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A45668
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Idea longitudinis being a brief definition of the best known axioms for finding the longitude, or a more rational discovery thereof, than hath been heretofore published / by Edward Harrison ...
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Harrison, Edward.
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1696
(1696)
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Wing H888; ESTC R40549
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32,770
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99
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probable that these Magnetick Poles have a Motion from North to South but whether they move altogether with one Motion or with several whether equally or unequally whether Circular or Libratory if Circular about what Center If Libratory after what manner are Secrets as yet unknown to Mankind That which I call the more proper North Magnetick Pole Predominates at London and the Period of its motion as some say is about 370 Years and others write that the Period of the Motion of the American Pole is about 700 Years some think the Magnetical Inclinatory Needle always points directly to one of these Poles which I dare affirm to be an untruth and can prove it by Practice Experience and Infallible Demonstrations The mean Motion of the Variation here at London is about eight Minutes or somewhat more in one Year it is my Opinion that most places in the World have not altogether so swift an Alteration of the Variation as at London but in many other places it is a little slower especially near the Equinoctial for the nearer one of these Poles is to any place there encrease or decrease of the Variation may be swiftest As the Sun in the Tropicks differs his declination but slowly so the Horrizontal Needle is in its slowest Motion when it hath the greatest Variation East or West at London and whereas some places have above 22 or 30 Degrees Variation North-Westerly I doubt those places will not have above 22 Deg. Variation North-Easterly in one Revolution of the said Poles That all Magnetical Needles well toucht have the same direction exactly as Mr. Sellers hath proved by many Experiments see Ph. tr N. 26. p. 478. but if badly toucht they may vary sometimes one or two Degrees from the true Magnetical Meridian as often happens at Sea as our Marriners say that the Compass in the upper or lower beetackle sometimes varies near half a point from each other I have often taken notice of their expressions and tryed to find the cause and when I found no Iron near them or the like Obstructions that might draw me off the Compasses I have found that one and sometimes both had a a weak touch or Old and Rusty and good for nothing except to throw over-board c. Five Hundred such Compasses I believe at this present writing may be found belonging to the Navy c. A true Knowledge of the Variation is very requisite in keeping a Sea reckoning it troubles me to think what Ignorant Persons are Masters of Ships it is scarce two Years since in my company some Masters of Ships complained of a wrong Course steered by their Comodores who in the Night by a fair Wind were carryed too near the Borlings even so near that some of them were oblig'd as I was inform'd to go between the Borlings and the Main This Error happened for want of good understanding Masters in the Comodores Ships they knew not the Variation else forgot to allow for it for though the true Course from the Lizard to Cape Finister be S. S. W. the Course to be Steered by Marriners Compass at present is S. S. W. ½ W. and S. W. by S. It is scarce one Year ago to this present writing since a Master of a Rate Ship told his Captain that he observ'd 14 or 15 Deg. Variation about 6 Leagues off of Malhago by the Stars there is about 6 Deg. If I may compare a Man to a block that same Man knew little more how to find the Variation by the North Star or any other Star than a Buggilug or Bracket on that Ships Quarter I doubt whether or no he ever saw an Azimuth Compass When you would observe the North Star to find the Variation in the Night consider its distance East or West from the Pole a Nocturnal may be of good use in this Case England does not know how many losses hath happened for want of a better Knowledge in the Variation of the Compass through only a Pilot of a Ship suppoie between the Bunt-Head and the K. K. or Gunfleet if he understands not the Variation he may in my Opinion as well deserve a Rope as a Branch But the Extra -Pilots will blame me for mentioning Variation or Course in Pilot Water where the Strength and Course of Tides and depths of Water is of much more Consequence My Lord's Commissioners of the Admiralty c. You have been pleased to give us a Form to keep our Journals at Sea there is in my Opinion two Columns wanting in that Form that Column for Longitude If you please my Lords let it be accounted from our first Meridian may there be added one Column called Meridian Distance by plain Chart and accounted from the last Land seen and another for the Variation of the Compass I have belong'd this War to Six several Rates in the Navy and never saw an Azimuth Compass Aboard any of them my Judgment tells me that every Ship of War requires at least three such like Instruments to observe the Variation though they ride only at an Anchor at the Nore or in the Downs the Commissioners of the Navy have taken Care to provide such Instruments and I believe the Masters of Men of War may have them for asking for for the Boatswains Indent for them I believe they are ashamed or ought to be for some of them knows not what to do with them the Instruments if well used may be a means to help to save a Ship and Ships Companies lives they are good to be used for the Instruction of Ships Company and Youth there is Education in the Navy little enough that is good Navigation seems to decay or decline I wish it be not a Crime for some Gentlemen in the Navy to understand it Now I will say something to my Honourable Company Masters little do they know how many Ships have been lost for want of a better Knowledge of the Variation I believe I could name some and several in great danger by report but dare not fear of the Law Captain William Wildey understood the Variation very well in my Opinion I believe it was almost as good to him as the Longitude between St. Hellena and any part of the East-Indies When I was in East-India I understood what Variation there was in most Adjacent Parts so well that I have offered in Discourse in Company to go in a Ship that set Sail from any part of the Coast of India bound any way two three or four Hundred Leagues I would keep no Account of her way for a Week or ten Days time and any fair Day when I could have Reasonable Observations I would a told them the place where the Ship was as well as they that kept the most exact Reckoning almost as well if you please provided they had not seen the Land since I saw it and this I must have done by the Lattitude and Variation observ'd October the 30th 1688. in the Offing at Cape Bona Esperance the Cape
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