Page:Proceedings of the Royal Society of London Vol 4.djvu/168

Rh 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