NORTH MAGNETIC POLE AND NORTHWEST PASSAGE. 259 



greater from them. On a series of pomts within the tropical zone 

 the inclination is 0° ; that is to say, the dipping needle places itself 

 exactly horizontally, and that line which Ave may imagine as drawn 

 throngh all these points is called the " magnetic equator," It is 

 situated partly above, partly beneath, the earth's geographical 

 equator. 



The force of terrestrial magnetism w^orks, as will be understood, 

 with its Avhole strength in the direction given by the dipping needle, 

 and it may be asked. How great is this force in the ditt'erent places? 

 In order to discover this Ave' must imagine the force dissolved into 

 tAvo parts, one part Avorking horizontally, and one part Avorking 

 A'ertically. It is evident that it is the horizontal part of the force 

 Avhich causes the needle to take a set position, and if Ave Ioioav all 

 about this force — " horizontal intensity," as it is called — and at the 

 same time knoAv the inclination, it is easy, by a simple calculation, to 

 find the collectiA^e strength, the total intensity. For the determina- 

 tion of horizontal intensity Iavo methods are adopted, either inde- 

 pendentlA', preferably, for the sake of comparison, simultaneously. 

 One method consists in placing a magnetic l)ar by the side of a needle 

 at a giATU distance from it, and observing hoAV many degrees the 

 needle moves aAvay from its original position. It is clear that the 

 Aveaker the horizontal intensity the greater the oscillation of the 

 needle, and Avhen the strengih of the magnetic bar is knoAvn, it is 

 possible, by the aid of the angle of oscillation and the distance, to 

 calculate the horizontal intensity. 



The other method is to note the time of oscillation of a magnetic 

 bar suspended by a thread in such a manner that it can reA'oh'e in the 

 horizontal plane. When the magnet is allowed to be at rest it sets, 

 under the influence of horizontal intensity, in the direction of the 

 needle. Brought out of equilibrium by a little push, it Avill swing 

 backAvard and forAvarcl, and the stronger the horizontal intensity the 

 sooner it wall come to rest again, or, in other Avords, the shorter Avill be 

 the time of each individual oscillation. When the strength of the 

 oscillatory magnet is knoAvn and observation is made of how many 

 seconds are necessary for an oscillation, the horizontal intensity can be 

 calculated. 



Maps are constructed to giA^e an idea of the A^alue of horizontal 

 intensity, expressed in so-called electric units, on the different parts of 

 the earth, A line passes through all the places Avhere the horizontal 

 intensity is the same. The horizontal intensity decreases toward the 

 magnetic poles. It is therefore matter of consequence that terres- 

 trial magnetism here, where the inclination is 90°, acts with its whole 

 strength vertically downward, and thus can not have any effect in a 

 horizontal direction. 



Although the magnetic maps are very dissimilar, they are alike in 



