28o 



NA TURE 



[January i8, 1894 



Clarke in 1841, thous^h its existence would appear to have been i 

 known as early as 1823. In 1844, without being aware of these 

 discoveries, Sir Roderick Murchison pointed out the similarity 

 of the rock structure of the eastern Cordillera of Australia to 

 that of the Ural Mountains, and predicted the occurrence of 

 gold. Subsequent events afforded a proof that geology, like the 

 more exact sciences, is capable of advancing philosophical in- 

 ductions to very important results. But the precious metal was 

 not commercially di-covered, so to speak, till 185 1, by Har- 

 greaves, who had spent some of his earlier years as a stock- 

 raiser in Eastern Australia ; in 1849 he was gold mining in 

 Cal fornia, and his experiences there gained convinced him of 

 the similarity in structure of the auriferous rocks of California 

 and cer'?.in districts in New South Wales. He revisited New 

 South Wales early in 1S51, to put to the test his geological 

 instinct and the accuracy of his observations ; in this he suc- 

 ceeded, and ultimately, under Government direction, the gold- 

 field of Ophir in the district of Bathurst was declared open. 

 He was awarded ^10,000 for his discovery, and in 1876 a pen- 

 sion was granted him. He died in 1 891, at the age of 75 years. 

 The practical discovery of gold proved a source of an enormous 

 amount of wealth to New South Wales, and was soon followed 

 in the same year by the di.-covery of much richer goldfieldsin 

 A^ictoria, which had just then been separated into an indepen- 

 dent colony, and thus added a powerful factor to the economic 

 and scientific advancement of the continent. The consequent 

 stimulus to a higher intellectual culture resulted in the founda- 

 tion of the Universities of Sydney and Melbourne, and the 

 establishment of systematically organised geological surveys. 

 By the concurrence of the memorable events just alluded to, the 

 history of geological progress enters a new period. Up to 1854 

 our exact knowledge of the sedimentary deposits, as derived 

 from the organic remains, was confined to the Carboniferous, to a 

 late Tertiary (represented by the Diprotodon period), and a more 

 recent ^Eolian formation ; no distinct identification to prove 

 the existence of Upper Silurian, Devonian, or Eocene had been 

 forihcoiaing, though it was implied, whilst the only evidence of 

 a Mesozoic epoch was a single imperfect example of a Belemnite. 

 Restricted means of communication in a vast extent of country 

 was the main cause which retarded advancement in geological 

 investigation ; with increasing population this barrier is gradu- 

 ally being removed. Expansion of our pastoral occupation, and 

 the opening out of new trade routes bring new fields within the 

 horizon of geological vision. It is, therefore, not a matter for 

 surprise that in the next decade great and rapid advances were 

 made in establishing a comparison on paloeontological grounds 

 with corresponding geological systems of Europe. The history 

 of geological progress in the second half-century is mainly that 

 of the geological surveys, and the chronological treatment of my 

 subject must be abandoned at this stage. 



It is a general impression that Australia is a very old con- 

 tinent ; undoubtedly it is, because it presents an equal range 

 of the geological record as other continental masses. But this 

 impression is based on illogical deduction, derived solely from 

 the fact that certain characteristic types of the Jurassic fauna of 

 the northern hemisphere still linger in the Australian area, 

 *uch as Trigonia, Ceratodus, and Marsupials among animals, 

 Cycads and certain Conifers am ^ng plants. But the physio- 

 graphic aspects of Australia have not always been absolutely 

 continental. Since Upper Devonian times there have always 

 been land-surfaces, at any rate in Eastern Australia, where 

 partial interruption to an absolute continuity (and the area locally 

 affected is not relatively greal) was frequent during the deposi- 

 tion of the Carboniferous series, which is, however, in a large 

 measure littoral. It may safely be asserted that Australia, 

 <:er!ainly so far back as the deposition of the extensive marine 

 Cretaceous occupying the low level tracts of the interior, pre- 

 served the aspect of a vast archipelago. At the close of that 

 epoch the various insular masses became welded together, so 

 that the antiquity of Australia as a whole is only post-Cretaceous. 

