NA TURE 



177 



THURSDAY, JANUARY 4, 1872 



BRITISH PREPARATIONS FOR THE AP- 

 PROACHING TRANSIT OF VENUS 



IN nearly all those countries of Europe in which Astro, 

 nomy is nationally cultivated, preparations are being 

 made for thorough observation of the first of the coming 

 Transits of Venus, which will occur on December S, 1S74. 

 In Russia, whose territory presents many favourable 

 points for observation of the phenomenon, a committee, 

 organised by Professor Struve, has had under considera- 

 tion during the past two years the establishment of a 

 chain of observers at positions 100 miles apart along the 

 region comprised between Kamschatka and the Black 

 Sea. The principal astronomers of Germany have held 

 two conferences, each of several days' duration, which 

 have resulted in a decision to furnish four stations for 

 heliometric observation of the planet during its transit : 

 one of these will be in Japan or China, and the others 

 probably at Mauritius, Kerguelen's and Auckland Islands ; 

 and three of these, with the addition of a fourth station 

 in Persia, between Mascate and Teheran, will be equipped 

 for photographic observations also. A French commis- 

 sion on the subject sat before the war, and reported to the 

 Bureau des Lonf,itudes that it was desirable for their 

 government to provide for observing stations at Saint 

 Paul's Islands, and Amsterdam, Yokohama, Tahiti, Nou- 

 mea, Mascate, and Suez. Since the close of the war the 

 subject has been revetted to, and lately the Academy of 

 Sciences applied to the Government for the requisite 

 funds ; but these could not be granted till next year, the 

 budget for 1S72 having been disposed of. 



The British preparations, to which we shall chiefly 

 conline our remarks, are, we believe, in a more advanced 

 state than those of any other country. This forwardness 

 may probably be ascribed to the circumstance that they 

 have from the first been directed by a single mind, and 

 have thus been freed from the inevitable delays of a 

 committee. The Astronomer Royal first called attention 

 to the Transits in 1S57 and again in 1S64. In 186S he 

 commenced to shape definiteplans, selected the observing 

 stations which were in all respects most suitable for 

 British occupation, and opened communications with the 

 Government upon the financial requirements of the under- 

 taking. 



Presuming a general acquaintance with the pheno- 

 menon under notice, and its availability for determina- 

 tion of the paralla.x of Venus, and that of the Sun (a 

 subject that has been well popularised), we merely remark 

 that there are several methods by which observers at 

 opposite points on the earth may measure the parallactic 

 displacement of Venus upon the Sun's disc : (i) by dura- 

 tions of Transit (Halley's method) ; (2) by absolute local 

 times of ingress and egress (Delisle's method) ; (3) by 

 heliometric measures of the planet referred to the limbs of 

 the sun ; (4) by similar measures obtained from photo- 

 graphs of the sun with the planet on his disc. The first 

 of these has been considered disadvantageous for the 1S74 

 Transit, which is the one that immediately concerns us. 

 The third and fourth are of recent suggestion, and have 



VOL. V. 



been regarded as of doubtful accuracy, especially the fourth, 

 whose reliability is still the subject of experimental inquiry. 

 The second was the one which demanded foremost atten- 

 tion. The Astronomer Royal, therefore, as a first step, 

 set down the stations best available for its application. 

 These had to be selected in order to combine a sufficient 

 altitude of the sun witli the maximum attainable accele- 

 ration of ingress and retardation of egress on one side of 

 the earth, and retardation of ingress and acceleration of 

 egress on the other side of the earth. And after weeding 

 the lists for each pliasc of such stations as were ex- 

 pected to be provided for by foreign governments, and of 

 those already occupied by established colonial observa- 

 tories, it was found that there were five stations which it 

 was desirable that England should prepare to equip. 

 These were Woahoo (for observation of accelerated in- 

 gress), Kerguelen's and Rodriguez Islands (for the retarded 

 ingress), Auckland in New Zealand (for the accelerated 

 egress), and Alexandria (for the retarded egress). 



Now, as at all these places the absolute local time of 

 the phenomenon is required, it is indispensable that the 

 longitude of each be very exactly known. In no one 

 case does a sufficiently accurate determination of this 

 element exist, and provision must therefore be made in 

 each case for obtaining it. This vastly increases the ex- 

 tent of preparations for the instrumental equipment of 

 the stations, and renders necessary a three or four months' 

 sojourn of the observers at each. Of the methods for 

 determining longitude which were open to choice, the 

 Astronomer Royal decided to employ that by vertical 

 transits of the moon, and for observing these he resolved 

 upon supplying altitude instruments with fourteen-iach 

 circles and telescopes of twenty inches focus. For time 

 determinations he proposed three-inch transits, of thirty- 

 six inches focus, with clocks of moderately high class 

 For observing the phenomenon he elected to employ at 

 each station one six-inch equatorial and one four- inch 

 portable telescope. For these an observatory of three 

 rooms was required. With the exception of one altazi- 

 muth, two clocks, and two or three four-inch telescopes, 

 which the Greenwich Observatory could furnish, all the 

 specified instruments and the observing rooms had to be 

 specially provided. An estimate for their purchase and 

 construction, amounting to 2, 154/,, was therefore submitted 

 to the Admiralty. Some needful chronometers and 

 meteorological instruments were available from home 

 stores. To the above estimate for material requirements 

 were added others, prepared by Admiral Richards, for the 

 personal expenses, the conveyance, residence, pay, and 

 contingencies, of the observing parties. These amounted, 

 for the Woahoo detachment, to 2,500/., for the Rodri- 

 guez and Kerguelen's parties to 2,000/. each, for the Auck- 

 land parly to 1,000/, and for Alexandria to 750/, making 

 a total of 8,250/ The grand total of 10,500/ was asked 

 of the Treasury in May 1869, and immediately granted. 



The construction of the requisite instruments and clocks 

 was forthwith commenced, by Messrs. Troughton and 

 Simms and Messrs. Dent. Three six-inch equatorials hap- 

 pening, however, at the time to come into the market, they 

 were at once purchased ; one of the three being that which 

 is known to fame as the " Lee Equatorial," and is the 

 instrument used by Admiral Smyth in the preparation of 

 his " Celestial Cycle." The observatories were put in 



