ought thus to be Corrected that the further you advance towards either Pole their Motion is swisaer and the further they are carryed towards the Equator their Motion is retarded c. The cause wherefore these Instruments may move swifter nigh the Poles then near the Equinoctial cannot proceed from the Oval Form of the Earth as some think the cause of their swifter Motion nigher the Poles in my Opinion is cold Weather and Frosts I believe that in England in Frosty Weather our Clocks move swifter then in Summer for in Frosty Weather Springs may contract a little and grow stiffer consequently have more force to make a swifter Motion and in Hot Weather they may extend and be a little more pliable therefore the Motion may not prove altogether so swift as in Frosts There are other Causes also that may hinder the regularity of their Motions see Ph. tr N. 47. p. 951. and 976. Some may desire to know my Reasons for Springs contracting in Frosty Weather take a piece of Cold Iron or Steel one or two Foot long about ½ or ¾ of an Inch Square heat it red hot keep heating and well Hammering it hot an Hour or two when you have done measure its length exactly before it be Cold lay it aside in the Air where the Frost may ting it a Winters Night and if you measure it in the Morning you will find it considerably shorter Experience teacheth that there are few Watches that have a regular Motion for it seems impossible that the Springs in all their parts should be so exactly Hammered as to draw always equally and by what I can learn from Watch-makers a ballsnce Watch that requireth winding up every 24 Hours keepeth time more exact than one that goeth a Week before it need winding up About Seven Years ago my curiosity carried me to Gresham Colledge when the Assembly was seated the President told me they had caused several Experiments to be made with these Instruments and thought fitting not to commend them for Common Practice at Sea for fear that the Errors that might happen in their use should prove greater then the Errors in the common Practice of Navigation but now let us handle our Subject a little more closely and suppose that three or four of the said Watches being good work well made and carryed to Sea in one Ship under the Command of Ingenious Men may prove very useful towards the keeping a true Account of a Ships way on the Sea not for finding the Longitude from our first Meridian but to help to find your difference of Longitude every 24 Hours or every two or three Days as often as you observe the Latitude provided they be not too much tampered with keeping the Indexes always moving but never move them with your Fingers except extraordinary occasion require it desire not that they should always shew you the true time of the Day you may if you please keep one for that purpose besides but learn by continuance of time to find the Motion of each Watch in a Year or in a Month or Day and Hour and having the true Theory of each Watches Motion with their equalities and inequalities you may by Calculation find at what time of the Day or Night the Indexes of each Watch for I suppose they may not all move just alike will point to such or such an Hour Degree or Minute in any Meridian keep them from being hurt by any Violent Motion as falls or knocking near them keep them from Winds from too moist Airs and stinking Fumes such as are when Salt Water Casks are emptied on Deck or in a Ships Well where Men have been Stifled keep them very clean from Salt Water or any Rust or Filth let there be a place in the Ship properly appointed for them to hang in Equilibrio I find in the Ph. trans N. 47. p. 937. Instructions concerning the use of Pendulum Watches for finding the Longitude at Sea together with a Method of a Journal for such Watches c. Brother Tar if these Automata's could speak as the Welsh Man thought they would tell thee they hate Novices and if thou art so they may not bite thee but they may happen to break thy Head or do the some other Mischief or in a more plain meaning I dare not approve of their use for the difference of Longitudes except only in an East or West Course or in Case of some unknown Currents use thy discretion The Watches that are now esteemed most useful have a Pendulum Spring to regulate the Motion of the ballance those that desire to know more of these Automata's and Mechanical Motions may read Bishop Wilkin's his Mathematical Magick Water-Glasses 24 Hour Sand-Glasses and such like Self-movers are of no use in this Science though I have mentioned them for Vulgar Satisfaction only but of what use Mercurii or Quick-silver may prove in keeping time true by the regularity of its Motion in Glasses I cannot yet inform you CHAP. V. Of or concerning Longitude by Magnetick Variation THE Variation of the Compass by which I mean the deflection of the Needle from the true Meridian is of that great concernment in the Art of Navigation that the neglect thereof does little less then render useless one of the noblest Inventions Mankind ever attain'd to I desire the Ingenious Seamen that the Knowledge of Magnetick Variation may be always in his great Esteem for without it Navigation cannot be perfect It is my Opinion that many Ships have been put by their design'd Ports and more Ships lost for want of knowing the Variation then hath been lost for want of the Longitude how many Ships in Sailing from the South-fâ—re-land to the Maese hath been put to the Northward of the Maese for want of minding the Variation for the like cause some Ships have missed the Island Barbadoes and met with other dismal Accidents Mr. Edward Wright an able Mathematician in his time in the latter end of his Correction of Errors in Navigation did write concerning the Haven finding Art by the Variation of the Marriners Compass but alas he knew nothing of the Variation of the Variation or as I call it of the Increase and Decrease of the Variation in the same place therefore that part of his Book proved very Erroneous by supposing that the Variation at any place never Altered now we know better There are many of Opinion at this present time that the Longitude may be found by the Variation of the Magnetical Horizontal Needle I doubt the possibility thereof from our first Meridian for many parts of the World will afford us no Practicable proportions for the difference of Variation between them according to their distance in Longitude nor can the quantity of the Earths Atraction be easily determin'd in all places besides the quantity of each Polar Atraction for there are four at least two of them lie in the bowels of the Earth and not in the Air as Mr. Pond saith it is
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
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