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III. The Variation of the Magnetick Needle is so commonly known, that Ineed not infist much on the Explication thereof ; ’tis certain that the true SolarMeridian, and the Meridian Ihewn by a Needle, agree but in very few placesof the World ; and this too, but for a little time (if a moment) together;the difference between the true Meridian and Magnetick Meridian, perpetuallyvarying and changing in all places, and at all times; sometimes to the Eastward,and sometimes to the Westward.

On which Account ’tis impollible to compare two Surveys of the fame place,taken at didant times, by Magnetick Instruments (such as the Circumferentur,by which the Dornt Survey, or Sir William Pctty s Survey of Irel/tnd was taken)without due Allouance be made for this Variation. To which purpofe, weought to know the difference between the Magnetick Meridian and true Meri-dian, at that timcof the Doxvn Survey , andthefaid Difterence at the time, when wemake a Nero Survey to- compare with the Davon Survey.

But here I would not be undcrstood, as if 1 propofed hereby to shcw, thata Map of the fame place, taken by Magnetick Instruments at never fo distanttimes, stiould not at one time give the fame Figure and Contents as at anothertime. This certaiuly it will do most exactly, the Variation of the Needle ha-ving nothing to do either in the Shape or Contents of the Survey. All that is af-fected thercby, is the Hearings of the Eines run by the Chain, and the Boun-daries between Neighbours. And how this may caule a confiderable Error (un-icis due Allowance be made for it) is what 1 fhall prove most fully.

Inorderto which, let us fiippofe that aboutthe Year 1657, (at which timethe Doxvn Survey was taken) the Magnetick Meridian and true Meridian did a-gree at Dublin , or pretty nigh ali over lreland ; that is to fäy, that therc wasno Variation. And indced. by Experiment it was at that time found, as I am.well affured, that at Dublin it was hardly half a degree.

Lerus fuppole that in the Year 169s, the Variation was 7 Degrees fromthe North to the Westward : that it was rcally so, I bclieve I am pretty well af-fured, from an Experiment rhereof made by my seif with aU diliger.ee. But this isnot material, let us now only fuppole it.

Let A, B, Reprefent the Survey of two Town Lands, one in the Possestion of A, and the other in the poffeffum of B, taken by the Doxvn Survey ,Anno 1657, when there was no Variation.

Let the Line N 8, runningthroughthe Point P, be the true Meridian, andconfequently the Magnetick Meridian also at that time,becaufe of the fupposod noVariation, and let this Line NS, be also the Boundary between the two-TownLands A, and B.

In the Year 1695, when the Variation is 7 Degrees from the North to theWestward, B having a Map of the Doxvn Survey , and being fufpicious that hisNeighbour A, had incroached on him by a Ditch P Q, imploys a Surveyor toenquire into the matter: The Surveyor finds by this Map, that the Bounda-ry between B and his Neighbour A, run from the point P, througha Mcadow^..directly according to the Magnetick Meridian 8 P N ; but obscrving the DitchP Q_ cast up much to the Eastward ot the present Magnetick Meridian, heconcludes that A has incroached upon B, and that the Ditch ought to have

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\Ah Error ofCommon Survey-0rt, in comparingSo rvcyt taljen atLang Intervals oftime with theMagneticl^tfedlfy,Demonßrated ^hy Mr. Willi-am Mol y neu x..n. 2 30 . p. 5-

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