March 24, 19 10] 



NATURE 



97 



In my former letter in Nature of January 6 I attempted 

 to prove that the arrangements of dots in a band would 

 occur even if the numbers of which the sums were taken 

 were entirely independent of one another, in which case 

 a forecast regarding one of the numbers could not possibly 

 be made from knowledge of the remainder. A forecast 

 could only be made if it were shown that the width of 

 the band were smaller than would be expected on the 

 hypothesis of pure chance, and this vital point has received 

 no consideration. 



The situation may be made clearer by reference to the 

 original letter in Nature of September i6, 1909. The 

 essence of the method is that, if we were forecasting for 

 1910, the dot the two rectangular coordinates of which are 

 the sums of data for thirty years up to 1909 and 1910, re- 

 spectively, will Ke near to a line through the origin at an 

 angle of 45° with the axes. Thus the sum of the data from 

 1880 to 1909 will be nearly equal to the sum of the data 

 from i88i to 1910, or the data for 1880 and 1910 will be 

 nearly equal. If the nearness to equalit}' has any value at 

 all for forecasting, this is equivalent to asserting that the 

 data in question tend ta bfe repeated after thirty years, or 

 have a thirty years' period; but as the same result could 

 be reached if 25 or 35, or any other comparable number, 

 had been substituted for 30, it will be seen that the reason- 

 ing cannot be free from error. 



That the nearness to equality is inadequate is clear from 

 the diagram in the original letter. The edges of the band 

 there intercept a length representing about thirty-six days 

 along any vertical ordinate. Hence all that can be inferred 

 in forecasting for 19 10 is that the number of hot days will 

 probably not differ by more than ± 18 from the nuniber of 

 hot days in 1880 ; and as the average number of hot days 

 in a year is stated as fifteen, it appears that a forecast so 

 entirely vague could be made without any analysis what- 

 ever. Gilbert T. Walker. 

 Kodaikanal, Februarv 21. 



SOME SCIENTIFIC CENTRES. 

 Xo. X\'. — The Mount Wilson Solar Observatory 



OF THE Carnegie Institution of Washington. 

 ■jV/rOUNT WILSON rises 6000 feet, almost abruptly, 



■'■*- from the plain in which lie the twin cities of 

 Los Angeles and Pasadena. From the mountain top 

 these cities appear at night as glittering star clusters ; 

 by day they are seen through a haze of dust which 

 the ascent of the mountain has put below- our feet. 

 Beyond is the vast Pacific ; above our heads the 

 glorious sky of California ; around us the buildings 

 of perhaps the best equipped observatory in the world. 



These words are written by anticipation. Prof. 

 Hale has invited the International Union for Solar 

 Research to hold its next meeting on Mount Wilson 

 on August 29, 19 10, and astronomers and physicists 

 from all parts of the world are eagerly looking for- 

 ward to the occasion. The present writer is not, 

 however, altogether a stranger to the scene ; he was 

 on Mount Wilson in 1904; but at that time the 

 observatory was in its infancy. It had not even been 

 decided on what scale it was to be designed. Prof. 

 Hale had realised the magnificent opportunities offered 

 by the climate and site, and he had made urgent 

 application to the Carnegie Institution for funds 

 adequate to deal with the serious diflficulties to be 

 overcome ; but he had also resolved that, if his applica- 

 tion was not granted, there should still be a solar 

 observatory on Mount Wilson, for which he would 

 himself provide the funds, so that he had already 

 commenced building operations. Nevertheless, the 

 utmost provision which he and his courageous wife 

 could afford to make would naturally fall far short 

 of what was needed for a suitable observatory, and 

 he was therefore anxiously awaiting the answer of 

 the Carnegie Trustees. Fortunately for astronomy, 

 it was favourable ; and since it was received one 

 marvel has followed another in rapid succession. The 

 visitors will be drawn to Mount Wilson as to the 

 NO. 2108, VOL. 83] 



main focus of astronomical enterprise and success 

 at the present moment. 



The first of the principal instruments to be com- 

 pleted was the great horizontal Snow telescope, origin- 

 ally constructed at the Yerkes Observator\-, with the 

 aid of funds given by Miss Snow, of Chicago. The 

 concave mirror, of 24 inches aperture and 60 feet 

 focus, is fed by a coelostat with plane mirrors of 30 

 inches and 24 inches, the beam of light being sheltered 

 by a house specially designed to guard against 

 temperature effects. To this telescope can be attached 

 a spectrograph of i8-feet focus, or a 5-foot spectro- 

 heliograph. The heavy parts of the apparatus are 

 mounted on massive stone piers, built with great 

 labour, since it was found that the stone in the 

 neighbourhood was unsuitable, and that materials 

 had to be brought up from a lower altitude bv mules. 

 But, successful as this powerful instrument has 

 been, it has also ser\-ed to point the way to possible 

 improvements. Experience of its working suggested 

 that a vertical telescope might be in various wavs 

 better than a horizontal one; and accordingly a 

 " tower " telescope was constructed, with the coelostat 

 mounted on a tower 60 feet high, built as a skeleton 

 framework. This experiment was so successful that 

 a more ambitious one was at once projected, and a 

 tower 150 feet high is under construction. As wind 

 pressure will be much more serious on this new 

 structure, Prof. Hale has adopted the ingenious 

 device of building an outer tower for protection, sur- 

 rounding every bar of the inner tower bv a tube of 

 the outer. The lower parts of these towe'r telescopes 

 are contained in wells sunk many feet into the ground. 

 Thirdly, there is the beautiful" 5-foot reflector, made 

 by Prof. G. W. Ritchey. who has already proved his 

 skill in such work. The mirror was made at the 

 Yerkes Observatory some years ago, but has had to 

 wait until a mounting could be provided on Mount 

 Wilson ; and, indeed, there was a still earlier pro- 

 vision to be made; the track up the greater part 

 of Mount Wilson was originally only 3 feet wide — 

 a mere ledge in a precipitous descent^and up this 

 narrow track the materials and instruments were 

 carried, at first on mule back, later in a speciallv 

 designed carriage, with steering fore and aft, and 

 drawn by a mule. But the 5-foot mirror and its 

 mounting could not be taken up in this way, and it 

 was necessan,- to widen the track to 5 feet throughout 

 its whole length. This was not accomplished without 

 serious delays, owing to severe storms, which some- 

 times destroyed weeks of labour; but it was finallv 

 completed, the instrument was taken up and mounted, 

 and at the meeting of the Royal .Astronomical Societv 

 on December 10, 1909, were shown some photographs 

 of Mars taken with this great telescope which far 

 surpassed anything of the kind yet seen, and for 

 which the president was requested to convey a special 

 vote of thanks. 



.\ still larger telescope, with a mirror 100 inches 

 in diameter, is to be attempted on Mount Wilson, but 

 is not yet within sight of completion. Round the 

 existing three great instruments are grouped a 

 number of other buildings ; first and foremost a 

 ph\'sical laboratory', so indispensable now in astro- 

 physical work ; also an astrophysical museum, and a 

 variometer house ; and then such necessar\- accom- 

 paniments as a power-house, a pump-house, storage- 

 houses, and dwellings. The establishment is not 

 adapted for ladies and children, and the chief resi- 

 dence is called the " Monaster\\" Distressing news 

 reached us recently that the " Monaster},- " had been 

 burnt down, owing to the carelessness of a temporary 

 ser\-ant. Fortunately it contained no original photCK 

 graphs or records, and most of the books had been 



