xlviii 



THK PROGRESS OF 



I MI- since it has been slowly diminishing. In 

 1821 it was 70 3'. 



From numerous observations made in different 

 (.in- of the torrid zone, there is reason to believe 

 lli.it the line of the dip or the magnetic equator 

 is distant at a maximum about 12' from the ter- 

 resirial equator. It is a great circle of the 

 sphere, and cuts the equator on two places in 

 about 113 west longitude, and (57* east longitude. 

 As we advance north or south from this magnetic 

 equator, the dip slowly increases, the south pole 

 dipping if we go south, and the north pole if ue 

 go north. 



The first person who attempted to account for 

 the declination of the needle, was Dr Halley. He 

 considered the globe of the earth as constituting 

 one great magnet, having four poles, two to the 

 north, and two to the south, at considerable but 

 unequal distances from the poles of the earth. 

 Two of these poles he considered as fixed, and 

 two as movable. This subject was taken up by 

 professor Hanstein of Christiania, who has had 

 the advantage of another century of observations. 

 He has adopted the Halleyan hypothesis, with 

 some modifications. One of the north and one 

 of the south poles is weaker than the other. One 

 of the north poles revolves round the pole of the 

 earth in 1740, the other in 860 years. The 

 stronger north pole is at present situated in 

 north latitude 73 53', and west longitude 87*; 

 the weaker in north latitude 85 12', and east 

 longitude 140 6'. The stronger south pole is in 

 south latitude 68 52', and east longitude 132" 

 SS 7 ; the weaker in south latitude 78 16', and 

 west longitude 135* 59'. 



In 1722, Mr Graham discovered the daily va- 

 riation of the needle. The subject was prosecuted 

 by Mr Carters, and colonel Beaufoy determined 

 the amount of the daily variation for a number 

 of years with indefatigable accuracy. It was 

 taken up at last by Mr Barlow and Mr Christie. 

 They all concur in showing that the daily varia- 

 tion is owing to the action of the sun on the nee- 

 dle altering the intensity somewhat by the effect 

 of his heat. 



Mr Barlow ascertained that the intensity of the 

 action of iron on a magnetic needle is propor- 

 tional to its surface, not to its solid contents ; so 

 that a hollow sphere of iron, however thin, has as 

 great an effect upon a needle as a solid iron 

 sphere of the same diameter. 



It was supposed at first that iron or substances 

 containing iron, were alone susceptible of the 

 magnetic influence ; but cobalt and nickel were 

 afterwards found capable of being converted into 

 magnets ; though less powerful than those of 

 steel. Coulomb showed that all bodies were to a 

 certain extent susceptible of magnetic influence. 



A\ Iiile hot, iron has no effect upon the magnetic 

 needle ; but as the iron cools it begins to act, and 

 it acts most powerfully at a blood-red heat. 31r 

 Harlow and Mr Charles Bonnycastle observed 

 that between a bright-red and a blood-red he.it 

 it acts in the opposite way from what it does 

 when cold. If it attract the north pole of the, 

 needle while cold, it will attract the south pole 

 at the temperature above specified. 



Mr Barlow found that if a magnetic needle be 

 placed at a certain distance from a mass of iron 

 the needle is deflected when the iron is put into 

 rapid motion. 



About the year 1824, M. Arago discovered th:;t 

 when plates of copper and other substances .in- 

 set in rapid volution beneath a magnetized nee- 

 dle, they cause it to deviate from i(s direction, 

 and finally to drag it along with them. This cu- 

 rious fact was repeated and verified in 1825 bj 

 Messrs Herschell and Babbage. They mounted a 

 powerful compound horse-shoe magnet, capable of 

 lifting 20 Ibs., in such a manner as to receive a 

 rapid rotation about its axis of symmetry placed 

 vertically, the line joining the poles being hori- 

 zontal, and the poles upwards. A circular 

 disk of copper, 6 inches in diameter, .and 0'85 

 inch thick, was suspended centrally over it by a 

 silk thread without tortion, just capable of suit- 

 porting it. A sheet of paper properly stretched 

 was interposed, and no sooner was the magnet 

 set in rotation, than the copper commenced re- 

 volving in the same direction, at first slowly, but 

 with a velocity gradually and steadily accelerat- 

 ing. The motion of the magnet being reversed, 

 the velocity of the copper was gradually destroy- 

 ed ; it rested for an instant, and then began to 

 revolve in the opposite direction. 



The rate of rotation was not diminished, by 

 interposing between the copper and the revolv- 

 ing magnet, plates of paper, glass, wood, copper^ 

 tin, zinc, lead, bismuth, or antimony. But it 

 was greatly diminished when the substance in- 

 terposed was iron. One sheet of tin plate re- 

 duced the rate of rotation to about ^th part, and 

 two sheets nearly destroyed it. 



When disks of other metals were substituted 

 for copper, they were found also capable of 

 being made to rotate, though with different de- 

 grees of rapidity. The order of the different 

 metals, and the intensity of the force of each, 

 that of copper being reckoned 1, was as follows: 

 Copper . . . 1' 



Zinc .... 0-90 

 Tin .... 0-47 

 Lead .... 0-25 

 Antimony . . . O'll 

 Bismuth . . . OD1 



Wood . . 0-00 



