( 628 ) 
Thus for inftance in the Cafe before us, knowing that 
the Magnetick Variation has cau(ed the prefcnt Magne- 
tick Meridian to fall in the Line nq T s, 7 Degrees from 
the North to the Weftward ;to Reduce this to the Mag- 
netick Meridian at the time of the Down Survey, 1 mull 
make the Meridian of my Map to fall 7 Degrees to the 
Eaftward of my Magnetick Meridian as we fee the 
Line P Onfalls 7 Degrees to the Eaftward of the Lme 
What is here faid on fuppofition that the Magnet had 
no Variation at the time of the firft Survey taken , and 
that it had 7 Degrees variation Weftward at the time of 
the fecood Survey, may eafily be accommodated to the 
fuppofal of any other Variations at the firft and ftcond 
Surveys, Mutatis mutandis^ for knowing the Variations 
we know their Difference ; and if we know their Diffe- 
rence, this gives us the Angle (2. P ?) by which we re- 
duce them to each other. 
The beft way therefore to make Maps invariable, 
conftant, and everlafting, were for the Surveyors, who 
ufe Magnetick Inftruments to make always allowance for 
the Magnetick Variation, and to protraft and lay down 
their Plats by the true Meridian. This the wary Sailor 
is fully convinced of; and therefore in Steering his 
Courfe, he conftantly allows for the prefent Variation , 
which he obferves by the Azimuth Compafs, or elfe he 
would mifs his appointed Harbour oftner then he would 
hit it : For no two Points on the Globe keep the fame 
Bearing to each other by the Magnetick Meridian for 
any time together. And though the Variation be flow, 
yet in a long Courfe, or in times pretty diftant, itmay 
caufe vaft Errors, unlels allowed for. Thus for inftance, 
Suppofe in the Year 1660. a Sailor had fleered from 
the Lands-end of EnglanJ to Cape Femfter in Spain ^ by 
his Magnetick Compafs a dired South Courfe 5 and 
that 
