710 REPORT— 1903. 



Shortly, we may define 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 

 largely dependent upon the direction of her head whilst building and the part of 

 that field she occupied at the time ; partly upon her position in the magnetic 

 field she traverses at any given time during a voyage. 



For the reasons I have given, magnetic charts are a necessity for practical 

 purposes and in the following order of value. That of the magnetic declination 

 or variation which is constantly in use, especially in such parts of the world as the 

 St. 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 useful when correcting the ship's 

 compass, but are required in the analysis of a ship's magnetism both as regards 

 present knowledge and future improvements in placing compasses on board. 



If astronomers have for a very long time been able to publish for several years 

 in advance exact data concerning the heavenly bodies, is it too much to hope* that 

 magneticians will before long also be able to publish correct magnetic charts to 

 cover several years in advance of any present epoch ? It this is to be done within 

 reasonable time there must be a long pull, a strong pull, and a pull all together of 

 magnetic observers in all lands, and accumulated data must also be discussed. 



On Magnetic Instruments for Travellers. 



Travellers in unsurveyed countries, if properly instructed and equipped, can do 

 good service to science by observing the three magnetic elements of declination, 

 inclination or dip, and force at as many stations as circumstances will permit ; 

 hence the following remarks. 



For the purpose of making the most exact magnetic survey the best equipment 

 of instruments consists of the well-known unifilar magnetometer, with fittings for 

 observing the declination, and a Barrow's dip circle. To some travellers these 

 instruments might be found too bulky, and in some regions too delicate as well as 

 heavy to carry. 



Of suitable instruments made abroad, those used by M. Moureaux in his 

 survey of France may be mentioned, as they are of similar type, but much smaller 

 and lighter than the instruments above mentioned. 



Another form of instrument, called an L.C. instrument, for observing both the 

 inclination and total force, is shown in the instrument before you. Originally 

 designed for observations on board ships at sea where the ordinary magnetic 

 instruments are unmanageable, it has also been found to give satisfactory results 

 in a land survey, where greater accuracy is expected than at sea. Thus, during a 

 series of observations extending from the north side of Lake Superior to the 

 southern part of Texas last year, comparisons were made between the results 

 obtained with an L.C. instrument and those of the regular unidlar magnetometer 

 and dip circle, when the agreement was found satisfactory. 



I am therefore of the opinion that a traveller furnished with a theodolite for 

 land-surveying purposes, but fitted with a reversible magnetic needle, can at any 

 time he observes a true bearing obtain a trustworthy value of the declination. 

 Dismountino: the theodolite from his tripod, the latter will serve for mounting an 

 L.C. instrument with which to observe the inclination and force. Thus, by 

 adding to his ordinary equipment an instrument weighing in its box about 21 ib., 

 he can obtain valuable contributions to terrestrial magnetism, and at the same 

 time give useful assistance to geological investigations. 



Concluding Remarks. 



Although a great subject like terrestrial magnetism, even to exhibit our 

 present knowledge of the science, cannot be brought within the compass of an 

 address — for it requires a treatise of many pages- I have brought some of the 



