SCIENCE-GOSSIP. 



49 



GREENWICH OBSERVATORY. 



SATURDAY, June jth, 1S97, was the day for the 

 annual visitation of Greenwich Observatory, 

 when a goodly company, together with the official 

 Board of Visitors,- attended. The Astronomer- 

 Royal's report stated that the north wing and 

 central dome of the Physical Observatory was 

 completed in September, last. This building 

 contains the largest telescope in the world devoted 

 to photography, presented by Sir Henry Thompson. 

 On the same equatorial stand, of the German 

 pattern, but constructed by Sir H. Grubb, is at the 

 one end of the declination axis, the 26-inch photo- 

 graphic telescope, with the izj-inch Merz telescope, 

 formerly mounted as the great equatoreal, mounted 

 as the finder below it, whilst above it is the 9-inch 

 Thompson photo-heliograph. At the other end of 

 the axis is a 30-inch Cassegrain reflecting telescope 

 to which is attached the photographic spectroscope, 

 whilst the 6-inch Hodgson telescope is attached as 

 a guiding telescope. This instrument was a great 

 centre of attraction. Amongst other visitors was 

 Sir Henry Thompson himself. Just outside this 

 dome, upon the terrace roof, is mounted the 

 Dallmeyer photo-heliograph, with which photo- 

 graphs of the sun have been taken on 222 days, 471 



es being retained for preservation, as well as 

 twelve with double images of the sun for determina- 

 tion of the zero of the position-angle. These 

 photographs, together with those taken in India 

 and the Mauritius, give a series on 360 days out of 

 366. Many of these photographs, together with 

 the apparatus for their measurement, and also 

 some photographs of the moon, one plate of Saturn 

 and one of Jupiter, were on view in a sort of museum 

 below, under the charge of Mr. E. W. Maunder. 



The new Altazimuth was supposed to be ready 

 in September last, but its four reading microscopes 

 gave discordant results, varying according to the 

 last position of the telescope. Examination showed 

 that the axis was too weak ; this was strengthened, 

 but the fault was not wholly done away with, even 

 though the position of the friction rollers was 

 altered, and a system of levers substituted for 

 springs. Mr. Simms has, however, discovered an 

 unsuspected source of error in the transit circle, of 

 similar construction, made for the Perth Observa- 

 I "wing to the method of giving a helical twist 

 to the grinder in grinding the pivots, it was found 

 that these had a tendency to act as screws, intro- 

 ducing a longitudinal force varying in direction 



ng to which way the telescope was moved ; 

 the effect being to slightly move the iron standards 

 carrying the bearings and microscopes, and varying 

 the position of the latter with respect to the 

 graduated circles. A few circular turns of the tool 

 cured the Perth instrument, and as the (ireenwich 

 one suffered from the same complaint, similar 

 treatment was adopted, and it is confidently I 

 that the instrument will shortly be in working 

 order. 



Last year, as it: : ears, the sun, 1 



planets and fundamental Mars have been regularly 

 observed at the meridian with the transit circle 

 The annual cad bserved U 



n * 3-454 star* With the 13-inch astro- 

 1 17 ni^hr.. 1 \H plat< 



■ ■ ■ f I hi m 



t<e rejected. Th( iphic map 



heavens, many of wbl 1 re very 



■ ingress 



only to measure the plates face upwards, but to 

 reverse them and go over them again, so doubling 

 the work, but at the same time increasing its 

 accuracy. The entire work is expected to be com- 

 pleted in nine years, at the present rate of progress. 



With the 2S-inch equatoreal, 195 double stars 

 have been measured, each on an average on two 

 nights, and the satellite of Neptune on four nights. 

 By reversing the crown lens of the objective, it 

 has been proved to be an efficient photographic in- 

 strument, excellent photographs of the moon and 

 close double stars having been obtained. 



The glass ball used hitherto as a sunshine re- 

 corder has been found to give results latterly much 

 shorter for Greenwich than the instrument in 

 Bunhill Row. Since January 1st another glass 

 ball has been mounted close to it, and the records 

 of the two show differences equal to nine hours for 

 January, and as much as 21-7 hours for March. 

 The reason of the falling-off of the older instrument 

 is supposed to be due to some change in the glass. 



The total number of chronometers and deck 

 watches received at Greenwich for rating was 

 x,22o, those issued, 1,124, an d 519 were sent for 

 repair. Ninety-seven chronometers were entered for 

 the trial for twenty-nine weeks in temperatures 

 varying from 42 to 107°, and the Admiralty 

 purchased fifty-four of them for the Navy. 



On five days the Greenwich time-ball was not 

 raised owing to the wind, and that at Deal on ten 

 days for the same reason. On thirty- one days the 

 automatic signals from the clock at Westminster 

 failed. On one occasion the error of this clock 

 amounted to four seconds, but on fifty-fivepercent.of 

 the days of observation it did not exceed one second. 



The mean declination of the magnet for the 

 year was 16° 56'5' west. No great magnetic 

 disturbances were recorded during the year. 



The mean temperature of the year 1S96 was 

 50'i°, being 07° above the average. The rainfall 

 for the year ending April 30th, 1S97, was 20 '83 

 inches, 229 inches above the fifty years' average. 

 During the same period the mean daily horizontal 

 movement of the air was 2S9 miles, which is eight 

 miles above the average of the preceding twenty-nine 

 years. 



F. C. Dennett. 



THE PHOTOSCOPE. 



AMATEUR photographers, especially ladies, 

 who do not care to be burthened with heavy 

 camera and attendant tripods, have offered to 

 them this season valuable prizes for an interesting 

 competition. It has been organized by Messrs. 

 Ross and Co., of rn, New Bond Street, London, 

 for the two best series of twelve negatives taken by 

 their new photoscope, or field-glasses and camera 

 combined. The conditions of the competition are 

 simple, and open to all owners of the Ross instru- 

 ment, competitors having to state from whom it 

 was purchased. The twelve negatives must be 

 accompanied by prints made by any printing 

 . and mounted. The winning series to 

 become the property of the prize-givers. The 

 first prize is valued at £ 10, being a best quality 

 suitable (or either out-door observa- 

 tion or theatre, fitted with photoscope at i.i'hninit ; 

 the second prize 1 afini pair oi binocular gig es 



for field work or theatre. The negative! I 



prints must be 1 Ross and Co. 



[1 Si ptember next. 

 Ross 1 'hoto h 1 1 ■• I ' elaboration of the 



