I48 PHYSICAL GEOGRAPHY. 



never be depended on for such purposes, and should, in all cases, be used 

 with caution, and only when extreme accuracy is not required. 



It is well known that a bar (not magnetic) suspended from its centre of 

 gravity will remain in any position in which it may be placed, unless dis- 

 turbed by some extraneous force ; but if the bar be made of steel, and 

 magnetized, it will assume a definite direction, and, when disturbed, will 

 invariably return to the same direction when the disturbing force ceases. 

 This directive property of the magnet was known to the Chinese, and 

 probably in Europe, as early as the twelfth century; and the magnetic 

 needle has from that time been used to guide ships upon the seas, and 

 for exploring and other purposes upon the land. This needle consists of 

 a slender magnetized steel bar, balanced upon a pivot at a point consider- 

 ably above its centre of gravity, that it may retain its horizontal position ; 

 and, when left free to turn upon its pivot, it comes to rest, by the action 

 of the earth's magnetism, approximately in the plane of a meridian : hence 

 one end is called the north pole, and the other the south pole of the 

 magnet, and a vertical plane through the needle is termed the magnetic 

 meridian. It is not certain at what time the deviation of the magnetic 

 from the true meridian (called the declination of the needle) first became 

 known, but it is evident that it could not have been long after the 

 directive property itself was discovered. There is, however, no reliable 

 record of any experiments to determine the amount of this declination 

 prior to the discovery of America, although it is probable such experiments 

 were made. It seems likely, also, that this declination was previously 

 supposed to be constant, or nearly so, for all times and places, as Colum- 

 bus and his sailors were not a little surprised, and some of them alarmed, 

 on the 13th of September, 1492, to find that the needle, which at the 

 commencement of their voyage pointed east of north, had changed to 

 west of north. Since that time the interest in terrestrial magnetism, 

 among scientific men, has been increasing ; and observations, at first with 

 instruments rudely constructed, but more recently with those of extreme 

 delicacy, have revealed facts, a knowledge of which is important to every 

 one using the magnetic needle. 



To make the statement of these facts plain, let us recur to our magnet- 

 ized bar which we supposed to be suspended from its centre of gravity. 

 This magnet, if left free to turn about the point of suspension in all 



