C ) 
But here I would not be underflood , as if I propo- 
fed hereby to (hew, that a Map of the fame place , ta- 
ken by Magnetick Inftruments at never fo diftant times, 
fliQuid not at one time give the fame Figure and Con- 
tents as at another time. This certainly it, will do mod: 
exaftly, the variation of the Needle having nothing 
to do either in the Shape or Contents of the Survey. All 
that is affected thereby, is, the Bearings of the Lines 
run by the Chain , and the Boundaries between Neigh- 
bours. And how this may caufe a confiderable Error 
( unlefs due allowance be made for it ) is what I fliall 
prove moft fully. 
^ In order to which, let us fuppofe that about the Year 
1657. (at which time the Down Survey was taken ) 
the Magnetick Meridian and true Meridian did agree 
at Dulltn, or pretty nigh all over Ireland ; that is to 
fty, that there was no Variation. And indeed by Expe- 
riment it was at that time found, as I am well affur^d, 
that at DulUn it was hardly haif a Degree. 
Let us fuppofe that in the Year 1695. the Variation 
was 7 Degrees from the North to the Weft ward : that it 
was really fo, I believe I am pretty well afTured, from 
an Experiment thereof made by my felf with al| dili- 
gence. But this is not material, let us now only fup- 
pofe ir. 
Let A B reprefent the Survey of two Town Lands, 
one in the poflefiion of and t'other in the poflefTion 
of^, which we will call A Town-Land and B Town- 
Land, taken by the Down Survey, Anno 1657. when 
there was no Variation. 
Let the Line iSTS running through the Point P be the 
true Meridian, and confequently the Magnetick Meri- 
dian alfb at that time, bccaufe of the fuppofed no Varia- 
tion,and let this Line N S alio the Boundary between 
chc two Town- Lands -^4and B. 
