524 



NA TURE 



\_April 27, 1875 



whilst the upper is formed of a confused mass of matter 

 which belongs chiefly to the third element, but is inter- 

 spersed with the round balls of the second. The passages 

 in the intermediate region he conceives to be so grooved 

 that the fluted columns entering from one side cannot 

 return again by the same passages, but when opposed in 

 their straight course are forced back through the air or 

 upper portions of the earth to those openings by which 

 they entered, whilst those from the other side make 

 similar circuits. He considers that magnets contain 

 passages the same as those first mentioned, and such is 

 the inclination of the fluted columns to enter these pas- 

 sages, that even if the poles are not turned to receive 

 them they will push aside all opposing particles, till, if not 

 restrained by still stronger bodies, the magnets are forced 

 to assume those positions in which their poles point 

 oppositely to those of the earth. ^ ouch is the hypothesis 

 of Descartes, ingenious rather than plausible, and inte- 

 resting chiefly as exhibiting the speculative mind of its 

 author. 



In 1683 the celebrated Halley presented a paper of 

 great importance to the Royal Society of London, entitled 

 " A Theory of the Variation of the Magnetical Compass." 

 In this communication he states that the " deflection of 

 the magnetical needle from the true meridian is of that 

 great concernment in the art of navigation, that the 

 neglect thereof does little less than render useless one of 

 the noblest inventions mankind ever yet attained to," and 

 gives as the result of " many close thoughts " the follow- 

 ing explanation of the variation of the compass. " The 

 whole globe of the earth is one great magnet, having four 

 magnetical poles or points of attraction, near each pole 

 of the equator two ; and in those parts of the world which 

 lie near adjacent to any one of those magnetical poles, 

 the needle is governed thereby, the nearest pole being 

 always predommant over the more remote." He remarks 

 that the positions of these poles cannot as yet be exactly 

 determined from want of sufficient data, but conjectures 

 that the magnetic pole which principally governs the 

 variations in Europe, Tartary, and the North Sea is 

 about 7° from the north pole of the earth, and in the 

 meridian of the Land's End, whilst the magnetic pole 

 which influences the needle in North America, and in the 

 Atlantic and Pacific Oceans, from the Azores westward to 

 Japan, is 15° from the north pole, and in a meridian 

 passing through the middle of California. The variation 

 in the south of Africa, in Arabia, Persia, India, and from 

 the Cape of Good Hope, over the Indian Ocean to the 

 middle of the South Pacific, is ruled by the most powerful 

 of all these magnetic poles, which is situated 20° from the 

 south pole of the earth, and in a meridian passing through 

 the island of Celebes ; in the remainder of the South 

 Pacific Ocean, in South America and the greater part of 

 the South Atlantic Ocean, it is governed by a magnetic pole 

 16° from the south pole, in a meridian 20° west of the 

 Straits of Magellan. On this hypothesis Halley explains 

 the variation observed in different places, and among 

 others cites the two following instances. On the coast of 

 America, about Virginia, New England, and Newfound- 

 land, the variation was found to be west, being above 20° 

 in Newfoundland, 30° in Hudson Strait, and 57° in 

 Baffin's Bay. On the coast of Brazil, on the contrary, it 

 was found to be east, being 12° at Cape Frio, and increasing 

 to 2o|° at the Rio de la Plata, thence decreasing to- 

 wards the Straits of Magellan. Thus, almost in the 

 same geographical meridian, we find the needle at 

 one place pointing nearly 30° west, at another 20;}° east ; 

 this is explained by the north end of the needle in 

 Hudson Strait being chiefly attracted by the North 

 American magnetic pole, whilst at the mouth of the Rio 

 de la Plata the south end is attracted by the south mag- 

 netic pole, situated west of the Straits of Magellan. 



' Descartes designates the south pole of the magnet that which turns to 

 the north pole of the earth, and 'the north pole of the magnet that which 

 turns to the south pole of the earth. 



