338 HISTORY OF SCIENCE. 



P'IG. 165. DR. HALLEV. 



point, not north and south, but in the direction of the curve which 

 passes through that place. It will be noticed that these lines converge 

 to two points, one in the extreme north of America, the other in the 

 circum-polar seas of the Southern Ocean. These are the magnetic poles, 

 and at them the horizontally-poised needle would not be turned mag- 

 netically at all. The other set of curves are in this chart drawn to form 

 right angles with the magnetic meridians, to which they have the same 

 geometrical relation in this respect as the geographical parallels of 

 latitude have to the geographical meridians. But the so-called " mag- 

 netic parallels" have no physical significance, and both the sets of 

 lines are on modern charts replaced by two other sets, which are re- 

 spectively lines of equal variation, and lines of equal dip. These are 

 formed by drawing lines through the places where the declination is 

 the same; and similarly for places where the inclination is o, 5, 10, 

 etc. Thus, for example, all the places where the needle does not dip 

 at all are situated near the earth's equator, and the line including them 

 all would be not unlike the " magnetic meridian" shown in the figure. 

 As we leave this line to go northwards, the north-pointing end of the 

 needle dips ; and as we leave it to go towards the South Pole, it is the 

 south end of the needle which dips. Graham, the celebrated instru- 

 ment-maker, discovered in 1722 that the direction of the magnetic 

 needle was subject to a small daily variation of about half a degree. 



The law of the decrease of magnetic force by distance occupied 

 the attention of Newton and others, who made many experiments to 

 discover it. Some experimenters drew erroneous conclusions from 

 their results, through neglecting to consider the joint action of the four 



