1869.] The Transit of Venus in 1874. 377 



accelerated by 9^ m., or in all transit shortened by 18^ m. If 

 we take Victoria Land, in south latitude 70^ (say)j and east longi- 

 tude 172^, we get ingress retarded by 6 m. and egress accelerated 

 by 11^ m., or in all transit shortened by 17^ m. If we take 

 Enderby Land, in east longitude 50^, we get ingress retarded by 

 llf m. and egress accelerated by 8^ m., or in all transit shortened 

 by 20^ m. These four southern stations, combined with the 

 northern station before considered, give a total difference of dura- 

 tion of 36|- m., 34^ m., 33^ m., and 36^ m. respectively. Also, as 

 it would not be well to trust to a single northern station, it may 

 be noticed that any part of the nearly circular region extending 

 from Lake Baikal to Saghalien, and from north latitude 40'^ to 

 north latitude 60^, might be used for observing the increased dura- 

 tion without important disadvantage as compared with the station 

 already considered. Also, Crozet Island, Kerguelen's Land, and 

 other parts of the Antarctic continent besides those considered, give 

 abbreviated transits of considerable value. Thus for Crozet Island 

 the abbreviation is no less than 17 m.; for Kerguelen's Land, 16 m. 

 Even Macquarie Island, Eoyal Company Island, Hobart Town, 

 and parts of New Zealand, might serve as useful subsidiary stations. 



And now to compare the value of the transit of 1874 with that 

 of 1882. We see that by the method of durations we get a dif- 

 ference of more than 36 m., whereas the maximum difference is 

 50i m. The Astronomer Eoyal has shown that for the transit of 

 1882 it is possible to take positions for observation (not by any 

 means more favourable than those above considered) which give at 

 the outside a difference of duration bearing to the maximum the 

 proportion of 341 to 400. The maximum difference in the case of 

 the transit of 1882 is only 32 m. 48 s., in place of 50 m. 12 s. as 

 in 1874. Eeducmg 32 m. 48 s. in. the proportion of 341 to 400, 

 we obtain the period 27 m. 57 s. in place of the difference of 

 36^ m. which the most favourable situations in 1874 will give. 



If we assume that the value of a transit is not to be estimated 

 according to the magnitude of the observable difference, because the 

 rate with which the planet crosses the sun's hmb is diminished in 

 exactly the same proportion, and the error of observation corre- 

 spondingly increased, we have the relative values of the transits of 

 1874 and 1882 as 



m to 2^ 

 50^ 32? 



or almost exactly as 6 to 7. But this extreme result, although as 

 it stands it is altogether opposed to the theory of the utter value- 

 lessness of the transit of 1874, is obtained on an assumption which 

 is unsupported by evidence. Mr. Stone has shown that the forma- 

 tion and breaking of the black Hgament connecting Venus with the 



