On Irregularities in the Compass Needles. Ill 



first years of the present century. Tlie survey of the coast of 

 New Holland being carried on in a considerable measure bv the 

 intersection of compass bearings taken from the deck of the 

 Investigator, so much embarrassment and perplexity were found 

 to arise from the effects of local attraction, that much of Captain 

 Flinders's atteiition and thoughts were necessarily devoted to a 

 consideration of some means of remedying the inconvenience. 



On his return to England, he obtained permission from the 

 Lords Commissioners of the Admiralty to make a course of ex- 

 periments in shijos under their direction at the principal sea-ports, 

 with a view to ascertain if compasses were similarly affected in 

 other ships, and to try the general applicability of rules which he 

 had found useful in correcting the errors in the Investigator. 

 These rules, with the observations and reasonings on which they 

 were founded, were published in a short paper in the Philosophical 

 Transactions, and in a more detailed form in Appendix No. II. 

 in the Voyage to Terra Australis. There are three points in these 

 statements chiefly worthy of attention, from their practical im- 

 portance ; and on which it seems desirable, therefore, to notice 

 how far his observations have been confirmed by those made in 

 the Isabella and Alexander. 



First ; he found that in every ship a compass would differ very 

 materially from itself, on being removed from one part of the ship 

 to another. Experience of this source of irregularity had in- 

 duced him, early in his voyage, to confine the use of the compass, 

 with which his survey was carried on, to one particular spot. 

 The place he selected was determined by convenience in other 

 respects ; it was on the biimacle, and exactly amidsiiips. 



The Isabella and Ale::ander had not completed half their voy- 

 age across the Atlantic, before it was found that the binnacle 

 compasses of the one ship differed very materially, in indicating 

 the course steered, from those of the other : namely, one point, 

 or 11 {°. No dependance whatsoever could be placed on the 

 agreement of compasses in different parts of the ship, or of the 

 same compass with itself^ if removed but a few inches : even in 

 the neighbourhood of the binnacles the variation, as observed 

 amidships, was from 8° to 10° greater than the result of azimuths 

 taken by a compass placed between two or three feet on the lar- 

 board side ; and an almost equal difference in a contrary direc- 

 tion took place on removing the compass to the starboard side, 

 rendering it a matter of some trouble and dithculty, to make the 

 azimuth compass agree with those in tiie binnacle by which the 

 ihip was steered, and for which it was therefore necessary to de- 

 termine the variation. 



As the ships ascended Davis's Strait, these latter compasses be- 

 gan to traverse so sluggishly, that it was licccssary to shake the 



S y binnacles 



