i:i6 



XATURE 



[June 6, 189 = 



enable us to determine the fraction of the sun's light which it ] 

 receives, and correctinj; fur theallwilo, it is easy to calculate the 

 brightness of the sun in terms of ihal of the planet, the exact 

 stellar magnitude of which can be found by direct measurement. 

 Thus. Mr. C'lore finds that the apparent diameter of Mars in 

 opposition, as seen from the sun, is 6"" 17, so that the area of the 

 disc is 29-9 square seconds. Divitling the number of square 

 seconds in a hemisphere by the latter, it is found that il the 

 surface of Mars were a perfect reflector, the sun as seen from 

 Mars would be 8.940,450,000 times brighter than Mars apjwars 

 to us when in opjwsition. 



According to Zijllner. the reflecting ixjwer of Mars is only 

 0'2672, so that the pre\ious number must be raised to 

 33,459,768,000. This, however, is for mean distance i -5237. so 

 that when reduced to the earth's distance (by multiplying l)y the 

 square of 0-5237 ). we gel the light of the sun .is seen fnmi the 

 earth to be 9.174,668,385 times the light of Mars when in 

 opposition ; this numlnrr. on the Irasis of a light ratio of 2512 

 corresponding to a difference of I m.agnitude, represents 24-9 

 magnitudes. l'r<if. Pickering's photometric measurements show 

 that the stellar magnitude of Mars a^ mean opposition is 2-25, 

 so that the deduced stellar magnitude of the sun is -27 '1 5. 

 Similar calculations from the data relating to Jupiter give .a value 

 of -27-17, and from Saturn -27-11. Though .ngreeing so 

 remarkably among themselves, these new values differ very con- 

 sidenibly from the value hitherto adopted, namely - 25-5. The 

 new value, however, receives confirmation in the fact that il is 

 very nearly equal to ihe magnitude which o Centauri would 

 assume if it were brought to the sun's distance from the earth, 

 .assuming the jiarallax to be o"-76, the specinmi of this star 

 resembling the spectrum of the sun. 



THE GREENWICH OBSERVATORY. 



TlIK Report of the .Vstronomer Koy.al to the Hoard of 

 Visitors of the Royal Observatory, (ireenwich, was read at 

 the annual visitation on Saturday. .\ few of the developments 

 made during the year covered by ihe report, and some observa- 

 tions of interest, are referred to in the subjoined extracts. 



Provision has l)een made in the Navy Kstimates for the erec- 

 tion in ("ireenwich Park of a magnetic [Mvilion for absolute 

 ileterminations of the magnetic elements, and the i)lans are now 

 lieing preiared in the Director of Works' Department. It is 

 projiosed to establish this station in the immediate neighbour- 

 hcMxl of the Observatory, ani.l at such a distance that there 

 would lie no suspicion of di-slurlance from On- ir.in in the 

 buildings. 



Work with Eiju.vroRiAi-. 



The flint and crown discs for the new photographic telescope 

 of 26 inches a|ierture, the gilt of Sir Menry 'Thompson, have 

 l)cen received at the Obscnatory. The details of llie design for 

 the mounting have been carefully worked out, and good progress 

 has iK-cn made with the mechanical work. 



The 28-inch refractor has Ijeen in use throughout the year, and 

 is i|uitesatisfacloty. 1 1 moves easily in R..\. and Declination, 

 the new slow motion screws work successfully, the water clock 

 in general drives it with great precision, and the performance of 

 ihe object glass under good atmospheric comlitions is admirable. 

 Various improvements in the accessories of the inslrumeni have 

 lieen carried out in the ])ast year. \ sj)eclro.scoiic si>ecially 

 arlapted to photography, for use with this refraclur, is lieing 

 lua^le. 



Micrometer measures r>f sixlylhree double stars have been 

 in.idi- ; in 27 of these the distance of the com|xments w.as under 

 I . Old in 13 il was o"-5 or under. The most remarkable of 

 lir..- inexsures are those of x Pegasi (18989). The components 

 ..f ihissiar, ihough only o"-l4 aixirl, were distinctly separated 

 with a |Kiwer '»f 1030. 



■: ..f Ihejiositions of salelliles nf Mars near elongation 



ri tw. nights. Several attempis were also made In 



,1. , , iilil) satellite, bul the results obtained were 



■f measures of ihe )K)lar and equatorial 



• A his satellites wius ma<le. .Measures of 



.ii.'.ol .s,iiurn and his rings and ihe ))osititms of the 



vc alwi Itcen made, and are being continued. 



