66 



NA TURE 



[May 28, 1874 



7 HE COMING TRANSIT OF VENUS * 

 VI. 



HAVING now discussed all the methods to be em- 

 ployed, and the chief difficulties to he encountered, 

 it is time to examine what has actually been done. What 

 method or methods ought to be chosen ? What stations 

 are most suitable, taking into account the chances of 

 good or bad weather and good or bad anchorage ? What 

 preparations have been made by the various Governments 

 and by private individuals ? And are the arrangements 

 satisfactory ? 



As to the choice of method, the observation of contacts 

 was the only kind originally contemplated. The employ- 

 ment of photography and heliomclcrs is a comparatively 

 new idea, and will be spoken of later. The observation 

 of contacts is applicable to three nn-thods, for each one 

 of which different stations must be chosen ; these aie 

 Halley's method, the method of durations, and De I'lsle's 

 method. We will consider these in order. 



1. Halley's method fails totally in the transit of 1874, 

 but Jiiay perhaps be applied in 1SS2, though not under 

 good conditions. On referring to I-"ig. 13 in Article 111., 

 it will be noticed that Sabrina Land is a station where in 

 1882 the transit will commence just before sunset, and 

 end just before sunrise. Hence during the transit this 

 station and another placed in America will be moving 

 in opposite directions, thus fulfilling the conditions re- 

 quired by Halley in his communications to the Royal 

 Society. By referring to Fig. 12 it will be seen that no 

 such stations exist in 1S74. 



2. The method of durations may be successfully 

 applied, so far as mere geometrical position is con- 

 cerned, in either of the two transits. This method is 

 really combined of two parts, and includes Halley's as 

 a particular case. The lessening of the duration of the 

 transit depends partly upon the diminished motion of one 

 of the stations, or upon the fact that it moves in the 

 opposite direction to the other ; and partly on the fact 

 that in one case the planet seems to trace a path on the 

 sun farther from his centre (and therefore shorter) than in 

 the other. The difference in this last case is greatest 

 when tire path of Venus is far from the sun's centre. But 

 in transits like the coming ones, where this is the case, 

 the motion of Venus towards the sun's centre at the time 

 of contact is very much slo xer than when she describes a 

 larcre chord upon the sun. This has been well pointed 

 out "oy Mr. .Stone, f and from his paper we learn that the 

 method of durations depending upon two such observations 

 at each of the two stations will not be so satisfactory as we 

 might otherwise have expected. But other very serious 

 objections present themselves to a method like this re- 

 quiring four observations of contact, when we carefully 

 consider the circumstances. In applying this method, 

 one station must be chosen in high southern latitudes. 

 Now diligent inquiries have been made upon this subject, 

 and it appears veiy improbable that the weather at any 

 suitable station will be such as to give much hope of 

 observing both the ingress and egress in a satisfactory 

 manner. Hence if we depended upon this method there 

 would be a great probability of the expedition proving a 

 failure. The method of De I'lsle requires the observation 

 of only one contact at each of the two stations. For 

 these reasons hardly any expedition will use this method 

 except as secondary to De I'lsle's, the photographic, or 

 the heliometric method. 



3. De I'lsle's method. The accuracy with which this 

 method can be applied depends upon the certainty of 

 longitude operations. From what was said in the last 

 article, it will be seen that this is no easy matter; but it is 



* Continued from p. 52- 



t Monthly Notices of the R. A. S., vol. 



. p. 250- 



absolutely necessary that it must be done if this method 

 is to be employed. Sir George Airy says that longitudes 

 can be determined with an error of not more than one 

 second by lunar observations ; and observers will receive 

 orders to remain at their stations until they have a suffi- 

 cient number of observations to accomplish this. The 

 lunar observations will be supported, where practicable, 

 by telegraphic determinations of longitude, and also by 

 the transport of chronometers. The Russians, whose 

 stations lie mainly along the whole length of Siberia, will 

 employ a telegraphic line over that region, with branch 

 lines to the subsidiary stations. The English will pro- 

 bably fix the longitude of Alexandria by submarine cable. 

 They will employ chronometers to group together all 

 stations neighbouring each other. The station at Rodri- 

 guez will be thus connected with Lord Lindsay's station 

 at Mauritius, and with the French station at Reunion. 

 Lieut. Corbet, R.N., will connect by chronometers the 

 various islands occupied by the Germans, Americans, and 

 French in the neighbourhood of the two English stations 

 on Kerguelen's Island. The three English stations on the 

 Sandwich Islands will likewise be connected by chrono- 

 meters ; and it would be very desirable to connect these 

 islands with San Francisco on the one hand, and Yoko- 

 hama on the other. The longitudes of both these places 

 will have been compared with Greenwich by telegraph. 

 It would be a matter of the utmost interest to complete 

 the chain round the world by the transport of chrono- 

 meters across the Pacific. M. Struve says that with the 

 aid of an uncompensated chronometer this might be done 

 with great accuracy. The Germans have also made 

 valuable suggestions for comparing the longitudes of 

 the observing stations of all nations ; and the French will 

 also probably help in this matter. Thus it is likely that 

 the longitudes of all the stations of different countries 

 suitable for the application of De I'lsle's method will be 

 very accurately known. 



It will be noticed that the accuracy of De I'lsle's 

 method depends upon two longitudes and two observa- 

 tions of contact ; while that of durations depends upon 

 four observations of contact. Neglecting all considera- 

 tions of climate the two methods are, so nearly as the 

 somewhat vague data at our command can tell us, very 

 nearly equal. But the uncertain climate of soutlicrn seas 

 renders the chance of many contact observations doubtful 

 and throws the balance in favour of De I'lsle's method. 

 Add to this that before long all the stations except the 

 Kerguelen group will soon have their longitudes deter- 

 mined absolutely by telegraph, and recollecting that the 

 coming observations are to servo astronomers until the 

 next transit of Venus in 2004, by which time even the 

 Kerguelen group may perhaps be chronometrically deter- 

 mined : recollecting all this, there is little doubt that 

 astronomers have been wise in settling upon De I'lsle's 

 method for the main observations of contacts. 



It will be well, before going further, to mention the 

 stations which have been chosen by different nations for 

 the observation of the coming transit. 



I. — The British, having selected for the reasons above 

 mentioned the method of De I'lsle, originally fixed upon 

 the following stations : — 



Alexandria, Sandwich Islands, Rodriguez, Kerguelen's 

 Island, and New Zealand. No alteration has been made 

 in the choice of these stations. -Supplementary ones 

 have, however, been added. Thus at Kerguelen's Island 

 there vill be two expeditions : one at Christmas Harbour 

 in the north, and the other in the south of the island. In 

 the Sandwich Islands there will be three stations : one at 

 Honolulu, a second on the island of Hawaii, and a third 

 on the island of Kauai, sometimes called by English 

 writers .Xtooi. The station at Alexandria will be supple- 

 mented by a second one at Cairo, and a private one by 

 Col. Campbell, of BIythswood, under the Astronomer 

 Royal's direction at Thebes. The New Zealand station 



