144 



position determined by the previously-ascertained numerical values, 

 a large bar magnet to neutralize the effects of the permanent mag- 

 netism of the ship, and a roll of soft iron on one side of the compass 

 to counteract the disturbance arising from induced magnetism. That 

 this correction vi'as effective appears from the very small amount of 

 unconnected disturbance then observed in the compass. 



The obsei-vations of the compasses at stations II., III., IV., are 

 similarly discussed : the disturbing force arising from the permanent 

 magnetism of the ship being in like manner determined, a comparison 

 is instituted between the observed and computed disturbance of the 

 compass ; and the results of this comparison, vdth. the exception of 

 the observations at Station IV., are found to be in perfect accordance 

 with the theory. Attempts are made to correct the compasses at 

 these stations in the same manner as at Station I., but owing to the 

 imperfection of the compasses they did not succeed so perfectly. 



The observations made with the dipping needle are next discussed, 

 and the values of the constants are deduced from them. The gene- 

 ral agreement of those determined from the observations when the 

 needle vibrated in the direction of the ship's keel, with those de- 

 duced from the observatioDs when the needle vibrated transversely, 

 is pointed out, and is considered an additional proof of the general 

 correctness of the theory. 



Observations on the disturbance of the compass in the iron-built 

 sailing-ship Ironsides are next described. These are similar to 

 those in the Rainbow, but not so extensive ; and they are discussed 

 on the same principles. From this discussion it is considered that 

 the theory is in perfect accordance with the facts observed both with 

 regard to the deviations and the intensities. The correction of one 

 compass was effected by a tentative process, which the author consi- 

 ders likely to be of the highest value in the correction of the compasses 

 of iron- ships in general. The ship's head being placed exactly north, 

 as ascertained by a shore compass, a magnet was placed upon the 

 beam from which the compass was suspended, with the direction of 

 its length exactly transverse to the ship's keel : it was moved upon 

 the beam to various distances till the compass pointed correctly, and 

 then it was fixed. Then the ship's head was placed exactly east, 

 and another magnet, with its length parallel to the ship's keel, was 

 placed upon the same beam, and moved to different distances till 

 the compass pointed correctly, and then it was fixed. The correc- 

 tion for induced magnetism was neglected, but there would have 

 been no diflttculty in adjusting it by the same process, placing the 

 vessel's head in azimuth 45° or 135° or 225° or 315°. 

 In conclusion Mr. Airy makes the following remarks : — 



The deviations of the compass at four stations in the Rainbow, 

 and at two stations in the Ironsides, are caused by two modifications 

 of magnetic power ; the one being the independent magnetism of 

 the ship, which retains, in all positions of the ship, the same mag- 

 nitude and the same direction relatively to the ship ; the other being 

 the induced magnetism, of which the force varies in magnitude and 

 direction when the ship's position is changed. In the instances 