 In early Eocene or late Cretaceous times, the flora was of a 

 cosmopolitan type, consisting of an admixture of generic forms, 

 some of which are now proper to the temperate and sub-tempe- 

 Tate parts of the northern hemisphere, such as oaks, birch, alder, 

 <S:c., and others exclusively Au-tralian, such as eucalypti, bank- 

 sias, Araucarias, &c. The differentiation of the Australian flora 

 has therefore been brought about during the post- Eocene times. 

 The antiquity of Australia, as inferred from its almost exclusive 

 marsupial types, is erroneous, because there is every reason to 

 <loubt the correctness of the s;atement thereby implied that 



marsupials originated in Australia. Despite the recurrences of 

 land surfaces from late Palaeozoic times to the present day — and 

 it is not improbable that some of them may have been per- 

 manent throughout, or for a greater part of that long interval — 

 yet no marsupials as old as those of Europe and North America 

 have yet been found ; neither its coaly strata nor its ancient 

 lake basins have yielded any of the higher types of flitviatile or 

 terrestrial vertebrates. Indeed, the only instance of a fossil 

 representative of the Marsupialia older than Pliocene in the 

 AuNtralian area is that of a diprotodontoid in the Eocene beds 

 at Table Cape, Tasmania ; whereas we must look for a poly- 

 protodontoici as the early ancestor of the class. Recent 

 researches point io South America as the area from which the 

 Australian marsupial fauna has probably been derived, which 

 possesses in the Eocene marsupial fauna close alliances with 

 certain existing polyprotodontoid types in Australia. 



A DYNAMICAL THEORY OF THE ELECTRIC 

 AND LUMINIFEROUS MEDIUM} 



II. 



T^HE next stage in this mode of elucidation of electrical phe- 

 nomena is to suppose, once the current is started in ournon- 

 dissipative circuit, that both the condensers are instantaneously 

 removed, and replaced by continuity of the wire. \Ve are now 

 left with a current circulating round a complete perfectly con- 

 ducting channel, which in the absence of vi-;cous forces will 

 flow round permanently. The expression for the kinetic 

 energy in the field is eajily transformed from a volume integral 

 of the magnetic force, which is represented by the velocity of 



the medium — (|, tj, ^), to an integral involving the current 

 dt 



— (/'<?> ^')' which is in the present case a line integral round 



dt 



the electric circuit. The result is Franz Neumann's celebrated 



formula for the electromagnetic energy of a linear electric 



current, 



T = \C- \ ^ icos idsds; 



or we may take the case of several linear circuits in the field, 

 and obtain the formula 



T = ^Si- I f ^ cos € ds ds + 2i|i2 I i '' ^ cos e ds^ ds^, 



which is sufficiently general to cover the whole ground of 

 electro-dynamics. 



Our result is in fact that a linear current is a vortex ring in 

 the fluid selher, that electric current is represented by vorticily 

 in the medium, and magnetic force by the velocity of the 

 medium. The current being carried by a perfect conductor, the 

 corresponding vortex is (as yet) without a core, i.e. it circulates 

 round a vacuous space. The strength of a vortex ring is, how- 

 ever, permanently constant ; therefore, owing to the mechanical 

 connections and continuity of the medium, a current flowing 

 round a complete perfectly conducting circuit would be un- 

 affected in value by electric forces induced in the circuit, and 

 would remain constant throughout all time. Ordinary electric 

 currents must therefore be held to flow in incomplete conduct- 

 ing circuits, and to be completed either by convection across an 

 electrolyte, or by electric displacement or discharge across the 

 intervals between the molecules, after the manner of the illus- 

 tration given above. 



Now we are here driven upon Ampere's theory of magnetism. 

 Each vortex-atom in the medium is a permanent non-distipative 

 electric current of this kind, and we are in a position to appre- 

 ciate the importance which Faraday attached to his discovery 

 that all matter is magnetic. Indeed, on consideration, no other 

 view than this seems tenable ; for we can hardly suppose that 

 so prominent a quality of iron as its magnetism completely dis- 

 appears above the temperature of recalescence, to reappear 

 again immediately the iron comes below that temperature ; 

 much the more reasonable view is that the molecular rearrange- 

 ment that takes place at that temperature simply masks the 

 permanent magnetic quality. In all substances other than the 



1 A paper read before the Royal Society on December 7. 1S93, by Dr. 

 Joseph Larmor, F.R S., Fellow of St. John's College, Cambridge. (Con- 

 'inucd from p. 262 ) 



NO. I 264, VOL. 49] 



