394 TERRESTRIAL MAGNETISM. 



This work of the royal na\y and the Indian marine is one of great 

 importance, l>oth in the interests of practical naviojition and of science; 

 for besides the equipment of instrmnents for absolute determinations 

 of the declination, dip, and horizontal force supplied to certain of our 

 surveying ships, every seagoing vessel in the service carries a landing 

 compass, specially tested, by means of which the declination can be 

 observed with considerable accuracy on land. 



Although observers of many other oljjects may still speak of their 

 ''heritaoe, the sea," as a mine of wealth waiting for them to explore, 

 unfortunately for magnetic observations, we can no longer say "the 

 hollow oak our palace is,"" for wood has been everywhere replaced by 

 iron or steel in our ships, to the destruction of accurate ol^servations 

 of dip and foi'ce on board of them. Experience, howevei-, lias shown 

 that verv useful results, as regards the declination, can be obtained 

 every time a ship is ''swung," either for that purpose alone or in the 

 or(linar\' course of ascei'taining tlie ei'rors of tho compass due to the 

 iron or steel of the sliip. 



As an example of this method, th(^ cruise of the training scjuadron 

 to Spitzbergen and Norway in isnr) may ])e cited, when several most 

 useful observations were made at sea in regions })ut seldom visited. 

 Again, only this year a. squadron of our ships cruising together near 

 Madagascar separated to a distance of a mile ai)art and "swung" to 

 ascertain the declination. 



I would here note that all the magnetic observations made by the 

 officers of Her Majesty's shi[)s during the years 189»)-1 !♦()() have ])een 

 published in a convenient form by the hydrographic departiuent of 

 the Admiralty. 



The fact remains, howe^■er, that a great portion of the world, other 

 than the coasts, continues unknown to the searching action of the 

 magnetic needle, while the two-thirds of the globe covered b}' water 

 is still worse otf. Among other regions I would specify Africa, 

 which, apart from the coasts, Cape Colony, and the Nile valley to lat- 

 itude oi'^ north, is absolutely a new field for the observer. 



]Moreov(M', the elaborate surveys I have mentioned show how much 

 the results depend upon the nature of the locality. I am therefore 

 convinc(Hl that travelers on land, proAided witii a proper eipiipment 

 of instruments for conducting a land survey of the strange countries 

 which they may visit, and mapping the same correctly, can, with a 

 small addition to the weight they have to carry, make a valuable con- 

 tribution to our knowdedge of terrestrial magnetism, conunencing with 

 ol)servations at their principal stations and tilling in the intermediate 

 space with as man}- others as circumstances will permit. 



