44 



DISCOVERY 



problem, and for a period of thirteen years he kept the 

 planet under observation, following it for long periods 

 in the daytime. His conclusion was a startling one — 

 that, so far from the period being of 23 or 24 hours, it 

 synchronised with the period of revolution — 225 days. 

 This result indicated that Venus turned one face con- 

 stantly toward the Sun, and that in one hemisphere 

 there is perpetual day and in the other everlasting 

 night. This conclusion was confirmed in 1895 b}' a 

 second series of observations by Schiaparelli himself ; 

 and by Perrotin at Nice, Tacchini at Rome, Cerulli at 

 Teramo, and in addition by Lowell at Flagstaff, Arizona. 

 Nevertheless other competent observers, including 

 Flammarion, were satisfied that the rotation was 

 performed in a period of about 24 hours. 



About twenty years ago, the spectroscope was applied 

 to the solution of the problem. By means of Doppler's 

 principle, Belopolsky, the Russian astronomer, arrived 

 first at a period of 12 hours and later of 24 hours, while 

 Lowell and his assistants at Flagstaff found that " the 

 evidence of the spectroscope is against rotation of short 

 duration, and so far as its measure of precision admits, 

 the investigation confirms a rotation of 225 days." 



Last summer, Professor W. H. Pickering announced 

 the results of his observations on Venus during the 

 favourable appearance of the planet in the previous 

 winter. In December 1920 Pickering detected two 

 dark spots near the terminator, and by following these 

 spots — their appearances and disappearances — he 

 reached the" conclusion that the planet rotates in a 

 period of 68 hours, and that the axis of rotation lies 

 very nearly in the plane of the planet's orbit. So far 

 this is the last word on the subject. If Pickering's 

 rotation period prove to be accurate, it may be possible 

 to reconcile the discordant results reached by dis- 

 tinguished observers. 



That dark markings and dusky shadings appear from 

 time to time on the dazzling disc of Venus admits of no 

 doubt. It is, however, difiicult to speak with any 

 confidence as to their nature. They may represent 

 irregularities in the surface, as has been generally 

 maintained ; or, as Pickering suggests, they may be 

 due to breaks in the cloud-laden atmosphere. White 

 spots have been suspected from time to time, and 

 occasionally brilliant markings have been seen, and 

 interpreted as polar caps or as the snow-clad summits 

 of high mountains ; but there is no certainty concerning 

 the reality of these appearances. 



Mercury 



If little is known of Venus, still less is known of 

 Mercury, the smallest of the four dwarf planets. 

 Mercury is situated much closer to the Sun than Venus ; 

 it is a small world — only 3,000 miles in diameter — and 

 it is comparatively at a great distance from us. The 



first systematic series of observations was made by 

 Schroter at the beginning of the last century. This 

 patient though not brilliant observer discovered that 

 the southern horn of the Mercurial crescent presented a 

 blunted appearance — a fact which he attributed to the 

 existence of a high mountain or mountains. He 

 also reached the view that the planet's rotation was 

 performed in a period of about 24 hours. This 

 estimate was not received with any degree of con- 

 fidence ; yet Schiaparelli's announcement in 1889, at 

 the close of his seven years' study of Mercury, that 

 the period of rotation equalled that of revolution, was 

 received with a certain degree of scepticism. His 

 conclusion, however, was abundantly confirmed by 

 Lowell at Flagstaff, and it is now generally conceded 

 that Mercury turns always the same face to the Sun 

 just as the Moon does to the Earth. This conclusion, 

 in the case of Mercury, is theoretically confirmed by 

 Sir George Darwin's hypothesis of tidal friction. 



Mercury, in fact, seems to resemble the Moon more 

 than any other celestial body. The balance of evi- 

 dence would seem to favour the view that the little 

 planet has no appreciable atmosphere, and that its 

 surface is rugged and mountainous. According to 

 Lowell, whose observations were prolonged and ex- 

 haustive, the dark markings which revealed themselves' 

 to his search merely represent huge cracks in a barren 

 and lifeless world ; " the surface of Mercury," he 

 concluded, "is colourless — a geography in black-and- 

 white." Mercury, then, would seem to be a slightly 

 larger edition of the Moon — a world in the last stages of 

 planetary decrepitude. 



(To he continued) 



The Courtship of the 

 Red-throated Diver 



By Julian S. Huxley, M.A. 



Fellow of New College^ Oxford 



Bird photography is a splendid sport. In so far as 

 it is a sport, it is an end in itself. Looked at objec- 

 tively, it has the further great advantage that it does 

 not destroy but helps to preserve ; those who have 

 watched a bird for days together from a hiding-tent 

 will never wish to kill the creature whose inmost life 

 has been displayed before their eyes. Further, like 

 any sport, it brings with it knowledge of its objects. 

 But while the bird photographer carmot help acquiring 

 much intimate knowledge of the birds he photographs, 

 the acquisition of knowledge is not his main aim, and 

 too often his opportunities for enlarging science are 

 wasted. 



