Royal Society. 501 



mentioned, the effect of the former force was found greatly to exceed 

 that of the latter. 



It appears that experiments and observations similar to those 

 applied in the above cases are sufficient to obtain with accuracy the 

 constants on which at any one place the ship's action on the hori- 

 zontal needle depends, namely, 



-r^^+tand\N, r-^s' ^I' andP; 

 I cos 1 cos d 



and that by placing a magnet so that its action shall take place in a 

 direction opposite to that which the investigations show to be the 

 direction of the ship's independent magnetic action, and at such a 

 distance that its effect is equal to that of the ship's independent 

 magnetism, and by counteracting the effect of the induced mag- 

 netism by means of the induced magnetism of another mass, accord- 

 ing to rules which are given, the compass may be made to point 

 exactly as if it were free from disturbance. 



It appears also, that by an easy tentative method, the compass 

 may now be corrected without the labour of any numerical investi- 

 gations or any experiments except those of merely making the 

 trials. Although the uniformity of the induced magnetism under 

 similar circumstances is to be presumed, yet the invariability of the 

 independent magnetism during the course of many years is by no 

 means certain. 



These statements suggest the following as rules which it is desi- 

 rable to observe in the present infancy of iron-ship building. It 

 appears desirable that 



1. Every iron sea-going ship should be examined by a competent 

 person for the accurate determination of the four constants above- 

 mentioned for each of the compasses of the ship, and a careful re- 

 cord of these determinations should be preserved as a magnetic 

 register of the ship. 



2. The same person should be employed to examine the vessel at 

 different times, with the vieAv of ascertaining whether either of the 

 constants changes in the course of time. 



3. In the case of vessels going to different magnetic latitudes, 

 the same person should make arrangements for the examination of 

 the compasses in other places with a view to the determination of 

 the constant N. 



4. The same person should examine and register the general 

 construction of the ship, the position and circumstances of her build- 

 ing, &c., with a view to ascertain how far the values of the magnetic 

 constants depend on these circumstances, and in ])articular to ascer- 

 tain their connexion with the value of the prejudicial constant M. 



5. The same j)erson should see to the pro])er iipjjlication of the 

 correctors and the proper measures for preserving the permanency 

 of their magnetism. 



The most remarkable result in a scientific view from the experi- 

 ments detailed in the present pajicr is, the great intensity of the per- 

 manent magnetism of the malleable iron of which the ship is com- 

 posed. 