W li.d 595 plates, with a total of 



1450 Of these. 162 have been re- 



jectol '. .is: partially foggeil pl.ates ; 



because (he reticules were not clearly printed ; Iwcnuselhc images 



NO 1336, VOL. 52] 



were too faint to show gth magnitude stars with a twenty-seconds- 

 exposure : for faidts in tlevelopmeiU ; for mistakes of setting : and 

 for miscellaneous defects. It is hoped that a much smaller 

 number of pl.ites will need to be rejected in future for these causes. 

 The total number of celestial fields photographed since the 

 commencement of work for the chart is 422, and the total 

 number of fields photographed for the catalogue is 617. Only 

 half as many fields for the chart and catalogue have been 

 photographed this year as during last year. This is due parllyto 

 the unfavourable weather, and [jartly to the telesco]>c being out 

 of use for two months while the shutter of the dome was being 

 repaired. 



SpECTROSCOI'K AM) 1 lliLIOCKAl'llK: OUSERVA tlONS. 



Since 1S94 December 19, when the spectroscope was brought 

 into adjustment, 98 measures have been made of the displace- 

 ment of the I-" line in the spectra of 13 stars, and 16 of the >' line 

 in the spectra of four stars. Some exix^riments have also been 

 made in photograi^hing stellar sjK'ctra. ttt give data as to the 

 work to be done with ihe new photographic s|iecttoscope. 



Photographs of the sun were taken with the Dallmeyer photo- 

 heliograph on 199 days, and of these 375 have been selecteil for 

 preservation, liesides iS photographs with double images of the 

 sun for determination of zero of position-angle. 



The 9-inch photographic telescope i)resented by Sir Henry 

 Thompson, which has been mounted on the I.assell equatorial, 

 was also in regular use as a phoioheliograpli up lo Ocl>)bcr 15, 

 when the progress of the building operations jirevented its 

 further use. Photographs of the sun had been obtained with it 

 by that time on 80 days, of which 121 have been selected for 

 preservation. In all, with one pholoheliograph or the other, a 

 record of the state of the solar surface h.as been secured on 213 

 days during the year. 



The mean daily spotted area of the sun was only sligluly 

 smaller in 1894 ihan in 1893, the marked falling off in the spring 

 of 1894 noted in the hist rejiorl being followed by an increase 

 during the summer months. Ihe number of sun-spots was 

 greater than in 1893. rii<^ spring of this year has shown a 

 decline both in the number and area of spots. 



Macnetic Observations. 



The variations of magnetic declination, horizontal force 

 and vertical force, and of earth currenls liave been registered 

 lihotograiihically, and accompanying eye observations of 

 absolute declination, horizontal force, ami dip, have been 

 made as in former years. Increased magnetic acliviiy was 

 shown in the year 1894, and great disturbances occurreil on 

 July 20 and .\ugusl 20: the spot of light of the vertical force 

 magnet, on the former dale, and the spots of light of the hori- 

 zontal force and vertical force magnets, on the latter, having 

 passed beyond the range of the registering shecls for some hours. 

 In luly and August ihe dislurbances in the earlh-current 

 registers caused by the South London Kleclric Railway showed 

 a gre.al increase, which is presumably due to the experiments 

 then being maile in Ihe use of motors <m the carriages of the 

 railway instead of sejiarale locomotives. 



The following are Ihe principal results for the niagneiic 

 elements for 1S94 : — 



Mean declination . . 



Mean horizontal force 



.... i7''4'-6 Wesl. 

 f 3-9661 (ill lirilish units). 

 1^ 1-8287 (ill iiielric unils). 

 1 67° 16' 5" (by 9-incli neeillesl. 



Mean ilip 67° 17' 8' (by 6-inch needlesl. 



/ 67° 18' 43' (iiy 3-inch neeillesl. 



In the year 1S94 there were ten days of great magnetic dis- 

 turbance and thirteen other days of lesser disturbance. Tracings 

 of Ihe photographic curves for all of these days are being made, 

 and will be publisheil in the annual volume according lo the 

 arrangements made with M. Mascarl. The calculation of diurnal 

 inecpialities from five typical quiet days in each month has lieen 

 continued. 



M 1; ii;nRoi.«>r.u-Ai. Observations. 



The regislralion of almospheric pressure, temper.aUire of ihe 

 air and of evaporati<in, pressure and velocity of ihe wind, 

 rainfall, sunshine, and atmospheric eleclricily has been con- 

 tinuously luainlaincd, excepi lhal during ihe winter ihe register 

 of almospheric eleclricily was inlerriipleil during the greater 

 pan of Kcbruary by freezing of the water in the exit piiie. 



