168 



KNOWLEDGE. 



[Septembkh 1, 1898. 



otherwise situated, the inequality reaching a maximum 

 when the same ellipses lie right athwart the visual ray. 



The fundamental part of M. Duner's theory of Y Cygni 

 may be accepted without the smallest hesitation. His 

 research leaves no doubt that the star really consists of 

 two equal components describing, in a period of nearly 

 three days, elliptical orbits situated in the same plane with 

 our line of vision. The application of the spectroscope 

 will give the dimensions of the ellipses in linear measure, 

 whence the mass of the system can be at once deduced. 

 And this unconditionally, because the stars being both 

 bright, their relative velocity — if not their separate absolute 

 velocities— can be determined directly from a photographic 

 plate, while the mass of a bright and dark combination 

 such as Algol can be fixed only on some precarious 

 assumption regarding mean density. 



The striking inequality between the alternate light- 

 periods of Y Cygni is then fully explained by the ellipticity 

 of their orbits; while the view that the changes in tliat 

 inequality are due to a revolution of the line of apsides, 

 although highly plausible, must be regarded as still on its 

 trial, "a comparison during the next few years of the 

 predicted and observed dates of minimum ought to prove 

 decisive as to its truth : and the test will be made more 

 stringent if the spectrographic method can be satisfactordy 

 turned to account. For differences in the velocities of the 

 stars in the line of sight, at the elongations preceding 

 alternate minima, should, on this hypothesis, correspond 

 strictly with the varying time-intervals separatmg the 

 minima, and should disappear with their equalization. 



A further criterion is provided by the duration of the 

 eclipses, which, by the nature of the case, must depend 

 upon the quicker or slower movement of the stars. 

 When, accordingly, owing to the line of apsides being 

 pointed towards the earth, they occur by turns at apastron 

 and periastron, the phases at successive minima must be 

 considerably lengthened out and hurried up. In inter- 

 mediate positions of the line of apsides, moreover, the 

 phases should progress unsymmetrically, the loss of light 

 being retarded at one side of the orbit, the recovery of 

 light at the other. 



These irregularities have not yet been definitely observed. 

 They may, however, eventually serve to measure the 

 eccentricity of the ellipses pursued by the components of 

 Y' Cygni. And the same element is of course involved in 

 the extent of opposite deviations from the mean epoch of 

 what we may call complementary minima. M. Duner has 

 indeed already shown that the eccentricity must exceed 

 0-1, while falling short of 0-2. It would then be small 

 for a telescopic binary, although it appears large for a 

 spectroscopic revolving pair. On the basis of Dr. See's 

 theory of the growth, through an effect of tidal friction, of 

 stellar orbital eccentricity, the stars of Y Cygni can.be 

 inferred to be much more widely separated and materially 

 further advanced in development than the components of 

 Algol. And since the period of both systems is nearly the 

 same, the former ought to prove greatly more massive than 

 the latter, its inferiority in brightness being probably due 

 to its much greater distance from the earth. 



Satisfactory progress has now been made towards in- 

 vestigating the inequalities of two Algol -variables. The 

 results are noteworthy, both for their likeness and for 

 their unlikeness. A third sensibly dark body appears to 

 be included in each combination, and in neither has any 

 evidence been gathered of variation in the circulatory 

 period of the occulting pairs. But the phases of Algol 

 are believed to be afi'ected by a light-equation consequent 

 upon revolution in a wide orbit ; while the irregularities 

 conspicuous in those of Y Cygni depend upon their im- 



symmetrical relations to the elliptical paths of the mutually 

 eclipsing pair. Moreover, the irregularities themselves are 

 subject to change, probably through a revolution of the 

 apsidal line, with the position of which they are intimately 

 connected. Here, then, the presence and power of a dis- 

 turbing body appear to be betrayed. But the unseen 

 third member, if really there, must form a revolving 

 system of a higher order with the close pair. And the 

 plane of the large orbit is unlikely to deviate widely from 

 that of the small ones. If this be so, however, the eclipses 

 of Y Cygni must show the effects of a light-equation 

 similar to that of Algol. They have yet to be detected, and 

 may prove inconsiderable. They are almost necessarily 

 so, if the mass of the bright stars be large as compared 

 with that of the dark body which is reasonably supposed 

 to influence their motions. 



Dr. G. B. Lougstatf, in an interesting paper on " Rural 

 Depopulation," read before the Statistical Society, brings 

 together evidence to show that the flow of population from 

 rural districts to towns has been taking place not only in 

 the United Kingdom, but also in France, Germany, 

 Norway, Austria, Hungary, Switzerland, Italy, Spain, 

 Canada, the United States, Victoria, and New South 

 Wales. The rural depopulation amounts, as a general rule, 

 only to a small fraction of the initial population, while the 

 towns and urban districts have been increasing in popula- 

 tion more rapidly than the whole community. Thus the 

 total loss of population in the Welsh counties varies from 

 0-9 per cent, in Carnarvon to 4-0 per cent, in Flint and 

 16'7 per cent, in Montgomery. Dr. Longstaff shows that 

 the phenomenon of rural depopulation is not due to any 

 faulty system of government, for it is as prevalent under 

 republican institutions as under military despotisms and 

 constitutional monarchies. It does not seem to be due to 

 any system of land tenure, for it is as marked in France 

 as in England, and under the land systems of the United 

 States and Canada, as under those of the continent of 

 Europe. Freeholds and leaseholds are alike affected ; 

 large holdings and small. The peasant proprietor of 

 France, on his much treasured scraps of land, feels the 

 impulse no less than the yeoman of Ontario. Dr. Long- 

 staff' conceives that many causes contribute to the flow of 

 the rural population into towns. Agricultural implements, 

 waggons, tools, which were formerly made in the village 

 smithy, are now more cheaply made in the larger manu- 

 facturing towns. In the last century locomotion was slow, 

 inconvenient, and expensive ; it is now rapid and cheap. 

 Men, by the press, learn where there is a demand for labour, 

 and they easily migrate to it. The dream of the free- 

 trader is fast being realized — we are learning to do in 

 each place that for which each place is most advantageously 

 circumstanced ; and, above all, the towns, with their gas 

 and life, and excitement, possess greater attractions than 

 ever to the countryman. 



Mr. Ernest Hart was emphatic upon the nature of 

 cholera before the British Institute of Public Health. For 

 all practical purposes he asserts cholera is an exclusively 

 water-carriecl disease, and caused only by water poisoned 

 from human sources. " You may eat cholera and drink 

 cholera, but you cannot catch cholera." He traces all 

 cholera to Mecca, which he styles the " nursery of 

 cholera," and he makes a series of suggestions to meet 

 the dangers to the world arising from the Mahommedan 

 pilgrimages thither. 



