TRANSACTIONS OF THE SECTIONS. 21 



On the ineans adopted for correcting the Local Magnetic Action of the 

 Compass in Iron Steam-ships By G. B. Airy, F.R.S., Astrono- 

 mer Royal. {In a letter to Rev. Prof. Whewell.) 



In this communication, the author states some of the principal results 

 of a series of observations and experiments (made at the request of the 

 Admiralty) for correcting the local magnetic action on the compass in 

 the steam -ship the Rainbow. 



" The compass was placed in four different stations near the deck, 

 and in four stations about IS feet above the deck ; and for each of these 

 the ship was turned round, and the disturbance observed in many posi- 

 tions. The disturbances even at the upper stations were great, but at 

 all the lower stations they were very great, and at the station next the 

 stern they were enormous. The whole amount there was 100° (from 

 — 50° to + 50°) ; and on one occasion, in turning the vessel about 

 24°, the needle moved 74° in the opposite direction. I should have 

 perhaps found some difficulty in reducing these to laws if I had not 

 made some observations of the horizontal intensity at the four lower 

 stations in different positions of the ship. From these I was able to 

 infer the separate amounts of disturbance due to the permanent mag- 

 netism of the ship and to the induced magnetism, and to construct cor» 

 rectors. These correctors I tried yesterday, completely at the sternmost 

 station, and imperfectly at two others. The correction at the sternmost 

 station was (speaking generally) complete ; the extreme of deviation, 

 which formerly exceeded 100°, did not, with the corrector, exceed 1°, 

 At the other stations I had not leisure to adjust the apparatus : but I 

 fully expect to-morrow to produce the same accordance at them. This 

 result is, I should think, important in a practical sense. Some theo- 

 retical results which I did not anticipate are also obtained. At the stern 

 position, the disturbance is produced almost entirely by the permanent 

 magnetism, the inductive magnetism producing only gV of the whole 

 effect. Going towards the head, the effect of the permanent magnetism 

 diminishes, and that of the inductive magnetism increases, till the latter 

 produces about ^ of the whole effect. The resolved part of the per- 

 manent magnetism transverse to the ship varies little (increasing 

 somewhat towards the head): the part longitudinal to the ship decreases 

 rapidly from the stern to the head (where it is less than the transverse 

 part)."* « G. B. AiBY," 



A Statement of the Progress made towards developing the Law of 

 Storms ; and of what seeins further desirable to he done, to ad- 

 vance our hnotoledge of the subject. By Lieut.- Colonel Reid, Royal 

 Engineers. 



Having been ordered, in the course of military duty, to the West 

 Indies in 1831, the author arrived at Barbadoes immediately after the 



* A memoir containing the full investigatim of this subject has been presented to 

 the Royal Society, and is expected to appear in the forthcoming volume of the Philo- 

 sophical Transactions. 



