232 



REPORT 1866. 



displacement of o being in the line o o', the libration of the centre of the appa- 

 rent disk /T will be W in longitude and N in latitude. It is easy to see that 

 the path of the point of intersection of the equator and first meridian, a short 

 time before and after the epoch of mean libration, wiU be in a very narrow 

 ellipse, the line o' o" being the major axis, which does not, however, retain its 

 position on the apparent disk, but revolves around the central point. 



25. This eUipse opens out and undergoes changes of form proportional to the 

 interval elapsing from the epoch of mean libration until the epoch when the 

 greatest excursion of libration in longitude towards the east (of the point of 

 intersection of the equator and the first meridian) coincides with the passage 

 of the ascending node when the equator is represented as a straight line 

 across the apparent disk and the first meridian by the curve cEm in fig. 10^ 



Fig. 10. 



where the libration of the centre of the apparent disk is nothing in latitude, 

 but west in longitude. When the first meridian returns to its normal posi- 

 tion, the eqiiator is represented by the curve E" N q (fig. 10), and the point of 

 intersection is situated at o" (nearly) ; the libration of the centre in this case 

 is nothing in longitude but south in latitude. 



26. At this epoch, intermediate between two of mean libration, the path 

 of the point of intersection of the equator and first meridian may be re- 

 presented by the four diagonals, of which o' o" (fig. 10) is one, or, perhaps more 

 correctly, by a wavy ellipse ; for as the values of the two librations diiFer in 

 amount, the circle W E N S is not a true representation of the excursions of 

 the intersecting point E and W, N and S ; so when the greatest deviation 



