284. JMifceUanea Curiofa. 



at that time of the Down Survey, and the 

 fa id difference at the time when we make a 

 new Survey to compare with the Down 

 Survey. 



But here I would not be underftood as if 

 I propofed hereby to mew ., that a Map of 

 the fame place, taken by Pvlagnetick Inftru- 

 ments at never fo diftant times, mould not at 

 one time give the fame Figure and Contents as 

 at another time This certainly it will do 

 moft exa&ly, the variation of the Needle 

 having nothing to do either in the Shape or 

 Contents of the Survey. All that is affecled 

 thereby, is, the Bearings of the Lines run by 

 the Chain , and the Boundaries between 

 Neighbours. And how this may caufe a con- 

 fiderable Error ( unlefs due allowance be 

 made for it) is what I fhall prove molt 

 fully. 



In order to which, let us fuppofe that about 

 the Year 1 6^57. (at which time the Down 

 Survey was taken) the Magnetick Meridian 

 and true Meridian did agree at Dublin^ or 

 pretty nigh all over Ireland ; that is to fay, 

 that there was no Variation. And indeed 

 by Experiment it was at that time found, as 

 lam well affur'd, that at Dublin it was hardly 

 half a Degree. 



Let us fuppofe that in the year, 1695. tne 

 Variation was 7 Degrees from the North to 

 the Weftward \ that it was really fo, I be- 

 lieve I am pretty well affured , from an Ex- 

 periment thereof made by my felf with all 

 diligence. But this is not material, let us 

 now only fuppofe it. 



Let 



