MAGNETISM. 411 



placed, they diminish and destroy each other's 

 power. Magnetic bars should therefore be always 

 left with the opposite poles laid against each other, 

 or by connecting their opposite poles by a bar of 

 iron. The power of a magnet is increased by let- 

 ting a piece of iron remain attached to one or both 

 of its poles. A single magnet should therefore be 

 always thus left. 



The mariner* s compass consists of three parts; 

 the box, the card or fly, and the needle. The 

 box is made of a circular form, and either of 

 wood, brass, or copper. It is suspended within a 

 square wooden box, by means of two concentric 

 circles called gimbalds, so fixed by cross axes to 

 the two boxes, that the inner one, or compass-box, 

 shall retain a horizontal position in all motions of 

 the ship, whilst the outer or square box is fixed 

 with respect to the ship. The compass-box is co- 

 vered with a pane of glass, in order that the 

 motion of the card may not be disturbed by the 

 wind. The card is a circular piece of paper, 

 which is fastened upon the needle, and moves with 

 it. Sometimes there is a slender rim of brass, 

 which is fastened to the extremities of the needle, 

 and serves to keep the card stretched. The outer 

 edge of this card is divided into 360 degrees, and 

 also into 32 equal parts or arcs, which are called 

 the points of the compass, or rhumbs 3 each of 

 which is often subdivided into quarters. The let- 

 ters &c. are annexed to those rhumbs, to denote 

 the north, north-east, &c. 



The needle is a slender bar of hardened steel, 

 having a hole in the middle, to which a conical 

 piece of agate is adapted, by means of a brass 

 piece into which the agate case is fastened. The 

 apex of this hollow cap rests upon the points of a 



