226 ME M O IK S of the 
inftance, J^JfW 1^)57, when there is fuppofed no vamtioii; 
let the line K S, running thro' the point P, be the true meri- 
dian, and conrcquently the magnetic meridian alfo at that 
time^ and let this line NS, be alio the boundary between the 
two lands A and B j in 1695, when the variation is fuppofed 
to be 7 ° from the north wellwards, B having a map of the 
former furvey, and fufpeding that his neighbour A had 
encroached upon him, by a ditch PQ^, employs a furveyor to 
enquire into the matter^ the furveyor finds, by his map, that 
the boundary between B and A runs from the point P thro* a 
meadow, diredly according to the magnetic meridian SPN; 
but obferving the ditch PQ^thrown up much to the eaftward 
of the prefent magnetic meridian, he concludes, that A has 
encroached upon B, and that the ditch ought to have been caft 
up along the line P^y, the angle QJ*^ being an angle of 7°, 
that is, the variation of the needle, and the line Pq the mag- 
netic meridian 5 for which variation, the furveyor, not making 
any allowance, pofitively determines, that B has all the land 
in the triangle Q_P<^, more than he ought to have, and that 
his ditch fhould run along the line Vq. It is true indeed, if 
the furveyor go the whole round of the land A and B, he will 
find their figure and contents exactly agreeable to the map 5 
but then the bearings of the lines are all 7° different from the 
bearings in the map, and they will run in and out upon the 
adjacent neighbouring lands, and caule endlefs difputes between 
their poflefibrs; as is manifell from the figure, wherein the 
pricked lines reprefent the difagreement in the bearings of the 
fines, prorradcd from the point P- and we fee A encroaching 
on his neighbours to the weft, as he himlclf encroaches upon 
B5 whereas, by a due allowance for the variation of the 
needle, all this confufion is avoided. \N hat is here fiid on 
fuppofition, that the magnet had no variation at the time of 
the firft furvey, and that it had 7° variation to the weft, at the 
time of the fccond furvey, may eafily be accommodated to any 
other variations; for knowing the variations, we know their 
difference, and if we know their difference, this gives us the 
angle (^P^, by which we reduce them to each other; the 
beft way therefore to make maps invariable, were for the fur- 
vcyors, who ufe magnetic inftruments, to make always allow- 
ance for the magnetic variation, and to protra61 and plot by the 
true meridian. 
Perhaps it may be objected, that furveys may be taken 
without magnetic inftruments, and that therefore this error. 
arifing 
