5 1 Mifcellanea Curiofa. 



Magnetick Syftem is referv'd for remote Pofteri- 

 ty • all that we can hope to do, is to leave be- 

 hind us Obfervations that may be confided in, 

 and to propofe an Hypothecs which after Ages 

 may examine, amend or refute. Only here Imuft 

 take leave to recommend to all Mailers of Ships, 

 and all others, Lovers of Natural Truths, that 

 they u(e their utmoft Diligence to make, or pro- 

 cure to he made, Obfervations of thefe Variati- 

 ons in all parts of the World, as well in the North 

 as South Latitude (after the laudable Cultom of 

 our Eaft India Commanders) and that they pleafe 

 to communicate them to the T{pyal Society, in or- 

 der to leave as compleat a Hiftory as may be to 

 thofe that are hereafter to compare all together, 

 and to compleat and perfect this abftrufe The- 

 ory. 



And by the way it will not be amifs to a- 

 mend a receiv'd Error in the Practice of ob- 

 ferving the Variation, which is, to take it by 

 the Amplitude of the Rifing and Setting Sun,; 

 when his Center appears in the vifible Ho- 

 rizon ; whereas he ought to be obferv'd when 

 his under Limb is ftill above the Horizon a-- 

 bout j of his Diameter, or twenty Minutes, 

 upon the fcore of the Refraction, and the height 

 of the Eye of the Obferver above the Surface 

 of the Sea .* Or elfe they are to work the 

 Amplitudes as they do the Azimuth, reckon- 

 ing the Suns Diftance from the Zenith 90°; 

 36' ; This, though it be of little confluence 

 ne^r the ^Equinoctial, will make a great Error 

 in high Latitudes, where the Sun riles and fets 

 obliquely. 



But to return to our Hypothecs, In order to 

 explain the Change of the Variations, we have 

 adventur'd to make the Earth hollow, and to 



plac€ 



