136 



KNOW 1.1 : DC ".!•:. 



AiMdi. 1912. 



the surface of (lie Kroiind is intensely heated clurinK the middle 

 hours of the day, the atmosphere as a whole beinK more or 

 less at rest. This unstable state of a healed lower layer (jives 

 rise to niunerous minute whirlwinds, locally known as " Dust 

 Devils." varying in size from a few feet to perhaps a himdred 

 y.irds in diameter, and In duration from about ten seconds to 

 as many minutes or longer. Their .ippcar.uice on first forminp 

 is that of a wliirlitiK colmun of dust, mixed with leaves, straws, 

 and so on. which, after continuing for some time. K'adually 

 lose their sharp outline and appear to be more or less 

 dissipated, but the consequent uprush of air can be traced for 

 long afterwards by the cloud of dead leaves hovering far above 

 the surface of the earth. The action is, of course, similar to 

 the formation of waterspouts at sea. 



Soaring birds, such as Vultures, Kites and Storks, are 

 common ; indeed, if is of very rare occurrence that the sky 

 can be searched for two mimites without discovering at least 

 one Vulture. These, however, are usually at such an elevation 

 that they are useless for the present observations. 



.•\ low-flying bird is required, and one is chosen flying across 

 country, perhaps two or three hundred feet above the ground 

 and straight as the proverbial crow. 



Observed with good glasses, the wings and tail are seen to 

 be practically motionless as a whole, but with a constant 

 adjustment of the plane angles. 



The loss of elevation as it proceeds is clearly noticeable, 

 and if it cannot find an external source of energy it must flap or 

 come to the ground. It need not proceed far, however, before 

 encountering a dust devil uprush, on meeting which the 

 straight flight is changed to a circular one, on which it mounts, 

 still without flapping, to a limit either at which the uprush of 

 air is no longer strong enough, if the dust devil be small, or at 

 which the Vulture chooses to start off again on its straight 

 flight. 



It should be understood that the birds do not circle on a 

 dust devil in active operation, as the motion is then extremely 

 violent, but after breaking, as noted above, the vertical current 

 becomes suitable for the birds' purpose. Numerous dead 

 leaves may occasionally be noticed hovering amongst the 

 circling birds. 



At times the dust devil movement does not begin by the 

 intense motion as previously stated, but a gentle uprush 

 vortex motion sets in. which gradually drifts across the 

 country. These are frequently occupied by twenty or thirty 

 Vultures, which continue to circle as long as visible. 1 he 

 upward current is detected by the radially inward draught 

 felt when the movement passes near the observer. 



In the early morning or late afternoon, when the dust devils 

 do not occur, the Vultures invariably proceed by alternate 

 flapping and soaring flight. 



The extent of the bearing power of these upward movements 

 of the air is evident from the phenomena of dead leaves drop- 

 ping out of the still evening sky, as late as can be observed 

 up to and after sunset, the last dust devil having broken up 

 some two or three hours previously. 



Melbourne. ' S. TrLLOCII. 



THI£ SUN'S PATH IN SPACE. 

 To the Editors o/ "Knowledge." 



Sirs, — The direction of the Sun's path in Space has been 

 calculated with some degree of certainty by observations ot 

 " Star Drift." Could not its path be ascertained with a 

 greater amount of accuracy by calculations of the diflering 

 positions of the centre of gravity of the Solar System, which 

 must change to some very slight but still appreciable degree, 

 with the relative changes in the positions of the planets ? 



If, for instance, when the larger planets are in conjunction, 

 or in opposition, or at their perihelion, or at their aphelion, a 

 sufficient number of calculations were made of the consequent 

 varying positions of the centre of gravity of the Solar System 

 in Space, some definite resultant might be found, shewing the 

 direction in which it is travelling. 



G. R. GIBBS. 



BOURNK.MOUTH. 



ASTRONOMY. 

 To the lidititra of " Knowli'.dgi;." 



Sirs, — I believe I am right in saying that while some 

 account for the origin of the moon by .saying that it at one 

 time formed part of the earth, — an offshoot from the earth, 

 and so on, — others explain it as being an old planet. 



The reasoning for this latter explanation seems to be that a 

 body goes through the following processes: First, the sun; 

 second, the sun cooled and become a planet, fit for the abode 

 of living creatures ; third, the planet cooled and become a 

 moon, a dead world. 



To take as an example the sun and the earth. The 

 supposition would be that the earth was previously a sun, but 

 had now cooled and become a planet. As the sun gets 

 cooler, so, naturally, will the earth, until the latter eventually 

 reaches the phase of a moon. 



It seems to be rather on these lines that Mr. Proctor bases 

 his theory of Jupiter's moons being inhabited ividc " Expanse 

 of Heaven"), Jupiter being put forward as a sort of sun, with 

 his moons really planets (Jupiter in this way might be regarded 

 as illustrating the half-way stage in the evolution of a sun to 

 a planet fit for living creatures). 



At a lecture I attended not long ago the lecturer happened 

 to mention the theory of the moon being an offshoot from the 

 earth, and at the close of the lecture I mentioned the other 

 theory (just described).. ind asked him which of the two was the 

 more universally accepted. He seemed never to have heard 

 of the second theory at all. 



Can you tell me which of these two theories finds most 

 support from astronomers of to-day ? 



"INQUIRER." 



Catford, S.E. 



ON THE ROTATION OF VEXL'S. 

 To the Editors of " Knowledge." 



Sirs, — I write this letter in answer to the doubts expressed 

 by your correspondent. Mr. Harrison, in the hope that 

 my reply may be of general interest to your readers. 



Mr. Harrison first contrasts the conclusions of Belopolsky 

 and Slipher as to their spectroscopic evidence. 



It is to me amply evident that in this matter Slipher's con- 

 clusions are the stronger, founded as they are on photographs 

 taken on a more favourable opportimity and with a more 

 powerful spectroscope than that used by Belopolsky. It is a 

 striking fact that Slipher's results indicate no rotation at all — 

 for the final mean radial velocities deduced are conflicting in 

 sign, and either pr.actic.illy equal to or less than the probable 

 errors of observation. Tlie evidence is therefore negative. 



Referring to the Lowell Observatory Bulletins Nos. 3 and 

 4. I find the following statements: — 



" \ rotation period of twenty-four hours svould in the case of 

 Venus imply an inclination of 15' to the normal in the planetary 

 lines. The probable errors of determination of the inclination 

 of the lines are. in the case of Venus, somewhat less than 1' in 

 the final result." 



" When Mars' spectrum w;is measured to test the method — 

 an inclination ov b''b was found — indicating a period of 

 revolution within an hour of the known one." (These quotations 

 are not verbatim but condensed.) 



It is then apparent that had \'enus any such period the 

 latter could be determined with more than double the accuracy 

 possible in the case of Mars. It is thus clear that Venus' 

 period is much longer than tliat of the Earth. 



Now for positive evidence as to how umch longer the period 

 may be, visual observations are our only resource ; and thanks 

 to the purifying influence of modern research, that source is 

 now reliable. I have myself been able to corroborate the 

 markings seen by Schiap.arelli, Lowell and others, and I am 

 convinced not only of their reality on the planet, but of their 

 fixity with regard to the terminator. They indicate without 

 question that the planet turns always the same face to the 

 Sun. 



The observations of Schroeter and De Vico, quoted by Mr. 

 Harrison, were interpreted by them to indicate a period about 