Sailing north-west from St. Helena to the equator, the 

 variation is always in the same direction, and slightly east. 

 Here the South American is the chief governing pole, but 

 its power is opposed by the attraction of the North 

 American and Asian south poles ; the balance as you 

 recede from the latter being maintained by approach 

 to the former. 



Nine years later Halley made another communication 

 to the Royal Soceity, in which he endeavoured to meet 

 two difficulties he had always felt in his former explana- 

 tion ; one, that no magnet he had ever seen or heard of 

 had more than two opposite poles ; the other, that these 

 poles were not, at least all of them, fixed in the earth, but 

 slowly changed their positions.. The following obser- 

 vatioms are cited by Halley in proof of the motion of the 

 magnetic system. At London, in 1580, the variation was 

 11° 15', east; in 1622 it was 6° east, in 1634 it was 

 4° 5' east, and in 1657 there was no variation ; whilst in 

 1672, it was 2° 30' west ; and in 1692, 6° west. At 

 Paris the variation was 8° or 9° east in 1550, 3° east 

 in 1640,0° in 1666, and 2° 30' west in 1681. At Cape 

 Comorin it was 14° 20' west in 1620, 8° 48' west in 

 1680, and 7° 30' west in 1688. Halley considered the 

 external parts of our earth as a shell, separated by a 

 fluid medium from a nucleus or inner globe, which had 

 its centre of gravity fixed and immovable in the common 

 centre of the earth, but which rotated round its axis a 

 little slower than the superficial portions of the earth. 

 The nucleus and exterior shell he regarded as two dis- 

 tinct magnets, having magnetic poles not coincident with 

 the geographical poles of the earth. The change observed 

 in Hudson's Bay being much less than that observed in 

 Europe, Halley concluded that the North American pole 

 was fixed, while the European one was movable ; and, from 

 a similar observation on the coast of Java, he considered the 

 Asian south pole as fixed, and the pole west of the Straits 

 of Magellan to be in motion. The fixed poles he regarded 

 as those of the external shell, and the movable those of the 

 inner nucleus. Of these latter, the one placed by him in 

 the meridian of the Land's End was ascertained, in the 

 present century, to have moved to Siberia, in 1 20° east long., 

 and that placed by him 20° from the Straits of Magellan 

 to have moved between 30° and 40° west of this position ; 

 while those poles regarded by Halley as fixed were found 

 but slightly altered in position since his time. It is ex- 

 tremely interesting to find that not only modern obser- 

 vations of declination, but also those of dip and magnetic 

 intensity, have received their best explanation on the 

 assumption of four magnetic poles. Much, however, that 

 is mysterious remains unsolved, and H alley's remarkable 

 words may even now with truth be quoted : " Whether 

 these magnetical poles move altogether with one motion 

 or with several ; whether equally or unequally ; whether 

 circular or libratory ; if circular about what centre, if 

 libratory after what manner, are secrets as yet utterly un- 

 known to mankind, and are reserved for tlae Industry of 

 future ages." K. 



THE POTATO DISEASE 



IN the Journal of the Royal Agricultural Society oj 

 England, Second Series, vol. xii., Part I., No. xxiii.,| 

 1876, Prof. A. De Bary of the University of Strasburg has| 

 published a paper entitled " Researches into the Nature ^ 

 of the Potato Fungus." 



De Bary's essay treats of the Peronosporeas, Artotrogus j 

 (in its plain and echinulate forms) and Pythium, These ■ 

 fungi are described by De Bary as four distinct plants,1 

 whilst I, in common with several other observers, believe] 

 the first three (if indeed not all four) to be mere condi-j 

 tions of one and the same fungus, viz., the Pe7-onospora\ 

 infestans of Dr. Monlagne. In replying to De Bary'sj 

 remarks it will be convenient (especially as the potato- 

 fungus appears to be somewhat imperfectly understood)^ 



