Sun will be this Year 1696 March the 9th Day 3 h. 21′ 20″ P. M. it cuts the Equinoctial in two opposite points in the beginning of ♈ and ♎ it s greatest Obliquity from the Equinoctial in these our Days is 23° 29′ it ends where it begins The Theory of the Planets and Stellary Motions are accounted on the Arches of this great Circle because in their Revolution they respect the Ecliptick Poles as their Center and the Sun also and not the Equinoctial Poles except the Earth only in her Diurnal Motion Sydereal Longitude is accouted from the first Star of Aries on the Ecliptick also it begins where the Ecliptick is cut by a great Circle that passeth through the 1 ⚹ of ♈ and the Ecliptick Poles the difr Long. between the 1 point of ♈ and the 1 ⚹ of ♈ is this Year the 9th Day of March 28° 52′ 9″ which difr is the praecession of the Aequinox CHAP. III. A Definition of Time and Equation of Natural Days Atque in se sua per Vestigia Volvitur annus Virg. OLD Style or Julian Year is our English Account contains 365 d. 6 h. of Astronomical years there are divers kinds but I shall only mention two the Solar or mean Tropical Year is 365 d 5 h 49′ 1″ the Sidereal Year is 365 d 6 h 8′ 30″ the natural Day in most parts of Europe is divided into 24 h the Suns or Earth mean motion in the Ecliptick in 24 h is 59′ 8″ the longest natural Day is 361° 1′ 15″ the shortest is 360° 57′ 10″ the difr in one Day is but little but in a Months time is considerable each difference adjusted is called Aequation of natural Days the shortest natural Day will be this Year the 18th Day of June when the Sun is in 7° of Cancer or thereabouts for then the ☉ and ♠are in their Apogaeon or Auge Apoge Aphelion Absis have all one Signification in Astronomy the 17th Day of December they will be in their Perigaeon or Perihelion or nearest distance this Inequality of Natural Days seems to me to be the cause why our Summer half Year is now in our times almost Eight Days longer than our Winter half Year as also it is the cause of the Impossibility of any Clock or Watch. though they move very regularly always to shew the true time of the Day Mr. Parker in his Almanack teacheth us when to Add or Substract the Time aequated to or from the mean time to accord with the Apparent Time In most Books of Astronomy there are Tables for our purpose Vide Astr Ang. lib. 3. p. 84 and 85. Canones Aequationum Dierum Naturalium Canones Equationis dierum Naturalium duabus praecedentibus Composit nostro huic Seculo citra errorem sensibilom subservien And Ph. trans N. 214. pa. 248. Tabulas aequationis Dierum cum Solis loco adeund c. All which Tables you must well understand before you can attain to Practical Longitude except Magnetick Variation it may seem necessary here that I show how to find the time of the Day or Night by Observation there are many ways Published in several Mathematick Books but I shall reduce them to one for I think Mr. Molineux's Sciotericum Telescopium will do us little or no service right way and commend you to Colson's Callender see his Astronomical Problems Prob. 9 and 10. instead of Hours and Minutes work by Degrees and Minutes and whereas I said that 360° 59′ 8″ was equal to 24 Hours you may work only by 360° for when you observe in the Night it is supposed you take the Sun 's right Ascension exactly true at that very Time and Place of your Observation allowing for the Suns Diurnal difference of R. A. also allowing for your difference of Longitude from our first Meridian else all your trouble may prove good for nothing As yet I have never seen any Vniversal Ring-Dial or other Sun-Dial that shewed the true Time of the Day except about Noon our Dialists thinking the Suns Refraction not worth their notice but those that pretend to find the Longitude must duly consider Refractions and Parallaxes It seemeth not proper in this Chapter but I think it convenient to add something concerning Instruments common Practice and Experience teacheth that a Forestaff is a useful Instrument both for backward and forward Observations I have also had Experience of it my self and for forward Observations I like it not and for backward Davis's Quadrant is better for in its use there happen several Errors hard to Correct as the Excentricitie of the Eye and end of the Staff a true Horrizon cannot be found by it in the Night but only guessed at besides the height of the Eye above the Water and the great difficulty of seeing two Objects at once that is the Sun or Star and the Horizon for if the Star be high whilst you are a looking for it at one end of the Vane you