Royal Society. 29 
lerved by tlie fuperficial parts of the globe : As to tbe quantity 
of this motion, it is ahiaoft impoffible to define it, both from the 
nature of this kind of obfervation, which cannot be very accu- 
rately performed • as alfo, from the fmall time theie variations 
have been obferved in, and their changes diicovered : The period 
of this motion appears by all circumltances, to coniift of many 
centuries of years, and confidering the change of the place, where 
there was no variation, by realbn of the equilibrium of the two 
fouthern magnetical poles, viz^ from cape d'Agidhns to the me- 
ridian of St. Helena^ (which is about 23° in about 90 years) and 
of the place where the wellerly variation is in its greateft deflec- 
tion, being about half fo mucn 5 viz. from the ifland of 'Die^j 
Roiz to the fouth-weft parts of Madagafiar^ we may with lbn>e 
realbn conjedlure, that the American pole was moved weftwards 
45° in that time, and that the whole period thereof is performed 
in 700 years, or thereabouts ; fo that the nice determmarion of 
this, and of feveral other particulars in the magnetic fyilem is 
referved for later pofterity : By the way, it will not be amifs to 
correct a received error in the praftice of oblerving the variation, 
which is, to take it by the amplitude of the rifing and fettinglun, 
when his centre appears in the vifible horizon, whereas he ought 
to be obierved, when his under limb is ftill above the horizon 
about -f of his diameter, or 20 minutes, on account of the refrac- 
tion, and the height of the eye of the obierver above the furface 
of the lea- or elie the amplitudes are to be wrought as the azi- 
muths, reckoning the fun's diitance from the zenith 90° 56'^ this, 
tho' of little confequence near the Equator, will make a great 
error in high latitudes, where- the fun riies and fets obliquely : 
But to return to our hypothefis, in order to explain the change 
of the v>ariation>, we have adventured to make the earth hollow, 
and t0 place another globe within it^ and no doubt it Vv'iil be 
obje6^ed, that there is no iniiance in nature of the like thing ; 
that ii there were fuch a middle globe, it would not keep its 
place in the centre, but be apt to deviate therefrom, and might 
poffibly fnock againft the concave fhell, to the ruin, or at leait 
endamaging thereof 5 that the waters of the lea would perpetually 
Ibak thro', unleis we fuppofe the cavity full of matter j that were 
it pofiible, yet it does not appear of what ufe fuch an inward 
jpliere can be of, being Aiut up in eternal darknefs, and therefore 
unlit for the produ^lion of animals or plants : Briefly to anfwer 
thele and all other objcdtions, and firlt, the ring invironing the 
globe o^ Saturn is a notable inftance, as having the fame common 
,;;ntre, and moving along with the planet, without leniibly 
approach- 
