April 2"]^ 1876] 



NA TURE 



523 



and larger than that formerly obtained by Puluj from 

 transpiration-experiments with capillary tubes (§). The 

 exponent for hydrogen is somewhat smaller than that 

 for air. Not a little interesting is the remarkably large 

 exponent for carbonic acid, in which the friction appears 

 nearly to follow the law of temperature, to which Max- 

 well's new theory of gases leads (which, as is known, 

 proceeds from the supposition of a repulsive action at a 

 distance, inversely proportional to the fifth power of the 

 distance of the molecules). 



From these experiments it clearly appears that the 

 friction does not, in all gases, vary luith the temperature 

 in the same ivay. The theory of gases must still undergo 

 modification, in order to afford us a satisfactory explana- 

 tion of this molecular process. S. W. 



THE EARLY HISTORY OF MAGNETISM 



'T'HE earliest references to the properties of the magnet 

 •*• occur in the annals of the Chinese nation, who used 

 it as a means of guiding the wayfarer over the vast and 

 trackless plains of Eastern Asia, long before it was applied 

 to maritime purposes. To the Emperor Hoang-Ti, who 

 lived 2,000 years before our era, is attributed the invention 

 of a chariot, upon which stood an elevated figure pointing 

 to the south, independently of any position of the chariot. 

 Nearly ten centuries later, we find the learned Tchdou* 

 Koung presenting and teaching the use of the tchi-nan- 

 kiu, or chariot indicating the south, to some envoys from 

 Youd-tchang, a southern maritime province. The com- 

 pass, or, as it is even now called in Chinese, tchi-ndn, 

 appears to have been first used at sea by this remarkable 

 nation about the third century of our era, during the Tsin 

 dynasty. 



When the compass became known in Europe is dis- 

 puted ; Gilbert refers its introduction to Marco Polo about 

 1260,^ but it is probable that earlier accounts of it were 

 brought from the East by the Crusaders, an accurate 

 description of it occurring in a poem entitled " La Bible," 

 written by the minstrel Guiot de Provence about the year 

 1 190. A Latin letter ascribed to Peter Adsiger, 1269, 

 preserved among the manuscripts of the university 

 of Leyden, contains the following remark on the declina- 

 tion of the needle :— " Take notice that the magnet, as 

 well as the needle that has been touched by it, does not 

 point exactly to the poles, but that part of it which is 

 reckoned to point to the south declines a little to the 

 west ; and that part which looks towards the north in- 

 clines as much to the east. The exact quantity of this 

 declination I have found, after numerous experiments, to 

 be five degrees." 



The discovery of the dip of the needle is due to Robert 

 Norman, a nautical instrument maker at Wapping, near 

 London, who is described by Gilbert as "a skilful sailor 

 and ingenious artificer." He found that after being 

 touched by a magnet the needle always appeared heavier 

 at its northern end, and making an instrument to deter- 

 mine the greatest angle formed with the horizon, he ob- 

 served the inclination in 1576 to be 71° 50'. 



In the early part of the following century, the variation 

 of the declination was clearly asceitained, and was attri- 

 buted by Bond, a teacher of navigation in London, to the 

 motion of two magnetic poles. 



In the year 1600 was published the celebrated treatise 

 " De Magnete," by Gilbert of Colchester, who was pro- 

 nounced by his great contemporary Galileo, to be " great 

 to a degree that might be envied."' Gilbert regarded our 

 globe as a great magnet, and its centre as the centre of 

 the magnetic motions of the earth. Variation he defines 

 to be the arc intersected between the point where the 

 meridian of the place cuts the horizon, and that point to 



^ "Scientia Neutlcae pyxidulae traducta ^-idetur in Italiam per Paului.i 

 Yenetum, qui circa aniwun M.CCLX. apud Chinas artem pyxidis didicet. 

 De Magnete, p. 4. 



which the magnetic needle looks ; the length of this arc 

 varying with the place of observation. He states that 

 from the coast of Guinea to the Canary Islands, and 

 thence throughout Spain, Gaul, England, Germany, and 

 Norway, the magnetic needle turns towards the east, that 

 on the opposite shores of North America it turns to the 

 west, whilst near to the Azores it points exactly north 

 and south ; nor does he fail to observe that from the 

 north of Brazil, along the coast of South America to the 

 Straits of Magellan, the southern end of the needle points 

 west of the true meridian. He rejects the vulgar opinions 

 of variation depending upon magnetic mountains or 

 magnetic rocks, upon the poles of the zodiac, or the 

 positions of certain fixed stars, but ascribes it in some 

 measure to the configuration of sea and land on the 

 surface of the earth ; chiefly, however, to irregularities in 

 what he terms the magnetic globe and true earth, which 

 he conceives to be more considerable under the con- 

 tinents than below the depths of the ocean. He devotes 

 the fifth book of his work to a full account of the dip of 

 the needle, termed by him declination, with a minute 

 description of the instruments used in its measurement. 



Descartes attributes variation to the irregularities of 

 the earth's surface, considering magnetic attraction 

 strongest wherever iron and loadstone are most abun- 

 dant. To account for the variation of the compass, he 

 asserts that the amount of iron in certain localities 

 constantly changes, partly because man draws it from one 

 place to transport it to another, and partly because new 

 iron is formed in some districts where there was none 

 before, whilst in others old iron becomes corrupted and 

 disappears entirely. To explain his theory of magnetism 

 it is necessary to state briefly the hypothesis he formed 

 respecting matter in general, an hypothesis for which he 

 does not claim absolute truth, but one from which deduc- 

 tions may be made in conformity with experience. The 

 universe he supposed was formed originally of one uniform 

 material, divided into equal parts having equal move- 

 ments. These movements he considered to be twofold, 

 each part revolving on its own axis, and several together 

 revolving round fixed centres, and thus forming distinct 

 vortices. As he deemed no void possible, it followed that 

 these parts being equal, could not at first have been 

 round, but might eventually become so, their angles as 

 they met together being rubbed off and the intervening 

 spaces filled with the dust or debris. 



Descartes considered these two forms of matter as two 

 elements of the universe, the first consisting of the debris 

 and the second of the little spheres. The less agitated 

 parts of the first move chiefly in straight lines from the 

 poles to the centre of each vortex, and in passing through 

 the triangular spaces often left between contiguous balls of 

 the second element, they assume the form of fluted, spiral 

 columns. On the disposition of their channels the force 

 of the magnet principally depends. His third e'ement is 

 formed by the union of the less subtile matter of the first, 

 including the fluted columns. From the centrifugal force 

 of the round parts a central space is left within each 

 vortex, composed purely of matter of the first element ; this 

 Descartes supposed to form an extremely subtile body, 

 such as he conceived the fixed stars to be, and even con- 

 sidered that the earth formerly occupied such a centre till 

 the less subtile matter collecting on its surface changed 

 into that of the third element, and thus fonned clouds and 

 other obscure bodies. As each new layer was added, the 

 force of the containing vortex diminished, and more matter 

 escaped into the surrounding vortices than returned to 

 occupy its place ; finally, the earth, enveloped in its atmo- 

 sphere, descended to the position it now occupies in the 

 powerful vortex around the sun. He divides it into three' 

 regions, the lowest consisting entirely of matter of the first 

 element ; the middle, of an opaque solid body containing 

 passages sufficiently large to admit the fluted columns of 

 the first, but not the spheres of the second element, 