loose the Horrizon at the other end of the Vane or a looking for one you lose the sight of the other therefore to avoid such Errors for taking Altitudes in the Night I commend you to a Quadrant with a Pendulum from its Center see the Figure of it in Moxon's Tutor to Astr Prob. 13. p. 48. This Instrument may seem very troublesome at first for want of Use and Custom use a weight of at least two or three Pound with a small Wire or Catts-Gut for taking small Heights or Distances you may use an Almicanter Staff for ordinary Uses but if your Observation requires greater exactness Vide Tichonis Brahe Astronomiae Instauratae Mechanica or John Heuelii Machinae Caelestis pars prior c. instead of Instruments that resolve Mathematical Questions as Sectors upon Quadrants Analemmas c. Stick close to the Doctrine of plain and Spherical Triangles and to Logarithmical Tables but on better Consideration the Quadrant in Moxons Tutor to Astr aforesaid is not very fitting for Sea Uses without some Alterations therefore I have contriv'd a Quadrant something like the former with weights that Commands it so that it will always continue in the same position you direct it to It may be contriv'd to hang like a pair of Scales or Ballance having an Axis in its Center of Gravity on which it is equally poised without the weight to turn round having a Swifle and Ring to hang or hook to any convenient place in the Ship or those that please may have a Stand purposely for it there must be a small Line or slippery Cord or Cords fastened somewhat slack from Corner to Corner on the Circumference of the Quadrant on which Cords hang the weights of 20 or 30 Pound or more or less as you find most convenient for the Substance or Radius of the Quadrant from the said weights or rather from that part that slides on the Cords may come two Indexes one to point to the Altitude on one side of the Quadrant and the other to the
Paw or only my labour for my Pains future Ages may be more Competent Judges but my Mind will be easier when this Book is Published because therein I have done my Duty and eased the burden of my Brain If in this Treatise have seemed to reflect on any Person it is my Dialect I desire their Excuse I Envy no Men Envy and Ingratitude is most natural to some whose Duty it is to understand these things and their Capacities cannot reach them If any Person is minded or can or will undertake to Correct or Amend what I have writ I wish them good success I desire them to own what they borrow and not to reflect without just cause it is difficult to write such a Book as this is I think it is the first of this nature without Faults it is likely I my self may find some in a Years time that if I were to write over again I could amend I dare affirm that this Book contains some estimable Expressions and Methods fit for Seamens Knowledge and Practise and such as have not been formerly Published there are some Persons in England whose Duty it is being paid for it to improve Navigation and Astronomy and from whom much is expected and little or nothing of purpose appears It is my Opinion that the appearances of Comets may be found by Calculations if any Person capable think it worthy his trouble Concerning this Book the common Vogue will be viz. but will it do is it the Longitude I say Yes it is the Longitude begun and will do with Industry and Encouragement else not for it is contrary to Seamens Vulgar sayings that when the Longitude is found then there will be abundance of Artists Navigation will be so easie No no Navigation will not be so easie at present but much more certain and true The Learned Bacon said the Longitude was worth a Kingdom to a Wise Man but before I conclude to return to my Instrument the Description I have given thereof availeth little without my further assistance till then the Reader if he pleaseth may compare to Foxes staring on Bunches of ripe Gripes and cannot get at them then they are too sowr for them for so my Dream intimateth when I was among a Crowd of Foxes aw'd at my Presence and after I had looked on them they vanished and left me other Objects The most Ingenious of Humane Inventions is far less facile then the ordinary performances of Truth and Nature I love my King the Church of England and my Country and I thank my God I know my self capable and worthy to serve them though too much debased for want of Friends in Court If this meet with its deserved Encouragement the rest may follow in good time but I must make haste and conclude my attendance and dependance being required in our Navy Royal which Duty I dare not neglect for fear of being Excommunicated or which is as bad Excluded from Preferment Quamvis bene merenti Nil tam difficile est quod non Solertia Vincat Te Deum Laudamus FINIS ERRATA PAge 2. l. 9. r. plane p. 17. l. 8. r. Zulichem p. 20. l. 6. r. cert is l. 9. r. Industria l. 19. r. an Idea p. 28. l. 3. r. Bond. p. 29 l. 5. r. the encrease p. 30. l. 3. for me r. one l. 17. for by r. with p. 31. l. 22. r. though l. 23. r. suppose p. 36. l. 12. add when Northerly p. 37. l. 10. r. preceding p. 59. l. 27. for P. M. r. A. M. add Schema Solis ad Exitum Mercurii St. Hellene Anno 1677. Octob. 