536 



NA TURE 



[Oct. 2, i! 



that pari of their producing gas may be even far outside the 

 usually considered limits of everything belonging to the earth. 



M. Thollon, indeed, gives a view of the "B" region, "as," 

 he says, "it viould appear if observed outside the earth's atmo- 

 sphere " ; and therein he shows every line constituting our 

 magnificent earthly constellation "Great B," absolutely wiped 

 out of existence— a few ultra-laSx&, accidentally intruding solar 

 metallic lines alone excepted. But how has the eminent savant 

 obtained that view ? Not by ascending in a balloon, or up the 

 sides of a high mountain above all the grosser atmosphere, and 

 seeing that it was so, but merely by observing some small amount 

 of difference of effect, at two slightly different degrees of large 

 zenith distance, viz. 60' and 80°, at the Observatory of Nice. 



Two points, however, alone, will never enable a curve to be 

 drawn on their sole authority ; and as a curve of effect is what 

 the investigation now requires, M. Thollon's hitherto merely 

 duplex observations will acquire a far greater power of convic- 

 tion for other men's minds, if he will kindly supplement them 

 with others at 20 Z.I)., or as near to it as the latitude of Nice 

 will allow him at time of summer solstice. Still more would he 

 make us all his debtors if he would repeat those three angular 

 directions at three successive stations at greater hypsometric 

 altitudes ; duly remembering that while every one knows that 

 water-vapour and oxygen (the gaseous parentage of " B" accord- 

 ing to the grand experiments of M. Egoroff) do exist in the 

 earth's atmosphere, that does not, by itself, therefore render 

 them impossible in greater or le-s quantity to the outer region of 

 the sun's envelopes ; or, in a highly attenuated degree, to the 

 92,000,000 of miles of space between. C. PlAZZI-SMYTH 



15, Royal Terrace, Edinburgh, September 27 



Shifting of the Earth's Axis 

 With reference to the letter of Mr. W. M. Flinders Petrie 

 (Nature, September 25, p. 512), I would remark that there 

 has been no sensible change in the latitude of Greenwich (as 

 found by observations of circumpolar stars) during the last forty- 

 seven years, a period nearly twice as long as that covered by the 

 Pulkowa observations which M. Nyren has discussed. In a 

 paper on the " Systematic Errors of the Greenwich North Polar 

 Distances" (Mem. R.A.S. vol. xlv.), I have exhibited the results 

 for the co-latitude of the centre of the Greenwich transit-circle 

 for each year and for groups of years from 1836 to 1877 reduced 

 to the same refractions throughout (Bessel's), and corrected where 

 necessary for index-error of the thermometer, and the accordance 

 of the individual results is as close as can be expected, when allow- 

 ance is made for the systematic errors to wdiich all observations are 

 liable, but which are usually ignored in estimating theoretically 

 the probable errors of mean results. It may suffice if I here 

 give the results for co-latitude for three periods of years : — 

 1836-49 mean co-latitude 38° 31' 2i"'85 

 1851-65 2l"-87 



1S66-S3 2i"-8s 



The first and last results are identical and are absolutely incon- 

 sistent with Mr. Petrie's supposed increase of the Greenwich co- 

 latitude of 1" or move in a century. 



W. H. M. Christie 

 Royal Observatory, Greenwich, S.E., September 27 



The Sky-Glows 



From the great purity of the sky this evening, and from the 

 flatness of the horizon westwards, on the line of the Great 

 Northern Railway between Huntingdon and Hitchin, the sun- 

 set glow was of a very beautiful description. At five minutes 

 before six (watch-time, three or four minutes slow) the sun Set ; 

 and it was no sooner hidden than a parhelion-like patch of white 

 light, 6° or 8° in diameter, brighter than the rest of the sky, 

 occupied a place io° or 15 above the sunset point of the horizon, 

 and continued shining there with pearly brightness for about ten 

 minutes. The horizon-line became edged at the same time with 

 bright red, melting abruptly away upwards into orange, and 

 higher up into a field of yellow light round the lucid spot. At 

 6.5 this spot's white light began to acquire a rosy tinge, and 

 during the next ten minutes, until 6.15, it became intensely rose- 

 coloured, preserving its definite place unchanged in the up] er 

 expanse of yellow ; a vivid golden oriole-yellow stripe some 

 degrees broad divided it from the red fringe along the horizon, 

 the dazzling gold colour shading exquisitely into the fiery red 



below and rosy red above, and deriving itself from the latter a 

 bronze-like greenish cast in its bright golden hue by contrast. 



