30 LIGHT SCIENCE FOR LEISURE HOURS. 



the announcement of my result, the news arrived that 

 the French astronomer, Puiseux, had obtained almost 



siderably increased at a suitable northern station, notwithstanding the 

 undoubted partial cancelling which takes place from the cause indicated. 

 Not to favour one side or the other, we go direct to the Nautical Alma- 

 nack for 1874. We take Nertchinsk, the place pointed to by Mr. Proc- 

 tor so far back as March 1869 ; and we note that he then assigned to 

 this station a lengthening of the duration of transit by 15^ minutes (a 

 very considerable amount, much more in fact than at the most favourable 

 station in 1882). Now, what says the Nautical Almanack for 1874 ? At 

 page 434, it states that the mean duration of transit is 3 hours 42 min. 2 

 sec. At page 20 of the appendix, it states that at Nertchinsk the duration 

 is 3 hours 57 min. 6 sec., exceeding the former duration by 15 min. 

 4 sec. This is very close indeed to Mr. Proctor's result, and shows 

 how nearly the values obtained by his graphic constructions accord 

 with those deduced by rigid calculation. (Moreover, a part even of the 

 slight difference is due to a difference in the adopted value of Venus's 

 diameter.) Here, then, instead of that complete cancelling of the value 

 of the northern station which Sir G. Airy too hastily assumed, we have 

 a lengthening of the transit period by more than 15 minutes, which in 

 this problem is an unusually large amount. To show that this is so, 

 and how slightly the northern station is affected by the peculiarity which 

 Sir G. Airy had hastily regarded as introducing a fatal objection, we 

 have only to remark that at Possession Island, the most favourable 

 southern station (where the two conditions conspire, instead of opposing 

 each other^, the shortening of the transit amounts only to 17^ minutes. 

 Combining this shortening with the lengthening at Nertchinsk, we have 

 a difference of duration of fully 32^ minutes. And now observe how 

 greatly this result differs from Sir G. Airy's anticipation ! He thought 

 the difference of duration would probably " not be half of that in 1882 " ; 

 but his own estimate of the greatest difference of duration in 1882 (to 

 be obtained only by seeking an inaccessible station, where the sun will 

 be but four degrees high at egress) amounts only to 28 minutes. In- 

 stead of being less than half, the difference of duration in 1874 is 

 greater in the proportion of about 7 to 6. Add to this that in 1874 the 

 solar elevation, both at ingress and egress will exceed twenty degrees, 

 and the importance of having a station at Possession Island becomes 

 manifest. Eussia has occupied Nertchinsk, and it is Great Britain's 

 duty (and that of no other country) to occupy Possession Island. If 

 she shrinks from this duty, it will be no answer to the reproach which 

 she will hereafter incur, that she occupied stations in other respects ad- 

 vantageous. Other countries are occupying these stations, the Papelotte 



