(57*) 
formed , as alfo from the fmall rime tfaefe Variations' 
have been obferved, and their change Hifcovered. It 
appears by all Circumftances, that its period is of many 
Centuries of Years, and as far as may be colle&ed from 
the Change of the Place, where there was no Variati- 
on by reafon of the Equilibre of the two Southern 
Magnetical Poles, viz. from Cape d* Jgulhas to the Me- 
ridian of St. Helena (which is about 23«' in about 90 
years) and of the place where the Wefterly Variation 
is in its d%im or greateft Deflexion, being about half fo 
much, viz, from the Ifle of Diego Roiz to the South 
Weft parts of Madagascar. We may with fome Rea- 
fon conje&ure, that the American Pole has moved 
Weft wards 46 degrees in that time, and that the whole 
Period thereof is performed in 700 Years, or therca 
bputs} fo that the nice Determination of this and of 
feveral other particulars in the Magnetick Syftem is re- 
ferved for remote Pofterity 5 all that we can hope to do 
is to leave behind us Obfervations that may be confi- 
ded in, and to propofe Hypothefes which after Ages 
may examine/amend or refute. Only here I muft take 
leave to recommend to all Mafters of Ships and all o- 
thers, Lovers of natural Truths, that they ufe their uc- 
moft Diligence to make, or procure to be made, Obfer- 
vationsof thefe Variations in all parts 1 of the World, as 
[ well in the North as South Latitude (after the laudable 
cuftom of our Eafi- India Commanders) and that they 
pleafe to communicate t hem to the Royal Society, in or- 
der to leave as compleat a Hiftory as may be to thofe 
that are hereafter to compare all together, and to com- 
[ pleat and perfe& this abftrufe Theory. 
And by the way it will not be amifs to amend a recei- 
ved Error in the Praftice of obferving the Variation, 
\ which is, to take it by the Amplitude of the Rifihgand 
Setting Sun, when his Centre appears in the vifible Ho- 
) rjzon 3 whereas he ought to be obferved when his under 
1 R Limb 
