HISTORY OF DISCOVERY RELATIVE TO MAGNETISM. 295 



the whole. From the comparison of these observations with those of Graham 

 it was found that in Europe the needle is furthest to the east in the morning from 

 eight to nine o'clock, and furthest to the west in the afternoon from one to two 

 o'clock, when it again proceeds eastwardly imtil eight or nine o'clock in the 

 evening, when it either remains stationary for a few hours, or makes a small 

 movement of a few minutes back towards the west. During the night it gener- 

 ally moves somewhat towards the east, so that in the morning at eight o'clock 

 it is found a little more to the eastwaid than in the evening. 



About the year 1756, John Canton, in London, made observations on the 

 daily deviations, or of the variations, as they were called, from which the 

 result was deduced that the regular daily motion about the time of the sum- 

 mer solstice is nearly twice as great as at the winter solstice. In the first 

 instance, it amounts to about ^, in the latter to about |- of a degree. Can- 

 ton endeavored to explain the daily. western and the subsequent eastern varia- 

 tion of the needle by referring it to the influence of solar heat on the mag- 

 netism of the earth. He supposes, since magnetism is weakened by heat, that, 

 if iii the forenoon the sun warms the eastern parts of the earth, the needle 

 will be more attracted towai'ds the western parts, and in a similar manner in 

 the afternoon, when the sun has weakened the western side, the greater in- 

 fluence of the eastern will draw the needle more towards that direction. 



Before proceeding further in the exposition of this subject, we are obliged to 

 take a step backwards and direct our attention to an individual who produced 

 an epoch in the theory of the magnetism of the earth. We allude to Dr. 

 Edmund Halley, of England, who in 1683 published his theory of terrestrial 

 magnetism, which, in some particulars, still forms the basis of our present 

 theories. He advanced the hypothesis that there were four magnetic poles, 

 two in the vicinity of each geometrical pole of the earth, so that in difierent 

 parts of the earih the needle always directs itself in such a manner that 

 the influence of the nearest poles overcomes that of the more distant one. He 

 further assumed that the pole which at that time was nearest to England was 

 situated on the meridian of Cape Landsend, at the distance of seven degrees 

 from the north geometrical pole, and that the other magnetic north pole was on 

 the meridian of Calitbrnia, at the distance of 15 degrees from the north geometrical 

 pole. He placed one of the two magnetic south poles 16 degrees from the geo- 

 graphical south pole, and 95 degrees Avest from London, and the other, the 

 strongest of the four, at the distance of 20 degrees from the south pole, and 120 

 degrees west from London. 



In order also to explain the successive variations of deviations, he advanced 

 the remarkable hypothesis that our earth is a hollow sphere within which is a 

 solid globe ; that the two revolve around the same centre of gravity in nearly, 

 though not in exactly the same time ; and furthermore, that the solid globe is 

 separated from the exterior hollow shell by a liquid medium. He also supposed 

 that the internal globe, as well as the external shell, have each two magnetic 

 poles, and that the changing deviation of the needle was produced by the want 

 of perfect simultaneousness in the rotation of the two spheres. According to 

 this hypothesis the magnetic poles of the external shell, while they do not 

 coincide with the geometric poles of the same shell, always retained the same 

 position, and, therefore, if the needle was only affected by them, the variation 

 would always remain the same at the same place ; but the needle being also 

 acted vipon by the magnetic poles of the interior globe, and as these slowly 

 change their position relative to those of the exterior shell on account of the 

 difference of velocity in the revolution of the two spheres, a change in the 

 direction of the needle on all points of the earth's surface must be constantly 

 going on. 



Also after a complete rotation of the exterior within the interior sphere the 

 variation must again become the same. This hypothesis created at the time a 



