June i6, 1892] 



NATURE 



157 



The total number of observations made with the alt- 

 azimuth in the year ending 1892 May 10 is as follows : — 



Azimuths of the moon and stars 345 



Azimuths of Mark 1 162 



Azimuths of Mark II 182 



Zenith distances of the moon 166 



Zenith distances of Mark 1 164 



Zenith distances of Mark II 176 



These numbers are slightly greater than in recent years, 

 owing to the fact that during August and September, 

 when the transit-circle was under repair, the observations 

 of the moon with the altazimuth were made throughout 

 the lunation instead of being confined to the first and last 

 quarters. 



WMth regard to clocks and chronographs, we may 

 mention that the daily rate of the sidereal standard clock 

 underwent a very considerable disturbance, changing 

 from a daily gain of ros. to that of 2-os. The cause of 

 this difference was due to some workmen who were fixing 

 a new shelf, the necessary hammering setting up vibra- 

 tions in the building. 



With the reflex zenith tube, eighteen double observa- 

 tions of 7 Draconis have been made, but owing to the 

 pressure of work the reductions are not yet complete. 



Ten occultations of stars by the uneclipsed moon (8 

 disappearances and 2 reappearances) and 48 phenomena 

 of Jupiter's satellites have been observed with the equa- 

 torials, or with the altazimuth. These observations are 

 completely reduced to 1891 December 31. On the occa- 

 sion of the partial eclipse of the moon on 1892 May 11, 

 7 disappearances and 3 reappearances were observed of 

 the faint stars in a list prepared by Mr. Crommelin ; and 

 the times of transit of the shadow over some principal 

 craters were also noted. But it is to be regretted that, 

 although favoured by fine weather on this occasion, the 

 Observatory was seriously crippled in their instrumental 

 equipment,the 1 3-inch refractor of the south-east equatorial 

 and the Lassell 2-feet reflector being both dismounted. 



With the photographic equatorial, 301 plates with a 

 total of 1 190 exposures have been taken on 112 nights, 

 many of these being taken for special investigations. Of 

 these, 62 plates were taken to determine the relations 

 between diameter of image, length of exposure, and 

 brightness of the star, the results of which have already 

 appeared in the Monthly Notices for January of this 

 year. The discussion indicated that, through a range of 

 exposures corresponding to 8 magnitudes, "the square 

 root of the diameter increases as the logarithm of the ex- 

 posure ; and further, that for equal photographic effects 

 duration of exposure should vary inversely as the bright- 

 ness of the star." These results were based on as many 

 as 2200 measures of 150 star images. The rcscaux seem 

 to have given much trouble, the silver film developing 

 pin-holes, the images of which resemble on the photo- 

 ,'raphic plates those of stars. M. Gautier is now supply- 

 ng the Observatory with two more, coated this time with 

 a film of collodion, in the hopes that it may be freed from 

 the deficiencies mentioned above. The catalogue which 

 has been undertaken at Greenwich of the guiding stars 

 for the zones + 60" to the pole, -f 25' to + 29', and — 3" 

 to - 5^, is very near completion. The catalogues of 

 places (epoch 1900) are complete for the Greenwich 

 zones + 65'' to + 80' (the reductions for the circumpolar 

 region being deferred), also for the zones + 60° to -f 64^ 

 to be photographed at Rome, and for the Oxford zones 

 -|- 25^ to -f- 29^. The stars for the San Fernando zone 

 ( - 3° to - 5 ) have all been selected, and their places 

 have been computed for those between R.A. I2h. and i8h. 

 Spectroscopic and PJiotographic Observations. — The 

 observations of the displacement of the lines in stellar 

 spectra for the determination of their motion in the line 

 of sight have not this year been regularly continued ; a 

 preliminary discussion of the former observations sug- 

 gesting that they were affected to some e.xtent by the 



NO. I 1 8 I , VOL. 46] 



position of the spectroscope, Vega and Altair were ob- 

 served during the summer and autumn at as wide a range 

 of hour-angle as possible, and with the spectroscope set to 

 each of the four positions o'',9o^, i8o^ and 270^ ; the slit 

 being parallel to the declination circle at o^ The numbers 

 of observations obtained of the F line in the spectrum of 

 Vega are : at o', 39 ; at 90=, 42 ; at 180°, 36 ; and at 

 270', 39 ; and of the F line in the spectrum of Altair : 

 at o', 30 ; at 90, 32 ; at 180°, 26 ; and at 270"^, 29. The 

 measures are now under discussion, and give clear indi- 

 cations of the existence of the systematic error referred 

 to. The observations were interrupted by the dismounting 

 of the I2f-inch telescope on 1891 November 19. 



At the appearance of the new star in Auriga the south- 

 east equatorial was unfortunately dismounted, but the 

 object-glass presented to the Observatory by Sir Henry 

 Thompson was mounted as quickly as possible on the 

 Thompson telescope ; but alterations of the telescope tube 

 were found necessary to bring the spectrum to focus on 

 the photographic plate, and before these could be 

 completed, the Nova had become nearly too faint for 

 observation. 



For the year 1891, 360 out of 365 photographs of the 

 sun's surface have been selected for measurement; 136 

 of these were sent to the Solar Physics Committee from 

 India and Mauritius. 



The solar activity has increased in a remarkable man- 

 ner during the past year. While there were 175 days 

 without spots in the year 1890, there were only 21 such 

 days in 1891, and since 1891 March 28, the sun has not 

 been free from spots on a single day on which it has been 

 observed. The number of groups visible on the disk at 

 the same time, and their average size and complexity, 

 have all greatly increased during the past twelve months, 

 the group of February 5-18 being the largest ever photo- 

 graphed at Greenwich. This group has had an unusually 

 long life, appearing first on 1891 November 15, and 

 persisting till 1892 March 17. 



Magnetic Observations. — The continuous register by 

 photography of the magnetic elements has been satisfac- 

 torily maintained. It has been found that serious dis- 

 turbances of the earth-current registers is due to the 

 trains of the City and South London Electric Railway, 

 situated at a distance of 2\ miles from the nearest earth 

 plate, and about i,\ miles from the Observatory. The 

 change of potential takes place every two or three minutes, 

 varying in amount from " a small fraction of a volt to 

 one-third of a volt or more." 



The following are the principal results for the magnetic 

 elements for 1891 : — 



Mean declination (approximate) 17° 23' West. 



Mean horizontal force ... | 3*9587 (in British units). 

 \ I "8253 (m Metric units). 

 i 67' 19' 49" (by 9-inch needles). 



Mean dip | 67''2i' o" (by 6-inch needles). 



\ 67° 23' 22" (by 3-inch needles). 



In the year 1891 there were five days of great magnetic 

 disturbance, but there were also about twenty other days of 

 lesser disturbance, for which tracings of the photographic 

 curves will be published ; these days having been selected 

 in concert with M. Mascart according to the arrangement 

 mentioned in the last report. The calculation of diurnal 

 inequalities from five typical quiet days in each month, 

 commenced in 1889 at Prof. Riicker's suggestion, has 

 been continued. 



From February 13 to 14 a very large disturbance was 

 recorded, commencing a day after the large sun-spot was 

 on the central meridian. Considerable magnetic disturb- 

 ances also occurred on March 6, 11, and 12. Other dis- 

 turbances occurred on 189 1 September 9, 1892 April 

 25-26 and May i, and may perhaps " be connected with 

 spots then on the sun's disk." 



Meteorological Observations.— The mean temperature 

 of the year 1891 was 48"-4, being I'-i below the average 



