30 Experimental Philosophy. [Lecture 3. 



earth ; for the magnetic needle, when it is so sus- 

 pended as to move freely in a vertical plane, ge- 

 nerally assumes a position with one of its poles 

 elevated and the other depressed. This, how- 

 ever, varies in different latitudes: near the equator 

 it is in a position almost horizontal ; as it ap- 

 proaches the northern regions, the south pole is 

 depressed, or drawn towards the earth ; and on 

 the other side of the equator, in the southern la- 

 titudes, the. north pole is depressed. This is 

 called the dip of the needle, and is subject to 

 periodical variations. In 1720, the dip at Lon- 

 don was 75 10'; in 1775, it was 72 30'; in 

 1805, 70 20' ; now, in 1819, it is 70" 32'. 



Iron may acquire the magnetic virtue by other 

 means than communication with a magnet. 1st. 

 If a bar is kept for a long time in a vertical po- 

 sition, or, still better, in the direction of the 

 dipping needle. Thus old iron bars in windows 

 are often found strongly magnetic. 2d. If iron 

 is heated and suffered to cool quenched in water, 

 holding it in the position of the dipping needle, 

 the same effect is produced. 3d. If it is rubbed 

 hard in the same position by any steel instru- 

 ment. 4th, A few strokes of a hammer, first at 

 one end of a bar, and then at the other, while 

 held in the position of the dipping needle, will pro- 

 duce the effect. 5th. A shock of electricity passed 

 through the bar will gf ten render it magnetic. 



Many entertaining experiments are performed 

 by means of magnetism. In the shops, little 



