Page:Proceedings of the Royal Society of London Vol 4.djvu/169

Rh 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

-T-^.+tana.N, r-^s> andP;

I cos 6 1 cos C

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 view of ascertaining whether either of the constants changes in tbe 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 particular to ascer- tain their connexion with the value of the prejudicial constant M.

5. The same person should see to the proper application of the correctors and the proper measures for preserving the permanency of their magnetism.

The most remarkable result in a scientific view from the experiments detailed in the present paper is, the great intensity of the permanent magnetism of the malleable iron of which the ship is composed.