28. 2 h. 4 P. M. p. 55. l. 10. r will be p. 63. l. 27. r. with p. 70 l. 2. r. Sun and Stars
at the Azores and will be more in time If the Magnetick Needle were very high up in the Air it would have no respect to the Earth or to any Poles but its own or Center this is my Opinion The cause of the quick Alteration and great Variation at New-found-land and some other places I would also tell you my Opinion but I am afraid of seeming tedious with Tautologies In using Azimut Compasses take care the Chart Librate well in the Box and that the point of the Brass Needle on which the Chart Librates be as exact as may be in its proper place or Center of the Box trust not to one Observation when you can have the Medium of 5 or 6 or more nor to one Amplitude when you may have the mean of 3 or 4 Azimuths mind to substract the Suns Refraction c. Suffer no great Guns or other Iron too near your Compasses those are best that have the quickest Motion you may try the points of the Needle by seeing what Iron they will take up but be very careful and use it not often for you may spoil the Needle With a Needle and round Loadstone may be perfom'd many Natural and pleasant Conclusions but to think that such a Terrula as they call it hath or represents the like Magnetick Force or Power in all Species as the Earth is a weak and ridiculous Opinion whereas it may be thought in these parts of Europe there is no occasion for carrying Azimuth Compasses to Sea because it may be supposed most Masters knows what Variation there is in most Adjacent places it is a mistake for the Major part nay â…” of them I believe knows little or nothing concerning it as I could prove by manifest Examples If I were to declare what I know and sometimes been an Eye witness to but Truth ought not to be spoke at all times besides I am not willing to disoblidge some Persons that might think I reflect too much on them those Masters that have understood something of the Variation often forget it for want of Azimuth Compasses or for not being obliged to give an Account thereof I have known some Masters of Ships that knew by hear say from others that in such places there was about half a Point West Variation I have asked them how they accounted it from what point of the Compass pretending my self Ignorant they have answered I might Account it from N. or S. E. or W. or from any point it was always Westerly neither do they know how to estimate the Variation in their Reckonings but it may be objected how can the Masters be so Ignorant that go and come well from Port to Port I answer Thickie West-Country-Man lock zure Life and Soul Man can carry a Ship to New-found-land zure and near a word a Book zure this is by long Experience and Custom not by Art I have seen the Blind lead the Blind a Begging I knew two Blind Men in Cochin that would go very readily to any House in that Town and I have sent one of them an Errand two Miles into the Country but they were both born Blind therefore by long Experience and Custom they naturally know most Adjacent places As to Masters or Pilots their carrying Ships in and out of Harbours and for the Coasting part when they can see the Land I have nothing to say against them And if a mischance happens you will rarely find any of them without a lying Excuse to salve their Ignorance But it may be said the Masters make Observations and keep a Reckoning yes so they do and a very bad one oftentimes I will acquaint you how they do that knows not how to allow for the Variation as they ought it is likely they can tell whether they have Sailed between the South and the East or S. and W. or N. and E. or N. or W. having wrought their Days Work by Log and difference Lattitude and Departure they find what dist the Ship hath run by Log. so having that dist and diff Lattitude by Observation they can find as they think the Departure without allowing for Variation and this kind of Reckoning is the cause of so many great Mistakes in their Easting and Westing