NATURE 



99 



THURSDAY, JULY 28, 1910. 



FhAi^ETOhOGV. 

 The Evolution of Worlds. By Prof. Percival Lowell. 

 Pp. xiii+262. (New York: The Macmillan Com- 

 pany; London: Macmillan and Co., Ltd., 1909.) 

 Price 105. 6d. net. 



ONLY by the minutest study of that which is can 

 finite man of a finite epoch hope to draw even 

 the roughest sketch of the antecedent, or form the 

 vaguest speculations as to the ultimate ; the truth of 

 the sketch and the soundness of the speculation in- 

 crease or decrease in proportion to the knowledge 

 acquired by the observer. Therefore we welcome a 

 volume in which Prof. Lowell sets out a scheme of 

 evolution which embodies a proper sequence of con- 

 gruities and is based on the results of years of careful 

 observation. 



Planetology is defined as the astronomy 



"which deals with the evolution of worlds. It treats 

 of what is general and cosmic in that evolution, as 

 geology treats of what is terrestrial and specific in 

 the history of one member of the class, our own earth." 



On these lines Prof. Lowell develops a scheme of 

 evolution wherein the observational evidence wrested 

 from what is, leads to the speculations as to what 

 was and what will be. 



The matter originally formed the subject of a 

 university course of lectures, but a larger public de- 

 manded it, and hence the present volume. 



There are seven chapters in which the probable life- 

 histories of a planet and of a planetary system are 

 vividly portrayed, and a perusal of the first, " The 

 Birth of a Solar System," impresses very forcibly the 

 idea how finite is man, how infinite matter and 

 time. The inception of a new system is but the 

 death-knell of its predecessor. 



The problem of the birth was solved when Good- 

 ricke, the deaf mute of York, divined the cause of 

 Algol's demoniacal winkings — a dark sun; on this is 

 based the whole story. The descriptions of a number 

 of Novse support the statement that cataclysms are 

 not unknown, and, by several lines of reasoning, the 

 production of spiral nebulae — the matrices of new 

 worlds — is shown to be the result. The planetesimal 

 hypothesis is probably now too well known to call 

 for further elaboration here of Prof. Lowell's storv 

 of the birth, but before proceeding to the second chapter 

 he draws a wonderfully awful picture of the conditions 

 which would precede and attend the incursion of a 

 second dark body, which, by its powerful perturbativo 

 action, would once more produce chaos from our 

 present orderly system. 



In the second chapter we are given descriptions of 

 the existing proofs of this cataclysmic birth. The 

 existence of myriads of meteors in interplanetary 

 space, their common motions, and their likeness to 

 terrestrial material, are cited as evidence of a common 

 origin in one rotating mass. The mathematics for 

 this — mathematics being "a precise reasoning appli • 

 usually to the discovery that a pet theory will not 

 work," but more fruitful in this instance — are, with 

 NO. 2126, VOL. 84] 



other more or less profound items, relegated to a 

 series of notes at the end of the book. The physical 

 evidence for this common origin reaches a climax with 

 a statement of the varying densities of the planets ; 

 those torn off earlier by the tidal stress are less dense 

 because they formed the upper layers of the parent 

 dark body. Thus the hydrogen envelopes of Neptune 

 and Uranus, demonstrated by Dr. Slipher's spectro- 

 grams, are evidences of primogeniture. 



In the chapter on inner planets, many old "facts" 

 are reorganised, on the evidence of the careful observa- 

 tions made at Flagstaff, and, as they have not yet 

 appeared in their new forms in the ordinary text- 

 books, their statement here is a useful addition to 

 astronomical literature. Among others, the correc- 

 tion to the diameter of Mercury, so dramatically con- 

 firmed, independently, by Newcomb, is an example of 

 the value of such careful attention to observing con- 

 ditions and observations as has been paid at Flag- 

 staff. Students of astronomy will also experience a 

 feeling of relief that the rotation periods of Mercury 

 and Venus now appear to be placed beyond question. 

 The following description of Mercury is so character- 

 istic of Prof. Lowell's graphic style as to be worth 

 quoting : — 



"Two antipodal hemispheres divide the planet, the 

 one of which frizzles under eternal sun, the other 

 freezes amid everlasting night." 



The persistent observations at Flagstaff also dis- 

 pelled the idea of a cloud-covered Venus, replacing 

 it by a diaphanously-clad body on which the strong 

 winds sweep up enough dust to account for the 

 planet's high albedo. The radial streaks depicted on 

 the accompanying drawing of the planet are supposed 

 to be "runs" produced by winds which have con- 

 sistent, and persistent, directions. An interesting sug- 

 gestion is that the " earth-light " sometimes seen on 

 Venus 's dark limb is but a darkened vision of the 

 ice which for countless seons has been hoarded up 

 on that side of the planet which never sees the sun. 

 " Monotony eternalised " is Prof. Lowell's apt descrip- 

 tion of the Mother of Loves. 



It is with something like a shock that the reader 

 finds but about a page and a half devoted to Mars ; 

 but the author opines that he has already treated 

 adequately of the subject elsewhere. Phobos and 

 Deimos are dealt with, however, and, according to 

 Prof. Lowell's observations, they are larger than 

 hitherto supposed ; he gives 36 miles and 10 miles 

 as their probable, respective, diameters. The dilata- 

 tion of "Fear" and "Panic" by observations made at 

 the Flagstaff Observatory may not appear to some 

 conservative tenants of "facts" as a novelty. 



Coming to the minor planets, evidence is cited to 

 show that Olbers's theory of an exploded planet is 

 untenable, and that this congeries of fragments repre- 

 sents what would have been a planet had not the 

 giant Jove prevented the agglomeration. By plotting 

 the major axes of their orbits in the form of a spec- 

 trum band, the wave-lengths scale being replaced by 

 one of astronomical units. Prof. Lowell shows how 

 commensurability of period with the period of Jupiter 

 has determined the location of the asteroids. The 



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