MAGNETISM. 



CHAPTER I. 

 General Facts and Principles. 



(1.) THE attractive power of the load- 

 stone for iron was known in times of 

 very remote antiquity, and has been, in 

 all ages, a subject of curiosity and of 

 wonder. It is a property which seems, 

 at first sight, so unconnected with every 

 other, as to form of itself a separate 

 class among- natural phenomena: and, 

 although an immense mass of knowledge 

 relating to Magnetism has been accu- 

 mulated by the labours of successive 

 generations, and embodied into a science 

 of high rank and importance, yet the 

 field it comprises is of comparatively 

 limited extent. This arises from the 

 great simplicity which characterises both 

 the phenomena and the laws that govern 

 them ; a quality, however, which pecu- 

 liarly invites a philosophic mind to 

 undertake their investigation. A still 

 more powerful motive to this inquiry 

 will present itself when we reflect on 

 the signal benefits mankind has derived 

 from magnetism as applied to the pur- 

 poses of navigation. The discovery of 

 the compass, by the aid of which, the 

 manner, however distant from land, 

 amidst cloudy skies, or in the darkest 

 nights, is enabled, at all times, to steer 

 his course with certainty, and traverse 

 in all directions the wide expanses of 

 ocean which separate the countries and 

 continents of our globe, must unques- 

 tionably rank among the great discove- 

 ries that have essentially contributed to 

 advance the civilization of the human 

 race. 



(2.) The term MAGNETISM expresses 

 the peculiar property occasionally pos- 

 sessed by certain bodies, more especially 

 by iron and some of its compounds, 

 whereby, under certain circumstances, 

 they mutually attract or repel one 

 another, according to determinate laws. 



(3.) This property was first noticed 

 in a mineral substance called the native 

 magnet, or the loadstone, which is an 

 ore of iron, consisting chiefly of the 



two oxides of that metal, together with 

 a small proportion of quartz and alu- 

 mina. Its colour varies in different spe- 

 cimens, according to minute differences 

 in the proportion of the oxides, and the 

 nature of the other substances with 

 which they may be found united : but it 

 is usually of a dark grey hue, and has 

 a dull metallic lustre. It is found in 

 considerable masses in the iron mines 

 of Sweden and Norway, and also in 

 different parts -of Arabia, China, Siam, 

 and the Philippine Islands. Small load- 

 stones have occasionally been met with 

 among the iron ores of England. 



(4.) There are several modes in which 

 a piece of iron may be rendered mag- 

 netic, or converted into what is called 

 an artificial magnet; and for all pur- 

 poses of accurate experiment such a 

 magnet is much to be preferred to a 

 loadstone. The following is a simple 

 and ready method of obtaining artificial 

 magnets with a view to the investigation 

 of the magnetic properties. 



Let a straight bar of hard tempered 

 steel, devoid of all perceptible magnetism, 

 be held in a vertical position (or still 

 better, in a position slightly inclined to 

 the perpendicular, the lower end deviat- 

 ing to the north,) and struck several 

 smart blows with a hammer ; it will be 

 found to have acquired, by this process, 

 all the properties of a magnet. 



(5.) These properties are the four fol- 

 lowing: viz. 1. Polarity. 2. Attrac- 

 tion of unmagnetic iron. 3. Attraction 

 and repulsion of magnetic iron. 4. The 

 power of inducing magnetism in other 

 iron. These we shall now explain and 

 illustrate. 



1. Polarity. 



(6.) If a bar, which has been ren- 

 dered magnetic, be supported in such a 

 manner as to have entire freedom of 

 motion in a horizontal plane, and be re- 

 moved from the neighbourhood of all 

 ferruginous bodies which might influ- 

 ence it, it will spontaneously turn round, 

 and, after a few oscillations, will finally 

 B y 



