134 ANNUAL OF SCIENTIFIC DISCOVERY. 



found the westerly deviation decreased ; and on placing the tripod of 

 the compass directly over the rudder-head, easterly deviation was pro- 

 duced ; and hence it folloAved that there must be a position somewhere 

 between the last two places of observation where there would be no 

 deviation while the ship's head remained in the same direction. This 

 position I practically discovered by moving the compass a few inches 

 at a time, till it indicated the correct magnetic direction of the ship's 

 head. The question which now remained to be proved was, to what 

 extent the deviations of the said compass had been lessened (or what 

 they actually were) when the ship's head was placed upon different 

 points ; and I was gratified to find that after swinging the vessel, and 

 observing upon the eight principal points, the compass placed as before 

 described, proved to be correct within a quarter of a point. It is 

 necessary to mention, that the Trident has wooden beams under the 

 quarter deck, aud therefore it remains to be seen to what extent such 

 observations may be useful in vessels which have iron beams. It will 

 also be requisite to ascertain, by actual observation, how far a position 

 so selected shall prove advantageous when the ship changes her geo- 

 graphical position ; and as the Trident is about to proceed to the 

 southern hemisphere, and is amply provided with instructions and the 

 means of ascertaining such changes, and as I shall swing her again on 

 every point before she leaves, we may hope for much useful informa- 

 tion on this important subject. I must observe, that it may not always 

 be practicable to find the position of no deviation, or where the influ- 

 ences of the iron in the ship upon the magnetic needle are equalized, 

 because such a point might be found in a most inconvenient position, 

 or be too near movable ironwork, machinery, &c. ; but if we succeed 

 in approximating towards it, and thereby reduce the deviations within 

 moderate limits, a point of great practical importance will be gained 

 in navigation. 



Mr. John Gray, of Liverpool, has published a letter, in which he 

 proves, by the example of the Sarah Sands, that the compass can be 

 as accurately adjusted in iron vessels as in those of wood. He says : 

 " This steamboat has been a most valuable agent for the determination 

 of a mooted point now being investigated, whether iron ships undergo 

 a very important change after crossing the equator, or not. For years 

 I have entertained the opinion that, for all practical purposes, the 

 adjustment on Professor Airy's principle will answer equally well in 

 both north and south latitudes, and which this vessel has demonstrated 

 beyond all doubt. Simultaneous bearings were taken by Captain 

 Thompson and his chief officers, in various parts of the Straits of 

 Magellan, and the result clearly showed that no deviation whatever 

 took place. " 



RELATION BETWEEN THE SPOTS ON THE SUN AND THE MAG- 

 NETIC NEEDLE. 



ACCORDING to observations made by M. Rodolphe Wolf, Director 

 of the Observatory at Berne, it appears that the number of spots on 