By 6. 15 the rosiness of the bright spot had extended upwards 

 and outwards from its centre, and was now blended in its colour 

 and confines with the yellow band and red fringe below it, until 

 the whole presented a conflagration or red aurora-like outburst 

 of light in the west, 20 or 25° high, and extending 45° or 50° in 

 base along the horizon. 



The yellow belt was fading out of this glory, and the bright- 

 ness and rosiness of its upper part was fast disappearing, when 

 at 6. 15 there appeared, with extreme quickness in the brighter 

 base, dark intervals dividing it into upward radiating diverging 

 beams of light, which rapidly acquired such fixed breadths and 

 distinctness that I easily counted six or eight separate beams 

 nearly equidistant from each other, and of equal lengths and 

 strengths, marking out plainly by their divergence the sun's 

 place, and the considerable depth to which it had already sunk 

 below- the horizon. The two outer ones only of the beams, on 

 the northern side, were a little confused together, and marred 

 the symmetry which the whole presented, but the full number 

 of their display was several times counted over during the ten 

 minutes — until 6.25 — that they continued visible. They were 

 about 15 long from their base*, and extended across and usurped 

 to themselves the light of what had been the golden-yellow belt ; 

 but they gradually shortened and became dull red when at the 

 latter hour the horizon assumed the red appearance which it 

 usually pre-ents some time after sunset. The above striking 

 phases of the glow, — the white spot, the rose-red one, and the 

 streamers, — occupied just ten minutes each, and the unusual 

 aspect of the sky ceased entirely just thirty minutes after sunset. 



The patch of whitish light observed this evening had all the 

 appearance of a true, but extended and diffused, mock sun of 

 some description ; and I have noticed the peculiarity before, in 

 the sunset glows of last winter and spring, whenever I had an 

 opportunity to see the sky and watch their early phase just 

 after sunset ; but crystals of ice being then plentiful in cirrus, 

 the evidence of the mock sun's formation by non-aqueous dust 

 in the atmosphere was not so strong as now, when it has 

 recurred after a long continuance of a summer temperature 

 which has been unusually high. It is also singular and curious 

 that the rosy tint began in the white-glow spot, and spread 

 evenly outwards from it as a centre, as the extent and intensity 

 of this remarkable colour grew and increased. 



Appearing as the white spot does, when I have seen it, at a 

 pretty fixed height of not less than about 12° from the horizon as 

 soon as the sun has set, it seems difficult to reconcile its presence 

 at such an altitude with a theory of its production by reflection 

 of the sun's horizontal rays from fine films or lamina? of floating 

 glassy dust, unless descending equatorial currents, perhaps, in 

 those extremely stable heights may have a sensible inclination 

 downwards from the west, and may tilt the films' under sides 

 in a direction corresponding with that of the current slightly 

 towards the sun ? 



With regard to the diverging beams, they are also, perhaps, 

 not quite ordinary, irregularly produced straight lines of radia- 

 tion ; but seemed by their symmetry to be connected, at least in 

 the origin of the shadow-gaps which formed them, with evenly 

 ruled stripes or pleats of the cirrus, and of loftier haze, in this 

 case directed, it seemed, nearly east and w est. With such haze- 

 bands and stripes directed rather more sou h-westerly, or about 

 towards the point of winter sunset, and intersected also with 

 slightly slanting systems of striation, I constantly noticed the 

 sky in Newcastle-on-Tync, during the prevalence last winter and 

 spring of the repeated sunset glows, to be for weeks and months 

 more or less constantly and uniformly, but in general weakly 

 and dimly, streaked and furrowed over. Either an unusually 

 steady current was prevailing in the upper air ; or else a perma- 

 nent current there, and long lines of aerial disturbances troubling 

 its streams were made more visible and conspicuous than ordinary, 

 by exceptional radiation, or some other unusual refrigerating and 

 haze-engendering cause, depriving the upper air of its transpar- 

 ency, during the time of the sky's pre-enting such unusual 

 appearances in what Quetelet named and considered to be the 

 " stable" or untempestuous upper regions of the atmosphere. 



Whatever may be the explanation of the streamers and of the 

 white glare-spot, observations may perhaps be made of them 

 under even more favourable conditions than occurred this even- 

 ing, and they would then possibly give a little help towards 

 arriving at some further conclusions both as regards the crystal- 

 line or other nature of the haze-causing substance, and as to the 



