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
bearing from us about North distant 6 or 7 Leagues the mean Variation by three Azimuth Compasses and Nine Observations was 10° N. Westerly the same Year November the 16th Variation at St. Hellena 1° 4′ N. Westerly the difference is 8° 56′ the the Meridian distance by plain Chart is 20° difference of Longitude is 21° 54′ in Sailing directly between the two aforesaid places we raised or layed Sea Terms the Variation somewhat gradually therefore having the difference of Longitude and the difference of the Variation given between the two said places it may be supposed that the difference of Longitude between the places may be found by Observation of the Variation I answer yes it may be nearly guessed at and I know how to make proportions for finding the difference of Longitude by the Variation not only between the two aforesaid places but in many other places But it is a Method I do not approve of and advise the Industrious Seamen not to use it except in Cases of great Necessity but rather to learn by former Journals or from some Experienc'd Artist what Variation is at such places where you are bound to and all along your Voyage as near as you can notwithstanding the Variation of the Variation I believe the difference of the Variation between any two places in East-India may be always near the same and not only there but in most other parts of the World near the Isles Azores excepted that are not above 20 or 30 Degrees distant asunder this I recommend as a Secret worthy Observation But there are several places that will not allow of any proportions like the former as the Downs the Chanel of England and several Degrees Westerly from Vshant there is but little difference in the Variation The Variation also alters apace in Sailing N. or S. under the same Meridian not always gradually but in some places faster then in others It is found by Experience and Practice that the Morning Variations are the greatest as I have found sometimes near 30 Minutes and my Opinion is that this difference happeneth not from the Oval Form or Eliptical Motion of the Earth but that the Suns Refraction is considerably greatest in a Morning if the Suns greatest Refraction be but 34′ as our Astronomers affirm how came it to pass that some English-men and Hollanders that Wintered in Green-land and Nova Zembla saw the Sun five Days sooner then was expected according to the Latitude of the place but to prevent the former difference or Error substract the Suns Refraction from the observed Altitude allowing the Refraction when the Sun is in the Horizon to be a few Minutes more in a Morning then in the Evening I cannot at present Certifie you how many Minutes the difference is because I have lost my former Amplitudal Observations It is a Fault in our East-India Commanders that they have not used to Substract the Suns Refraction c. they commonly take the Suns Declination from the preceeding Noon which is also a great Fault Vide Astr Ang. page 94. Tabula Refractionum Triplex But near the Poles it is probable the Suns Refraction is greater then near the Equinoctial If the Variation of the Compass were better known and Tables of the Variation for all the most noted parts in the World as Headlands Capes Islands c. The Tables Calculated to a certain Year because of the Increase and Decrease of the Variation and the difference of the Variation between those places then the Knowledge thereof would be almost as good as the Longitude known some few places excepted and if there be 10 or 20 Years difference in laying or setting down the Variation in some places it matters not provided the Year be set down when the Variation was so much at such places and whether Increasing or Decreasing as for Example suppose in the Year 1708 one should happen to be 10 or 12 Leagues South off of C. B. Esperance and find the Variation 12° 28′ as I told you before the difference of the Variation between the said Cape and St. Hellena is 8° 56′ by which Rule when you are at the said Cape you may know what the Variation is at St. Hellena viz. 3° 32′ both N. Westerly increasing the like Examples may be consider'd of most other places in the World that are not above 20 or 30° distant asunder except about the Azores I present you with another Method viz. When we have such or such Variation at London thereby to know what Variation there is in any other part of the World for if at this present writing I know exactly what Variation there was in all parts of the World I believe it would be no hard matter then by knowing the Variation at London to find what Variation there was in other parts in future Ages This Method I recommend to the Consideration of the Ingenious as also to see if they can find the Lattitude and Longitude of each Magnetick Pole but some People with think I have writ nothing to the purpose if I do not tell them wherefore there is little or no Variation about the Azores the quantity of the Earths Attraction is but little in some places two Degrees or three may be the most and in most places at Sea the more solid parts of the Earth have no Attractive quality for then the Needle is without the Command of the more Magnetick Vigor of the Earth as for Example in the Year 1688 about 8 or 10 Leagues at Sea in the Offing at C. B. Esperance when we had about Ten Deg. Variation I was told by Experienced Artists that there was not above 7½ or 8 Deg. at most Variation Ashoar or in the Harbour at the Cape which is a very quick Alteration for any where thereabouts above Ten Leagues off at Sea in Sailing on any point of the Compass 10 Leagues you will not alter your Variation above 12 or 14 Minutes at most The like Experiment hath been proved on the Coast of India and in the Meriterranean Therefore it is evident that Promontories Capes and Head-lands draw the Compass a little and that only when you are near them within 10 or 12 Leagues besides the Earths quality of hindering it from pointing to its Poles there are four Poles at least which have a Sympathetical Virtue with the Magnetical Needle to which it naturally inclines two of them that are furthest within the Earths Orb I call its more proper Poles and the other two in the Cortex of the Earth the Magnetical Poles these four Poles draw the Needle some one way and some another so that they hold it amongst them each requires a share but that Pole Predominates most to which it is nearest The Isles of Azores I account lies almost between the two proper Poles which two Poles always for ought I know bear N. and S. from those Islands for there it is without the Attraction of the Magnetick Poles but I believe there is some Variation
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