April, 1003.] 



KNOWLEDGE. 



S3 



matters of fact. But, on the one hand, so far from his 

 having led the way in pointing out these facts, they have 

 been fully considered by every astriinomer who has treated 

 of the Galaxy at all ; and, on the other liaud, he gives to the 

 two facts a greater jsrecisiou than he is warranted in doing. 

 The Gralaxy marks out roughly a great circle in the sky ; it 

 is far too irregular an object for anyone to be able to 

 declare that its axis hes precisely along a great circle. 

 But it is exceedingly convenient to treat it as if it did, and 

 no error can arise from such a convention except when an 

 argument like Dr. Wallace's is seriously based upon it. 

 But with regard to our sun being placed in its centre, the 

 estimate of the distance from us of the mean mass of the 

 Milky Way is roughly three hundred light-years ; a 

 " light-year " being the distance which it takes Ught a year 

 to traverse, nearly six millions of millions of miles. Our 

 distance from Alpha Centauri is a little over four light- 

 years, so that we have no right to say that we are nearer the 

 centre than this twin sun of ours ; nor indeed would it be 

 safe to assert it of any of the stars whose parallax can be 

 considered really well-determined. From the nature of 

 the case, a distance of over thirty light-years involves a 

 parallax too small for really satisfactory handling, and yet 

 makes but an inconsiderable fraction of the diameter of 

 the ring of the Milky Way. 



More than that, our sun is itself travelling at a pace 

 sufficient to bridge the distance to Alpha Centauri in 

 sixty-five thousand years, a mere moment in oiu' world's 

 complete life-history. If this pace has been maintained 

 in a straight line, five million years ago we were deep in 

 the actual stream of the Milky Way ; five million years 

 hence we shall have completely crossed the gulf which it 

 encircles, and again be a member of one of its constituent 

 groups, but on the opposite side. And ten million years 

 are regarded by geologists and biologists as but " a 

 trifle on account " to meet their demands upon the bank 

 of Time. 



The paragraphs on " The Earth as Adapted for Life " 

 are rather for biologists to criticise than for astronomers, 

 but the conclusion of the paper contains several statements 

 which almost lead us to doubt whether we have not been 

 mistaken in supposing the article to be a serious one, and 

 whether it was not intended as an elaborate skit on 

 astronomical cosmogonies. How else can we regard the 

 statement that " we can actually see beyond the outer 

 boundaries" of the material universe, "a limited universe 

 of matter and ether." To see bejoud the lumiuiferous 

 ether reminds one of the inventor who discovered a 

 universal solvent, but did not know of what to make the 

 bottle in which to hold it, so that the precious liquid was all 

 lost. Dr. Wallace compares the stars of the Milky Way to 

 the molecules of a gas, and suggests that "a certain pro- 

 jwrtion of them would continually escape from the attrac- 

 tive power of their ueighbuurs, and wandering into outer 

 space soon become dead and cold and lost for ever to the 

 universe." The process, he not obscurely intimates, will 

 be continued indefinitely, until this earth of ours, from 

 being tlie centre of the universe, will become the centre of 

 a space from which the universe has all departed. He 

 thus offers to our j)oor planet only the cold comfort which 

 Polyphemus tendered to Odysseus, that he should be eaten 

 last. Dr. Wallace further intimates that " at any con- 

 siderable distance beyond the central portion of the universe 

 gravitation would vary in intensity in different directions," 

 and gravely suggests that this variation may possibly bo 

 detected by means of the motions of remote binary stars. 



To sum up, the little in Dr. Wallace's paper which can 

 fairly be said to lie demonstrated fact is anything but 

 new, and that which is new. whether true or not, is as 

 yet but speculation. His conclusions are, at the best. 



premature, and lie in a region which, from its very nature, 

 must probably always be outside the bounds of our 

 knowledge. The Milky Way, which forms the chief 

 ])ortion of the structure of that sidereal system in which 

 we are placed, no doubt is finite in extent, and we can 

 make certain statements with regard to it. But we are 

 not yet in a position to say that all of the objects revealed 

 to us by the telescope are included in that system ; nor 

 can we legitimately assert that what is apparently true of 

 such of it as we can see is absolutely true of the entire 

 material universe. Dr. Wallace's underlying error is, 

 indeed, that he has reasoned from the area which we can 

 embrace with our limited perceptions to the Infinite 

 beyond our mental or intellectual grasp. We are on the 

 earth, and can only reason, only guess, from our earthly 

 experience of the laws, of the materials, of the conditions 

 elsewhere. Our eyes have limited powers of vision, our 

 mental grasp is confined, our days are but few on the 

 earth, and our experience small ; all these boundaries 

 limit for us the universe, however vast it may actually be, 

 and inevitably tend to make our view-point seem to be the 

 centre of our horizon. 



ILtiXtx^. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



THE VISIBILITY OF THE CRESCENT OF 



VENTJS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — With regard to this discussion, perhaps I 

 may be allowed to quote from Sayce, in "Fresh Light 

 from Ancient Monuments," 4th edition, page 120, where 

 he says that " Ahaz had introduced into Judah the study 

 of astronomy, for which the Babylonians were renowned, 

 and had set up a gnomon 0;i' sundial in the palace court 

 (2 Kings, sx, 11). It is possible .that some of the astro- 

 nomical literature of Babylonia, which has been recovered 

 from the cuneiform tablets now in the British Museum, 

 was introduced at the same time, with its multitudinous 

 observations and predictions of eclipses, its notices of the 

 appearance of comets, of the movements of the planets 

 and fixed stars, of the phases of Venus, and even of spots 

 on the sun. It is also possible that the Assyrian calendar 

 and the Assyrian names of the months now fii'st became 

 familiar to the Jews." 



I have also seen it stated that Layard found at Nineveh 

 a rock crystal lens, which, however, in Dr. Brewster's 

 opinion, could only have served as a burning glass. 



4, East Broughton Place, W. T. Mackie. 



Edinburgh. 

 llHh February, 1903. 



[The oldest Babylonian calculations of the moon and 

 the planets are to my knowledge the Inscriptions of the 

 eighth year of Cambyses, which I had published in the 

 "Babylonian Inscriiitions of Cambyses," cp. "Un annuaire 

 astronomique Babylonien traduit en partie en Grec par 

 Ptolemce," par M." J. Oppert, "Journal Asiatique." 1890. 

 That system of calculation had been variously improved by 

 different astronomical schools during the reign of the Aehae- 

 menian Persian kings, the Seleucides and the .\rsacides, 

 down to the first century b.c. These calculations are mainly 

 cvclical : — The Saros period for the Moon ; for Jupiter a 

 period of 83 yeai-s, or of 83 ± 1 2 years ; for Venus a period 

 of ei<'ht rears ; for Mercury a period of 4t> years ; for 

 Saturn 59 yeprs, and for Mars a period of 32 or 47 years. 



