VARIATION. 



change. The fame thing took place at Jouth, for the two 

 attraftions were ftill in the fame continued line, though on 

 oppofite fides of the compafs ; and throughout the voyage, 

 I found the variation, taken with the head at north and 

 fouth, agreed very nearly in themfelves, and with tlie ob- 



fervations themfelves near the fame place, when fuch obferv- 

 ations were not affeAed by local attraftions." 



The following table contains a few of the inllances, 

 where the change in the variation was obferved by captain 

 Flinders. 



Table of Variations obferved in a Voyage of Difcovery to Terra Auftralis, in i8oi and 1802, by Captain Flinders, 



in His Majefty's Ship Invefligator. 



After various other obfervations, and much examination, 

 (for the needle was not always deflefted the fame quantity, 

 when the pofition of the fhip was the fame,) captain Flinders 

 foimd that the trrors had a clofe conneftion with the dip. 

 When the north end of the needle had dipped, it was the north 

 point of the compafs that had been attrafted by the iron of the 

 ftiip ; and as that dip diminifhed, fo had the attraftion, until 

 at the magnetic equator ; where the dipping-needle Hands 

 horizontal, there feemed to have been no attraiSion ; and, 

 upon the whole, it feemed probable that " the error pro- 

 duced at any direftion of the (hip's head would be to the 

 error at eaft. or weft, at the fame dip, as the fine of the angle 

 between the (hip's head and the magnetic meridian was to 

 the fine of eight points or radius." 



After captain Flinders's arrival in England, he made ap- 

 plication to the lords commifTioners of the Admiralty to have 

 experiments tried on board fome of his majefty's (hips, that 

 the obfervations made during his voyage might be verified ; 



and a feries of obfervations was accordingly made on board 

 iive different (hips at Sheernefs and Portfmouth, which fully 

 eftabli(hed the accuracy of his former conclufions. 



Should this rule, upon farther trial, be found to anfwer 

 under all circumftances, we muft confider it as a moft im- 

 portant acquifition to our prefent knovv'ledge of navigation ; 

 but thofe who wi(h fully to appreciate all the confequences 

 of this difcovery, (hould confult Bain's treatife on the 

 " Variation of the Compafs," to which work we have been 

 much indebted in the compofition of this article, and where 

 they will find every information of a praAical kind con- 

 nefted with the fubjeft. 



Theory of the Variation of the Needle — Dr. Halley, as we 

 have already ftated, was the firft who attempted any theory 

 relative to the variation of the compafs ; and from the ob- 

 fervations which he collefted, many of which are included 

 in our preceding remarks, he conceived " that the whole 

 globe of the earth is one great magnet, having four mag- 



netieal 



