[34^3 
at ail. And it may fb happen that in the fame place 
where the Year before an .Obfervation was made, if in 
the next, the Iron Inftruments be found otherwife placed 
than they were the time before, either in the fame Shipp 
or another, the Needle will fhew a Variation much dif- 
fering from that found the firfl: time. And this fort of 
Caution not being Obferved at Sea may be caufe of very 
great Errours in the Obfervations of the Needles Variation 
tho' not affefting the Courfe of the VelTel. For the Nee- 
die being drawn after a certain manner will conftantly ob- 
ferve the fame fituation in refped of the North, provid- 
ed the Iron round about it be notftirrd: And you fhall 
not failetoftear true upon any point of the Compafs, if 
this falfe Variation be oblerved after the ufual Manner by 
the Amplitudes of the Sun. We cannot therefore hope 
to befecure of any thing from the Obfervations we have 
at prefent, and elpecially from thoft made at Sea, which 
are the moft confiderable. This put me upon finding out 
fome means independent from Obfervations to difcover 
the Variation at Sea ; but having confidered that ieveral 
learned Men of this age had propofed divers ways of mak-* 
ing Magnetical Needles, which fliould not be fubjeft to 
Variation, and that all thefe propofitions had had no ef- 
fed ; I judged that after all that they had done by means 
of the Loadftone^it wasnot tobe hoped to draw any farther 
advantage from it ; fince the Stone it lelf, as far as might 
be gueffed from the Experiments hitherto made, was fub-* 
jed to the fame Variation : 
I had quite given over this Enquirie, when there acci- 
dentally fell into my hands a TerrelU or Spherical Load- 
ftone, of three Inches Diameter ; with which being mind- 
ed to make fbme Experiments, with a little Needle whofe 
foot might eafily be placed upon the Stone, I fbon Obfer-* 
ved that which hath been already noted by feveral, vizc 
that this Globe of Magnet caufed the Needle to have the 
fame changes which are found in the Compafs in diffe** 
ren| 
