March 1, 1898.] 



KNOWLEDGE 



57 



position of the cloud belt ia not coincident with, but lies 

 a few degrees north of, the Equator, and, as has been 

 mentioned before, it oscillates over a zone more than 

 three times its width in a year. 



The cause of this annual oscillation of the belt is that 

 during the winter of each hemisphere the earth's surface 

 and atmosphere becomes much colder than it is in the 

 other hemisphere, and consequently the atmospheric 

 volume is considerably less, and there is a pressure 

 gradient above by which the air of the higher strata flows 

 from the warmer hemisphere to the colder, L^lving rise to 

 a counter flow of air below, from the colder to the warmer 

 hemisphere. The conse<juence is that the stronger system at 

 this season encroaches somewhat upon the territory of the 

 other, causing the middle of the equatorial calm and rain 

 belt (which is the dividing line between the two systems) 

 to be displaced from its mean position. There is there- 

 fore an annual oscillation of the calm and cloud belts, such 

 that the most northerly position is in midsummer and 

 the reverse in midwinter of the northern hemisphere, or, 

 in other words, as the sailors say, " The cloud belt follows 

 the sun." 



Wet and dry seasons are thus produced in districts 

 which fall within the range of oscillation of the rain-cloud 

 belt, where it is well defined and not afl'ected by abnormal 

 disturbances, but is somewhat as in the ocean and on 

 level countries. Such is the case with the Orinoco and 

 great Amazonian basin. Humboldt says : " As in the very 

 North the animals become torpid with cold, so here, under 

 the influence of the parching drought, the crocodile and 

 the boa become motionless and fall asleep, deeply buried 

 in the mud. At length, after the drought, the welcome 

 season of rain arrives, and then how suddenly is the 

 scene changed !" In ponds from which, but a week before, 

 the wind blew clouds of sandy dust, tlie reanimated fish 

 may be seen swimming about, deciduous trees become 

 verdant, and scarcely a week elapses before the plants 

 are covered with the larvre of butterflies, the forest is 

 murmuring with the hum of insects, and the air is har- 

 monious with the voice cf birds. 



The rain at these periods excites the astonishment of a 

 European. It descends in almost continuous streams, so 

 close and dense that the level ground, unable to absorb it 

 sufficiently fast, is covered with one uniform sheet of 

 water ; and down the sides of declivities it rushes in a 

 volume that wears channels in the surface. In the towns 

 many of the houses are built on raised causeways, so that 

 the roadway is able to act as a river bed during these 

 tropical downpours. Perhaps in some subsequent number 

 of KNowLEDtiE we may reproduce a street scene under 

 these conditions, with half-submerged carts, floating 

 barrels, and a rushing stream carrying all before it. 



The effects of these alternating seasons can readily be 

 imagined, and to obviate this — or, rather, to have a supply 

 of water for irrigation and other purposes during the dry 

 season — some of the provinces in South America thus 

 affected are constructing large reservoirs; «.'/., in the 

 province of Cearfi they are damming up the end of a large 

 valley at Quixada, thus forming a large artificial lake in 

 the wet season, which will be distributed gradually over 

 the parched land throughout the stcn, or dry season. 



Space will not here permit me more than to point out 

 that the most conspicuous features of the members of the 

 solar system larger than the earth are their dark belts, 

 whilst in the case of those planets smaller than ours these 

 bands are scarcely traceable. ^Yhether in this respect the 

 earth marks a different condition of things existing on the 

 giant planets to that on the smaller ones (which constitute 

 less than one-hundredth of the planetary mass) is only ccn- 



jacture ; but one would expect that the appearance of the 

 cloud belts on the earth, as seen from some other planet, 

 would, on account of the great reflecting power of clouds 

 and mists, be not in the form of dark but of bands brighter 

 than the general surface. 



Seeing, tjien, that all the large planets are so striated, 

 should we not expect the central and largest body of our 

 system — the sun — to exhibit these characteristics :' And, 

 indeed, it does, for are there not two zones of maximum 

 " spotted area " on either side of the equator, along which 

 concentrated portions of cloud belts move across the sun .' 

 Hence, if we imagine these gregations of umbrae to be 

 disseminated as penumbraj along the spot zones, we should 

 have presented to us a phenomenon closely resembling 

 that of the " cloud belts " of the larger planets. From 

 these few considerations it may be gathei-ed that " cloud 

 belts " play an important part in the cosmogony of the 

 solar system. 



A NEW THEORY OF THE MILKY WAY. 



By C. Easton. 



TOWARDS the end of a previous article on " Richard 

 A. Proctor's Theory of the Universe " I suggested 

 that, if we confine ourselves to those facts known 

 to-day with suHlcient certainty, we can only 

 attirm, with respect to the structure of the Milky 

 Way, that we there see marked irregularity of details, and 

 some traces of a regularity at least partial in the principal 

 features of the phenomenon. Before venturing to go a 

 little further 1 must sum up the facts and considerations 

 on which this opinion is founded. Want of space compels 

 me in an article of this kind to direct in some cases the 

 reader to the sources of information. 



Now that photographs of the Milky Way are so wide- 

 spread, there is no need to insist on the great irregularity 

 that we observe (in projection) in the distribution of 

 the stars, so long as we confine ourselves to a relatively 

 small portion of the galactic zone. It follows, moreover, 

 from the evidence of all the results recently obtained in 

 the study of the galactic phenomenon, that the manner 

 of distribution of stars //( spur, varies, even between limits 

 that are relatively large : in this part of space the stars 

 are widely scattered, in this other part they are gathered 

 together into veritable stellar agglomerations. But, a 

 priori, that does not by any means exclude a fairly marked 

 regularity of the Milky Way, tdhn as a uhoh-. Suppose 

 that the Milky Way has the form of the well-known elliptic 

 nebula in Lyra; unless we admit that its borders are defined 

 by this figure, and a perfect regularity of distribution 

 prevails inside this ellipse, we should see — we being situated 

 near the central portion, relatively void of stars — a " Milky 

 Way " enclosing the heavens in a fashion similar to the 

 one we see in reality. 



Besides, this theory of a Milky Way roughly annular or 

 elliptical recommends itself by its simplicity, and appears 

 to be the one most widely spread at the present day. 



Nevertheless, if one studies the phenomenon closely, 

 there are, in this theory of a galactic ring, several points 

 that require explanation. 



We see, it is true, the Milky Way forming a great circle 

 round the heavens, but, even apart from the irregularity of 

 detail, the galactic light is very unequally distributed on 

 the circumference of this ring. The Aquila part is much 

 more brilliant than the Monoceros part. This is not 

 only seen in the studies made with the naked eye, 

 but also in the star gauges ; and it is the case for the 

 southern hemisphere as well as for the northern. As for 



