Transportation on Water 373 



that runs through South Carolina and Michigan to the north 

 magnetic pole of the earth, which is located northwest of Hud- 

 son Bay, about longitude 97 west. The declination or de- 

 flection of the needle at all points east of this line is thus con- 

 stantly increasing, and that of all places west of the line de- 

 creasing. The declination of the needle at New York is about 

 ten degrees west. That at San Francisco is about sixteen 

 degrees east, and that at Seattle about twenty-two degrees 

 east. Local declination may be determined by the following 

 method. 



Exercise : Finding local declination. Determine first of all the 

 correct north and south line of your locality. This you may do by 

 observations on the north star. The orbit of the north star is about 

 the size of the circumference of the moon and is completed by the star 

 every twenty-four hours. Twice in each twenty-four hour period the 

 star is directly north. This occurs when it is directly above or below 

 the star in the crook of the handle of the Great Dipper. Observations 

 should be made at such times, although the error of observations at 

 any other times is very slight. The observations may be made more 

 accurately by using illuminated plumb lines. When you have deter- 

 mined the true north and south line, the angle which is formed by the 

 needle of the compass with that line gives the local declination. 



Steel ships and the compass. The construction of ships 

 with iron and steel gives greatly increased strength, size, and 

 safety, but it also makes uncertain and difficult the use of the 

 ordinary compass. The great mass of iron and steel not only 

 influences the magnetic needle, but itself becomes magnetized 

 through the influence of the magnetism of the earth. This 

 condition of the ship interferes directly with the needle and 

 deflects it from the direction it would take under the influence 

 of the earth alone. Compasses of the usual kind have to be 

 tested by astronomical observation so that the error may be 

 known and corrected. 



The influence of the magnetism of the earth upon iron and 

 steel may be observed experimentally. The experiment which 

 we shall describe was made by William Gilbert, physician to 



