195 



There is no doubt, however, that the hypothesis of the Astronomer 

 Royal is a nearer approximation to truth than that of Mr. Barlow, and 

 that practice resulting from his theory has, under certain circumstances, 

 been attended with beneticial results. 



Mr. Airy has introduced permanent magnets for the correction of the 

 compass instead of the inductive magnetism of soft iron. This system 

 was introduced into the service of the Cunard line from a very early 

 period, under the management of Mr. Gray, and I have satisfied myself 

 that the results have been very satisfactory. But it must be observed 

 that the voyages made by these steamers are not such as to test the 

 value of the correction in different hemispheres; in a voyage from 

 England to Boston, or New York, the magnetic dip does not vary to 

 any considerable amount. 



But the Astronomer Royal informs us, that '* it is satisfactory to add, 

 that the Ironsides has three times crossed the Atlantic to South 

 America, (approacliing the magnetic equator,) and that the compass, 

 corrected, has always been perfectly correct." Corresponding with this 

 report, I have been informed that the captains of two ii'on steamers 

 from Liverpool, one that weathered Cape Horn, and the other that 

 coaled at the Cape, reported that their compasses were there all right.* 

 I am sorry, however, to be obliged to state, that I am very sceptical of 

 these general reports from captains in the merchant service on their 

 compasses. From my own knowledge few, very few, are the ships that 

 proceed from this port, the compasses of which are ever compared with 

 the heavenly bodies. The practice is far too common to report the 

 compass ** all right," if the voyage be made in safety, but " all wrong," 

 if they meet with any accident. I made it my business to inquire of 

 one of the captains, upon whose dictum the accuracy of the theory of 

 the Astronomer Royal is said to be confirmed, what was the variation of the 

 compass at the time he determined that the deviation was not changed? 

 He had no record of his experiments ; but we are required to confirm 

 an important theory, by receiving these vague assertions. If, however, 

 we should be placed in the position of being convinced that iron 

 steamers have proceeded to the other hemisphere without experiencing 

 any change in the deviation of the compass, still we could receive this 

 as a fact, without being in the position to admit, generally, that iron 

 vessels proceeding to another hemisphere will experience no change in 

 the deviation of their compasses. 



It is admitted that the Astronomer Royal is correct in stating that, 

 under two conditions, the effect of inductive magnetism vanishes. 



* Since the above was written, both the captains above alluded to, have discovered that 

 their compasses have undergone a great change when they had paued from one hemisphere 

 to the other. 



