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XLV. On Irregularities olserved in the Direction of the Com- 

 pass Needles ofH. M. SS. Isabella and Alexander, in their 

 late Voyage of Discovery , and caused by the Attraction of the 

 Iron contained in the Ships. By Captain Edward Sabine, 

 of the Royal Regiment of Artillery, F.R.S. &c* 



J.T is proposed in this paper to sliow in what respects the effects 

 of local attraction, in the abovemcntioned ships, were conform- 

 able to the observations which had been made in preceding voy- 

 ages ; and how far the errors, which were found to take place on 

 different courses, and under different dips of the magnetic needle, 

 corresponded with the rules for calculating corrections, which 

 Captain Flinders had found useful in his own experience, and 

 which he had recommended for a more extensive trial. 



It may be desirable to premise, that the irregularities here al- 

 luded to, are not those accidental disturbances which may be 

 caused by iron placed inadvertently too near the compasses ; but 

 the permanent and constant effect of the mass of iron contained 

 in a ship, affecting its compasses at all times, and in a greater or 

 less degree, according as its influence is more or less povi'erful, in 

 comparison to the directive force of the earth's magnetism. That 

 errors have always existed from this cause, may be inferred, from 

 the uncertainty which experience has attached to the results 

 of azimuths observed in ships. The cause, however, appears to 

 have been very long unsuspected, whilst its effects had produced 

 a general impresssion, that the azimuth compass was in itself an 

 imperfect instrument, and only to be relied on within certain un- 

 defined and variable limits. 



It was reserved to the accurate observation, and the habit of 

 recording and comparing apparently trivial and accidental diffe- 

 rences in results, which distinguished the lale Mi. Wales, (astro- 

 nomer in the second voyage of Captain Cook,) to enable him to 

 lead the way to a knowledge of the nature and causes of these 

 errors : he remarks, " that in the passage of the Resolution and 

 Adventure to the Cape of Good Hope, and subsequently, the 

 greatest west variations had happened when the ship's head was 

 north and easterly, and the least when it was south and westerly, 

 differing very materially from one another with the ship's head 

 in different positions, and still more when observed in different 

 ships ;" thus manifesting that they were something more than 

 accidental. 



This voyage was the last in which Mr. Wales embarked, and 

 the investigation does not appear to have been pursued in this 

 country until tlie voyage of discovery to Terra Australis, in the 



* From the Philosophical Transactions for 1819, Part I. 



first 