for the Log. is but a false supposition to find the distance run as hath been Experienc'd in one Fleet where some have had 70 Miles on the board some 80 and others about 90 the same 24 Hours and all the while not above a League asunder Now I will acquaint you with the best known Common Method for keeping an Account of a Ships way on the Sea Be as curious as possibly you can in observing the Suns Meridian Altitude or for want thereof some known Stars to find the Lattitude by if you observe with Davis's Quadrant see your Vanes very carefully and exactly placed especially the Shade Vane when the Horizon is very clear observe somewhat close when hazy a little more open these cautions I give that you may observe every Day alike and thereby the better to Judge of your true difference of Latitude by Observation then work your Days work and find what Course the Ship hath made as near as possibly you can find it by allowing for Lee-way Variation c. or lying by or any other Impediments as Tides or Currents or a great Sea and having the true diff Latitude by Observation and the Course given you may easily find the distance run and departure by the common Method and difference of Longitude by Mercator c. It is my Opinion that the use of the Log. is good to help to find the Ships Course though there are some able Artists that use no Log. but now to our purpose again as for Mr. Bond 's Longitude found as he called it he limits his Hypothesis to the City of London affirming himself as he had a great deal of Reason that the same Calculus is not sufficient for other places whereby it appears that his Rule is far short of the so much desired general one The following digression may please some Capacities Famianus Strada in the Vein of Lucretius writes viz. Magnesi genus est lapidis mirabile cui si Corpora ferri plura Stylosve admoveris inde Non modo vim motumque trahent quo semper ad Ursam Quae lucet vicina Polo severtere tentent Verum etiam c. Outinam haec ratio scribendi prodeat usu Cautior citior properaret Epistola nullas Latronum verita Insidias fluviosque morantes Ipse suis princeps manibus sibi conficeret rem Nos soboles Scribarum emersi ex aequare nigro Cousecraremus Calamum Magnetis ad Oras But this Invention is altogether Imaginary having no foundation on any real Experiment The Royal Society in London have a great Load-stone found in Devonshire that will move a Needle at 9 Foot distance Some Persons may happen to Read this Chapter that understand not Navigation and think that most Ships are lost for
long which hath only two Glasses in it and I have been troubled at Sea to find an Object with a Perspective of 1 Foot and a half long which hath only two Glasses in it therefore at Sea for the better finding an Object we commonly use Telescopes with 3 or 4 Glasses in them of three or four Foot long the Telescopes through which I have often looked for Jupiter but never could see his Satellites through one of that length and what must we do for the Longitude when Jupiter rises and sets Cosmically or Acronically as in September this Year but we will suppose the best that the Weather should be so fair and Sea smooth that we may possibly get Observations two or three times in a Month indeed it would be a very great help and satisfaction to us if the Observations proved good Mr. Colson an able Mathematick Teacher told me he had near one Hundred Observation by him made at Sea with a Telescope of 6 Foot long which he contriv'd with a convenient apparatus for manageing it I have one 5 Foot long in which I have seen the Satellites keep your Glasses very clean especially the Object Glass from Mists or Dews I desire to know what we shall do for Tables of Jupiters Satellites for Years to come always for four Years at least Mr. Parker in his Ephemeris hath inserted a Catalogue of the Eclipses of Jupiters Satellites visible under the Meridian of the Observatory I suppose he means the King's Observatory in Greenwich-Park or near it in the Year 1696. But what must they do for such Tables every Year that are in the West and East-Indies a late Author in his Book Printed Anno 1695 hath inserted two Years Tables for the time past viz. 1693 and 1694. borrowed perhaps out of some old Ephemeris it is well he said something of the time past its likely he knew little or nothing of what was to come yet some of our able Astronomers revile and reflect on our Seamens Ignorance because the Seamen as they say cannot find the Longitude Indeed I think our Seamen have most cause to rail at our Mathematicians that have done nothing as I