404 TEKRESTRIAL MAGNETISM. 



If the nature of tlie whole of the iron or .steel used in construction 

 of ships were such as to become permanently magnetic, their naviga- 

 tion would ])e much simplitied, as our knovvledoe of terrestrial mag- 

 netism would enable us to pro^•ide correctors for an^^ disturbing effects 

 of such iron on the compass, which would then point correctly. But 

 ships, taken as a whole, are general!}' more or less unstable magnets, 

 and constantly sul)ject to change, not only on ciiaiige of geographical 

 position, but also of direction of the ship's head with regard to the 

 magnetic meridian. Thus, a ship st(»ering on an easterh' course ma}^ 

 be temporarily magnetized to a certain extent, ])ut t)n reyersing the 

 ship's course to west she would after a time ])ecome tem])orarily mag- 

 netized to the same amount but in the opposite direction, the north 

 point of tlie compass being attracted in each case to that side of the 

 ship which is southernmost. 



Shortly, w(^ in^i}" dehne the action of the earth's magnetism on the 

 iron of a ship as follows: The earth being surrounded by a magnetic 

 field of force differing greatly in intensity and direction in the regions 

 from the north pole to the equator and the equator to the south pole, 

 the ship's magnetic condition is largeh^ dependent upon the direction 

 of her head Avhile building and the part of that iield she occupied at 

 the time; partly upon her ])osition in the magnetic field she traverses 

 at any given time during a voyage. 



For the reasons J have given, magnetic charts are a necessit}^ for 

 practical purposes and in the following order of value: That of the 

 magnetic declination of variation which is constantly in use, especially 

 in such parts of the world as the Saint Lawrence and the approaches 

 to the English Channel, where the declination changes very rapidly as 

 the ship proceeds on her course. Next, that of the dip and force, 

 which are not only immediately usef id when correcting the ship's com- 

 pass, but are required in the anah'sis of a ship's magnetism ])oth as 

 regards present knowledge and future improvements in placing com- 

 passes on board. 



If astronomers have for a very long time ])een able to publish for 

 several years in advance exact data concerning the heavenly bodies, is 

 it too nuich to hope that magneticians will before long also be able to 

 pu])lish correct magnetic charts to cover several 3"ears in advance of 

 any present epoch? If this is to be done within reasonable time, there 

 nmst be a long pull, a strong pull, and a pull all together of magnetic 

 observers in all lands, and accuuudated datii must also be discussed. 



ON MACNKTIC INSTRUMENTS FOli THAVELKKS. 



Traveleivs in unsvu'veyed countries, if properly instructed and 

 equipped, can do good ser^'ice to science by observing the three mag- 

 netic elements of declination, inclination or dip, and force at as many 

 stations as circumstances will permit. Hence the following remarks: 



