MAGNETISM. 



115 



and inclining them at an angle of about thirty degrees to it, he drew them upon 

 it from the centre to the extremities, and repeated this process until the bar 

 was strongly magnetized. This method was modified by Mitchell, who placed 

 a series of bars to be magnetized in the same straight line, with their extremi- 

 ties successively in contact. He then placed two bundles of strong magnets 

 perpendicular to them, with their ends resting upon them, the northern end of 

 one bundle and the southern end of the other being downward. These two 

 bundles of magnets, being attached to each other, were moved over the series 

 of bars to be magnetized. 



In 1789, Coulomb directed his investigations to the processes of producing 

 artificial magnets. He showed that the susceptibility of bars of steel for mag- 

 netism depended conjointly on the temper of the steel and the force of the 

 magnets, and that there was a certain limit to the magnetic force which a bar 

 could receive. When a bar attained this limit, it was said to be magnetized 

 to saturation. 



The magnetic needles of ships' compasses being liable to great vicissitudes 

 of temperature, it was a question of considerable importance to navigation 

 whether heat affected the magnetic virtue. Gilbert was the first who observed 

 that a magnet lost all its power when raised to a white heat, and on being 

 cooled did not recover its magnetism. It was not, however, till a much later 

 period, that the influence of heat on magnetism was submitted to accurate in- 

 quiry. 



It was natural that the directive power of the magnet, and its application to 

 navigation, should engross a large share of attention ; and that the govern- 

 ments of maritime countries, more especially, should cause to be carefully and 

 accurately observed all those phenomena by which that property was affected. 

 The variation of the needle, and the changes periodical and local to which it 

 is subject, were questions of the highest importance to national and commer- 

 cial interests in every part of the world. So early as 1722, Graham had ob- 

 served that in a given place the needle was subject to a diurnal variation, 

 which was afterward ascertained with great precision in different parts of 

 Europe. It was observed by Wargentin, secretary to the Swedish Academy, 

 in 1750, and by Canton in London in 1756 ; and subsequently by Van Swie- 

 ten, with nearly the same results. From all these observations it appeared 

 that the north pole of the needle begins to turn westward at seven or eight 

 o'clock in the morning, and continues to deviate in that direction till about two 

 o'clock, when it becomes stationary, and soon begins to return eastward, ar- 

 riving at the position it had in the morning at the same hour in the evening. 

 Canton's observations showed that the amount of this deviation varied from 

 seven to thirteen or fourteen minutes, being greatest at midsummer and least at 

 midwinter, and increasing and decreasing gradually between these seasons. 



More recently the same phenomenon has been observed by Colonel Beaufoy, 

 Professor Hansteen, and others. 



Cassini, who observed the diurnal variation of the needle at Paris, found 

 that neither the solar heat nor light influenced it ; for it was the same in the 

 deep caves constructed under the Observatory in Paris, where a sensibly con- 

 stant temperature is preserved, and from which light is excluded, as at the sur- 

 face. In northern regions these diurnal changes are greater and more irregu- 

 lar ; while, toward the line, their amplitudes are gradually diminished until at 

 length they disappear. 



The investigation of the changes produced in the direction of the needle, 

 and in the intensity of the earth's attraction upon it, by change of place upon 

 the surface, being a matter vitally important to commerce and navigation, has 

 engaged the attention of all maritime and commercial countries, from an early 



