June 1 1, 1908J 



NA TURE 



137 



+ o"'i4. The mean error of the moon's tabular place, 

 determined from eighty-two observations and computed 

 from Hansen's tables with Newcomb's corrections, was 

 -0-3835. in R.A. and -o"-i5 in N.P.D. for 1906. From 

 104 observations in 1907 the error is — 0-4013. in R.A. 



Part i. of the Second Nine-year Catalogue, epoch 1900, 

 dealing with fundamental and zodiacal stars, is already in 

 the printer's hands, and part ii., giving the astrographic 

 reference stars, will be ready for the press shortly. 



The new method of illuminating the altazimuth field, as 

 described in the previous report, has proved very satisfac- 

 tory, and has now been adapted to the transit instrument. 

 The altazimuth was used for meridian and extra-meridian 

 observations throughout the year, and the observations of 

 the lunar crater Mbsting A, commenced in 1905, were con- 

 tinued whenever practicable ; these serve to connect the 

 observations of the first and second limbs made before and 

 after full moon, and, when discussed with the similar 

 observations that are being made at the Cape Observa- 

 tory, will provide data for the determination of the lunar 

 parallax. Forty-three observations of the N.P.D. and of 

 the R.A. of the crater were made during the year. That 

 both the transit-circle and altazimuth observations are 

 satisfactory is shown by their agreement. 



From the altazimuth observations in 1907 the mean 

 errors of the moon's tabular place are : — moon's limb, in 

 meridian, —0-365. ; Mosting A, in meridian, — 0-35S., 

 moon's limb, extra meridian, — 0-42S. 



The larger scheme of reflex-tube observations, mentioned 

 in the preceding report, was prosecuted throughout the 

 year, 1545 double and forty-four single observations being 

 made ; the total number of stars observed, including ;3 

 and 7 Draconis and i" Cygni, which are observed through- 

 out the year, was eighty-five. With the view of deter- 

 mining the variation of latitude, the discussion of the 

 observations from 1903 onwards has been commenced, but 

 owing to the variation of tlie instrument's scale value 

 from night to night, caused by minute alterations in the 

 distance between the mercury surface and the object-glass, 

 there are serious difficulties to be overcome in the dis- 

 cussion. This distance can be adjusted to within about 

 o-oi inch by means of the focussing rod, but an error of 

 that amount would introduce errors quite inadmissible in 

 the deduced zenith distances of the stars at a distance from 

 the centre of the field. For example, in the case of 

 8 Draconis, which is 535' from the zenith, the error would 

 amount to ±o"-s6. 



The 28-inch refractor was employed for observing double 

 stars, primarily those pairs discovered by Mr. Hough, and 

 measures were made on 105 nights as compared with 

 eightv-six nights last year ; t; Pegasi was measured on 

 fifteen, 5 Equueli on thirteen,- and 70 Ophiuchi on sixteen 

 nights. Complete sets of measures of the polar and 

 equatorial diameters of Jupiter were made, first with 

 the filar and then with the double-image micrometer, on 

 seventeen nights, whilst with the filar micrometer the 

 diameters of the satellites were determined on two nights. 



It was mentioned in the last report that a system of 

 twelve lignum-vita; wedges had been employed to fix rigidly 

 the mirror of the 30-inch Thompson reflector. Whilst the 

 method has proved very satisf.'ictory in fixing the mirror, 

 there is a tendency to produce slight distortion, so it is 

 proposed to introduce a further modification of the support 

 the next time the mirror is dismounted for re-silvering. 

 Cast-iron blocks, shaped to fit the steel supporting band 

 round the edge of the mirror, are to be introduced, the 

 pressure being applied by screws passing through the cell ; 

 in this way the strain may be adjusted as required. 



Thirty-one photographs of Neptune and its satellite were 

 secured with the 26-inch refractor, using the occulting 

 shutter as in previous oppositions, on sixteen nights; photo- 

 graphs of Saturn's, and of Jupiter's distant, satellites were 

 also taken. In regard to the latter, Mr. Melotte and the 

 Greenwich observers generally are to be heartilv congratu- 

 lated upon the discovery of Jupiter's eighth satellite, first 

 noted on a photograph taken on February 28. .'\ltogethcr 

 twelve photographs showing this object were secured 

 between January 27 and .\pril 24, and the measurements 

 show that the newly discovered satellite is very much more 

 distant from Jupiter than the sixth and seventh satellites, 

 and is perhaps not quite so faint as the seventh. Of J. vi. 



