74 PLEASANT WA YS IN SCIENCE. 



observing the transit of December 9, 1874, have been an- 

 nounced by the Astronomer Royal. It should be premised 

 that they are not the results deducible from the entire series 

 of British observations, for many of them can only be used 

 effectively in combination with observations made by other 

 nations. For instance, the British observations of the dura- 

 tion of the transit as seen from Southern stations are only 

 useful when compared with observations of the duration of 

 the transit as seen from Northern stations, and no British 

 observations of this kind were taken at Northern stations, or 

 could be taken at any of the British Northern stations 

 except one, where chief reliance was placed on photographic 

 methods. The only British results as yet " worked up " are 

 those which are of themselves sufficient, theoretically, to indi- 

 cate the sun's distance, viz., those which indicated the epochs 

 of the commencement of transit as seen from Northern and 

 Southern stations, and those which indicated the epochs of 

 the end of transit as seen from such stations. The Northern 

 and Southern epochs of commencement compared togethei 

 suffice of tJtemselves to indicate the sun's distance ; so also 

 do the epochs of the end of transit suffice of themselves foi 

 that purpose. Such observations belong to the Delisleaa 

 method, which was the subject of so much controversy 

 for two or three years before the transit took place. 

 Originally it had been supposed that only observations by 

 that method were available, and the British plans were 

 formed upon that assumption. When it was shown that this 

 assumption was altogether erroneous, there was scarcely time 

 to modify the British plans so that of themselves they 

 might provide for the other or Halleyan method. But the 

 Southern stations which were suitable for that method were 

 strengthened ; and as other nations, especially America and 

 Russia, occupied large numbers of Northern stations, the 

 Halleyan method was, in point of fact, effectually provided 

 for a fortunate circumstance, as will presently be seen. 



The British operations, then, thus far dealt with, were 

 based on Delisle's method ; and as they were carried out 



