Curiosities of Science. 201 



nets, different in position and intensity. Ampere set aside these 

 unsatisfactory hypotheses by the view, derived from his discov- 

 ery, that the earth itself is an electro-magnet, magnetised by an 

 electric current circulating about it from east to west perpen- 

 dicularly to the plane of the magnetic meridian, to which the 

 same currents give direction as well as magnetise the ores of 

 iron : the currents being thermo-electric currents, excited by the 

 action of the sun's heat successively on the different parts of the 

 earth's surface as it revolves towards the east. 



William Gilbert,* who wrote an able work on magnetic and 

 electric forces in the year 1600, regarded terrestrial magnetism 

 and electricity as two emanations of a single fundamental source 

 pervading all matter, and he therefore treated of both at once. 

 According to Gilbert's idea, the earth itself is a magnet; whilst 

 he considered that the inflections of the lines of equal declina- 

 tion and inclination depend upon the distribution of mass, the 

 configuration of continents, or the form and extent of the deep 

 intervening oceanic basins. 



Till within the last eighty years, it appears to have been the 

 received opinion that the intensity of terrestrial magnetism was 

 the same at all parts of the earth's surface. In the instructions 

 drawn up by the French Academy for the expedition under La 

 Perouse, the first intimation is given of a contrary opinion. It 

 is recommended that the time of vibration of a dipping-needle 

 should be observed at stations widely remote, as a test of the 

 equality or difference of the magnetic intensity; suggesting also 

 that such observations should particularly be made at those parts 

 of the earth where the dip was greatest and where it was least. 

 The experiments, whatever their results may have been, which, 

 in compliance with this recommendation, were made in the ex- 

 pedition of La Perouse, perished in its general catastrophe ; but 

 the instructions survived. 



In 1811, Hansteen took up the subject, and in 1819 pub- 

 lished his celebrated work, clearly demonstrating the fluctua- 

 tions which this element has undergone during the last two 

 centuries ; confirming in great detail the position of Halley, 

 that " the whole magnetic system is in motion, that the mov- 

 ing force is very great as extending its effects from pole to pole, 

 and that its motion is not per saltum, but a gradual and regular 

 motion. " 



THE NORTH AND SOUTH MAGNETIC POLES. 



The knowledge of the geographical position of both Mag- 

 netic Poles is due to the scientific energy of the same naviga- 



* Gilbert was surgeon to Queen Elizabeth and James I., and died in 1603. 

 Whewell justly assigns him an important place among tlie " practical reformers 

 of the physical sciences." He adopted the Copernican doctrine, which Lord Ba- 

 con's inferior aptitude for physical research led him to reject. 



