3. and at other times as 13. to 5. and that if there do not happen a change in the magnitude of theRing(as it is not likely there does) that must needs proceed from the Constitution of the air, or of the Glass's having more or lessAperture, or from the difficulty of making an exact estimate of their proportions. However it is not much wide (saith he) of two and a half, althoughCampanimake the length of theRingbut double to its breadth.
MonsieurAuzoutbelieves, that he was one of the first that have well observed this shadow ofSaturn'sBody upon itsRingswhich he affirms happened two years since; when, observing inJuly, for the first time, with aTelescopeof 21. and then another of 27. foot, he perceived, that theAngleof the obscure space on theright sidebeneath, was bigger and wider, than the three otherAngles, and that some interruption appear'dthere, between theRing, and theBody of Saturn; of which he saith to have given notice from that time to all his friends, and in particular, as soon as conveniently he could, to MonsieurHugens.
He confesseth, that he hath not had the opportunity of observingSaturnin hisOriental Quadrat; yet he doubts not, but that theshadowappears on theLeft-side, considering, that theExistenceof theRingcan be no longer doubted of, after so manyObservationsof theshadowcast bySaturn'sBody upon it, according as it must happen, following thatHypothesis; there being no reason, why it should cast the saidshadowon one side, and not on the other.
Concerning the Observation ofJupiterand itssatellites, the famousAstronomerofBononia,Cassinus, havingpublished, that on the 30. day ofJuly, 1664. at 2½ of the clock in the morning, he had observ'd, withCampani's Glasses, that there passed through the broad obscureBeltofJupitertwo obscurerspots, by him esteemed to be theshadowsof theSatellites, moving betweenJupiter& theSun, and eclipsing him, and emerging from the Occidental Brim thereof: ThisAuthourdid first conceive, that they were notshadows, but someSallies, orProminenciesin thatBelt; which he was induced to believe, because he perceived not, that thatProminency, which he there saw, was so black, nor so round asCassinihad represented hisspots; wherefore, seeing it but little differing in colour, from theBelt, and so not judging it round, because it did stand only about half its diameter out of theBelt, he persuaded himself, that it was rather aSally, orProminencyof theBelt, than a roundshadow, as that of aSatelliteofJupitermust have bin. But having been since informed ofalltheObservationsmade byCassiniandCampani, with theNewGlasses, and seen hisFigure, he candidly and publickly wisheth, that he had not spoken of thatSally, orProminency; advowing that he can doubt no longer, but that it was theshadowof theSatellitbetweenJupiterand theSun, having seen the other emerge, as soon as with a 20. foot Glass he made the Observation, and having not perceiv'd theseshadowswith a 12. foot Glass: But although he grants that they did ghess better than he, yet he doth it with thisproviso,vid.in case they madethatObservation on ofJuly30. not with their 36. but 12. or 17. footTelescope. If it be wondred at, that MonsieurAuzoutdid not see thisshadowmove, he allegeth his indisposition for makinglong Observations, and addeth, that it may be much more wondred at, that neitherCampaninor himself did see upon the obscureBeltthe Bodies of theSatellites, as parts more Luminous than theBelt. For (saith he) although theLatitudewasMeridional, it being no more than of 9. or 10. minutes, the Body of theSatellitesshould, thinks he, pass betweenusand theBelt, especially according toCampani, who maketh theBeltso large, and puts theshadowsfarr enough within the same. This maketh him conclude, that either they have not observed well enough, or that the motion of theSatellitesdoth not exactly follow theBelts, and is inclin'd unto them. Whereupon he resolves, that when he shall know that they are to pass betweenJupiterandus, and to be over against theBelt, thatthen he will observe, whether he can see them appear upon theBelt, as upon a darker ground, especially, thethirdof them, which is sensibly greater, and more Luminous, than the rest. He hopeth also, that in time, theshadowofSaturns Moonwill be seen uponSaturn, although we are yet some years to stay for it, and to prepare also for better Glasses.
From this rare Observation, he inferrs theProportionof theDiameterof theSatellitesto that ofJupiter; and judgeth, that no longer doubt can be made of the turning of these 4.Satellites, orMoonsaboutJupiter, as ourMoonturns about theEarth, and after the same way as the rest of the Celestial Bodies of ourSystemedo move: whence also a strong conjecture may be made, thatSaturnsMoon turns likewise aboutSaturn.
Hence he also taketh occasion to intimate, that we need not scruple to conclude, that if these twoPlanetshaveMoonswheeling about them, as ourEarthhath one that moves about it, the conformity of theseMoonswith ourMoon, does prove the conformity of ourEarthwith thosePlanets, which carrying away theirMoonswith themselves, do turn about theSun, and very probably make theirMoonsturn about them in turning themselves about theirAxis; and also, that there is no cause to invent perplex'd and incredibleHypotheses, for the receding from thisAnalogiesince (saith he) if this be truth, the Prohibitions of publishing this doctrine, which formerly were caused by the offence of Novelty, will be laid aside, as one of the most zealous Doctors of the contrary Opinion hath given cause to hope, witnessEustachius de Divinis, in hisTractagainst MonsieurHugen'sSystemeofSaturn,p.49. where we are inform'd, that that learned Jesuit,P. Fabry, Penitentiary of SPeterinRome, speaks to this purpose:
It hath been more than once asked of your Chieftains, whether they had a Demonstration for asserting the motion of the Earth? They durst never yet affirm they had; wherefore nothing hinders, but that the Church may understand those Scripture-places, that speak of this matter, in aliteralsence, and declare they should be so understood, as long as the contrary is not evinced by any demonstration;which, if perhaps it should be found out by you (which I can hardly believe it wil) in this case the Church will not at all scruple to declare, that these places are to be understood in a figurative and improper sence, according to that of the Poet,Terræque Urbesque recedunt.
Whence this Author concludes, that the saidJesuiteassuring us that theinquisitionhath notabsolutelydeclared, that those Scripture-places are to be understoodliterally, seeing that theChurchmay make a contrary declaration, no man ought to scruple to follow theHypothesisof theEarths motion, but only forbear to maintain it inpublick, till the prohibition be called in. But to return to the matter in hand, this Author, upon all these observations and relations ofCassiniandCampani, doth find no reason to doubt any more of the excellency of the Glass used by them, above his; except this difference may be imputed to that of theAir, or of theEys. But yet he is rather inclined to ascribe it to the goodness of their Glasses, and that the rather, because, he would not be thought to have the vanity of magnifying his own; of which, yet he intimates by the by, that he caused one to be wrought, of 150Parisianfeet; which though it proved none of the best, yet he despairs not to make good ones ofthat, and of far greater Length.