20 POPULAR LECTURES AND ADDRESSES. 



now see that the angle from point to point of 

 the compass is the eighth part of 90, that is to 

 say, i iJ. This is just two per cent, less than 

 one-fifth of the radian. 1 Hence nearly enough 

 for most practical purposes, you may reckon that 

 an error of one point in steering will lead you 

 wrong one mile in five. More precisely reckoned, 

 an error of a quarter-point will lead you wrong 

 one mile in twenty and a half. 



1 8. The magnetic north and south points are 

 the points of the horizon marked by the direction 

 in which a thin straight magnetised steel needle 

 rests when balanced on a point, or hung by a 

 fine fibre, so as to be very free to turn round 

 horizontally. A magnet of any shape or kind, 

 for example, a bar or horse-shoe of magnetised 

 steel, or a lump of loadstone, or even an electro- 

 magnet, if somehow supported by its centre of 

 gravity, but free to turn round it, will not rest 

 indifferently in all positions, but balances only 

 when a certain line of it, which is called its 



1 The " radian " is an angle whose arc is equal to the radius 

 it is 57'3, or thereabouts. 



