Nov. 18, 1881.] 



KNOWLEDGE 



49 



the course of the light to he illuminated. The beam is 



simply that long array of material particles which the light 



illuminates while leaving the particles in neighbouring 



I'uce in darknes.';. So understood, sucli a beam is seen to 



■ utterly unlike a comet's tail ; for, in the first place, we 



now of no matter beliiiid the head to be illuminated ; and, 



n the second, we know that light is falling on the regions 



il around the apparent array of illuminated particles, so 



tliat these surrounding regions sliould be as brightly lit up, 



which is not the case. 



If any further doubt could remain as to this theory, it 

 would be removed by — first, the circumstance that the 

 tail of a comet is generally curved ; and, secondly, the exist- 

 ence of several tails extending from the head of one and 

 the same comet. 



Professor Tyndall started a theory based on physical 

 experiments, and otherwise in better accordance with 

 scientific possibilities. Having found that certain gases, 

 even in an exceedingly attenuated form, form a luminous 

 cloud under the action of the electric light, he suggested 



through which the comet had passed much earlier. Such 

 luminous trails as were formed more quickly would 

 account, he considered, for the straighter tails. He 

 overlooked, I think, the circumstance, that the shape 

 of the luminous cloud - trail would not in reality 

 depend at all upon the length of time which the 

 cloud might take in becoming \isible. Light would pass 

 with the same velocity through the ditlerent kinds of 

 tenuous gas, and whether the cloud became visilile at once 

 along the space thus passed through, or did not become 

 visible for several seconds, or minutes, or even hours, it 

 would become visible at the farther end of its course only 

 just so long after it had become visible at the nearer end, 

 as light had taken in traversing the length of cloud so 

 formed. This interval of time would be the same for the 

 quickly-appearing as for the slowly -appearing luminous 

 cloud, and there Mould, therefore, be no diflerence between 

 their forms. It would be necessary to account in this way 

 for the curvature of the larger tail in the figure, as com- 

 pared with the straightness of the smaller tails, that the 

 curved tail should have been more slowly 

 extended from the head ; whereas the 

 theory gives the same rate of extension 

 for both, namely, the rate at which light 

 travels. 



We seem almost forced, by the phe- 

 nomena of such a comet as Donati's, 

 to the theory of the actual repulsion of 

 matter from the head of the comet into 

 the tails — matter repelled most swiftly 

 forming the straighter tails, while matter 

 repelled more slowly, and seemingly in 

 greater abundance, forms the great curved 

 tail. 



We shall proceed to consider in our 

 next paper the e\4dence which seems to 

 show that, strange though this theory 

 of material repulsion may be, it is in 

 point of fact the only admissible theory. 

 If this shall be established, we shall 

 have to admit the existence of a re- 

 pulsive force, whose action on the 

 grosser material of planetary bodies is 

 insensible. 



Fig. -■ — Donati's Cun\et, September 2(3, 1S5S, 



that a comet's tail may be a luminous cloud of this sort, 

 formed in the a?ther of space by those rays of sunlight 

 which have passed through the comet's head. The rays 

 which, without passing through the head, fall on the aether 

 f space, would not call into existence this visible cloud, 

 ■ cause their heating action would destroy what their 

 ' lii'mical or actinic action b;/ itgelf wou\d produce. And 

 a> fast as, by the comet's motion, the cloud formed behind 

 the head came under full solar action, it would be 

 <lestroyed. So the tail would always be behind the head. 



It appeared to Professor Tyndall that the curvature of 

 a comet's tail, or the existence of more tails than one, as 

 in Donati's Comet (Figs. 1 and 2), was not inconsistent 

 with this interpretation. For he noticed that, according 

 to the gas dealt with, the luminous cloud would take a 

 longer or shorter time in becoming visible. And he sug- 

 ijested that when the cloud formed slowly, the tail would 

 be curved, the part near the head being behind the position 

 which the head had recently passed through, while the 

 part near the end of the tail would be behind the regions 



" All knowledge, and wisdom which ia tho 

 seed of knowledge, is an impression of pleasuro 

 in itself." — Bacon. 



TiTE PopCLATioN OF THE Globe. — According to MM. Bohm A 

 Wagner's Bevolkerung der Erde, Europe has nsw a population of 

 315,939,000 inhabitants, Asia 831,707,000, Africa 205,679,000. 

 America 95,405,000, Australia and Polynesia 431,000, tlio Polar 

 regions 82,000, giving a total of 1,455,923,000, being an increase of 

 10,778,000, according to the latest known censuses. At the end of 

 1877 Germanv had a population of 43,943,000, Austria and Hungary 

 (1879) of 38,000,000, France (1S7C) of 36,900,000, 'IMrkey in 

 Europe of 8,l?60,000, Russia of 87,900,000. In Asia, China possesses 

 434,900,000 inhabitants, Hong Kong 130,144, Japan 34,300,000, 

 according to the census of 1878. The British possessions in India 

 number 2U),200,000 people (an estimate made before tho census of 

 this vcar), the French possessions 280,000, Cochin China 1,600,000, 

 the East Indian Islands 34,800,000, the islands of tho South Sea 

 878,000. Tlie area of Africa is estimated at 29,383,000 square 

 kilometres, divided as follows : — Forests and cultivated land 

 6,300,000, savannahs 6,235,000, steppes 4,200,000, deserts 10,600,000. 

 The inhabitants of British North America number 3,800,000, of 

 the United States 50,000,000, of Mexico 9,485,000, and of Brazil 

 11,100,000. The Polar regions extend round the Arctic Circle 

 with an area of 3,859,000 square kilometres, and the Antarctic 

 regions about 600,000. The population of the former is small, 

 with the exception of Iceland, which has 72,000, and Greenland 

 10,000.— Times. 



