NATURAL PHILOSOPHY. 155 



Great Britain, on this subject. In this communication, published in the 

 London At! e ucuni, he says : 



I have deep satisfaction in remarking that the great principles upon 

 which I founded the method of correcting the compass are entirely recog- 

 nized by Dr. Scoresby, and even that some minor modiiications of those 

 principles (which, as will appear in the remarks below, I had anticipated 

 as probable) have now been established by Dr. Scoresby 's beautiful experi- 

 ments. In the estimation of the actual extent and rapidity of the changes 

 produced by these modifications, I may perhaps differ in some measure 

 from Dr. Scoresby, and I may be disposed to recommend a practical 

 course slightly different from, that which he would propose. Still I am 

 happy to find that upon the fundamental points of the theory we are in 

 complete accordance. 



1. It may perhaps be advantageous to give a few steps of the history of 

 this subject. The law, that the greater part of the disturbance of the 

 compass produced by an iron ship depends upon its polar, and not upon 

 ts induced, magnetism, (in the ordinary sense of the word induced,) was 

 established by me in a paper printed in the " Philosophical Transac- 

 tions" for 1839. The experiments themselves had been made in 1838. In 

 page 212 I observe : " The most remarkable result, in a scientific view, 

 from the experiments detailed above, is the great intensity of the perma- 

 nent magnetism of the malleable iron of which the ship is composed. It 

 appears, however, that almost every plate of roiled iron is intensely 

 magnetic." (It is to be noted that I used the term permanent magnetism 

 as equivalent to polar magnetism.) I then allude to experiments on the 

 magnetism of plates of wrought iron ; and these experiments were the 

 last with which I had any acquaintance until I saw some of Dr. Scoresby's 

 beautiful illustrations of the change of magnetism of iron plates. In page 

 213 I remark: "It seems sufficiently probable that the independent 

 [polar] magnetism of the ship will change with time. This consideration 

 enforces strongly the necessity of perio'dical examination as suggested 

 above." This is all that was printed by me in reference to the change of 

 the polar magnetism of ships and their occasional examination ; but it is 

 not the only instance in which I endeavored to bring them before the 

 notice of the proper authorities. In 1839, July 20, I submitted a memo- 

 rial to the Board of Admiralty on the advantage of a supervision, by the 

 government, of the correction of the compass in iron ships, in which 

 occur the following remarks : " There is no reason for presuming that 

 the magnetic state of the ship (especially in the case of steam-ships) will 

 remain invariable for many years ; and there is reason for supposing that it 

 will vary." " Experiments of various kinds and in various localities 

 should be made on the same ship, for ascertaining whether there is sensible 

 change in different parts of the earth." And with regard to the magnets : 

 "The important results lately arrived at by Mr. Scoresby, and wholly 

 unknown to the persons commercially engaged in the fabrication of 

 magnets, show that attention to those points on which the permanency of 