know of to any purpose for the Encouragement or Improvement of Navigation for almost 20 Years past except a Tide Table as I shall prove by Sellers's Practical Navigation the most useful Book for Seamen yet extant and the Impression worse now then it was almost 20 Year ago compare the Tables of the Suns place and Declination in the 7th Edition of the Practical Navigation with Tables of the Suns place and Declination in Parkers Ephemeris for this Year and you will find considerable difference and that the Impression was better 18 or 20 Years ago I prove by Pag. 330 viz. A Table of the Variation of the Suns Declination to be applied for Years to come c. If the Tables were the same as in Anno 1680 or thereabouts we might by the said Table of Variation have Corrected the Tables of the Suns Declination but there is imposed on us new dates of Years c. The aforesaid Book wants Correction very much there is wanting in it a larger Table of difference of Latitude and departure such a one as is in Colson's Kalendar the Tables of the Suns right Ascention ought to be more large and Calculated to Degrees and Minutes to a certain Year c. Pardon this Digression now to our purpose again since we are unfurnished with fitting Tables of all the said Satellites I recommend you to the best I know of viz. See Ph. Trans N. 214 p. 237. to 256. Monsieur Cassini his New and Exact Tables for the Eclipses of the first Satellite of Jupiter reduced to the Julian Style and Meridian of London c. But in Calculating the Eclipses of the Satellite by the said Tables there somtimes happens an Error of 3 or 4 Minutes of time I believe I know how to Correct that Error though it hath baffled the Ingenious and would a done it here if it had no required Figures or Cuts for Demonstration Saturn hath also five Satellites attending him but are not accounted so worthy our notice for the Longitude as the former therefore I pass them in Silence The Sun Jupiter and all the rest except the Moon of the Planets have a Circumrotation in their own proper Vortex as Telescopial Observations testifie by several spots that appear in their Phases Perchance it may be said why have not I furnished this Book with regular Tables of the Moons motion and of Jupiters Satellites It would be a very unreasonable demand to require them from a Man that hath wanted both Books and Instruments till now also conveniences and opportunities having only a small Cabbin sometimes not high enough to stand upright in nor my length to lie down in insomuch that I have been obliged to borrow both Books and Instruments for the common Practice of Navigation besides the Circumstances of my Imploys and many other Inconveniencies that might and did attend such Studies and Observations I will conclude this Chapter with the following words which I take to be the Ingenious Mr. Flamstead's viz. As by Lunar Eclipses so also by the Eclipses of Jupiters Satellites the difference of the observed Moments of the Occultation or Emersion of a Satellite from his shadow noted carefully in two distant places will be the difference of Meridians betwixt these two places in time but I cannot hope that this Method shall prove of much use to the Ingenious Seamen because the Observations require long Telescopes which in a Ship will hardly be manageable nor can we expect to find the difference of Meridians by one only Observation of a Satellite Eclipse as we have hopes we may by a Lunar by reason that as yet the inequality of their motions and the time required for the transmission of Light from the Planet to our Earth are unknown nay their mean motions are scarce so exactly stated but that we may Justly suspect them Erroneous omitting these therefore the best Method for the discovery of the Longitude will be in my Opinion I am dubious he is now of another Opinion by the Moons Apulses to or observed distances from fixed Stars upon which Accoun I would recommend the Improvement and Correction of her Theory with the Doctrine and Construction of Apulses to the Study of the Ingenious Astronomer and Seamen Humanum est Errare I had almost forgot Mr. Newton hath contriv'd an Instrument for his own use to see Jupiters Satellites by reflection on well polished Mettle the Instrument is about 15 Inches long as some say other 9 or 10 Inches and call it a reflecting Telescope I am informed it is of little or no use I have not seen any of them therefore can give you no better Description I also add To tell some Masters of of Jupiters Satellites is like telling a Story to a Bear they know better how to manage a Cann of