NO. 2015, VOL. 78] 



and vii., respectively, thirty-eight and twenty-two photo- 

 graphs were secured during the opposition. 



The 30-inch reflector was also used to photograph fifty- 

 four minor planets, and comets 1907^ and e. Several 

 long exposures on comet 1907^ (Daniel) produced negatives 

 in which the structure of the tail is of great interest. 

 Four long exposures were made in an unsuccessful search 

 for Halley's comet, and this search, for which an 

 ephemeris, based on the perturbations calculated at Green- 

 wich, has been prepared, will be resumed during the 

 coming autumn. 



With the astrographic equatorial, 128 satisfactory plates 

 were taken during the year to replace plates which, 

 although satisfactory in other respects, are unsuitable for 

 reproduction of large prints. Positives have been made 

 and passed for 192 plates which cover the zones 75°— 78°, 

 and 109 plates in zones between 79° and the pole. Only 

 fifteen chart plates remain for reproduction, and these 

 have to be replaced by more suitable negatives. 



The work of the Greenwich section of the Astrographic 

 Catalogue is complete so far as the publication of the 

 measured rectangular coordinates and the data necessary 

 to convert them into Right .■\scension and Declination is 

 concerned. The conversion of the coordinates of such stars 

 as are in Carrington's Catalogue has been commenced. 

 Vol. ii. of the Greenwich .'\strographic Catalogue was pub- 

 lished during the year, and contains 98,738 stars. The 

 report contains an interesting table, too large to reproduce 

 here, comparing the number of stars which appear on the 

 plates for each zone and for the three different exposures, 

 and also comparing these numbers with the number of 

 stars shown in the same zones of the Bonn Durch- 

 musterung ; the total number of stars shown on the forty- 

 minutes' plates is 719,088, or 344-4 per square degree. 



The perturbations of Halley's comet, which are being 

 computed by Messrs. Cowell and Crommelin, and the data 

 necessary for determining the time of next perihelion 

 passage, are nearly complete, but some further investiga- 

 tion" of the close approaches of the comet to Jupiter in 

 1834 and 1837, when the perturbations appear to have 

 been considerable, is necessary; 19 10 April 8 appears to 

 be the most probable date for the occurrence of the next 

 perihelion passage. Mr. Crommelin has confirmed Dr. 

 Hind's identifications of the comet with three e.xceptioni 

 (1223. 912, and 837), and the perturbations have been 

 carried back to 760. 



Photographs of the sun were taken with the Thompson 

 photoheliograph on 210 days, and with the Dallmeyer 

 photoheliograph alone on two days. 



Some remarkable fluctuations of the solar activity during 

 1907 are reported. From July onwards an increased activity 

 occurred, and several naked-eye groups were observed. 



The usual magnetic observations were carried out 

 during the year, and the principal results for the magnetic 

 elements for 1907 are given as follows : — 



Mean declination 15° 59' -8 West 



\ 4'oi95 (in British unitO 

 ■ / I '8533 (in metric units) 

 Mean dip (with 3-in. needles), 66° 56' 4" 



In 1907 there was one day of great magnetic disturb- 

 ance and sixteen of lesser disturbance. 



The meteorological results show that the summer of 

 1907 was exceptionally windy, and that the mean tempera- 

 lure of the year was 49°-4, or o°-2 below the average of 

 the sixty-five years 1841-1905. The rainfall of the year 

 ending 1908 April 30 was 23-14 inches, being 0-9S inch 

 below the sixty-five-year average. 



The testing of chronometers and chronometer watches 

 showed a serious falling off in the performance of the 

 former and an improvement in that of the latter instru- 

 ments. 



The danger which threatened the observatory from the 

 working of the L.C.C. generating station on the northern 

 meridian has been averted to a great extent by the two 

 years' working agreement with the Council. Apparently 

 the close, double-star observations with the 28-inch re- 

 fractor are not affected prejudicially, but it is still desirable 

 that the trouble arising from the outflow of heated gases, 

 which interfere with the observation of northern stars on 

 the meridian, should be mitigated. 



Mean horizontal force 



