298 



KNOWLEDGE. 



August, 1913. 



two hundred to one thousand three hundred million 

 years ; though since the geological positions of the 

 rocks examined are not available the figures do not 

 convey much. 



Considered broadly, the evidence of radioactivity 

 is to assign to the earth an age considerably greater 

 than results from any other method. This either 

 means that the accuracy and, perhaps, the funda- 

 mental assumptions, of the other methods are 

 impugned, or that the radioactivity method itself 

 is founded upon hyphotheses which are not justifi- 

 able. There is even some reason for believing that 

 the latter is not altogether out of question. Thus, 

 though no acceleration in the rate of disintegration 

 of uranium has yet been observed in the laboratory, 

 it is not beyond the pale of possibility that some 

 conditions did prevail in the earlier stages of the 

 earth's history which are not experimentally realis- 

 able to-day, but which then determined an increase 

 in the rate of decay. It would be unwise, however, 

 to pursue speculation further, notwithstanding an 

 innate desire to cast doubt upon a method which, by 

 disturbing the gradual maturing and stabilisation of 

 our views before its appearance, has thrown us back 

 into what Professor Schuster describes as " the 

 primitive state where no opinion is absurd and every 

 hypothesis justifiable." Until further evidence of a 

 more convincing nature is forthcoming, we may 

 provisionally assign an age of not less than one 

 hundred million years to this earth of ours. 



Of course, this estimate takes us back only to the 

 consistentior status. Whether, before that, the sun 

 was still pouring his beneficent rays upon the earth, 

 so that the present estimate of the earth's age is not 

 merely an estimate of the sun's activity, or whether, 

 instead, after solidification of the outer crust, the 

 earth rolled through space as a dead world shrouded 

 in darkness save " where her volcanoes glowed red in 

 the eternal night," cannot be answered decisively. 



From present evidence it appears very likely that 

 the sun shone brightly long before the earth came 

 into existence, and the one hundred million years 

 would not then have to be very materially increased 

 in order to take us back to the very birth of the 

 earth. On the other hand, it may be that the 

 discrepancies between the results derived from a 

 consideration of denudation and of radioactivity 

 are to be interpreted as pointing to long aeons of 

 time during which the earth slept as she rolled 

 through space. 



It would be an interesting diversion to consider 

 whether the whole scheme of evolution outlined by 

 Darwin and others could possibly be worked out 

 within such a comparatively short period as we have 

 accepted. Are one hundred million years time 

 enough for that evolution of the organisms which, 

 starting with a shapeless mass of protoplasm, deter- 

 mined its development through countless structures 

 of increasing complexity, until finally man himself 

 was reached ? At this stage only the briefest of 

 answers can be given. Eminent biologists declare 

 that the period is ample and the ordinary doubts 

 of the layman may therefore be laid aside. 



The period of one hundred million years, then, 

 which have been conceded to the earth, is from most 

 points of view a reasonable estimate. It cannot be 

 expected, however, that finality is attained. This 

 estimate, like its predecessors, may have to be dis- 

 carded if further investigation requires it, though 

 as yet there seems no valid reason for abandoning it. 

 Nevertheless, our ideas on the subject must be 

 pliant and versatile, our minds ready to recognise the 

 meaning of revolutionarydiscovery. The best attitude, 

 perhaps, is that which regards the one hundred 

 million years as the earth's minimum age, and is 

 prepared, if occasion demands, to consider im- 

 partially an estimate of greater — it may be much 

 greater — magnitude. 



CORRESPONDENCE. 



SPURIOUS DIAMETERS OF STARS. 



To the Editors of " Knowledge." 



Sirs, — Reading recently the papers of Sir W. Herschel, 

 published last year by the Royal and Royal Astronomical 

 Societies, 1 was surprised to see his statement that increase 

 of magnifying power with the same aperture of object glass or 

 mirror causes a decrease in the size of the spurious discs of 

 stars. I was fully under the impression from reading modern 

 works that the reverse was the case. I know that increase of 

 aperture means decrease of diameter of spurious discs, but 

 the other was news to me. Herschel's words are : " By many 

 observations their spurious diameters are lessened by increasing 

 the power, and increase when the power is lowered." In 

 Vol. II., page 303, again he says that the diameter of spurious 

 disc of <t Aurigaewith power 227 was 2"- 5, and that of a Tauri 

 with power 460 was 1" 46'", and with 932 it was 1" 12'", while 

 that of o. Lyrae with power 6450 was 0"-3553. I should be 

 glad if some of your astronomical readers would reconcile 

 these extracts with present-day ideas. 



THEODORE B. BLATHWAYT. 

 Cape Town. 



STELLAR DISTANCE UNITS. 



To the Editors of " Knowledge." 



Sirs, — I notice some comments in your July issue regarding 

 names for the planetary and stellar distance units. In my 

 opinion the names suggested are quite impossible — ugly of 

 look and ugly of speech. The names for any unit of measure 

 should assuredly be short and easy to pronounce. Dr. 

 Crommelin's suggestion of " astron " for one is quite 

 acceptable. But why bother about the inner significance of 

 the words at all ? An arbitrary adoption would do quite as 

 well. Why not follow the course adopted in electrical measure- 

 ments ? Here we have " Watts " and " Amperes " ; in 

 Astronomy we might well have " Keplers " and " Newtons." 

 Let the former stand for the shorter or earth-sun unit, and 

 the latter for the longer " light-year." In order to destroy the 

 personal look about the terms they might well be abbreviated 

 to " keps " and " newts." As alternatives the names of the 

 men who (a) first determined the earth-sun distance and 

 (6) who first used the " light-year " of a unit of measure might 

 be used. 



A. J. H. 



Aberystwyth. 



